Mixed protection devices for motors

[john8791]

I am trying to determine the correct feeder protection device size for two 3-ph 480V motors. Easy enough, but we will be using an inverse time delay breaker for the feeder protection and class CC fuses for the branch protection. As class CC fuses are allowed to be up to 400% of FLC, can I still calculate the feeder protection as if they were sized 250% FLC?

Thanks

 

[__dan]

Just going from memory, group motor loads only allow 125% of the largest motor plus 100% of the remaining motors. There is no upsizing the OCPD beyond that for group starting. Motor starts would have to be staggered, non simultaneous. Single motor should be 175% for time delay fuse and 250% for thermal magnetic breaker.

If you want to provide a reference, I'll look in 430, but I don't recall something allowing 400% at the fuse size.

 

[don_resqcapt19]

The size of the motor feeder conductors is per 430.24. The feeder short circuit and ground fault protection is found in 430.62. The type of protection used does not change the required size of the conductors.

 

[john8791]

Just going from memory, group motor loads only allow 125% of the largest motor plus 100% of the remaining motors. There is no upsizing the OCPD beyond that for group starting. Motor starts would have to be staggered, non simultaneous. Single motor should be 175% for time delay fuse and 250% for thermal magnetic breaker.

If you want to provide a reference, I'll look in 430, but I don't recall something allowing 400% at the fuse size.

Basically, because even though type CC's are time delay, sometimes it is not enough for the starting current there is an exception:

430.52(C)(1) Exception #2a "... a time delay Class CC fuse shall be permitted to be increased but shall in no case exceed 400 percent of the full-load current"

 

[augie47]

As don noted, check 430.62 (Exception).
You still use values based on Table 430.52.

 

[john8791]

The size of the motor feeder conductors is per 430.24. The feeder short circuit and ground fault protection is found in 430.62. The type of protection used does not change the required size of the conductors.

I agree with you on the conductor size, but determining the value of the feeder protection device does. 430.62A says the feeder "shall be provided with a protective device having a range or setting not greater than the largest rating or setting of the branch circuit short circuit and ground fault protective device for any motor supplied by the feeder [based on the maximum permitted value for the specific type of protective device ..."

My situation is two 480V 3-ph motors, one with 34A FLC, and the other with 4.8A FLC. Feeder size per 430.24 is (34A x 1.25) + 4.8A = 47.3A, so I use 8ga wire. The feeder protective device will be a breaker. The branch protection will be the class CC fuses. The commentary for 430.62 in the NEC handbook says "For the purposes of sizing the feeder protection device, it is assumed that the same type of protective device is being used fo rthe feeder and branch circuits" This is where I'm totally confused. Am I just reading too much into it?

 

[david luchini]

I agree with you on the conductor size, but determining the value of the feeder protection device does. 430.62A says the feeder "shall be provided with a protective device having a range or setting not greater than the largest rating or setting of the branch circuit short circuit and ground fault protective device for any motor supplied by the feeder [based on the maximum permitted value for the specific type of protective device ..."

My situation is two 480V 3-ph motors, one with 34A FLC, and the other with 4.8A FLC. Feeder size per 430.24 is (34A x 1.25) + 4.8A = 47.3A, so I use 8ga wire. The feeder protective device will be a breaker. The branch protection will be the class CC fuses. The commentary for 430.62 in the NEC handbook says "For the purposes of sizing the feeder protection device, it is assumed that the same type of protective device is being used fo rthe feeder and branch circuits" This is where I'm totally confused. Am I just reading too much into it?

If your feeder protective device is an inverse time c/b and your branch protective devices are fuses, you would base the feeder protection on the c/b values.

34*2.5=85 ==> 90A c/b.

90+ 4.8 = 94.8 ==> 90A c/b.

 

[john8791]

If your feeder protective device is an inverse time c/b and your branch protective devices are fuses, you would base the feeder protection on the c/b values.

34*2.5=85 ==> 90A c/b.

90+ 4.8 = 94.8 ==> 90A c/b.

OK, I think I just wasn't understanding the commentary right. Just out of curiosity, why did you take the calculated 85 to 90 before adding the 4.8A? Wouldn't you round down since table 430.52 is a "maximum rating", ie 85 + 4.8 = 89.8 ==> 80A c/b? Thanks!

 

[david luchini]

OK, I think I just wasn't understanding the commentary right. Just out of curiosity, why did you take the calculated 85 to 90 before adding the 4.8A? Wouldn't you round down since table 430.52 is a "maximum rating", ie 85 + 4.8 = 89.8 ==> 80A c/b? Thanks!

The max. branch circuit protection for you largest motor is 90A. 34*2.5 = 85 ==> 90A (next size up per 430.52(C)(1) ex.1.)

 

[john8791]

The max. branch circuit protection for you largest motor is 90A. 34*2.5 = 85 ==> 90A (next size up per 430.52(C)(1) ex.1.)

If it were only a single motor circuit I would agree, but you took the calculated the 85 up to 90 before adding the FLC from the second motor. Sorry if I'm beating a dead horse, just want to make sure I understand it.

 

[augie47]

Unless I'm mistaken, he (David) did exactly what 430.62 directs. Determine the breaker that you would use on the largest motor. Add to that the FLA of the other loads.

 

[john8791]

Unless I'm mistaken, he (David) did exactly what 430.62 directs. Determine the breaker that you would use on the largest motor. Add to that the FLA of the other loads.

Sorry. I see it now. Thanks guys.