NEC 430-24 Conductor Sizing

[ked]

I am sizing a fused main disconnect for several motors. Total sum of the FLA of all motors is 441 Amps including a 100hp motor. The 100hp is the largest motor (all run continuously). My question is sizing the service. My understanding is I add 25% of the largest motor FLA to the sum of the motor FLA's. See 430-24. Any help would be appreciated.

Which of the following formula's are correct:
(Sum of all motors FLA + (0.25 * largest motor FLA)) = Minimum size of service OR (Sum of all motors FLA + (1.25 * largest motor FLA))

 

[bphgravity]

Re: NEC 430-24 Conductor Sizing

The second formula is correct. You want to increase the largest motor by 25%, not use only 25% of it as the first formula would produce.

 

[charlie b]

Re: NEC 430-24 Conductor Sizing

I agree with BPH’s intent, but disagree with the answer. The first formula is correct. It first adds the FLA of all motors, and this includes the FLA of the largest motor. It then adds the extra 25% for the largest motor. By comparison, the second formula would count the largest motor plus the largest motor plus 25% of the largest motor.

 

[jmc]

Re: NEC 430-24 Conductor Sizing

Lets see if I can simplify the answer.The NEC requires that you multiply the largest motor FLA by 1.25 plus the sum of the OTHER motor,s FLA. Remember the FLA is base on Table values that may be higher than actual namplate values.
Amperes = (1.25 x Largest motor FLA)+ (FLA of all other motors)

 

[dellphinus]

Re: NEC 430-24 Conductor Sizing

The first formula is correct. The second incorrectly adds the largest motor twice.

 

[frenchelectrican]

Re: NEC 430-24 Conductor Sizing

i do agree with dell the first formila is correct but use the second one is not the correct way because you will count large motour twice but i go one step futher is most of time i use mulply by 1.5 and add 25% to get correct size i use the same formila for med size saw mill they use 100 hp motour also ( wired with 480 wye system ) genreally it will be ok but check with poco system too see if their trans is big engough to handle the load

merci marc

 

[iwire]

Re: NEC 430-24 Conductor Sizing

The first formula is correct but I think jmc's way of expressing it is clearer

jmc stated

Amperes = (1.25 x Largest motor FLA)+ (FLA of all other motors)

 

[jmc]

Re: NEC 430-24 Conductor Sizing

Thanks iwire
john

 

[hbendillo]

Re: NEC 430-24 Conductor Sizing

Does it matter that all of the motors run continuously as Ked stated? Seems like Formula 2 would be more appropriate. Amps are amps motor or not.