Motors

[memyselfandI]

Question: I have a 1/2 HP 480V 3PH motor for a garage door with contoller. The motor has overload protection. I need to feed this motor from an existing 30A 3PH 480 circuit that is available. After doing all my calcs I came to the conclusion that my BC,SC, and GF protection has to be fuses which are set at a value of not more that 2 amps. However 2 amps is not a standard fuse size and I must go up to 3 amp. No data on the controller, only what is on the motor. Is this correct and if not what and where am I doing wrong?

 

[ramsy]

If the Mfg. lists or Motor nameplate shows max BC, SC, or GF less than 3A, you can't exceed it per 430-52(C)(2). You would be left with 430-52(C) Exception 2, and purchasing an instantaneous-trip 15A CB for use with a listed combo controller that coordinates overload and fault protection.

If start-up current was not an issue, (ie) soft starter or VSD, you're allowed up to 3.3A fuses for that motor. Look at table 430.52, see the 300% FLC value for non-time delay fuses.

If the 10A start-up current is an issue, (ie) table 430.151B, you can use a duel element, time-delay fuse at 225% FLC of Tbl. 430.150. Not sure if you're allowed the next higher STD ampere rating (ie) 2.5 to 3A. See 430.52(C) Exception 1 & 2b.

 

[davidr43229]

While a 2 amp fuse is manufactured in a Class J, CC, RK-1 & RK-5, The Bussmann SPD says to go to a 3 amp, providing you have an FLA of 1.1 amps.
Just my $.02