125% of largest motor load
125% of largest motor load
(OP)
I've always been confused on this. When calculating the FLA of a piece of equipment or panel for purposes of feeder wire and disconnect sizing etc., the code states to add in 125% of the largest motor FLA. My question is: what if two or more of largest motors are the same size? Our pump systems often have two main motors and several smaller loads. Do I take 125% of both main motors or just one? The main pump motors may dip into the service factor intermittently.
E.g. (2) 75HP, (2) 20HP, and (1) 10HP motor on the same feeder.
E.g. (2) 75HP, (2) 20HP, and (1) 10HP motor on the same feeder.





RE: 125% of largest motor load
The breaker rating will be different, 250% (max) of the largest plus 100% of the rest, for a thermal magnetic, for example.
Rafiq Bulsara
http://www.srengineersct.com
RE: 125% of largest motor load
RE: 125% of largest motor load
The way I think it is supposed to work is that you size the feeder conductors, and by default the breaker supplying them, per 430.24, but AT THE CONTROL PANEL, the main can be sized per 430.110. Not sure why one would want to, but 430.110 is minimum, so if you did it at 125% there that would be OK too.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: 125% of largest motor load
66.4.1 Field wiring terminals for supply connections shall be sized based upon the sum of:
a) 125 percent of all heater loads;
b) 125 percent of the largest motor load; and
c) The full-load current ratings of all other motors and other loads that are simultaneously
operable.
RE: 125% of largest motor load
Rafiq Bulsara
http://www.srengineersct.com
RE: 125% of largest motor load
David Castor
www.cvoes.com
RE: 125% of largest motor load
Also the UL 508a reference made my kiljoy above, appears very much in agreement with NEC.
Rafiq Bulsara
http://www.srengineersct.com