Special permission?

[wwhitney]

continuous load is the driving element behind both conductor and CB sizing, per NEC

Certainly the rules as written do not distinguish. But the reasoning behind those rules goes in this order:

- A non-100% rated breaker may nuisance trip at a continuous load exceeding 80% of its rating (not an immediate safety issue, but a problem).
- Therefore the breaker needs to be increased in size by a 25% factor. Now the conductors are no longer protected properly (a safety issue).
- Therefore the conductors get increased in ampacity .by a 25% factor as well.

You are missing the point of scenario (B), the load is 1000A continuous in all scenarios. There is no safety issue (overloading/overheating anything), other than the breaker tripping when we don't want it to, and whatever knock-on effects that could have.

And as a practical matter, in mild ambient temperatures, in not heavily loaded panels, a breaker may hold indefinitely at its nameplate rating. In which case there would be nothing wrong with scenario (B). It is simply the uncertainty as to whether the breaker would hold that causes the NEC to prohibit scenario (B).

And that's all a 100% rated breaker is--a breaker alone in an enclosure designed to ensure that it doesn't trip below 100% of its rating under continuous loading conditions. In free air, at 40C ambient, all breakers are tested to hold continuously at 100% of their rating.

Cheers, Wayne

 

[Isaiah]

Certainly the rules as written do not distinguish. But the reasoning behind those rules goes in this order:

- A non-100% rated breaker may nuisance trip at a continuous load exceeding 80% of its rating (not an immediate safety issue, but a problem).
- Therefore the breaker needs to be increased in size by a 25% factor. Now the conductors are no longer protected properly (a safety issue).
- Therefore the conductors get increased in ampacity .by a 25% factor as well.

You are missing the point of scenario (B), the load is 1000A continuous in all scenarios. There is no safety issue (overloading/overheating anything), other than the breaker tripping when we don't want it to, and whatever knock-on effects that could have.

And as a practical matter, in mild ambient temperatures, in not heavily loaded panels, a breaker may hold indefinitely at its nameplate rating. In which case there would be nothing wrong with scenario (B). It is simply the uncertainty as to whether the breaker would hold that causes the NEC to prohibit scenario (B).

And that's all a 100% rated breaker is--a breaker alone in an enclosure designed to ensure that it doesn't trip below 100% of its rating under continuous loading conditions. In free air, at 40C ambient, all breakers are tested to hold continuously at 100% of their rating.

Cheers, Wayne

“You are missing the point of scenario (B), the load is 1000A continuous in all scenarios. There is no safety issue (overloading/overheating anything), other than the breaker tripping when we don't want it to, and whatever knock-on effects that could have”

Absolutely incorrect. According to this statement section 215.2(A)(1) has no reason to exist.


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[david luchini]

Conceptually I maintain the need to upsize the cable based on continuous load. The real question is what exactly is the continuous load, 960A or is it 1200A?

The load is neither 960A or 1200A. The load is what is calculated from the rules in Article 220. The size of the c/b or mcc bus doesn't determine the load.

If it’s 960A based on the 80 percentile of the MCB then the cable is OK. If it’s 1200A then the cable needs to be increased to 1500A. If the CB is 100% rated then nothing has to change

If the load is 1200A continuous then BOTH the cable and the c/b size need to increase to 1500A. You can't use the 1200A feeder that you described for a 1200A continuous load.

 

[Dale001289]

The load is neither 960A or 1200A. The load is what is calculated from the rules in Article 220. The size of the c/b or mcc bus doesn't determine the load.

If the load is 1200A continuous then BOTH the cable and the c/b size need to increase to 1500A. You can't use the 1200A feeder that you described for a 1200A continuous load.

only the design engineer knows what the load is so it’s not possible to use article 220. We’re talking code here not design. the maximum possible continuos load is still 960A with a 1200A MCB. This was established earlier in this thread by the OP.


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[infinity]

If the load is 1200A continuous then BOTH the cable and the c/b size need to increase to 1500A. You can't use the 1200A feeder that you described for a 1200A continuous load.

I agree and if it's non-continuous, (which it is) as long as the calculated load is 1200 amps or less the 1200 amp feeder is code compliant.

 

[don_resqcapt19]

only the design engineer knows what the load is so it’s not possible to use article 220. We’re talking code here not design. the maximum possible continuous load is still 960A with a 1200A MCB. This was established earlier in this thread by the OP.


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The word possible should be changed to permitted.

As stated a number of times in this thread, most recently by Wayne in post 61, a standard breaker may carry 100% of its load forever where the ambient at the breaker is less than 40°C.

 

[Dale001289]

I agree and if it's non-continuous, (which it is) as long as the calculated load is 1200 amps or less the 1200 amp feeder is code compliant.

Mcc loads are nearly always non continuous since many motor loads cycle on/off within a three hour window. You’d be very hard pressed to find a 1200A mcc with 960A of continuous loading, but is it possible: yes.


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[infinity]

Mcc loads are nearly always non continuous since many motor loads cycle on/off within a three hour window. You’d be very hard pressed to find a 1200A mcc with 960A of continuous loading, but is it possible: yes.

I agree the definition of continuous load would require all of the motors to operate simultaneously for 3 continuous hours. Not impossible but not likely and given the information in this thread it would not apply to the feeder in question.

 

[Dale001289]

The word possible should be changed to permitted.

As stated a number of times in this thread, most recently by Wayne in post 61, a standard breaker may carry 100% of its load forever where the ambient at the breaker is less than 40°C.

Agreed. Permitted is a better word choice here


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[david luchini]

the maximum possible continuos load is still 960A with a 1200A MCB. This was established earlier in this thread by the OP.

Ya think?

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.

 

[Dale001289]

Ya think?

Oops, sorry about that. But regardless who said it the statement is correct.


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[david luchini]

Oops, sorry about that. But regardless who said it the statement is correct.

The load is what Article 220 says it is...but it's pretty much irrelevant to the discussion.

The original poster thinks he needs 1500A worth of conductor to supply the 1200A MCC. This is wrong, regardless of what the load is.

 

[Dale001289]

The load is what Article 220 says it is...but it's pretty much irrelevant to the discussion.

The original poster thinks he needs 1500A worth of conductor to supply the 1200A MCC. This is wrong, regardless of what the load is.

If you were feeding a fixed, 1200A continuos load how would you size the feeder? (Not a mcc load)


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[oldsparky52]

@Isaiah, your profile says you are an electrical inspector. I think it is very important for you to get this worked out in your mind. It is hard for a person to conclude that they are interpreting something incorrectly. With all the great minds on this board telling you that you misunderstand this part of the code, I hope you will take the time to sift through what is being said in this thread.

It is not fair to anyone if you try to enforce your opinion (which most of us believe is incorrect) on what the code says instead of what the code says.

Please do not take this as a personal attack, I do not mean it that way, but you are in a position of authority and it is important that you are correctly enforcing the NEC.

 

[david luchini]

If you were feeding a fixed, 1200A continuos load how would you size the feeder? (Not a mcc load)

10 sets of #1/0AWG

 

[Dale001289]

10 sets of #1/0AWG

you factored up by 125% since it’s a continuous load right? It would be the same if the mcc were continuous at 1200A


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[david luchini]

you factored up by 125% since it’s a continuous load right? It would be the same if the mcc were continuous at 1200A

You can't put a 1200A continuous load on a 1200A MCC.

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.

 

[Dale001289]

You can't put a 1200A continuous load on a 1200A MCC.

You mean it’s not ‘permitted’ - but you could still do it. And, the bus would not catch on fire or blow up.


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[d0nut]

you factored up by 125% since it’s a continuous load right? It would be the same if the mcc were continuous at 1200A

As stated before, all of the adjustments must occur before (or as) the equipment is selected. Once we have selected the equipment, there should be no more adjustment factors to apply.

 

[david luchini]

You mean it’s not ‘permitted’ - but you could still do it. And, the bus would not catch on fire or blow up.

You do realize that this is the NEC section of the Forum?