Breaker Protection for Wye-Delta Starter Arrangement

[mull982]

We have a new air compressor unit that has a 480V wye-delta starter. From a circuit breaker in the MCC we have one set of 4/0 cables going to to the main contactor in the compressor and the other set of cables going from the breaker and landing on the delta contactor of the compressor. So esentially the two sets of cables are actually part of the windings of the compressor motor and the line current is on the line side of the breaker. The compressor also has some auxiliary equipment that is tapped off of the delta contactor.

The breaker in the MCC that I am refering to is a 400A breaker, and I am now questioning if this breaker is adequate to protect this 4/0 cable. Assuming 4/0 is rated for 260A I could see that it would be possible for 260A to flow in these cables and thus not be protected by the breaker.

The part that is confusing to me is the fact that these cables are also part of the motor windings. The motor is only rated at 241FLA so the current that we should see in these cables is 139.3A, and I believe the auxiliaries are only a few hp. There is a motor overload set inside the compressor looking at the winding currents.

So even though these 4/0 cables I am referencing are actually part of the windings, and there is overload protection for the main motor is there still a code violation with having these 4/0 cables on a 400A breaker?

 

[Smart $]

... is there still a code violation with having these 4/0 cables on a 400A breaker?

I believe so. Conductors on the line side of the controller (contactors) must be sized to 125% of the FLC. The conductors in your installation are not parallel connected, nor are they part of the windings on the line side of the controller.

On the load side they can be 72% (58% ? 125%). Your OCPD-CB in the MCC is not the controller. In some manufactured [listed] wye start, delta run starter enclosures, a distribution block is used to split the feeder to smaller sized conductors between the block and the contactors. Wiring as such under NEC-only purview is not permitted.

 

[mull982]

I believe so. Conductors on the line side of the controller (contactors) must be sized to 125% of the FLC. The conductors in your installation are not parallel connected, nor are they part of the windings on the line side of the controller.

On the load side they can be 72% (58% ? 125%). Your OCPD-CB in the MCC is not the controller. In some manufactured [listed] wye start, delta run starter enclosures, a distribution block is used to split the feeder to smaller sized conductors between the block and the contactors. Wiring as such under NEC-only purview is not permitted.

I agree that the conductors leaving the breaker are not paralleled but I would think that technically (maybe not from a code standpoint) they are still part of the winding since they are still inside of the delta of this motor. Only on the line side of where these cables terminate on the breaker do we see the true motor FLA. Are you sayting that although these cables are electrically inside the delta the NEC does not recognize anything on the line side of the contactors as being part of the windings?

Is only wiring on the load side of the contactors permitted to be reduced while eveything on the line side must be rated for full capacity. Thus the breaker would have to be reduced to provided protection for this 4/0 cable?

 

[Smart $]

I agree that the conductors leaving the breaker are not paralleled but I would think that technically (maybe not from a code standpoint) they are still part of the winding since they are still inside of the delta of this motor. Only on the line side of where these cables terminate on the breaker do we see the true motor FLA. Are you sayting that although these cables are electrically inside the delta the NEC does not recognize anything on the line side of the contactors as being part of the windings?

I agree with you in that how you have your conductors installed, they would not see the full current of the motor. Personally, I feel your installation is fine, provided your breaker's load-side lugs are listed for 2 - 4/0 conductors. Yet, I interpret the Code as not permitting conductors on the line side of the controller (or should I say portions of the controller) to be installed in such a manner unless each had an ampacity equal or greater than 125% FLC (guessing 350kcmil Cu @ 75?C). The conductors as installed are technically not paralleled so by definition fail to meet the minimum required ampacity. Ref: 430.22(C)

Is only wiring on the load side of the contactors permitted to be reduced while eveything on the line side must be rated for full capacity. Thus the breaker would have to be reduced to provided protection for this 4/0 cable?

Need more details to be certain... but assuming an inverse-time breaker and permitted up to 250% of the FLC, the 400A rating is fine. Only the non-paralleled 4/0 conductors present the compliance problem.

 

[weressl]

We have a new air compressor unit that has a 480V wye-delta starter. From a circuit breaker in the MCC we have one set of 4/0 cables going to to the main contactor in the compressor and the other set of cables going from the breaker and landing on the delta contactor of the compressor. So esentially the two sets of cables are actually part of the windings of the compressor motor and the line current is on the line side of the breaker. The compressor also has some auxiliary equipment that is tapped off of the delta contactor.

The breaker in the MCC that I am refering to is a 400A breaker, and I am now questioning if this breaker is adequate to protect this 4/0 cable. Assuming 4/0 is rated for 260A I could see that it would be possible for 260A to flow in these cables and thus not be protected by the breaker.

The part that is confusing to me is the fact that these cables are also part of the motor windings. The motor is only rated at 241FLA so the current that we should see in these cables is 139.3A, and I believe the auxiliaries are only a few hp. There is a motor overload set inside the compressor looking at the winding currents.

So even though these 4/0 cables I am referencing are actually part of the windings, and there is overload protection for the main motor is there still a code violation with having these 4/0 cables on a 400A breaker?

In a motor circuit:

Breaker - short circuit protection
OL - overload protection