Short circuit amps
Short circuit amps
(OP)
if i'm specifying a new main distribution panelboard (MDP) for a residence, say 208v/1phase, and i have the available short circuit amps from the utility company, then all i have to do to protect my electrical installation from the short circuit amps is make sure the ampere interrupt capacity (AIR) of the MDP is greater than the available short circuit amps and all the downstream panels can be fully rated at 10KAIR or 22KAIR. Is this correct?
If not, what is a good reference to figure this out?
If not, what is a good reference to figure this out?






RE: Short circuit amps
Utilities go to great lengths to keep residential fault currents below 10kA until you get into large multi-unit buildings, so everything rated 10kA may be sufficient, but you need the utility numbers.
RE: Short circuit amps
RE: Short circuit amps
If it were my home, I'd ask what the intended transformer size is and calculate the maximum available fault currents based on the next larger standard size, at the minimum allowable tolerance value of the standard impedance (I'm assuming ANSI) and with an infinite supply bus (zero system impedance). Size the main breaker's interrupting rating accordingly. It may sound conservative but why would you shortcut interrupting ratings?
RE: Short circuit amps
I understand that panels and breakers are rated by "available fault current", which is defined as the transformer rated current divided by the transformer percent impedance, or the symmetrical component of the fault current.
Which do utilities report to customers?
Symmetrical current or the higher asymmetrical current?
Thanks
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Short circuit amps
I think that some of you may be surprised at just how conservative the utility's fault calculations are in the first place. How about fault calculations based on the largest possible transformer for the installation, with lowest impedance of any transformer of that size in stock, with infinite bus on the primary of the transformer. I've seen utility documents that show how underground services are to be routed to ensure sufficient secondary conductor to guarantee no more than 10kA at the meter base from 112.5kVA pad-mount transformers. If the utility tells you 10kA, it will always be 10kA or less; if they tell you 22kA, then don't try to get away with 10kA.
RE: Short circuit amps
JIM