conductor ampacity
- Thread starter SEIEC
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Re: conductor ampacity
Depends on whether you want to apply NEC 240.4(B) which allows the next standard overcurrent device size if ampacity falls between 2 standard sizes. However, some circuit breaker manufacturers warn that applying this rule voids the performance warranty as to how much current the circuit breaker can carry without nuisance tripping. For a residential single phase circuit 500 KCM is 1 size larger than required.
If you choose to use two 3/0 or 4/0 conductors they must be in separate conduits and cables. However, parallel conductor installations can have problems with equal conductor length if there are any changes in direction. The circuit will always run cooler and live longer if you use larger wire than required.
Also, the listing for 200% neutral panelboards also prohibits use of 240.4(B). For a 400 amp 20% neutral panelboard you would need 10 250 KCM copper plus equipment grounds wires in 2 couduits.
Depends on whether you want to apply NEC 240.4(B) which allows the next standard overcurrent device size if ampacity falls between 2 standard sizes. However, some circuit breaker manufacturers warn that applying this rule voids the performance warranty as to how much current the circuit breaker can carry without nuisance tripping. For a residential single phase circuit 500 KCM is 1 size larger than required.
If you choose to use two 3/0 or 4/0 conductors they must be in separate conduits and cables. However, parallel conductor installations can have problems with equal conductor length if there are any changes in direction. The circuit will always run cooler and live longer if you use larger wire than required.
Also, the listing for 200% neutral panelboards also prohibits use of 240.4(B). For a 400 amp 20% neutral panelboard you would need 10 250 KCM copper plus equipment grounds wires in 2 couduits.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: conductor ampacity
If the service were without the 200% neutral why couldn't the parallel 4/0's be in one conduit?
If the service were without the 200% neutral why couldn't the parallel 4/0's be in one conduit?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: conductor ampacity
But it did not completely or correctly answer your question. When you say, ?400 amp service,? you are speaking about the rating of the service equipment. That does not tell us the amount of load that the service will be called upon to supply.
First, you must perform a load calculation, in accordance with Article 220. If your answer is lower than 380 amps, then you can use the 500 MCM, and you can protect them with a 400 amp breaker.
But if your calculated load is above 380 amps, then you cannot use a 500 MCM. The conductor must have an ampacity that is at least as high as the calculated load.
But it did not completely or correctly answer your question. When you say, ?400 amp service,? you are speaking about the rating of the service equipment. That does not tell us the amount of load that the service will be called upon to supply.
First, you must perform a load calculation, in accordance with Article 220. If your answer is lower than 380 amps, then you can use the 500 MCM, and you can protect them with a 400 amp breaker.
But if your calculated load is above 380 amps, then you cannot use a 500 MCM. The conductor must have an ampacity that is at least as high as the calculated load.
Re: conductor ampacity
If the 400 amp service is three phase, four wire, and the neutral is considered current carrying, the derating factors of 310.15(B(2)(a) would apply. This would put the ampacity of the 500 kcmil conductors below 350 amps, making the 400 overcurrent device too big.
finhead
If the 400 amp service is three phase, four wire, and the neutral is considered current carrying, the derating factors of 310.15(B(2)(a) would apply. This would put the ampacity of the 500 kcmil conductors below 350 amps, making the 400 overcurrent device too big.
finhead
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: conductor ampacity
That would only be true if there were a significant percentage of non-linear loads. We know nothing about the types of loads being fed from this service.
twindragons
Member
Re: conductor ampacity
ok it is single phase,serves 1 225 amp panel and 1 100 amp panel the rest will be for future use.
my thought was it would only be good for 400amps if the wire size was good for 400amps which 500mcm is only good for 380amps.
ok it is single phase,serves 1 225 amp panel and 1 100 amp panel the rest will be for future use.
my thought was it would only be good for 400amps if the wire size was good for 400amps which 500mcm is only good for 380amps.
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