I'm installing some new drives, and am using an old existing cabinet that's already wired with 480VAC. The cabinet has a 250A breaker in it, fed by three 3/0 copper (90c) cables... which are fed from a 250A breaker in an MDP. The cabinet used to house a motor starter. The NEC says that 3/0 copper (90c) cables in conduit/raceway have an ampacity of 225A. Was the cable undersized from the beginning? Or was there some exception where this was allowed? The cabinet was probably installed around 1997.
Undersized breaker?
- Thread starter davaravid
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- Location
- Logan, Utah
Welcome to the forum.
You can't use the 90 degree column unless all terminal that the wire is connected to are also rated at 90 degrees which is very unlikely.(See 110.14(C))
So 3/0 copper at 75 degrees C is rated at 200 amps according to Table 310.16.
You did mention that the wire supplied a motor starter correct?
When sizing the branch circuit short circuit and ground fault protection you are permitted to use a larger sized overcurrent protection device in accordance with 430.52 for an individual motor circuit.
Chris
You can't use the 90 degree column unless all terminal that the wire is connected to are also rated at 90 degrees which is very unlikely.(See 110.14(C))
So 3/0 copper at 75 degrees C is rated at 200 amps according to Table 310.16.
You did mention that the wire supplied a motor starter correct?
When sizing the branch circuit short circuit and ground fault protection you are permitted to use a larger sized overcurrent protection device in accordance with 430.52 for an individual motor circuit.
Chris
Welcome to the forum.
You can't use the 90 degree column unless all terminal that the wire is connected to are also rated at 90 degrees which is very unlikely.(See 110.14(C))
So 3/0 copper at 75 degrees C is rated at 200 amps according to Table 310.16.
You did mention that the wire supplied a motor starter correct?
When sizing the branch circuit short circuit and ground fault protection you are permitted to use a larger sized overcurrent protection device in accordance with 430.52 for an individual motor circuit.
Chris
Hi Chris,
Are you saying that the MDP breaker doesn't have to be sized to protect the 3/0 cabling?
Thanks
Chris,
How does that work out for drives ? I know that the conductors supplying the drive must be 125% of the rated input power and the GFSC protection ahead of the drive must must exceed the manufacturer's nameplate maximum, but I have not checked enough drives to see if the same situation applies as with ACL motors.
In the OP's situation, assume the drive has a rated input current of 160 amps. The 3/0 conductor at 200 amps would be correct.
What max OCP would most likely be prescribed for the drive ?
Based on 430.120, I assume it could be larger than 200 amps if the manufacture allowed.
How does that work out for drives ? I know that the conductors supplying the drive must be 125% of the rated input power and the GFSC protection ahead of the drive must must exceed the manufacturer's nameplate maximum, but I have not checked enough drives to see if the same situation applies as with ACL motors.
In the OP's situation, assume the drive has a rated input current of 160 amps. The 3/0 conductor at 200 amps would be correct.
What max OCP would most likely be prescribed for the drive ?
Based on 430.120, I assume it could be larger than 200 amps if the manufacture allowed.
- Location
- Logan, Utah
I have not dealt with a lot of large drive systems so I am not sure what the manufactures typically will specify.
Chris
- Location
- Logan, Utah
Motors and drives are treated different than other loads as far as overcurrent protection is required.
Take a look at Part IV of Article 430 for motor branch circuit short circuit and ground fault protection for a standard motor and Part X for adjustable speed drive systems.
Chris
Chris,
That's what got me to thinking. Motor SCGF protection is not directly addressed in Part X. Part X refers you back to Part I-X
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