125% of largest motor load

[Kiljoy]

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.

 

[rbulsara]

Use only one of them. Also read closely, it should be 125% the largest motor plus 100% of the sum of the rest of the motors (FLA). (which is same as 100% of the all motors plus 25% of the largest) for the feeder conductors.

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

 

[Kiljoy]

Great. Thanks! I've always used 125% of both motors just to be on the safe side because our hydraulic guys tend to design around the service factor instead of the full load. I figured that I would err on the side of caution.

 

[jraef]

There is by the way a subtle difference between the minimum feeder conductor size and the minimum breaker size. I haven't thought about it enough to try to figure out why yet and I don't have my NEC on this PC. Maybe someone else can explain this. 430.110 says the breaker can be 115% of the largest + 100% of the rest, 430.24 says the 125% + 100% for sizing the feeder conductors.

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.

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[Kiljoy]

It gets better. In the UL508a section on indistrial machinery, it states:

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.

 

[rbulsara]

430.110 is for "disconnects", not the overcurrent protection or breaker.

Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

Article 430 is unusual in that there is no direct relationship between the feeder sizing requirements and the overcurrent sizing requirements. The two requirements are essentially independent so it is possible to end up with breaker/wire size combination that would not occur for a non-motor feeder or branch circuit.



David Castor

http://www.cvoes.com

 

[rbulsara]

Yes, motor circuit conductor sizing falls under special conditions as identified in Article 240.4(G), where overcurrent protection rating can exceed conductor ampacity.

Also the UL 508a reference made my kiljoy above, appears very much in agreement with NEC.

Rafiq Bulsara

http://www.srengineersct.com