Tap Rule for MCC Feeders

[philly]

I'm looking at at a couple of MCC feeder applications and I was wondering where the condtions of the "tap rule" would be applicble in these scenarios.

The first scenario involveds a 500MCM cable which is fed from an upstream 600A feeder breaker and terminates on a 400A main breaker on the downstream MCC. Obviously the 600A feeder breaker does not adequately protect this cable however could the tap rule be applied since the cable terminates in to a single OCPD and therefore the the 400A main breaker in the MCC will adequately protect the feeder cable?

The second scenario is similar to the first however we now have (2) 500MCM cables fed from a 1200A breaker and terminate into a 1000A main breaker in the MCC. Even if the tap rule were to be applied in this case the 1000A main breaker in the MCC would not adequatley protec tthe feeder cable. However if the MCC main breaker was an 800A breaker then with the tap rule applied the breaker would adequately protect the cables is this correct?

The last scenario is a case where a 600A feeder breaker feeds a MLO MCC and then off of the MCC bus there is a 500MCM cable tapped (no OCPD) that then feeds another downstream MCC with a 400A main breaker. In this case the tapped feeder from the MCC is not protected by the upstream 600A breaker, however can it be considered protected by the 400A main breaker in the downstream MCC which it feeds?

Thanks

 

[petersonra]

I'm looking at at a couple of MCC feeder applications and I was wondering where the condtions of the "tap rule" would be applicble in these scenarios.

The first scenario involveds a 500MCM cable which is fed from an upstream 600A feeder breaker and terminates on a 400A main breaker on the downstream MCC. Obviously the 600A feeder breaker does not adequately protect this cable however could the tap rule be applied since the cable terminates in to a single OCPD and therefore the the 400A main breaker in the MCC will adequately protect the feeder cable?

If the other tap rules are followed, and the calculated load is < 380A, I see nothing wrong with this.

The second scenario is similar to the first however we now have (2) 500MCM cables fed from a 1200A breaker and terminate into a 1000A main breaker in the MCC. Even if the tap rule were to be applied in this case the 1000A main breaker in the MCC would not adequatley protec tthe feeder cable. However if the MCC main breaker was an 800A breaker then with the tap rule applied the breaker would adequately protect the cables is this correct?

I think the second scenario is Ok as long as the calculated load < 760A.

The last scenario is a case where a 600A feeder breaker feeds a MLO MCC and then off of the MCC bus there is a 500MCM cable tapped (no OCPD) that then feeds another downstream MCC with a 400A main breaker. In this case the tapped feeder from the MCC is not protected by the upstream 600A breaker, however can it be considered protected by the 400A main breaker in the downstream MCC which it feeds?

I think it is OK tap wise anyway, as long as the bus is rated for at least 600A and the calculated load on the 500 kCM conductor < 380A. Whether it is legal to tap the bus itself is something else. I would say not.

Thanks

keep in mind there are requirements for taps other than just the OCPD rating downstream.

 

[lielec11]

I'm looking at at a couple of MCC feeder applications and I was wondering where the condtions of the "tap rule" would be applicble in these scenarios.

The first scenario involveds a 500MCM cable which is fed from an upstream 600A feeder breaker and terminates on a 400A main breaker on the downstream MCC. Obviously the 600A feeder breaker does not adequately protect this cable however could the tap rule be applied since the cable terminates in to a single OCPD and therefore the the 400A main breaker in the MCC will adequately protect the feeder cable?

The second scenario is similar to the first however we now have (2) 500MCM cables fed from a 1200A breaker and terminate into a 1000A main breaker in the MCC. Even if the tap rule were to be applied in this case the 1000A main breaker in the MCC would not adequatley protec tthe feeder cable. However if the MCC main breaker was an 800A breaker then with the tap rule applied the breaker would adequately protect the cables is this correct?

The last scenario is a case where a 600A feeder breaker feeds a MLO MCC and then off of the MCC bus there is a 500MCM cable tapped (no OCPD) that then feeds another downstream MCC with a 400A main breaker. In this case the tapped feeder from the MCC is not protected by the upstream 600A breaker, however can it be considered protected by the 400A main breaker in the downstream MCC which it feeds?

Thanks

If you look at 240.21(B) (and if you have the handbook, the commentary that follows) you'll see that the provisions of 240.4(B) do not apply to taps. For example a 500kcmil feeder as a tap (good for 380A) cannot be used to supply a 400A OCPD.

 

[petersonra]

If you look at 240.21(B) (and if you have the handbook, the commentary that follows) you'll see that the provisions of 240.4(B) do not apply to taps. For example a 500kcmil feeder as a tap (good for 380A) cannot be used to supply a 400A OCPD.

It does say that. I take back what I said above.

 

[philly]

If you look at 240.21(B) (and if you have the handbook, the commentary that follows) you'll see that the provisions of 240.4(B) do not apply to taps. For example a 500kcmil feeder as a tap (good for 380A) cannot be used to supply a 400A OCPD.

O.k so then none of the examples provided above would be ok because of 240.21(B). I'm guess they would be o.k. if the conducter was smaller than the upstream feeder breaker but met or exceeded the rating of the MCC main breaker. For example if scenario 1 was a 600MCM cable instead of 500MCM?

 

[lielec11]

O.k so then none of the examples provided above would be ok because of 240.21(B). I'm guess they would be o.k. if the conducter was smaller than the upstream feeder breaker but met or exceeded the rating of the MCC main breaker. For example if scenario 1 was a 600MCM cable instead of 500MCM?

I believe that is correct.