motor load calculations

[LMAO]

Hi, Sorry, new engineer here.

I think I know how NEC instructs to size motor feeders and disconnects; I have a 3phase, 10HP @ 208V.

1. So per table 430-250, the cable has to be 30.8 X 1.25 (about 40A).
2. motor Overload should be name plate FLC X 1.25 (about28A).
3. and per table 430-52, short circuit/GF breaker is 2.5 X name plate FLC (about 60A).

What if the overload is located near the load (not immediately after the breaker)? Can I still size the breaker higher than cable rating?

Thanks

 

[raider1]

Hi, Sorry, new engineer here.

I think I know how NEC instructs to size motor feeders and disconnects; I have a 3phase, 10HP @ 208V.

1. So per table 430-250, the cable has to be 30.8 X 1.25 (about 40A).
2. motor Overload should be name plate FLC X 1.25 (about28A).
3. and per table 430-52, short circuit/GF breaker is 2.5 X name plate FLC (about 60A).

What if the overload is located near the load (not immediately after the breaker)? Can I still size the breaker higher than cable rating?

Thanks

Welcome to the forum.

The location of the overload device from the short circuit and ground fault protective device has no bearing on sizing the short circuit and ground fault protective device.

Most of the time the overload device is located close to the motor and the short circuit ground fault protective device is located at the panelboard supplying the motor.

So simple answer is Yes, you can still size the breaker higher than the cable rating.

Chris

 

[LMAO]

Thanks....

 

[david luchini]

1. So per table 430-250, the cable has to be 30.8 X 1.25 (about 40A).
2. motor Overload should be name plate FLC X 1.25 (about28A).
3. and per table 430-52, short circuit/GF breaker is 2.5 X name plate FLC (about 60A).

Just a small point - the short circuit/GF breaker should be based on the table 430-250 FLC, not the name plate FLC.

See 430.6(A)(1)

 

[LMAO]

Just a small point - the short circuit/GF breaker should be based on the table 430-250 FLC, not the name plate FLC.

See 430.6(A)(1)

Thanks, i was not really sure about that. So you only use name plate FLC to size the overload, correct?

 

[raider1]

Thanks, i was not really sure about that. So you only use name plate FLC to size the overload, correct?

Correct, the nameplate FLC is only used to size the overloads.

Chris