NEC 430.24 - Several Motors

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line_noise

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This one has me stumped. NEC 430.24 (2002 Edition) states that I can size conductors supplying several motors at 125% of the largest motor plus the sum of the other motors. However, NEC 215.2 states that the minimum feeder conductor size shall sized at 125% or more of the sum of all continuous loads, plus 100% of the non-continuous loads.

NEC 215.2 also references NEC 220, which specifically references 430.24. The example in Annex D also seems to allow 430.24. It seems to me that NEC 430.24 shouldn't even exist if it couldn't be applied to a feeder.

I have a panel that I want to serve 3 motors out of. The panel and its feeder conductors are rated 400 amperes, three phase, 480V, and the three motors are (2) 124A, 77A, and 11A, per tables in 430. Which is correct?

125%*(124+124+77+11)= 420A = violation per NEC 215.2

or

125%*124+124+77+11= 367A = OK per NEC 430.24

I can also convince myself that this will work fine, since even though the NEC doesn't permit this, the nameplate amperes of energy efficient motors, plus the power factor correction means we have, will reduce the continuous loads below 320A.

[ June 17, 2005, 11:48 AM: Message edited by: line_noise ]
 
Re: NEC 430.24 - Several Motors

If you are looking at sizing the feeder conductor for the panel, you must reference 430.62.

Using this you would take the largest motor OCPD plus the sum of the full-load currents of all other motors. Assuming you're using an inverse-time CB in the panel, the 100 hp motor could have up to a 350 amp breaker (sized at 250% and rounded up). Using this, the feeder OCPD = 350+124+77+11=562A (rounded down to 500A). Remember, this would be the maximum size. Your answer will depend on how high you oversize your CB multiplier.

430.24 is for sizing motor branch circuit conductors.
 
Re: NEC 430.24 - Several Motors

Woops, I just re-read your post and realized that you are looking at the size of the feeder conductors - 430.24 is correct. Your second example is correct.
 
Re: NEC 430.24 - Several Motors

I'm not gonna try to back it up with a bunch of NEC references, but common motor applications are already set up for continuous operation.

I think most will agree that intent of NEC is & has been 430.24 for motor feeder conductors. So the minimum ampacity of feeder conductors to feed the (4) motors is 367A.

Max Therm/Mag feeder OCPD, based on that conductor value would be (250% X 124) which is 310, + 124 + 77 + 11. Drop back to a 500A OCPD protecting, say 500KCMIL (380A), would be fine without overkill.

Now whether your existing source has the guts to handle the load is another story.
 
Re: NEC 430.24 - Several Motors

You are not required to apply the 125% continuous load rule to motor loads. Use 430.24. 367 amps is correct.

[ June 17, 2005, 11:45 PM: Message edited by: john m. caloggero ]
 
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