Short circuit amps 5

[stevemechanical]

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?

 

[davidbeach]

If you don't want to do any downstream calculations, everything, would have to meet the utility fault current, but everything downstream of the main breaker can accomplish that through use of LISTED series ratings.

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.

 

[alehman]

david, many new homes in my area are being required to have 22kA rated panels.

 

[apowerengr]

david, I believe the safer statement for utilities in general is that utilities go to great lengths to keep their primary system fault currents under 10kA (typical cutout interrupting rating) but even relatively small distribution transformers with short services will provide fault currents in excess of the typical 10kA interrupting ratings on inexpensive residential panels/breakers. As for Stevedantonio, 208 1ph is normally supplied from a three-phase transformer which, if it fails, can be replaced during an outage with a larger three phase transformer depending on utility stock and supply at the time of the outage. The risk here is that the utility usually instructs new customers via their service and metering requirements that they reserve the right to increase the service transformer one standard size for emergency replacements, thus the fault current could increase significantly and without opportunity to make a correction in the building's infrastructure.

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?

 

[waross]

David, do the utilities report the actual maximum possible fault current, including DC offset?
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

 

[davidbeach]

All ratings, as far as I know, are in symmetrical 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.

 

[lansford]

One must exercise caution in assuming the fault amps even residential. It could be many multiples of 10 KA if it is in a city and served from a NETWORK.
JIM