ground wire size for multiple motors

[j_braddock]

i just ran into part of an installation in the plant that i question.

the contractor had wiring for 5 motors in a single conduit. motors were 1/2 - 2 hp each, 460 volts 3 phase. conduit run length was 100 ft and he used tee's at each motor location. he used #14 black wire for the motors and a #14 wire for the ground wire for all 5 motors.

i know to derate the wire for the temperature, derate the wire based on the number of current carring wires, but i am at a loss about the ground wire.

how do you determine the size of the ground wire for all 5 motors?

thanks in advance.
james

 

[don_resqcapt19]

Re: ground wire size for multiple motors

James,
250.122(C) permits a single EGC to be used for multiple circuits. The only requirement is that the EGC be sized for the single largest circuit in the raceway. In this case the common #14 EGC is permitted by the code.
Don

 

[pierre]

Re: ground wire size for multiple motors

There are times when the ungrounded conductors to a motor are permitted to be reduced in size considerably in proportion to the overcurrent protective device rating. 250.122 permits the EGC to be sized no larger than the ungrounded conductor. Is the impedance that the smaller conductor creates, thus reducing the fault current available the reason this is permitted?

Pierre

 

[bphgravity]

Re: ground wire size for multiple motors

Originally posted by pierre:
...Is the impedance that the smaller conductor creates, thus reducing the fault current available the reason this is permitted?

Pierre

I think it has more to do with the fact that even though the short-circuit and ground-fault protection device is sized so high to allow inrush currents, the overload device is never really more 125% to 140% the FLC current rating of the mpotor which the branch circuit conductors are based on. (250.122(D))

Im sure the overall total circuit impedance does come into consideration as well.