sub panel wire size

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I don't see why this feeder does not fit what is aplicable in 310.15(6)(B) .

Is this feeder to be considered a branch circuit as far as sizing it ??

I don't think so.
 
M. D. said:
Why would we not calculate the load to be served.??
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We would.

But it comes out much closer to the real load when it is focused on specific loads as in my HVAC example.


My real opinion is this.'

Get 310.15(B)(6) right out of the code...through it on the floor and walk on it.

If the Article 220 service calculations come out so high that the NEC knows it and adds a section allowing smaller conductors to feed these over sized services that says to me that Article 220 calculations should be adjusted to to lower levels.

Resulting in a service that is more accurately sized to the real load.
 
Just to clarify!

In my original post I gave two examples, the first was a 200 amp main panel in a dwelling that has a sub panel fed from the 200 amp panel, in this example I don't question using a 90 amp max breaker per 310.16 for a #2 AL ser cable. (I think we all agree on this.)

In the second example if the main panel is a 100 amp panel with a 100 amp main breaker and the service conductors are #2 AL then I think a #2 AL ser cable is allowed to be protected by a 100 amp ocpd citing 215.2 (A)(3) that the feeder conductors are not required to be larger then the service entrance conductors.

The reason for the question is last week I went to a house where a house inspector cited the main 100 amp 20 position panel because it had doubled up breakers, (two wires on breakers) this was an underground service where a double meter was on the pole (also off peak hot water) I proposed installing a 12 position sub panel next to it. they were selling the house and did not want to go through the expense of replacing the service as the power co will not allow new services to be mounted on there poles, plus many more variables. I installed a 100 amp breaker and ran #2 AL ser 2' over to the new panel. I could have used a 90 amp breaker but my supplier did not have one in stock so the 100 amp was installed . Again I'm citing 215.2 (A)(3) as making this legal. More opinions :D
 
Now that I have come to my senses, :eek:, I realize 215.2 has nothing to do with the ocpd for the feeder, only the conductor size, I don't believe that I did any thing unsafe as the heavy loads such as the stove, dryer & water pump are still in the original panel and what was moved was some 15 & 20 amp circuits, Thanks for the discussion and opening my eyes. ;)

I guess my slogan below doesn't always apply:eek:
 
wireman3736 said:
I didn't think it was that stupid of a question, I got two answers,
By the way Whats 215.2(A) (4)
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I didn't mean to imply your question was stupid, what would be stupid would be requiring that feeders from a service be larger than the service entrance conductors. Especially since the main power feeder (the first one into a dweling) can be sized per 310.15(B)(6) too.

The 215.2(A)(4) was from 2002. The section you mention (215.2(A)(3)) is the same words but from the 2005 NEC. I wish I had pure PDF of the 2005 code instead of the cumbersome free one from the NFPA site.

So I think you can do what you did as long as the main service was 100A on #2 AL. The more debatable question is whether all the feeders in a house can use the table 310.15(B)(6). But, we didn't go down that path again.
 
Wireman , I don't see a violation. For what it is worth. IMO if the original panel was a MLO with a back fed 100 amp breaker you could use the lugs to feed another MLO , no second breaker required the first breaker is already protecting those wires .

408.36 as long as the sub-panel board is rated at 100 amps or greater & the feeder is rated at 100-amps and the calculated load is less than 100 amps, no second ocpd required. Thats how I see it anyway.

Bob I'm not sure if in your example it is a small appliance and lighting panel and if it is ,then the a.c. units would be part of the calculation.

I understand what you are saying , I guess "they" feel if we do our job right and calculate the load to be served , then the main power feeders between the main disconnect and the lighting and appliance panel boards in an individual dwelling unit, can be sized according to 310.15 (B)(6) . I guess "they" feel there is enough diversity. In this case 10 amps worth. I could be wrong though?????

If the hvac equipment is fed from a power panel this table does not apply
 
suemarkp said:
So I think you can do what you did as long as the main service was 100A on #2 AL. The more debatable question is whether all the feeders in a house can use the table 310.15(B)(6). But, we didn't go down that path again.
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That is how I thought at first but I have changed my thinking because after rereading 310.15 (B) (6) it specifically states, feeder conductors that serve as the main power to each dwelling unit,, Without knowing the intent of the panel I have to go with the wording although I don't agree with it.:)
 
I think there are two issues with 215.2 -- you can size feeders that comply with all the restrictions of 310.15(B)(6), and you can install a feeder of whatever ampacity you want and not have to exceed the size of the service entrance conductors. You're doing the second and not the first. Note that if the calculated load of that feeder was truely 100A, then your service would most likely have to be upgraded because of all the other loads in the main panel. This could also be dangerous if you had multiple service disconnecting means instead of a single 100A main, as it would be easy to exceed the calculated load.

If you had a 125A service and wanted to run a 100A feeder with #2 AL, then you'd be into the debatable section of 310.15(B)(6) and most here seem to take the interpretation that this would not be allowed. But a 100A feeder on a 100A service using #2 AL should be OK assuming the service entrance conductors were #2 AL.
 
suemarkp said:
I think there are two issues with 215.2 -- you can size feeders that comply with all the restrictions of 310.15(B)(6), and you can install a feeder of whatever ampacity you want and not have to exceed the size of the service entrance conductors. You're doing the second and not the first. Note that if the calculated load of that feeder was truely 100A, then your service would most likely have to be upgraded because of all the other loads in the main panel. This could also be dangerous if you had multiple service disconnecting means instead of a single 100A main, as it would be easy to exceed the calculated load.

If you had a 125A service and wanted to run a 100A feeder with #2 AL, then you'd be into the debatable section of 310.15(B)(6) and most here seem to take the interpretation that this would not be allowed. But a 100A feeder on a 100A service using #2 AL should be OK assuming the service entrance conductors were #2 AL.
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I agree it should be ok and I don't believe the local ahj would have a problem but I guess it's up to interpretation.:)
 
iwire said:
Get 310.15(B)(6) right out of the code...through it on the floor and walk on it.

If the Article 220 service calculations come out so high that the NEC knows it and adds a section allowing smaller conductors to feed these over sized services that says to me that Article 220 calculations should be adjusted to to lower levels.
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I agree that 310.15(B)(6) is tremendously confusing, and should similarly be replaced by adjusting the calculations in article 220.

However rather than adjusting the results of article 220 down, I believe that the calculations should provide two separate numbers 1) the required conductor ampacity and 2) the permitted OCPD rating.

This would bring things more into line with for example sizing conductors and OCPD for motors or welders, where breakers are commonly 'oversized' for the conductors.

-Jon
 
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