I received the AIC ratings for XXX Yyyyy Street
Single Phase.
When we last spoke, you mentioned that you needed the ratings only, not an official letter. Let me know if that is still the case. If so, I will be closing this application.
The size of the transformer, not the service, is usually the deciding factor.Now what's confusing me is I'd expect a larger service to have larger wire to have larger AIC.
What circumstances could result in the opposite ?
This is a 12 unit.Is this for a single family dwelling service? If so that eliminates using any combination service entrance equipment which maxes out at 22K.
For most of the 400 amp (class 320) services we install the plans show ~18K
It's AFC at the service, if I understand correctly.Just to be clear, are they giving you the AIC requirement for your service equipment, or the AFC. If AFC, it that at the transformer or ???
Sad that a utility can’t use correct terminology.It's AFC at the service, if I understand correctly.
Here's a sample letter. The utility calls these determinations "AIC letters" so that's what I have to ask them for.
I suppose that should be read as "required AIC at the main panel". Either way it appears to be a lot of paperwork for
an unclear level of hazard reduction.
The sample given is for an eight unit apartment on California Street in Berkeley CA
Utilities usually use low impedance transformers, like 1.5%Z. To get 35kA this would mean the transformer would be capable of supplying 525A or be roughly 126kVA at 120/240V. In my experience that is very large residential transformer.Back to the original OP question: do the AFC / AIC numbers they gave me seem actionable and trustworthy, or should I push back?
Actually that sample letter never provides an AFC.It's AFC at the service, if I understand correctly.
Ah, good point. I stand corrected. The poco letter could indeed be more clear on that purpose.Actually that sample letter never provides an AFC.
Instead it tells you what AIC equipment needs to be provided so that future utility upgrades to not overduty any installed equipment. This is a very common utility practice.
The POCO policy covers their entire service area not just your particular area.I don't see how the poco could ever increase the transformer sizes.
In September 2025 this poco instituted a new policy of not issuing available interruption current guidance lettersThe POCO policy covers their entire service area not just your particular area.
Try to elevate your request within the POCO, to see if you can get more appropriate available fault current data for your specific installation.
Try using the excuse of needing to calculate Arc Flash information in order to comply with 110.24.
I wish you luck.
Or meter it yourselfI'll try your arc flash bs technique
You cannot meter the available fault current without creating a bolted fault.
But how good is a proportional extrapolation? If you impose a temporary load that creates say a 2% voltage drop at the point of interest, will the AFC be close to 50 times the current?You cannot meter the available fault current without creating a bolted fault.
The NEC requires the available fault current to be computed at the line side terminals of the equipment. This particular tester measures the current at the load side of the outlet. The technical data for this tester doesn't even mention this feature or its limitations. The marketing literature mentions using it to determine how much current might flow through a small branch breaker before that breaker trips. This would mean the tester includes the dynamic impedance of the OCPD in its "ASCC" result.But how good is a proportional extrapolation? If you impose a temporary load that creates say a 2% voltage drop at the point of interest, will the AFC be close to 50 times the current?
Thanks,
Wayne
Engineers rarely qualify, or train in PPE, to access equipment covers, much less meter energized parts, so infinate current may be assumed.In 50 years of performing power system analysis, I don't think I have ever seen a formal write up suggesting extrapolating
Until AI makes an App for that, engineering supervision still has a job.For sure it wouldn't work when sources contribute additional short circuit current during a fault, particularly in large commercial and industrial installs.
I worked with a team of 150 engineers performing power system studies including short circuit and arc flash. About 100 of us wore PPE and stood in front of open energized equipment on a regular basis.Engineers rarely qualify, or train in PPE, to access equipment covers, much less meter energized parts, so infinate current may be assumed.
Dangerous in what way? Is the danger being overstated or understated?There is no way we used infinite bus for our Arc Flash reports, as it provides unrealistic and dangerous values.
