110.9 Compliant?

[bphgravity]

I have a 208Y/120, 3? service to a restaurant. The engineer performed some calcs as followed:

He shows 26,559 as being the starting SC current with 53.78 A. motor contribution. From this,

F = 1.732 x 93' x 26,559 / 1 x 12,843 x 208 = 1.6014 ,

M = 1 / (1+1.6014) = .3844 ,

26,559 x .3844 = 10,209 A RMS.

1. Where did the 12,843 come from (the "C")

2. If the main breaker is rated at 10,000 and the feeder panel it serves only rated 10,000 AIC with 10,000 AIC rated branch circuit breakers, am I not in compliance with 110.9?

 

[don_resqcapt19]

Re: 110.9 Compliant?

Bryan,
It should come from Table 2 on page 8 of this document, but I don't see a 12,843 in that table. There is a 12,844 that would apply if the conductors are 2/0 in a magnetic conduit.
Don

 

[jim dungar]

Re: 110.9 Compliant?

Yes, you may have a problem. There are no tolerances specified for device short circuit ratings. You can not use a 10,000A device on a system with a level of 10,001A. Also there is nothing to prevent the use of 10kA device on a system of 9,999A either.

 

[bphgravity]

Re: 110.9 Compliant?

Thanks.

The plans specifically call for a 65,000 AIC rasted main so that is what I have requested them to change out to. The electrician is arguing that the actual calcualtions don't match this need, and therefore doesn't want to do it. He is hoping to get a letter from FPL indicating that the available fault current from the utility transformer is actually less.

 

[petersonra]

Re: 110.9 Compliant?

how about making the feed 2 feet longer? sounds less expensive to me.

 

[charlie b]

Re: 110.9 Compliant?

I knew of a power plant that played that game. And it worked. They had a problem (in the initial design) with excessive fault current. So they ran the feeders to the MCCs and unit substations "once around the block," before terminating the conductors. The extra length brought the fault current within limits of the equipment, without giving too significant a voltage drop.