# motors: brach circuit OPD to feeder OCD



## 360max (Jun 10, 2011)

430.52 (Rating or Setting of Individual Motor Circuit) also see table 430.52, and 430.53 (Several Motors or Loads On One Branch Circuit)


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## stonewhite (Aug 10, 2012)

thanks, mate!


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## erics37 (May 7, 2009)

For what it's worth, I use this cheat sheet a lot, and I give them to my apprentices as well (after making them do it the hard way first, of course) :whistling2:


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## LATTC (Feb 12, 2012)

stonewhite said:


> hello--
> 
> Would you know the NEC citation for calculating feeder overcurrent protection device for several motors?


feeder = largest motor FLA X 1.25 + all other motors FLA 430.24
feeder OCPD =Largest fuse + FLA of each additional motor (430.62)



stonewhite said:


> Specifically, one would determine the branch-circuit FLA for the largest motor, then factor this number by 250% (before adding this figure to the other motor FLAs). Where in the NEC is that 250% factor cited?
> thank you


The 250% is from Table 430.52. The percent value is determined by the type of OCPD you are using.
300% for Nontime Delay Fuse
175% for Dual Element Fuse
800% for Instantaneous Trip Breaker
250% for Inverse Time Breaker


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## stonewhite (Aug 10, 2012)

for practice: 460 v, 3 phase

motor on right: 10hp, on left: 5hp. 

FLC: 14 amps and 7.6 amps, respectively. (Table 430.250).

thermals: 21.84 amps (156%), 12.9 amps (170%). (Table 430.32)

branch conductors: 14 X 125% = 17.5 amps. Table 310.16: 14 AWG THW (rated 20 amps)

branch OCP: inverse time breaker, 250% of 14 = 35 amps. 35 amp OCP device (Table 240.6).

feeder conductor: 17.5 + 7.6 = 25.1. Table 310.16: THW 10 AWG (rated 35 amps).

feeder OCP device: branch OCP device plus other motor FLC: 35 + 7.6 = 42.6 amps. Table 240.6: 40 amp OCP device (use next smaller size).

thanks, folks!


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## stonewhite (Aug 10, 2012)

Eric:

the disconnect factor you've penciled in: 115% of 14 amps = 16.1 amps (as per Article 430.110 A)

disconnect rated for next higher amperage--that being?

thank you.


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## erics37 (May 7, 2009)

stonewhite said:


> Eric:
> 
> the disconnect factor you've penciled in: 115% of 14 amps = 16.1 amps (as per Article 430.110 A)
> 
> ...


Here's the article:

430.110 Ampere Rating and Interrupting Capacity.
(A) General. The disconnecting means for motor circuits
rated 600 volts, nominal, or less shall have an ampere rating
not less than 115 percent of the full-load current rating
of the motor.

It doesn't care what rating disconnect you use, as long as it's at least 16.1 amps. If you could purchase a 16.1 amp disconnect, then that would be the economical way to go. As is, you're stuck with whatever standard sizes your preferred manufacturer offers. I would probably go with a 30 amp disco. Make sure it has an appropriate horsepower rating.


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## frenchelectrican (Mar 15, 2007)

erics37 said:


> For what it's worth, I use this cheat sheet a lot, and I give them to my apprentices as well (after making them do it the hard way first, of course) :whistling2:


That a cool way to do it and is that listing is based on 2008 NEC ? 

And I was taught that always start at the motours first then work it way to the source ( supply or other means ) 

Merci,
Marc


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