motor load calc needed A.S.A.P.

[shaun]

Hey guys, I need the formula for motor branch circuit calc. I know the 125% rule of FLA. but, what about when you have a 208v 3ph 20 hp motor with run load amps @72 and a start capacitor @166 amps. Is there a different formula to find the correct wire size and size conduit? If so, can someone please help me?

 

[bob]

Re: motor load calc needed A.S.A.P.

The FLA you listed is closer to a 25hp per table
430.150. It doesn't matter. Handle this motor as
you would any other as far as wire and OC device
unless the mfg specs otherwise.

 

[jim dungar]

Re: motor load calc needed A.S.A.P.

You do not use any nameplate currents in your branch circuit calculations, just use 125% of FLA from the NEC.

Though, you may want to use the higher nameplate current in your voltage drop calculations to see how bad the lights will dim during starting.

 

[shaun]

Re: motor load calc needed A.S.A.P.

Hey bob and Jim, I came up with 4awg copper and a 100amp breaker.

 

[bob]

Re: motor load calc needed A.S.A.P.

Shawn
Show us your math. Use table 430.150 for you FLA.

 

[shaun]

Re: motor load calc needed A.S.A.P.

hwy Bob, give me to this evening and I will have the math for you, kind of working on something right now

 

[shaun]

Re: motor load calc needed A.S.A.P.

hwy Bob,

oops, hey Bob not hwy Bob. HeHe

 

[bphgravity]

Re: motor load calc needed A.S.A.P.

The effect of a capacitor in a motor circuit is only to correct the power factor of the lagging current of the motor. It is not to be considered when determining the conductor size. It may have a determing factor on overload sizing however.