Wye Start Delta Run Motor Wire Sizing

[gwpowell]

Please provide any information on wire sizing for wye start delta run motors. The 2002 NEC states in 430.21(c) that the conductors on the line side of the contactors should be sized based on the motor FLC, I assume that the 1.25 multiplier should be applied to the FLC before chosing the conductor size is this correct? NEC 430.21(c) then states that the contactor load side conductors should be based on 58% of FLC, should the 1.25 multiplier also be applied after the ampacity is reduced to 58%? I am working with a vendor to make sure we size everything correctly so any documentation that could be used for reference material would be appreciated.

Thanks

 

[Ed MacLaren]

Re: Wye Start Delta Run Motor Wire Sizing

gwpowell,

In wye-delta motor starter installations there are six wires between the starter and the motor. Each of these wires carries phase current (58% of the FL amps) only, not line current.

The point where the line current splits into separate phase currents is at the line terminals of the starter, not at the motor terminal box, as in a conventional motor/starter installation. (Sketch below)

Wire size between the starter and motor would be based on - FLA x .58 x 1.25

By the way, most manufacturers wire the overload relay in such a way that the setting/heater selection is based on the phase current as well.

ED

[ December 01, 2004, 07:09 PM: Message edited by: Ed MacLaren ]