I have a cabinet that controls two motors. M1: 100hp 460V 3phase. M2: 40hp 460V 3 phase. M1 uses 2/0 and controlled by VFD, fuse protected at 200 amps (Altivar recommended). M2 uses 6 AWG overload protection and motor starter, fused at 90 amps. The feeder comes from a 400A instantaneous breaker, 4/0 conductors into the cabinet disconnect rated at 400A (non fusible). Then goes through a distribution block and branches off to the 2 motors. Question: Is it ok to use 4/0 (wire sized for the load) with a feeder breaker 400A instantaneous? That breaker was all that was available and we even called GE to verify our application. Thanks for any help.
Feeder wire sizing for multiple motors
- Thread starter hurt33
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Unless your 400 amp breaker is part of a listed combination motor controller , it is not allowed to be an instantaneous trip { 430.52(C)(3)}, but at 400 amp I would bet it is an Inverse time breaker.
If you look at the motor HP alone, a 4/0 is sufficient size for the feeder (207 amps required), however with the drive the numbers might be different. A drive is required to be sized at 125% of the drive input.
Also, based on motor HP (not tsaking the drive in consideration) a 350 amp breaker would be the correct size.
From what you state, it seems the two motors are controlled from a motor control center, if so it should have a nameplate marking showing the required feeder ampacity and maximum GFSC protection..
If you look at the motor HP alone, a 4/0 is sufficient size for the feeder (207 amps required), however with the drive the numbers might be different. A drive is required to be sized at 125% of the drive input.
Also, based on motor HP (not tsaking the drive in consideration) a 350 amp breaker would be the correct size.
From what you state, it seems the two motors are controlled from a motor control center, if so it should have a nameplate marking showing the required feeder ampacity and maximum GFSC protection..
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petersonra
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- engineer
I don't see a problem with the feeder size, although I agree with the other poster that a 400A instantaneous trip CB is not appropriate.
I am not real sure that you could protect the 4/0 feeder circuit adequately with a 400A anything. It kind of depends on just what you have.
It's definitely instantaneous. I'm still learning and I helped design this cabinet. I wondered about the instantaneous breaker b/c I haven't seen anything in the NEC about using them for motor feeder ckts. There is no MCC. So my best bet would be to swap out this breaker with an inverse time breaker? If I have a PE EE mentoring me through this and making all the major decisions, I'm not too inclined to contest it. It wasn't until electricians brought it to my attention.
Since the instantaneous trip units are only allowed in listed controllers, you will not find anything about them as feeder protection except for a guide in sizing the feeder protection (see Exception in 430.62)
An Instantaneous trip breaker for the 100 HP would probably be sized above 1000 amps so I would be surprised to see the 400 work even if it were allowable.
You mentioned you help design the cabinet. Are there any protection devices ahead of the drive and motor starter (none were mentioned) ?
An Instantaneous trip breaker for the 100 HP would probably be sized above 1000 amps so I would be surprised to see the 400 work even if it were allowable.
You mentioned you help design the cabinet. Are there any protection devices ahead of the drive and motor starter (none were mentioned) ?
I would agree up to the point we added a drive. Without knowing the input rating of the drive, I'm not sure we can use the 100 HP FLA as a calculation basic.
petersonra
Senior Member
- Location
- Northern illinois
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- engineer
since when can you protect the 4/0 feeder with a 400A breaker?
incidentally, it seems quite possible that the 400A instantaneous breaker would not trip. the vfd will limit the startup current on the large motor, and it probably would not trip when starting the smaller one.
Possibly in many motor/HVAC applications. The scenario I was looking at would be a 100 hp & 40 hp motor (without drive)
The max GFSC protection for the 100 HP would be a 350 amp breaker, (124 x 2.4)
The required branch circuit would be a 2/0 (124 x 1.25)
The 40 HP would be a #6 Feeder (52 x 1.25) with a 150 amp breaker,
Motor feeder SCGF protection (430.62) would permit a breaker rated at the larger motor SCGF (350 amps) + the other motor FLA (52 amps) or a 400 amp breaker
Feeder conductors (430.24) would allow the larger motor FLA X 1.25 (124 x 1.25 = 155 amps) + the FLA of other motors
In this case 155 + 52 or 207 amps or a 4/0.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Where does it allow you to use this method when there is a drive involved?
- Location
- Illinois
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- retired electrician
I don't think it does. As far as I know the supply conductors to a drive have to be protected at or below thier ampacity.
I don't know that it does. As I mentioned in Post #2, the drive adds a whole different ingredient to the soup.
My post was simply to show the OP a 4/0 feeder on a 400 amp breaker was not necessarily a concern.
Any installation with multiple motors and drives I have encountered were MCCs with a nameplate giving the needed numbers.
If I were to encounter this situation, I would be asking for engineering input.
Don,
I agree and thats why I asked the OP about drive protection.
As noted above, I am unclear as how one sizes Feeder SCGF prpotection when drives are involved.
Augie,
The drive is protected with 200amp fuse, the motor starter 90amp fuse. The cabinet I helped design is already in operation, has been for a year now. Just didn't come to my attention until I had to do a second. So I know it works without tripping the breaker. I had a PE advise me through the first.
The drive is protected with 200amp fuse, the motor starter 90amp fuse. The cabinet I helped design is already in operation, has been for a year now. Just didn't come to my attention until I had to do a second. So I know it works without tripping the breaker. I had a PE advise me through the first.
I decided a few posts ago, that in my attempt to be helpful, I probably let my *** overload ** ***** 
Code-wide I can not come up with a rationale to allow a the 400 amp breaker,
Time for me to hush and learn
(I will stick to my guns than an instantaneous breaker in this situation does not meet Code)
Code-wide I can not come up with a rationale to allow a the 400 amp breaker,
Time for me to hush and learn
(I will stick to my guns than an instantaneous breaker in this situation does not meet Code)
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