Motor branch circuit

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marcopoloc19

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Can someone explain this to me a little better? The way I understand it is that the motor requires a minimum conductor size of #10 when protected by a regular fuse or thermal breaker. When protected by the branch-circuit short-circuit and ground-fault protection you need to to multiply the FLA by a certain multiple and then do you use the correct ampacity for the wire according to the ampacity of the inverse time breaker?


With the following question. What size branch-circuit conductor does a 71/2-hp, 230V, 3? motor need if the conductor terminals are rated 60?C (Figure 430-5)?
(a) 14 AWG
(b) 12 AWG
(c) 10 AWG
(d) none of these
Answer: (c) 10 AWG
22A x 1.25 = 27.5A, 10 AWG, rated 30A at 60?C.
Size the branch-circuit short-circuit and ground-fault protection device per 240.6(A) and 430.52(C)(1) Ex. 1. For an inverse-time breaker: 22A x 2.5 = 55A, next size up = 60A.
 
Re: Motor branch circuit

The #10 conductors will be protected by the motor overloads in the motor starter if properly sized. The circuit breaker provides only ground fault and short circuit protection. It must be sized large enough to handle motor starting current. Many times this will result in a value for the CB higher than the conductor ampacity. The #10 motor branch circuit conductors would be capable of carrying the fault current for a short time to trip the CB in case of a fault. Hope this helps.

[ January 30, 2004, 10:10 PM: Message edited by: electricman2 ]
 
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