I have yet another fun example of trying to get straight info out of the utility. After gaining access to the utility vault and looking at their equipment with their linemen, and waiting an obscene amount of time, AND getting an initial AFC letter for the incorrect address, I finally got a response:

"The anticipated available fault current for this property is 27,535 amps L-G, 480V at the collector bus. This value does not account for source impedance."

I just wanted to get your (unofficial) opinions on translating that. The service is a three-phase, 480V Wye service. Am I right to interpret this as saying 'It's a 480V service, but the AFC is calculated at 277V'? At first glance it looks like they're saying the L-G is 480V, but that's both incorrect and ridiculous.


[edit: I have to add that I'm not sure what it means that the value "does not account for source impedance". The collector bus they're referencing is literally 3' from the 12.5kV/480V transformer...how do they even calculate AFC without taking into account the transformer?]

All I can say is good luck with that one.

My take is they are giving you worst case AFC suitable for equipment sizing. Zero source impedance ahead of the transformer with a line to ground fault close in on the secondary side. This value won't change over the life of the installation no matter what changes the utility makes short of up-sizing the transformer.

If you wish to obtain data suitable for calculating incident energy, you must be specific regarding what you want and periodically check back to see if changes have occurred.

Agreed...this is becoming more apparent, especially with certain local utilities. My supervisor is often the one who sends the info request to the utility, and he doesn't do studies. I need to come up with a specific boilerplate request and/or script to give him, so that I'm not having to wait (for months, sometimes) just to find out I've been given nothing.

The thing that gets me is that since studies are now required and enforcement is ramping up, you'd think that the Utility companies would understand by now what is expected of them. I can only hope it improves as regulation becomes more standardized.

If I'm doing a study for a larger customer/ one that has a medium voltage service I've just started requiring the customer to obtain the utility data and I deal with them if it's not sufficient. I'm a nobody to the utility company and at times I've just been blown off, whereas the end user paying the light bill has more pull and gets answers quicker. The larger users usually have a power company rep they can call directly too.

It is not uncommon to find the L-G (single phase) fault current to be higher than the L-L-L (three phase) current when you are very close to the transformer. It is possible, though not common, that this worse case is what the utility was providing.

This comment still makes me chuckle. About 1-1/2 weeks ago, I got another letter back from a different Utility provider on a project I'm doing. I'd already finished most of the modeling, so I just needed to plug in the Utility info and start crunching numbers. However, I noticed that in spite of his email correctly stating we had a 120/208V 3-Phase service, the actual design letter showed a 120/240V Delta. It also showed it as a single-phase service. He was correct on the wire size, type, and length.

I sent a letter out right away, detailing the problems and asking for clarification. No response. I sent out another letter repeating the same request. I got a response today. It said, "Good catch, I did use the wrong wire size. I have attached the revised AFC for your use."

The wire size in the Design Letter was still correct, and he did correct the number of phase conductors and re-calculate the AFC, but it's still listed as a 240V delta system.