Sizing Feeder Breaker

[jtgrace87]

I am trying to size a feeder breaker that will feed (3) 25HP, 35 FLA motors. Each motor will be controlled by a drive. One of the motors and drives will not be used and will be installed as an online spare incase of a breakdown. Each drive contains a 50A Circuit Breaker on the Line Side, so one of these will be open and locked until needed. So for sizing this feeder breaker, should I size it using only two motors since they will be the only two pulling load on the circuit or should I size using all three motors?

I will be using NEC 430.24 to size my feeder breaker:

Option 1- Two Motors

(35*1.25)+35= 78.75A (use an 80A Feeder Breaker)

Option 2- Three Motors

(35*1.25)+35+35= 113.75 (Use a 125A Feeder Breaker)

 

[topgone]

I am trying to size a feeder breaker that will feed (3) 25HP, 35 FLA motors. Each motor will be controlled by a drive. One of the motors and drives will not be used and will be installed as an online spare incase of a breakdown. Each drive contains a 50A Circuit Breaker on the Line Side, so one of these will be open and locked until needed. So for sizing this feeder breaker, should I size it using only two motors since they will be the only two pulling load on the circuit or should I size using all three motors?

I will be using NEC 430.24 to size my feeder breaker:

Option 1- Two Motors

(35*1.25)+35= 78.75A (use an 80A Feeder Breaker)

Option 2- Three Motors

(35*1.25)+35+35= 113.75 (Use a 125A Feeder Breaker)

I would size the feeder using the specs of the drives.

 

[GoldDigger]

I would size the feeder using the specs of the drives.

That is what the NEC requires everyone to do for branch circuits and it is probably a good idea for feeders too.

 

[Smart $]

Note 430.24 is for conductor sizing... not breaker rating determination.

For breaker rating determination, see 430.53.


And if by "drive" you mean VFD, see Article 430, Part X. Adjustable-Speed Drive Systems to determine if there are any nuances...