Ocpd
- Thread starter boku0003
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- Massachusetts
Combine the ampacity not the kcmil.
Two 500 coppers gives you about 760 amps of capacity and 240.4(B) allows a 800 amp OCPD.
Two 500 coppers gives you about 760 amps of capacity and 240.4(B) allows a 800 amp OCPD.
jumper
Senior Member
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- 3 Hr 2 Min from Winged Horses
Is this the whole question? Iwire's response is based on a best case scenario. Copper wire and a load calc that allows you to use the next larger sized breaker.
- Location
- Massachusetts
Given the info provided it has to be copper.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Bob , gave you short answer. There are other variables, connected load, number of raceways, ambient temperature etc. Disregarding all of those (et al.), I would go with Bob's recommendation.
The heart of the question was, when you are paralleling conductors, how do you size your OCPD (trying to figure out the EGC, which you need the OCPD for)? Do you size it based on the equivalent ampacity of the parallel conductors (ampacity of the single conductor times the number of conductors) or find the equivalent size conductor and look up the ampacity in 310.16. The answer was use the first method, which makes sense because you won't always have an equivalent single conductor, like if you parallel two 600 kcmils, you get 1200 kcmil, which is not a wire size in 310.16.
Would any of these scenarios change this? I'm not talking about derating, I understand how that works (more than 3 conductors, temperature, etc). I didn't think these factors changed the answer, thus I tried to keep it simple by not adding "copper, 1 conductor at ambient temp, etc..."
Would any of these scenarios change this? I'm not talking about derating, I understand how that works (more than 3 conductors, temperature, etc). I didn't think these factors changed the answer, thus I tried to keep it simple by not adding "copper, 1 conductor at ambient temp, etc..."
the problem with this question is, you are putting the cart before the horse.
your wire size and ocpd is based on the load you are serving. the rating of the ckt is based on the size of the ocpd. your conductors do not have to be protected at the largest ampacity they are able to carry, only to the ampacity they will be carrying (based on the load).
your wire size and ocpd is based on the load you are serving. the rating of the ckt is based on the size of the ocpd. your conductors do not have to be protected at the largest ampacity they are able to carry, only to the ampacity they will be carrying (based on the load).
- Location
- New Jersey
- Occupation
- Journeyman Electrician
The EGC is sized according to 250.122 and is based on the OCPD protecting the parallel conductors. Each parallel raceway would contain the conductor size listed in that table. If the conductors are larger than required for any reason it may be necessary to increase the size of the EGC's in each raceway according to 250.122(B).
So two 500kcmil conductors can be protected with an 800 amp OCPD. That's what I was after. Thanks iwire.
I understand that 250.122 is used to size your EGC. If I increase my conductors from two 500kcmil to say three 500kcmil on the same 800 amp circuit, do I then need to increase the EGC accordingly? I thought it was based only on the OCPD, which is what the EGC is designed to trip in the case of a fault.
I also know that 250.66 is used for the GEC and in some cases, you use it as the minimum size for the neutral. Why then does 215.2(A)(1) say "...The size of the feeder circuit grounded conductor shall not be smaller than required by 250.122,..."? Shouldn't it be sized to 250.66? I get a little confused sometimes as to the terminology of the neutral, grounded, GEC and EGC.
I understand that 250.122 is used to size your EGC. If I increase my conductors from two 500kcmil to say three 500kcmil on the same 800 amp circuit, do I then need to increase the EGC accordingly? I thought it was based only on the OCPD, which is what the EGC is designed to trip in the case of a fault.
I also know that 250.66 is used for the GEC and in some cases, you use it as the minimum size for the neutral. Why then does 215.2(A)(1) say "...The size of the feeder circuit grounded conductor shall not be smaller than required by 250.122,..."? Shouldn't it be sized to 250.66? I get a little confused sometimes as to the terminology of the neutral, grounded, GEC and EGC.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
But you need to be after more. Specifically, you cannot use a pair of 500's, if the load is over 760 amps.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Yes. The EGC starts by being based on 250.122. But 250.122(B) would require a larger EGC, in the case you describe.
If I have a load of 760 amps, can I use two 500's paralleled with an 800 amp breaker?
I also know that 250.66 is used for the GEC and in some cases, you use it as the minimum size for the neutral. Why then does 215.2(A)(1) say "...The size of the feeder circuit grounded conductor shall not be smaller than required by 250.122,..."? Shouldn't it be sized to 250.66? I get a little confused sometimes as to the terminology of the neutral, grounded, GEC and EGC.
I also know that 250.66 is used for the GEC and in some cases, you use it as the minimum size for the neutral. Why then does 215.2(A)(1) say "...The size of the feeder circuit grounded conductor shall not be smaller than required by 250.122,..."? Shouldn't it be sized to 250.66? I get a little confused sometimes as to the terminology of the neutral, grounded, GEC and EGC.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
- Status