xfer switch/generator/subpanel feeder

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R Bob

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Chantilly, VA
I installed a 100a sub-panel in a single family dwelling fed from a auto xfer switch that is fed from the main service panel and a stand-by generator.

The conductors were sized based on T310.16. I used #1 cu THHN protected by a 100a CB in the main service panel and a 100a CB in the generator.

The inspector comes and says "nice work"....he gives me a green sticker.
He asks why I used #1 cu. I explained my rationale as I did above. He says that I could have used T310.15(B)(6) to size the conductors which would have been #4 cu. He directed me to Article 215.2(A)(4)[NEC2002] and 310.15(B)(6).

I think that Article 215.2(A)(4) is specifically referring to the feeder for an apartment/condo or mobile home and does not apply to my situation. (???)

310.15(B)(6) references the feeder conductors that serve as the main power feeder. The article goes on to define the main power feeder as the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboard(s).

To me, the main power feeder is the conductors originating at the service disconnect(s) and terminating at the panelboard(s). (???)

What do you guys think?

Worst case scenario...I over engineered it a bit!
 
If the 100 A Panel is not the panel that supplies the entire dwelling unit, then 310.15 (B) (6) should not be used.

Was the run to the 100A panel in cable or raceway?? if cable what type??
 
cpal said:
If the 100 A Panel is not the panel that supplies the entire dwelling unit, then 310.15 (B) (6) should not be used.

Was the run to the 100A panel in cable or raceway?? if cable what type??
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Flexible Metal Conduit
 
I guess I should qualify that the feeder conductors need to be sized for the calculated load

Also I think the 4 CU AWG in flex could have OCP at 90A if the load current does not exceed 85A. (Assuming 310.15 (B) (6) is not applicable).
 
R Bob said:
The conductors were sized based on T310.16. I used #1 cu THHN protected by a 100a CB in the main service panel and a 100a CB in the generator.

What do you guys think?
Click to expand...

Why not #3 cu. THHN?
 
Cow said:
Why not #3 cu. THHN?
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Used T310.16 60C Column.

I would have used the 75C Column (#3cu THHN) but I could not determine the temp rating of the terminals of the factory installed CB in the generator. (they placed a sticker near the terminals "FOR RESIDENTIAL USE")

I assumed 60C to be on the safe side.

Based on the inspectors view, I'm way on the safe side!
 
cpal said:
If the 100 A Panel is not the panel that supplies the entire dwelling unit, then 310.15 (B) (6) should not be used.
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I disagree because one could say the same thing about, for example, a premises that has two (or more) panels. Let's say we have a 320a service with two 200a panels: each feeder may be sized per 310.15(B)(6).

So, the feeder from the TS to the sub-panel must be 310.16-compliant, because it carries both utility power and generator power. But, the feeder between the genny and the TS must suit the genny's breaker and the load.
 
I would agree with your example if the feeders supplied two individual dwelling unit panel boards, but I do not believe the intent of this section applies to a single dwelling with two 200 A panel boards. ( diversity of load)

But thats just my opinion.
 
cpal said:
I would agree with your example if the feeders supplied two individual dwelling unit panel boards, but I do not believe the intent of this section applies to a single dwelling with two 200 A panel boards.
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And I agree with this as well (We Charlie's must stick together. :D )
 
R Bob said:
I installed a 100a sub-panel in a single family dwelling fed from a auto xfer switch that is fed from the main service panel and a stand-by generator.
Click to expand...
The conductors from the utility to the main service panel can be sized per 310.15(B)(6). Conductors further downstream cannot. But I will say that this question has been debated on this forum before, and at some length I believe.
 
Thanks Charlie!

Just trying to get a concensus from the forum.

I would have been better off if the inspector had said nothing. Now I'm doubting myself.

I guess this qualifies for an official interpretation fron NFPA.

It would be interesting to find out what some of the other inspectors on the forum think.

Come on guys...chime in!

Marc, how about you, what do think?

Many thanks again Charlie!
 
I'd side with the charlie's.

In your situation, I probably would have used #3. I haven't seen a breaker witha 60 deg terimination in decades.

also

I don't recall the results from this much discussed issue, but in the example of 2/0s from a 400 amp meter to two -200 MB's, I find that is normally allowed, but IMHO, it does not meet 310.15(B)(6) as worded--especialy in 2008 ("...the panelboard.")
 
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