The customer wants a 480v, 125a MLO Sq. D NF panelboard for the service on an agricultural building. The PoCo transformers are pole mounted, delta-delta. Let's assume we have three 50kva transformers.

How do I select the correct aic breaker? The available sizes are;
65,000 aic
35,000 aic
18,000 aic

thanks,
GJ

[This message has been edited by golf junkie (edited 03-23-2004).]

Take a look at http://www.ecmweb.com/ar/electric_basic_shortcircuit_current/
for an extremely basic introduction to short circuit calculations. Find out the transformer impedance, and presume every other impedance in the short is zero, to get a conservative approximate value.

-Jon

thanks jon, that was helpful

The utility should be able to provide that information for the service entrance location. Needed transfomer impedance data to estimate that value is not generally available to the electrician working downstream of the meter.

Also, that relieves the electrician of having to do short-circuit estimates for service-entrance equipment.

Determine the full load current on the secondary, divide that number by the percent impedance of the transformer, and that gives you a conservative value at the secondary stabs of the xfmr. Depending on the length of the secondary conductors, that will further reduce the available fault current.
For example, 150000/480/1.73/.02=9030A of fault current at the secondary stabs.
You would be safe choosing the lowest AIC of the group.

Warning - the Square D NF panelboard is not rated for use on 480V delta systems. The NF panel has a slash rating of 480Y/277, so it can not be used on circuits with more than 277V to ground. You need to switch to their I-Line panel instead.

Ron
That is good info that 'us' electricians can actually use.
JBD
That is very observant and a key pickup.
Thanks

Pierre