MCA and EGC sizing

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Dennis Alwon said:
I did not read that you must use the 60C rating, I inferred it based on this



60C is the minimum size required-- a #4 wire does not care if it has 60C or 90C temp rating for a fault. The capacity of the #4 is really limited because of the temp of the insulation. In theory we should always use the 90C rating but the statement above says the minimum size. I think the changes creates more trouble then it will fix.

I assume the reason we can use smaller conductors for the egc is because the conductors themselves can carry the faults and the insulation should not come into play. Just a thought.
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Wouldn't 75? C terminations at both ends and a 75? C conductor be the minimum size? Using 60? C would give you a larger size conductor.
 
Just out of curiosity why are we talking about a 50A circuit? The MCA is 43.9A so for continuous load the calc would be 54.9A and the OCPD is 60A which is what is designated on the equipment label. As far as I can see this is a 60A circuit. The EGC is not a current carrying conductor (only in a fault situtation) and therefore is not subject to a termination temperature rating as far as I know. The EGC minimum size is determined by the I^2T (withstand) of the conductor. The amount of fault current available and the trip characteristics of the OCPD are what determine if the levels/duration of the fault current are within the withstand rating of the EGC conductor. If you have excessive fault currents available then you may have to look again at sizing in which case this is usually calculated out by an engineer in the coordination study. The only requirement that I am aware of for upsizing from that listed in 250.122 is for when you upsize due to voltage drop which is appropriate but does not appear to be the case here. So per 250.122 a 60A circuit calls for #10 EGC. This would meet the requirements of NEC and I think discussion about the temperature rating of the current carrying conductor terminations are a bit off course. Am I missing something?
 
BPoindexter said:
Just out of curiosity why are we talking about a 50A circuit? The MCA is 43.9A so for continuous load the calc would be 54.9A and the OCPD is 60A which is what is designated on the equipment label. As far as I can see this is a 60A circuit. The EGC is not a current carrying conductor (only in a fault situtation) and therefore is not subject to a termination temperature rating as far as I know. The EGC minimum size is determined by the I^2T (withstand) of the conductor. The amount of fault current available and the trip characteristics of the OCPD are what determine if the levels/duration of the fault current are within the withstand rating of the EGC conductor. If you have excessive fault currents available then you may have to look again at sizing in which case this is usually calculated out by an engineer in the coordination study. The only requirement that I am aware of for upsizing from that listed in 250.122 is for when you upsize due to voltage drop which is appropriate but does not appear to be the case here. So per 250.122 a 60A circuit calls for #10 EGC. This would meet the requirements of NEC and I think discussion about the temperature rating of the current carrying conductor terminations are a bit off course. Am I missing something?
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The MCA is used to size the conductor and already has the 25% added by the manufacturer's calculation. So no additional 25% is required.
 
infinity said:
The MCA is used to size the conductor and already has the 25% added by the manufacturer's calculation. So no additional 25% is required.
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Thanks for that info! However we are still talking a 60A OCPD with #6's and a #10 EGC. Meets NEC and normal engineering requirements. Size determined by the OCPD.
 
BPoindexter said:
Thanks for that info! However we are still talking a 60A OCPD with #6's and a #10 EGC. Meets NEC and normal engineering requirements. Size determined by the OCPD.
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The question; by using #6 THHN are the #6 conductors increased in size which would trigger the requirements of 250.122(B)? Consensus is that since a smaller conductor, #8 THHN would be sufficient then by using a #6 you have increased the size of the ungrounded conductor which requires a proportional increase in the EGC.
 
infinity said:
The question; by using #6 THHN are the #6 conductors increased in size which would trigger the requirements of 250.122(B)? Consensus is that since a smaller conductor, #8 THHN would be sufficient then by using a #6 you have increased the size of the ungrounded conductor which requires a proportional increase in the EGC.
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Which makes 250.122(B) silly!

Sorry I just had to chime in.
I gonna start a new thread 250.122(B) if that is ok!:?
 
I looked at the new thread and I do not have an issue with upsizing the EGC as per 250.122(B). I do not believe it is being properly applied in this situation. According to the table #10 is good up to 60A. #8 is good for 50A at 75C and as long as you are using properly rated terminations at each end. Sizing at #6 may not necessarily be an upsize but again still within the range of 30-60A. If this were a long run or due to derating you had to upsize to say #2 on a 60A breaker then I would say you need to upsize the EGC. However in this case we are really not 'upsizing' as a typical 60A circuit would use #6's as the normal required size. This issue boils down to are you upsizing the conductors? I would say no because you are still in within the normal wiring range of the 60A OCPD. This goes back to fault clearing. As you increase the size of the conductors then you would decrease the impedance and therefore increase the amount of current during a fault thus possibly requiring a larger EGC to meet withstand requirements. However in the case of a #10 EGC it is good for a circuit with up to a 60A OCPD. Logic would dictate then that a #10 is sufficient for any conductor size rated for up to 60A would it not?

And yes I will join the discussion on the other thread ;)
 
BPoindexter said:
I looked at the new thread and I do not have an issue with upsizing the EGC as per 250.122(B). I do not believe it is being properly applied in this situation. According to the table #10 is good up to 60A. #8 is good for 50A at 75C and as long as you are using properly rated terminations at each end. Sizing at #6 may not necessarily be an upsize but again still within the range of 30-60A. If this were a long run or due to derating you had to upsize to say #2 on a 60A breaker then I would say you need to upsize the EGC. However in this case we are really not 'upsizing' as a typical 60A circuit would use #6's as the normal required size. This issue boils down to are you upsizing the conductors? I would say no because you are still in within the normal wiring range of the 60A OCPD. This goes back to fault clearing. As you increase the size of the conductors then you would decrease the impedance and therefore increase the amount of current during a fault thus possibly requiring a larger EGC to meet withstand requirements. However in the case of a #10 EGC it is good for a circuit with up to a 60A OCPD. Logic would dictate then that a #10 is sufficient for any conductor size rated for up to 60A would it not?

And yes I will join the discussion on the other thread ;)
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#8 THHN is the minimum size required. You use #6 THHN you've up-sized the ungrounded conductor which triggers 250.122(B). Can't get much simpler than that.
 
infinity said:
#8 THHN is the minimum size required. You use #6 THHN you've up-sized the ungrounded conductor which triggers 250.122(B). Can't get much simpler than that.
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I would respectfully disagree. I think that is an over simplification. Just one of the implications of looking at it like this is now you have MC-HL installations illegal where larger conductors are specified. As an example 4C #8 MCHL has a single #10 EGC. Per your reasoning I could not use it on a 30A circuit because the EGC is required to be a #8. As in the original case in this post you could always use the conduit for the ground but for an outdoor run this is not the best case. And most specs that I have used for installations with classified locations don't allow it and most engineers I have dealt with highly discourage it.

I don't believe this is the intent but I have been wrong before!
 
BPoindexter said:
I would respectfully disagree. I think that is an over simplification. Just one of the implications of looking at it like this is now you have MC-HL installations illegal where larger conductors are specified. As an example 4C #8 MCHL has a single #10 EGC. Per your reasoning I could not use it on a 30A circuit because the EGC is required to be a #8. As in the original case in this post you could always use the conduit for the ground but for an outdoor run this is not the best case. And most specs that I have used for installations with classified locations don't allow it and most engineers I have dealt with highly discourage it.

I don't believe this is the intent but I have been wrong before!
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I will respectfully say that you're wrong. In your example you would need a #8 EGC with a #8 ungrounded conductor on a 30 amp circuit. Not saying I like 250.122(B) but it is what it is. :roll:
 
BPoindexter said:
Logic would dictate then that a #10 is sufficient for any conductor size rated for up to 60A would it not?
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Nope. Consider the case of a light pole, 30A circuit, using #6 wire. You need a #6 EGC on that circuit.

I think we all understand the intent of the section. The problem is the words don't match the intent. Just what is the "normal" size wire in a 40A circuit? A 60A is even more complicated because most wires don't have an ampacity of 60 (it could be 55 or 60). So what is the normal size on a 60A circuit?
 
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