2017 Pricing

I am a fully licensed, bonded and insured California C10 Electrical Contractor.  I have been a San Francisco electrician since 1987.    I do most all work with my own hands.

"...A professional handles money and accounts very carefully..."

Service calls typically come to resolution or clear estimate in the first hour. Hourly rates are as follows:
  • Residential Service :$250 visit & 15 minute increments after
  • Commercial Service :$300 Call & 15 minute increments after
  • After 8p or emergency: Time *1.5
  • Minimum call: $100 minimum "no tools used"
  • Menu of typical prices 

    Allow some margin in appt times. I normally send an "in transit" text en route


San Francisco permit expediter [historically significant]

Bringing historically significant projects into compliance 
with the San Francisco Department of building inspection



Q2. Under what condition can a two-wire receptacle be replaced with a three-wire receptacle, when no ground is available in the box?
A. Where no equipment bonding means exists in the outlet box, nongrounding-type receptacles can be replaced with [406.3(D)(3)]:

  • Another nongrounding-type receptacle.
  • A GFCI grounding-type receptacle marked "No Equipment Ground."
  • A grounding-type receptacle, if GFCI protected and marked "GFCI Protected" and "No Equipment Ground."
Basically, if you only have two wires in the box you will need to replace the outlet with a GFI

Note: GFCI protection functions properly on a 2-wire circuit without an equipment grounding (bonding) conductor, because the equipment grounding (bonding) conductor serves no role in the operation of the GFCI-protection device.

CAUTION: The permission to replace nongrounding-type receptacles with GFCI-protected grounding-type receptacles doesn't apply to new receptacle outlets that extend from an existing ungrounded outlet box. Once you add a receptacle outlet (branch-circuit extension), the receptacle must be of the grounding (bonding) type and it must have its grounding terminal grounded (bonded) to an effective ground-fault current path in accordance with 250.130(C).

If you need a grounding wire for some other reason, say proper operation of electronics, sound system, some control board or other sensitive electronic equipment, you will have to run new compliant wire back to the grounded part of the premise wiring.  Before 1994 you could just grab the cold water pipe at any place in the system,

Under the 1990 NEC {250-50(b)Exception} and in earlier code cycles a bonding jumper was permitted to be run from the grounding terminal of the receptacle to any water pipe that was bonded in accordance with 250-80(a).
250-80(a) Metal Water Piping. The interior metal water piping system shall be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor where of sufficient size, or to the one or more grounding electrodes used.
But no longer
You cannot just hook to a water line anywhere. I have seen that done and there was plastic pipe coming in. No good.
See also :


Gradual Stiffening

...the actual building process is a creative act. It allows the building to be built up gradually. Members can be moved around before they are firmly in place. All those detailed design decisions which can never be worked out in advance on paper, can be made during the building process. And it allows you to see the space in three dimensions as a whole, each step of the way, as more material is added…

The essence of this process is very fundamental indeed. We may understand it best by comparing the work of a fifty-year-old carpenter with the work of a novice. The experienced carpenter keeps going. He doesn’t have to keep stopping, because every action he performs, is calculated in such a way that some later action can put it right to the extent that it is imperfect now. What is critical here, is the sequence of events. The carpenter never takes a step which he cannot correct later; so he can keep working, confidently, steadily.

The novice by comparison, spends a great deal of his time trying to figure out what to do. He does this essentially because he knows that an action he takes now may cause unretractable problems a little further down the line; and if he is not careful, he will find himself with a joint that requires the shortening of some crucial member – at a stage when it is too late to shorten that member. The fear of these kinds of mistakes forces him to spend hours trying to figure ahead: and it forces him to work as far as possible to exact drawings because they will guarantee that he avoids these kinds of mistakes.

The difference between the novice and the master is simply that the novice has not learnt, yet, how to do things in such a way that he can afford to make small mistakes. The master knows that the sequence of his actions will always allow him to cover his mistakes a little further down the line. It is this simple but essential knowledge which gives the work of a master carpenter its wonderful, smooth, relaxed, and almost unconcerned simplicity.

A Pattern Language
Book by Christopher Alexander, Murray Silverstein, and Sara Ishikawa

Chris was the sponsor for my Individual Major in Building Process UC Berkeley '92


Don't try this at home...

Maintain Electrical SafetyThe insulation on electrical wires can become damaged by wear, flexing, or age. Some clues that you may have an electrical problem are:

  • Flickering lights. If the lights dim every time you turn on an appliance that circuit is overloaded or has a loose connection.
  • Sparks. If sparks appear when you insert or remove a plug, that could be a sign of loose connections.
  • Frequent blown fuses or broken circuits. A fuse or circuit breaker that keeps tripping is an important warning sign of problems.
  • Frequent bulb burnout. A light bulb that burns out frequently is a sign that the bulb is too high a wattage for the fixture.
  • Avoid the use of extension cords.


Aging happens but fires don't have to...


It's more likely that an electrical fire will occur in a place you can't easily see, although overloaded cords in contact w combustibles can also present a hazard in plain sight.
Waste heat generated by electrical current can cause wiring hidden within a home's walls to expand and contract, eventually loosening it. Once that wiring is loose, the electricity can arc, with a heat output reaching 1,500 to 1,800 degrees Fahrenheit.  

The temperature of an electrical arc can be hotter than the surface of the sun.

That's more than enough to ignite wood or old insulation under normal circumstances, but in San Francisco,  Inside a house in dry months, the relative humidity within the walls can drop to that of the average desert, turning studs -- wooden wall supports -- into kindling, easily ignited by an arcing current.
Here we arrive at one of the problems with electrical fires: By the time you see smoke coming out of your outlet, a fire has most likely already begun and is spreading out of sight within your walls and up to your attic. It's easy for a homeowner who has turned off the power to a burning socket to think that they've taken care of the problem. But an unseen fire may already be building beyond the outlet.
Even worse, electrical fires can be particularly tricky to put out. Since they involve electricity, using water to put out the fire can cause electrocution. Chemical powders can cause the fire to smolder then reignite. 

TFCI thermal fault outlets protect "glowing connection".

BSafe® Technology – How It Works 
BSafe’s TFCI™ thermal cut-off technology outlet incorporates a multi-sensor thermal switch mechanism. It senses abnormal temperature at each plug outlet and wiring screw terminal on the device and shuts off electrical power to the load on the outlet when overheating occurs.

AFCI – effective in 2002 this type of circuit breaker was first required only in residential bedrooms. It's now required for most all circuits in homes. AFCI shuts down power in a fraction of a second when hazardous condition exists.

Top Causes of Arc Faults

  • Loose or improper connections, such as electrical wires to outlets or switches
  • Frayed appliance or extension cords
  • Pinched or pierced wire insulation, such as a wire inside a wall nipped by a nail or a chair leg sitting on an extension cord
  • Cracked wire insulation stemming from age, heat, corrosion, or bending stress
  • Overheated wires or cords
  • Damaged electrical appliances
  • Electrical wire insulation chewed by rodents

GFCI protects people from electric shock. AFCI protects against electrical fire. Both together provide very thorough  protection for the system. Adding TFCI (thermal fault as above) is the most complete protection available today. 


Electrical Outlets Safety

San Francisco / Marina
All 15- and 20-ampere receptacles in a dwelling unit shall be listed tamper-resistant.

TR receptacles have been mandated in hospital pediatric wards for over 20 years and have proven to effectively prevent electrical injuries.

"...Going against all the fire safety training I’d gone through growing up, I grabbed an industrial extension cord and power strip from the garage, and ran it through the dining room into the den. I finished setting up my laptop around 2 a.m. and went to bed.... ...After that, I got kind of lazy. I knew that I had to deal with the electrical problem, but I was dreading what it would cost to have an electrician come fix it. We lived with the extension cord running across the floor for about a week, hoping that no one would decide to drop by unannounced..."

Incandescent lamp => longevity


Trapeze track lighting [and Monorail] rev 2015

All connections on 12V cable and rail systems, including fixture connections, need to be re-tightened every year or so. They expand and contract with heat and also get loose when people change bulbs and wiggle a system... Loose connections cause heat build-up and ... you end up with melted and fried parts.

We have seen a lot of this over the years, and it comes down to a deferred maintenance/negligence problem.

"Trapeze" suspension lighting systems can have many troubles, even when installed correctly if they are not occasionally maintained. Basically, when you put in new lamps, you should check that nothing is loose.

Loose connections, xfmr "hum", short lamp longevity, expansion contraction, socket burnout are some findings even at correctly installed installations over time

Many troubling issues revolve around incompatible dimmers and loose mechanically bolted connections.

The price driven practice of installing the wrong dimmer creates the fairly common scenario: arcing failures with suspension systems. See the attached photo showing incandescent switch controlling magnetic low voltage trapeze system. Instead of a $140 switch the original installer often uses a $45 switch which works well for a few years until it smokes both the switch and the xfmr.

Remedies include:

  • tighten up connections
  • replace loose sockets / fixtures
  • ensure compatible xfmr / dimmer pairing 
  • replace magnetic xfmr w electronic or magnetic DC
  • swap halogen lamps for LED

Mismatched incandescent dimmer [cheap] to low voltage load - usually smokes the switch and the light - but it take s a couple of years.  The mismatch switch is $45 / the correct switch is $140.



Here are some photos of our work and
some photos of electrical interest!
Our service area is the 415.

Telephone: (415)877.1172

Posted by Hello

Device longevity

Some things get better with time.



How many things can you plug into an electrical outlet before it catches fire?

out of the 5,300 annual household electrical fires, about 2,000 of those occur over the holiday season. By getting a better idea of how household electricity works, you can avoid the danger and blown fuses caused by overloaded outlets.


LED drivers

LED's are supposed to last a very long time.
Require specialized tools and troubleshooting process.


nBeach? or Marina...



Remodeling ceiling fans

The best news:
Replacing an existing light fixture with a fan can be 
a simple, one-day task since the wiring is already in place. video

However—and this is very important—you often can't just hang the fan from the existing electrical box, unless the original install used a really strong junction - rate to support a ceiling fan or large chandelier

 A common plastic box is simply not strong enough to support the added weight and vibration of a fan. Presuming you don't have access to the ceiling fan from above, you must either use a specially rated hanger and box to mount the fan between joists or screw a fan-rated "pancake" box directly to a ceiling joist.

If you're installing a new electrical box, fasten it to a ceiling joist or a 2 x 4 wood brace between two ceiling joists. If there isn't a suitable joist or brace available, but you have access to the framing through the attic, add a brace that can support the fan.

The other option is shown in this photo from the SF Chronicle: installing an expandable ceiling fan hanger bar and electrical box. A hanger bar for "new work" fastens to the joists with screws. You need access to the joists — such as from the attic — to install it. You can install a hanger bar for "old work" from below the ceiling by fitting the bar through the rough opening. Extend the bar until it wedges securely between two joists.  The new electrical box must be grounded or attached to a ground wire in the ceiling.


Typical Prices

Most basic troubleshooting calls including repair and installation calls are resolved less than $500. We have developed this list to serve as a guideline and to provide "ball park" estimates for common items. These costs are for a typical small to medium house in California. Difficult access, commercial work or other factors could increase cost substantially. Please note: Recent construction cost increases may make some of these prices lower than you will find locally. Drywall repair and paint, as needed, done by others

Minimum service call 
  • Residential Service Call $250 per man/hr or menu 
  • Commercial Service Call $300 per man/hr or menu 
  • After hours, wknd or emergency/ urgent call at time and a half 
  • Minimum call (tools used) $100
  • SF DBI permit and onsite inspection meetings $800
  • Correct "all lights flicker" troubles starting at $750 typical [liaison w PGE troubleman add $200]
  • PGE Liason for upgrades: Job Card, Underground engineer, troubleman, estimator, surveyor meetings $1250
  • Bring Grounding System to compliance $1000 labor typical
  • Mobilize extra large ladder or staircase ladders $500 typical.  Scaffold/Lift is more
  • Well documented small Safety / Compliance Survey $600-$900 typical
  • New small Subpanel from $1800 plus AFCI breakers as needed
  • Upgrade small fusebox or FP panel to 100 amp panel from $2,000 plus SFDBI permits, PGE liaison, grounding
    • Double that to move FP panel out of closet and over to a nearby wall. [Drywall and paint by others]
  • Upgrade small Meter Main 200amp $5k typical plus permits, PGE liaison, grounding, AFCI cbs
  • Upgrade larger residential single family Meter Main to 400amp main $8k+ typical
    • plus sfdbi permits and pge liason
    • does not include trenching new underground
  • Replace existing Small lighting Fixture $125 typical bulk: 
    • undercab fluorescents and kitchen counter pendants @$300/ea plus parts
  • Install simple chandelier less than 25lbs / new Ceiling Fan / 8' length of track $500 typical
  • Replace existing single gang switch or outlet [bulk] $125 typical plus material cost 
    • for 2 gang and larger boxes - count each switch, regardless
    • [$20 commercial duty device material - dimmers or specialty switches more]
  • Replace existing 2prong or non compliant outlet with GFI outlet bulk]  $145 typical plus material [typ $45]
    • [+ $45 for exterior in-use cover; weather resistant device or combo device]
  • Plasma TV / Bidet install $1,800 typical plus cost of power circuit if needed [Drywall and paint by others]
  • Small Electric Vehicle or Water Heater $1500 less than 50' from panel of adequate capacity. 
    • adjust for panel condition, wire path and total distance
  • Bathroom remove and replace existing Exhaust Fan with adequate vent $1000 labor typical. [Drywall and paint by others]
  • Install new in-wall / baseboard heater at existing feed & thermostat.  Heater provided by others $450 typical
New Circuits
    • New, remodel, or extend switching  / lighting / power outlets: $1800 residential typical (distance & path)
      • $1150 first remodel light, $750 for each additional switch or light in series
      • $500  for new romex to nearby outlet typical  [interior residential] => back to back or open hatchway in wall [traverse structure (beams, corners) is more]
      • (hard pipe / metal cable as required) is more
      • Wall repair / painting to be done by others
    • New120-volt circuit starting from $1500 less than 100 feet
      • adjust for panel condition, wire path and total distance
    • New low amperage 240-volt circuit starting from $2000 less than 50 foot run
      • adjust for panel condition, wire path and total distance
    • Add $200 parts and labor for AFCI protection as needed.
    • Low voltage / control wiring / troubleshooting at hourly rate typical
  • Bathroom Remodel starting from from $1800 [size & finishes]
  • Kitchen Remodel starting from from $4500 [size & finishes]
Repairs [special ladders / materials / access]
  • Repair lighting ballasts / transformers bulk or Recessed cans 1 hr labor [$250 residential / $300 commercial] plus $85 material typical
  • T5 / dimmable fixtures 1.5hr + parts typical commercial duty BALLASTS & LAMPS Includes four year warranty parts and labor on materials we provide


What causes humming in audio systems?

Some articles claim that wiring and grounding problems account for up to 80 percent of all power quality related problems related with sensitive electronic equipments like audio/video systems.


Frequently Asked Questions


There are three possibilities. One brand of breaker shows a "red flag" when the breaker is tripped. Another brand trips all the way to the "off" position, you can't miss it. The majority of breakers trip to an "intermediate" position, and those can be tricky to locate. They trip to a position half way between "off" and "on". You'll have to look closely. Sometimes they barely move from the "on" position. When in doubt, and when it is safe to do so, reset all of your circuit breakers. You might find one that feels "soft" and you'll know you found it.

Click each circuit breaker
all the way
then all the way
That's two clicks per breaker


Circuit breakers have to be forced all the way to the "off" position first, before being turned to the "on" position, or they won't reset. First force the breaker all the way off, then all the way on.


Go to the affected area and determine what all doesn't work. Unplug every appliance or lamp plugged into a "dead" outlet. Switch off all lighting in the affected area. Disconnect everything you can find on the "dead" circuit. Then return to the circuit box and try again to reset the breaker. If it trips off again, I will need to visit your site to troubleshoot the problem. If the breaker now stays "on", return to affected area and start reconnecting everything you disconnected. Often times you'll find that you have a defective appliance that is causing the problem. If you plug in the toaster and the breaker trips, you need a new toaster, not an electrician.

Many outlets today are "GFI" protected, meaning there is a GFI outlet somewhere in the circuit that has tripped, & killed all power to the remaining outlets on the circuit.
Look for a GFI receptacle somewhere. They look different than a normal power outlet, they have a "Test " and "Reset" button on them. They will be found in bathrooms, basements, garages, outdoors, kitchens, or any area around a sink or water source.

Push the reset button on any GFI outlet that you find, and power will likely be restored. Press hard using a sharpie or a wooden spoon. This simple knowledge may save you a service call.


If you are using CFL light bulbs on a dimmer switch, you probably just ruined your dimmer and possibly the light bulb also. CFL's aren't compatible with most dimmer switches, and damage will occur if attempted. Either remove the CFL and re-install a regular bulb, or replace the dimmer with a regular switch. We can help you out with that.  Mismatched dimmers with loads are also very common.  Many times the correct dimmer for an application is expensive [ELV and MAG LOVO] and there is a much cheaper dimmer that will work fine for a little while.  These problems often show as smoke coming slowly over time out of the switch box and the wall above the switches becoming sooty.

A large part of my house is out of power, all the powers gone, resetting the main breaker won't help what should I do?  
First, If this is going on for a while... go ahead and run an extension cord to power up your fridge as necessary

Talk with your neighbors, see if anyone nearby has the same problem = perhaps this is a Pg&E issue. 
If you have a smart meter you can tell if it's a Pg&E Issue by looking at the digital display of the smart meter. Blank is definitely an indicator!
If you suspect utility problems call PGE 1800 PGE 5000

Gas pipe chandelier

High Level Faults:


Sparking outlets and wires esp knob and tube

EXCESSIVE THERMAL INSULATION: There are simple ways in which a fire can be created with an electric cord that is neither damaged nor subjected to a current in excess of its rated capacity—loop it up upon itself several times, or provide a high amount of external insulation, or both. Laboratory demonstrations have verified that ignition readily occurs; in one case, simply coiling the cord three times and covering with a cloth sufficed. A special form of this hazard occurs with the old knob-and-tube wiring, which was common in the US prior to World War II. This type of wiring uses two separate conductors which are not grouped into a cable, but are individually strung on widely-spaced porcelain knobs. The current-carrying capacity is dependent on there being unobstructed air cooling of the wires, and fires have occurred when the wires were buried in thermal insulation. 

STRAY CURRENTS AND GROUND FAULTS:Stray currents occur when circumstances cause current to flow through paths not intended to carry current. Ground faults are a well-known example. They can occur if a conductor is abraded or damaged and contacts metal siding, roofing, etc. Kinoshita et al. documented that only 5 A was required for ignition when a 3-conductor, PVC-insulated cable contacted a galvanized iron roof.  An unusual mode of ignition from a ground fault is where current flows through a gas line. The current causes overheating of the metal and eventually a failure occurs In cold climates, it is not rare for individuals to thaw a frozen water pipe by attaching a welding transformer and passing current through it. Fires have resulted due the very large currents that are involved. Sanderson studied a case where thawing activity did not ignite the house that was being worked on, but caused ignition in six neighboring houses fed from the same power utility connection.

video video


Insuring knob and tube

There's still lots of knob and tube electrical distribution in San Francisco.
Some people don't see any value in upgrading it... hopefully, they're not your neighbors!

Some electricians also have been working with this for a long time and have become inured to it.
It's difficult, costly and disruptive to abandon all the old knob and tube wire.

If your home is already insured the most practical way to upgrade is to

install modern arc fault breakers as possible,
add new code compliant circuits as needed,
starting with the kitchen, the bath and points of heavy use.
Thermally protected outlets as per need
Verify the integrity of the grounding system
Install smoke detectors
We usually recommend to abandon knob & tube incrementally, again this is if your insurance company has no problem with it!

An obsolete electrical system that is giving warning signals should be inspected and corrected as recommended by an experienced professional

Good Labeling!  Under the fixture :/

Modern electrical systems are very safe.

Junction buried in the wall and filled it with newspaper from 1963

Gas Chandelier support

good looking junction box


knobby tube

Security system power source


valenica corridor



Extension cords no substitute for permanent wiring

Improper use of extension cords and surge protectors can present a serious fire safety hazard in the workplace. According to the National Fire Protection Association (NFPA), electrical distribution equipment, such as extension cords were the second leading cause of fire deaths in the U.S. between 1994 and 1998. The most common cause of fires from extension cords is improper use and/or overloading, especially when the cord has multiple outlets. The 2007 California Fire Code Chapter 6 Section 605 addresses the use of extension cords, power taps, surge suppressors and multi plug adaptors. By following the guidelines below the risk of fire due to overloaded or improperly used extension cords or surge protectors can be greatly reduced.

Extension Cords Approved Extension Cords Must:

  • Be Underwriter Laboratories (UL) listed.
  • Be at least 12-14 gauge wire and not be less than the rated capacity of the appliance. 
  • Have a ground wire and cannot be bypassed or rendered inoperable. 
  • Be used only for temporary situations, such as laboratory experiments lasting no longer than 90 days, and not be used in place of permanent wiring. 
  • Connect to only one portable appliance. Portable means easily moved from one place to another during use. 
  • Connect directly into a wall receptacle. 
  • Be protected from exterior damage. Examples include 
    • environmental (weather) and 
    • physical (foot traffic). 
Extension Cords Must Not: 
  • Be longer than 100 feet. 
  • Have more than one receptacle on each end. 
  • Be frayed, deteriorated, spliced or modified. 
  • Pass through walls, doors or windows. 
  • Be stapled or attached to a floor, wall, or ceiling. 
  • Be connected in series. 
  • Create a tripping or other safety hazard. 
  • Be unprotected where exposed to foot traffic, moving wheels, or falling debris to minimize tripping hazards and damage to the cords. 
  • Be in a cable tray that is not intended for power 

In most cases, hard pipe and new circuits are required for safety. The bottom line is that extension cords are never OK , but temporary power taps Can Be OK under certain circumstances.

The problem with flexible cord used in extension cords it that the rubber used will deteriorate over time and become brittle and fall off. This can leave the conductors inside exposed to damage and possible failure of the insulation system which could lead to a fire.  For this reason flexible cord can not be used as a substitute for the use of other permanent wiring methods

Flexible Cords, Cables and Fixture Wire

Understanding the wiring requirements of Article 400, including guidelines for flexible cords, cables, and fixture wire

To prevent fires, be very careful with extension cords.
Avoid trip hazards.
Do not cover or walk on cords.
Plug extension cords into GFIs.
Test the GFI when you plug the cord in.
Always unplug cords at the source when not in use.
Do not use cord if damaged.
Inspect them as you roll them up and out.
No "Daisy Chain"
Do not plug one extension cord into another.
Completely connect plugs.
Push them in all the way.
Use three-wire extension cords with 3-pronged plugs

605.5 Extension cords. Extension cords and flexible cords shall not be a substitute for permanent wiring. Extension cords and flexible cords shall not be affixed to structures, extended through walls, ceilings or floors, or under doors or floor coverings, nor shall such cords be subject to environmental damage or physical impact. Extension cords shall be used only with portable appliances.

Regarding temporary power taps, the UL white book is quite clear,
"...Relocatable power taps are not intended to be permanently secured to building structures, tables, work benches, or similar structures, nor are they intended to be used as a substitute for fixed wiring.
The cords of relocatable power taps are not intended to be routed through walls, windows, ceilings, floors or similar openings”.

Remember the UL position on permanent securing. It’s often desirable to secure the RPT to avoid damage. Securing is permissible by the UL standard as long as no tools are required to remove it. Therefore they are not permanently secured.

Power Strip This device is not classified as a cord set or extension cord, but is referred to as a Temporary Power Tap (TPT). It is permitted to have six outlets because its over-­current protection will protect the #14 AWG cord at its rated value of 15 amps. A TPT is intended only for indoor use as an extension of the branch circuit. (UL 1363)

Temporary Power Taps are often supplied with supplemental over-current protection, which is not the equal of a listed circuit breaker for reliability or effectiveness. Switches, indicator lights, a transient voltage surge suppressor (TVSS) and/or an electromagnetic interference (EMI) filter may also be included.  The word "temporary" is broadly interpreted because of the special need for the extra outlets where we have a cluster of appliances such as a computer, printer or disc drive. Admittedly this assembly may be used for months or even years without moving, but its length is limited to a maximum of 25 feet. UL (policy) considers this usage to be within the intent of the cordbecause such equipment is moved occasionally, and because there is no other practical way to provide for this very common need.

The main difference between temporary power taps and extension cords lies in the intended use of the products. UL lists extension cords as cord sets under the category of Cord Sets and Power Supply Cords. A cord set can have a current rating less than the branch circuit rating (depending on the ampacity of the cord and the rating of the devices such as cord-connector body or switch, whichever is less). For this reason, a cord set must be selected in relation to the rating of the equipment with which it is used. Cord sets extend the appliance power supply cord so that the appliance can be ".... energized from a receptacle outlet" [Section 400-7 (b)]. Temporary power taps extend the branch circuit.

This information was originally published in “Straight Talk on Extension Cords, Power Strips and Daisy-Chaining” by Ernie Harper, CSP.


the let-thru energy

Why is AIC rating important? When transformers are large or close to a house this rating should be considered. Distance has a huge effect on the available fault current at the load end. Your service equipment must be suitable for the fault current that is available. The easiest way to achieve this is to use nothing less the equipment rated 22k aic when the xfmr is close to the house. This is just Ohm's law. If the voltage remains (relatively) the same, the amperage (fault current) will go down if the impedance is increased. The conductors between the transformer and the service introduce that impedance. The farther away from the transformer you are, the more impedance you have, and therefore the less fault current available you will have. You have to know the available fault current at the line side of your service equipment to select the correct equipment. In many cases the off the shelf stuff is fine, but if the available fault current is high, you are required to buy equipment suitable for that fault current. Look at NEC110.9 and 110.10. If the distance to the pole is small and there is any doubt you need to contact the utility company and/or find out the size and impedance of the transformer that will serve your load. You can specify equipment that will need to be properly rated for that fault current duty.

Available Short-Circuit Current
EE Articles Menu By Mike Holt, Published in EC&M Magazine
Available short-circuit current (SCA) is the current in amperes that is available at a given point in the electrical system. This available short current is first determined at the secondary terminals of the utility transformer. Thereafter the available short-circuit current is calculated at the terminals of the service equipment, branch circuit panel and branch circuit load.
The available short-circuit current is different at each point of the electrical system; it is highest at the utility transformer and lowest at the branch circuit load. The available short-circuit current is dependent on the impedance of the circuit, which increases downstream from the utility transformer. The greater the circuit impedance (utility transformer and the additive impedances of the circuit conductors) the lower the available short-circuit current.
Factors that impact the available short-circuit current at the utility transformer include the system voltage, the transformer kVA rating and its impedance (as expressed in a percentage). Properties that impact the impedance of the circuit include the conductor material (copper versus aluminum), the conductor size, and it's length.
Author's Comment: The impedance of the circuit increases the further from the utility transformer, therefore the available short-circuit current is lower downstream from the utility transformer.
Interrupting Rating. Overcurrent protection devices such as circuit breakers and fuses are intended to interrupt the circuit and they must have an ampere interrupting rating (AIR) sufficient for the available short-circuit current in accordance with Sections 110-9 and 240-1. Unless marked otherwise, the ampere interrupting rating for branch-circuit circuit breakers is 5,000 ampere [240-83(c)] and 10,000 ampere for branch-circuit fuses [240-60(c)].
Extremely high values of current flow (caused by short-circuits or line-to-ground faults) produce tremendous destructive thermal and magnetic forces. If the circuit overcurrent protection device is not rated to interrupt the current at the available fault values, it could explode while attempting to clear the fault. Naturally this can cause serious injury, death as well as property damage.
Protection of Electrical Components. In addition to interrupting rating for overcurrent devices, electrical equipment, components, and circuit conductors must have a short-circuit current (withstand) rating that will permit the circuit overcurrent protective device to clear a fault without extensive damage to any of the components of the electrical system [110-9, 110-10, 250-2(d), 250-90, 250-96(a) and Table 250-122 Note].
If the available short-circuit current exceeds the equipment/conductor short-circuit current rating, then the thermal and magnetic forces can cause the equipment to explode and/or the circuit conductors as well as grounding conductors to vaporize. The only solution to the problem of excessive available fault current is to
(1) Install equipment that has a higher short-circuit rating
(2) Protect the components of the circuit by a current-limiting protection device such as a fast-clearing fuse, which can reduce the let-thru energy.