MOTOR Control protection

[shackman]

I was instructed by plant engineering that motors were not required to have Overload relays as overload protection. I was instructed the fuses were protection enough for the NEC. Can anyone provide me with the Articles within the NEC to prove/disprove this engineer?

 

[jim dungar]

Re: MOTOR Control protection

The engineer is technically correct.

The NEC requires motors to have overload protection as well as short circuit protection. It does not care if two individual devices or a single is used.

However, the NEC does restrict the size of the short circuit protective if it is also used as the overload device (the overload setting/sizing takes precedence over any other setting).

The result for sizing fuses is typically a max of 125% FLA for overload, while 175% is often used for short circuit.

 

[harryg]

Re: MOTOR Control protection

The motor overload devices are often integrated into the motor starter. But, you can use a separate overload device such as a dual-element fuse (usually located near the motor starter, not the supply breaker). If you use fuses, you must provide one for each ungrounded conductor [430.36 and 430.55]. Thus, a three-phase motor requires three fuses. Keep in mind that these devices are at the load end of the branch circuit and they do not provide short-circuit and ground-fault protection.

Motors rated more than 1-hp (without integral thermal protection) and motors 1-hp or less (automatically started) [430.32(C)], must have an overload device sized per the motor nameplate current rating [430.6(A)]. You must size the overload devices no larger than the requirements of 430.32. Motors with a nameplate service factor (S.F.) rating of 1.15 or more must have the overload protection device sized no more than 125 percent of the motor nameplate current rating.