motor currents and wire sizing

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NEC 2002
from another thread i read about 430.52(B) - the motor branch circuit short circuit and ground fault protective device shall be capable of carrying the starting current of the motor.

Where would one find the starting torque of motors or would you use table 430.52 to compute this current?


table 430.151(B) gives the locked rotor current for motors when determining the disconnecting means.

does 430.52(b) and table 430.151(B) imply the wire to the motor also carry this current?

for example, a 3 hp motor at 460 volts is 4.6 amps and 50 ft from the starter. i have seen #14 wire used to power the motor. table 430.151(B) says the locked rotor current is 36.5 amps. i know that #14 is only rated for 15 amps.

we have a wide range of motors at the plant and i would like some guideance on this topic so i may reduce our inventory and assist maintenance with the disconnect sizing and wire size required.

thank you in advance
 

mwdkmpr

Member
Re: motor currents and wire sizing

The motor overloads will protect your wire from the high locked rotor current.
 

bennie

Esteemed Member
Re: motor currents and wire sizing

A motor circuit is not like my old socks, one size does not fit all.

The wire size, overloads, short circuit, locked rotor, and ground fault clearing devices, must be custom made for the specific motor size and power source.

[ July 18, 2003, 05:28 PM: Message edited by: bennie ]
 

iwire

Moderator
Staff member
Re: motor currents and wire sizing

14 AWG is rated 20 amps 60C or 75C, 25 amps at 90C 240.4(D) happens to require a 15 amp overcurrent device for most circuits but motors are different look at 240.4(G).

And as has already been pointed out the motor overloads will protect the branch circuit conductors from overload.
 

bennie

Esteemed Member
Re: motor currents and wire sizing

Motor FLA...Determines wire size.

Locked rotor current...Determines branch circuit overcurrent device and size of disconnect.

Motor overload prevention...Overload heater size.

Starting current...Determines necessary time before trip.
 
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