Special permission?

[Isaiah]

I'm looking at a feeder that terminates in a 1200A MCB with a 80% rating for a 480V MCC. The E&C firm did not add 125% to the feeder ampacity as required by section 215.2(A)(1)(a). The MCC is considered a 'continuous load' but its lightly loaded. Is it possible to use a 'special permission scenario' by the AHJ per 90.2(C) to avoid rework if the calculated load shows the MCC will never go beyond 80% of its overall capacity?

 

[don_resqcapt19]

I'm looking at a feeder that terminates in a 1200A MCB with a 80% rating for a 480V MCC. The E&C firm did not add 125% to the feeder ampacity as required by section 215.2(A)(1)(a). The MCC is considered a 'continuous load' but its lightly loaded. Is it possible to use a 'special permission scenario' by the AHJ per 90.2(C) to avoid rework if the calculated load shows the MCC will never go beyond 80% of its overall capacity?

I can't imagine that an MCC would ever be a continuous load.

Continuous Load.
A load where the maximum current is expected to continue for 3 hours or more. (CMP-2)

Are you saying that every load in the MCC will be running at full load at the same time for 3 hours or more?

 

[david luchini]

I'm looking at a feeder that terminates in a 1200A MCB with a 80% rating for a 480V MCC. The E&C firm did not add 125% to the feeder ampacity as required by section 215.2(A)(1)(a). The MCC is considered a 'continuous load' but its lightly loaded. Is it possible to use a 'special permission scenario' by the AHJ per 90.2(C) to avoid rework if the calculated load shows the MCC will never go beyond 80% of its overall capacity?

What size is the feeder conductor?

 

[infinity]

As asked we need to know the conductor size. From the OP if the conductors are sized for the OCPD there may not be any problem.

 

[Isaiah]

I can't imagine that an MCC would ever be a continuous load.



Are you saying that every load in the MCC will be running at full load at the same time for 3 hours or more?

The loading presently will not operate at full capacity for 3 hours, however, the MCC has multiple spare capacity. I'm concerned with what they may add in the future.

 

[Isaiah]

What size is the feeder conductor?

The MCC horizontal bus is at 1200A. The feeder is 600kCMIL, 3 per phase, per 310.16 column 75Deg C: 420A X 3 = 1260A feeder size.

 

[david luchini]

The MCC horizontal bus is at 1200A. The feeder is 600kCMIL, 3 per phase, per 310.16 column 75Deg C: 420A X 3 = 1260A feeder size.

1260A feeder ampaciy is more than 1200A bus rating (and more than the 1200A c/b rating.) I'm not sure what you think is missing.

 

[Isaiah]

1260A feeder ampaciy is more than 1200A bus rating (and more than the 1200A c/b rating.) I'm not sure what you think is missing.

What about the 125% feeder requirement? The MCB is not rate 100% - its rated 80%

 

[david luchini]

What about the 125% feeder requirement? The MCB is not rate 100% - its rated 80%

1260A is more than 1200A. If the breaker has the 125% in there (ie, 80% rated), then so does the feeder.

 

[Isaiah]

1260A is more than 1200A. If the breaker has the 125% in there (ie, 80% rated), then so does the feeder.

I don't think it works that way. The CB does not inherently have 125% capacity if its rated only 80%.
However, when the circuit breaker is listed for operation at 100% of its rating, the additional 25% requirement goes away. Instead, the device simply has to be able to handle the sum of the continuous load and the non-continuous load. I do believe however, the MCC will never see full load for three straight hours of operation even factoring in the spare buckets.

 

[don_resqcapt19]

I don't think it works that way. The CB does not inherently have 125% capacity if its rated only 80%.
However, when the circuit breaker is listed for operation at 100% of its rating, the additional 25% requirement goes away. Instead, the device simply has to be able to handle the sum of the continuous load and the non-continuous load. I do believe however, the MCC will never see full load for three straight hours of operation even factoring in the spare buckets.

Both the CB and the feeder have a continuous rating of 960 amps and a non-continuous rating of 1200 amps. I still can't imagine an MCC being a continuous load, and don't see any issues.

 

[Isaiah]

Both the CB and the feeder have a continuous rating of 960 amps and a non-continuous rating of 1200 amps. I still can't imagine an MCC being a continuous load, and don't see any issues.

In this particular case, I agree with you. MCC oneline diagram shows it being lightly loaded and, even with spare capacity factored-in, will never see 1200A for three straight hours.
Thanks to all for great feedback!

 

[david luchini]

I don't think it works that way. The CB does not inherently have 125% capacity if its rated only 80%.
However, when the circuit breaker is listed for operation at 100% of its rating, the additional 25% requirement goes away. Instead, the device simply has to be able to handle the sum of the continuous load and the non-continuous load. I do believe however, the MCC will never see full load for three straight hours of operation even factoring in the spare buckets.

If the circuit breaker is only rated 80% (960A), then a feeder rated for 1260A has 131% capacity above that 960A....which is more than 125%.

But that's not how it works either. How it works is that the 1200A c/b has to protect the feeder conductor at it's ampacity, per 240.4, which it does.

 

[Isaiah]

If the circuit breaker is only rated 80% (960A), then a feeder rated for 1260A has 131% capacity above that 960A....which is more than 125%.

But that's not how it works either. How it works is that the 1200A c/b has to protect the feeder conductor at it's ampacity, per 240.4, which it does.

Agreed on 240.4, but you also have to look at 215.2(A)(1)(a). If the theoretical max continuous load is 1200A (i.e. MCC horizontal bus size) the feeder must be sized-up to 1200A x 1.25 = 1500A. The difference here is the MCC loading is not continuous at 1200A, therefore increasing the feeder size is not justified, in this case.

 

[david luchini]

If the theoretical max continuous load is 1200A (i.e. MCC horizontal bus size) the feeder must be sized-up to 1200A x 1.25 = 1500A.

This is not correct. There is nothing in the Code which requires a 1500A feeder for a 1200A circuit breaker.

If you have 960A continuous load, you would need a 1200A c/b and a 1200A feeder.

If you have a 1200A non-continuous load, you would need a 1200A c/b and a 1200A feeder.

 

[Isaiah]

This is not correct. There is nothing in the Code which requires a 1500A feeder for a 1200A circuit breaker.

If you have 960A continuous load, you would need a 1200A c/b and a 1200A feeder.

If you have a 1200A non-continuous load, you would need a 1200A c/b and a 1200A feeder.

The non continuous load is 960 and the continuous is 1200A. Therefore you need 1500A on the feeder. Its the same for branch circuits. If you have a 20A CB rated 80% on a lighting panel, the cable needs to be min #12 or 20A x 125%, i.e. 25A, per 240.4. In this case the MCC will not see 1200A continuous so its a moot point.

 

[david luchini]

The non continuous load is 960 and the continuous is 1200A, which accounts for the 125%. Therefore you need 1500A on the feeder.

Well...no. You would need 1200A on the feeder if the the feeder c/b is 1200A. I'm not sure what is causing your misunderstanding about 215.2(A)(1)(a).

Try this...
You have a 960A continuous load. 215.2(A)(1)(a) says that the allowable ampacity of the feeder conductor shall not be less than 125% of the continuous load. 960Ax125%= 1200A. Three sets of 400MCM has an ampacity of 1260, so that complies with 215.2(A)(1)(a).

215.3 says the overcurrent protection for the feeder shall not be less than 125% of the continuous load. 960Ax125%=1200A. So the 1200A c/b complies with 215.3.

There is nothing there that would require a 1500A feeder for the 1200A c/b or the 960A continuous load.

 

[Isaiah]

Well...no. You would need 1200A on the feeder if the the feeder c/b is 1200A. I'm not sure what is causing your misunderstanding about 215.2(A)(1)(a).

Try this...
You have a 960A continuous load. 215.2(A)(1)(a) says that the allowable ampacity of the feeder conductor shall not be less than 125% of the continuous load. 960Ax125%= 1200A. Three sets of 400MCM has an ampacity of 1260, so that complies with 215.2(A)(1)(a).

215.3 says the overcurrent protection for the feeder shall not be less than 125% of the continuous load. 960Ax125%=1200A. So the 1200A c/b complies with 215.3.

There is nothing there that would require a 1500A feeder for the 1200A c/b or the 960A continuous load.

If 960A were in fact the continuous load, you'd be correct, but without knowing the precise loading per the one-line diagram, this is only an assumption. What if the MCC was heavily loaded? That's why the NEC requires you to use 1200A as the theoretical connected (continuous) load. In this case, the calculated load has proven far less than 1200A, so its a moot point and the feeder does not need to be upsized to 1500A. Also, three sets of 400kCMIL does not equal 1260A (See 110.14(C)(1) and Table 310.16, 75 Degree column).

 

[david luchini]

If 960A were in fact the continuous load, you'd be correct, but without knowing the precise loading per the one-line diagram, this is only an assumption. What if the MCC was heavily loaded? That's why the NEC requires you to use 1200A as the theoretical connected (continuous) load. In this case, the calculated load has proven far less than 1200A, so its a moot point and the feeder does not need to be upsized to 1500A. Also, three sets of 400kCMIL does not equal 1260A (See 110.14(C)(1) and Table 310.16, 75 Degree column).

It's irrelevant how lightly or how heavily the MCC is loaded. You cannot put 1200A of continuous load on the 1200A MCC. The theoretical maximum continuous load you can put on the MCC is 960A. The theoretical maximum non-continuous load you can put on the MCC is 1200A.

In either case, the feeder would never be required to have an ampacity of 1500A. It would only require an ampacity that is not less than 1200A, as in the case of your 3 sets of 600mcm (the 400 was a typo.)

 

[don_resqcapt19]

If 960A were in fact the continuous load, you'd be correct, but without knowing the precise loading per the one-line diagram, this is only an assumption. What if the MCC was heavily loaded? That's why the NEC requires you to use 1200A as the theoretical connected (continuous) load. In this case, the calculated load has proven far less than 1200A, so its a moot point and the feeder does not need to be upsized to 1500A. Also, three sets of 400kCMIL does not equal 1260A (See 110.14(C)(1) and Table 310.16, 75 Degree column).

Where does the code require that?