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- Location
- The Great Meme State
- Occupation
- Electrical Engineer
Specification says:
30.2.1 the disconnect shall not carry a FLC of more than 80% of its rated current.
30.2.2(c) the disconnect serving one or more motor loads and other loads shall be not less than 115% FLC of all motor loads plus FLC of all other loads.
31.3.1 the motor branch circuit protective device (for dual-element, time delay fuse) is 175% of motor FLC.
Ok, specifically, running a 30HP motor on 480VAC, 3-P with a 150VA control transformer powering all controls.
Control transformer circuit only needs .4A FLC.
Motor FLC is 40A.
40.4 is 80% of 50.5A
So according to 30.2.1 the disconnect can be a 60A model
115% of total FLC is 40.4A*1.15=46.5A
So according to 30.2.2(c) the disconnect can be a 60A model
175% of 40A is 70A
So according to 31.3.1 the motor branch circuit fuse should be 70A
What am I missing? Why does the code specify a fuse of 175% (or an inverse-time circuit breaker of 250%) of motor FLC, but the fused disconnect only needs to be 125% of motor FLC? It seems the 'motor branch circuit protection' will never blow/trip, when there is a smaller up-stream protection.
30.2.1 the disconnect shall not carry a FLC of more than 80% of its rated current.
30.2.2(c) the disconnect serving one or more motor loads and other loads shall be not less than 115% FLC of all motor loads plus FLC of all other loads.
31.3.1 the motor branch circuit protective device (for dual-element, time delay fuse) is 175% of motor FLC.
Ok, specifically, running a 30HP motor on 480VAC, 3-P with a 150VA control transformer powering all controls.
Control transformer circuit only needs .4A FLC.
Motor FLC is 40A.
40.4 is 80% of 50.5A
So according to 30.2.1 the disconnect can be a 60A model
115% of total FLC is 40.4A*1.15=46.5A
So according to 30.2.2(c) the disconnect can be a 60A model
175% of 40A is 70A
So according to 31.3.1 the motor branch circuit fuse should be 70A
What am I missing? Why does the code specify a fuse of 175% (or an inverse-time circuit breaker of 250%) of motor FLC, but the fused disconnect only needs to be 125% of motor FLC? It seems the 'motor branch circuit protection' will never blow/trip, when there is a smaller up-stream protection.
