AIC ratings

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electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Occupation
Electrician
I have recently started doing a number of 480 wye services, feeders, and panelboards and have discovered that when requesting equipment quotes, I get asked for the aic rating. I have read up on the topic and know what it is, but I am unclear on in the real world, how many calculations are typically done when running 480 services and feeders and how this is usually handled for smaller non-engineered jobs? Is it like load calcs for single family dwellings in that nobody does them for routine jobs? A few examples of the type of projects I am referring to: 1) Say you are doing a 400 amp 277/480 service. Do you ask the utility for their asc? Every time or only if the AHJ asks for it? Does most service equipment in this category cover most situations? 2) Say you are running a 100 amp 277/480 feeder to a panel board in a machine shop. What should the AIC rating be? To what extent to you go to figure it out or do you just spend a bit more on the equipment to make sure you are covered? Appreciate any feedback on how others handle this situation.
 
110.9 and 110.10, require you to apply equipment with either an AIC or a SCCR that is sufficient for the amount of available fault current.
These sections always apply to all installations.

These have been sections for more than 50 years.

When I perform arc flash studies, the most common error I find is improperly applied equipment.
I would not be surprised to find that customers spend more money replacing equipment for the available fault then they do on any incident energy mitigation.
 
Ok great Ill take a look at the mike holt fault current calculator. I am still not clear what happens in practice - there are a lot of calcs we are supposed to do in theory but never do due to experience or 'rules of thumb'. I dont think anyone checks AIC for that 200 amp residential service upgrade, so in general under what circumstances do you have to worry about this? Is it possible to make a statement something like: on systems of 250 volts to ground and above AIC should be calculated and checked but for systems under 250 to ground it is rarely and issue? Just curious where the line gets drawn and what the general circumstances are....I am sure there must be a lot of electricians who are not as good at math and calcs as me out there doing non-engineered 480 stuff.
 
There are lots of things being done by rules of thumb, but it shouldn't be. For example, it is possible for a row house fed by a 50kVA transformer (with other neighbors) at 240V to the home, where the transformer has a short secondary feeder to the home's service equipment, may have >10kAIC at the service panel. Someone does an upgrade and gets the cheapest thing available at their supply house, which is rated at 10kAIC, and it is wrong.
 
For service equipment, they Utility provides the available fault current. Many of them publish maximum values in their rules for services. In my area, its called the PECO Blue Book. (It's not blue, and it hasn't been a book for years, you read it on-line.)

Down stream of the service, you need to do a calculation.
 
They taught AIC calculations to high school seniors in the Electrical Trade School I attended. There are several steps:

1: assume an infinite bus for the feed to the distribution transformer.
2: obtain the impedance for the distribution transformer, ask the Power Company or use binoculars.
3: adjust the impedance down for the manufacturer's tolerance.
4: determine the length, size, and composition for the service drop or underground lateral.
5: for an underground service, calculate the AIC. This is what you need for the meter socket, or the meter disconnect in the case of many 480V direct metering arrangements.
skip to 8:
6: for an overhead service, determine the length, size, and composition for the service entrance conductors.
7: calculate the AIC. This is what you need for the meter socket, or the meter disconnect in the case of many 480V direct metering arrangements.
8. determine the length, size, and composition of the conductors from the meter socket to the Service Equipment.
9: calculate the AIC. This is what you need for the service OCPD.
10: Determine if the service OCPD and feeder and branch circuit OCPDs are series rated with the main OCPD, If not adjust their AIC as required.
11: if more than 10kAI available at feeder and branch OCPDs repeat AIC calculations for equipment downstream. [most modern CBs are rated at least 10kAI, minimum rating is 5kAI.]

Note that neutral to phase faults are often larger than phase to phase faults. Calculate both.

Equipment that is rated less than the actual AIC can fail dramatically and fatally.
 
In the most recent issue of AIEI Magazine there is a good article along these lines. I will say to the OP, this is not something to be taken lightly and in most cases you should have E.E. help. Guessing/ignoring can lead to bad things along the lines of injury, property damage and liability.
 
texie said:
In the most recent issue of AIEI Magazine there is a good article along these lines. I will say to the OP, this is not something to be taken lightly and in most cases you should have E.E. help. Guessing/ignoring can lead to bad things along the lines of injury, property damage and liability.
Click to expand...

I am not sure EE help is required to do simple SCC calculations. It is pretty much Ohm's law.

Arc flash calculations are somewhat more complicated, although there is generally not going to be any way to make those calculations without the use of software designed to do it in an economical way.
 
fmtjfw said:
They taught AIC calculations to high school seniors in the Electrical Trade School I attended. There are several steps:

Equipment that is rated less than the actual AIC can fail dramatically and fatally.
Click to expand...

You can never ever calculate the AIC. You actually calculate the available SCA (short circuit, amps).

You choose a device with an SCCR (your step 7: NEC 110.10) or an AIC (your steps 9 and 11: NEC 110.9) that is greater than your calculated SCA.
 
You are right

You are right

jim dungar said:
You can never ever calculate the AIC. You actually calculate the available SCA (short circuit, amps).

You choose a device with an SCCR (your step 7: NEC 110.10) or an AIC (your steps 9 and 11: NEC 110.9) that is greater than your calculated SCA.
Click to expand...

filler filler filler
 
electrofelon said:
Ok great Ill take a look at the mike holt fault current calculator. I am still not clear what happens in practice - there are a lot of calcs we are supposed to do in theory but never do due to experience or 'rules of thumb'. I dont think anyone checks AIC for that 200 amp residential service upgrade, so in general under what circumstances do you have to worry about this? Is it possible to make a statement something like: on systems of 250 volts to ground and above AIC should be calculated and checked but for systems under 250 to ground it is rarely and issue? Just curious where the line gets drawn and what the general circumstances are....I am sure there must be a lot of electricians who are not as good at math and calcs as me out there doing non-engineered 480 stuff.
Click to expand...

With McMansions, you could run into a problem easier than on most typical dwellings. The biggest thing reducing the available fault current at most dwellings and other small services is the the length of conductor composing the service drop or lateral. For years many have installed services to these places without doing any calculations and for the most part still are in compliance because even if the transformer has a high available fault current, it is reduced by the amount of service conductor before it hits the terminals of any equipment in the building.

Available fault current can be an issue with any voltage source, not just 277/480 volt sources. Even a 12 volt battery source could provide fault currents that are damaging to connected equipment.
 
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