Am I Crazy?

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Bob, I can see multiple "Main Feeders" as follows.

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[FONT=Verdana, Helvetica, sans-serif]posted December 20, 2002 10:38 PM [/FONT][FONT=Verdana, Helvetica, sans-serif]

400A service 2-200 amp lighting appliance branch-circuit panels.

Service lateral --400KCM copper per article 310.15(b)(6) to 400A OCPD.
We still have not made it to our lighting appliance branch-circuit panels yet.

For sake of coversation we install 2-200 amp enclosed breakers adjacent to this 400 amp OCPD and feed these from parallel lugs. We use 2/0 copper per 310.15(b)(6).
We still have not made it to our lighting appliance branch-circuit panels yet.

We feed 2-200 amp lighting appliance branch-circuit panels with 2/0 copper per 310.15(b)(6)and make all terminations.
We have finally made it to our lighting appliance branch-circuit panels.

Now 310.15(b)(6) has seen it's end and if we feed any sub panel, say a 100 amp, from one of these aforementioned lighting appliance branch-circuit panels, this 100 amp sub panel can not use 310.15(b)(6)
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Roger[FONT=Verdana, Helvetica, sans-serif]

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roger said:
Bob, I can see multiple "Main Feeders" as follows.[FONT=Verdana, Helvetica, sans-serif][/FONT]
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We are thinking the same.
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Now that we've danced around this topic again, does anyone know if there was a proposal for the 2008 NEC to reword this to mean what the CMP intended it to mean?
 
infinity said:
Now that we've danced around this topic again, does anyone know if there was a proposal for the 2008 NEC to reword this to mean what the CMP intended it to mean?
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Here is a post by Ryan that addresses it.

IMO the change for clarification is good, all it changes is the clarity.

It already says what it will say.
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Okay, gonna stir the pot for a moment. All interpretation aside, does this really make any logical sense? Two scenarios:

1) 1200A meter mod on large apartment complex. Meter can has 100A breaker. Feeder is #2 SER AL. So we have a #2 Aluminum Feeder cable routed all through this wood-frame building to the main panel of an apartment and it is protected by a 100A breaker. MEETS NEC

2) House with 200A main panel in utility room at one end of the house. Contractor installs a lighting and appliance branch sub-panel near other end of house to simplify the wiring system and needs the feeder circuit to be rated at 100A. He installs #2 SER AL for the feeder and feeds it with a 100A branch breaker in the 200A panel. DOES NOT MEET NEC.

Whats the difference????

IMHO, the rule should apply to any Residential MAIN OR SUB lighting and appliance branch panel. Or else it should be abolished all together!!! This would make logical sense to me.

Please blast me on this if I am missing something. Its how I gain more knowledge from all you great folks on this site! :)
 
The difference between those two cases, is one has the load of a dwelling unit, and the other has the load of who knows what. The whole point of 310.15(b)(6) is that you can use a smaller conductor than would otherwise be needed when serving a dwelling unit. When you are serving a subpanel, it may have a higher load on it in reality. Say the subpanel had 2 50A breakers in it for electric heat for example. I assume the idea behind 310.15(b)(6) is like utilities using smaller conductors--they know it will be OK for a dwelling unit. But it does seem like an odd rule in that it completely ignore the insulation rating of the conductor. A 60 and a 90 are treated the same.
 
paul32 said:
The difference between those two cases, is one has the load of a dwelling unit, and the other has the load of who knows what. The whole point of 310.15(b)(6) is that you can use a smaller conductor than would otherwise be needed when serving a dwelling unit. When you are serving a subpanel, it may have a higher load on it in reality. Say the subpanel had 2 50A breakers in it for electric heat for example. I assume the idea behind 310.15(b)(6) is like utilities using smaller conductors--they know it will be OK for a dwelling unit. But it does seem like an odd rule in that it completely ignore the insulation rating of the conductor. A 60 and a 90 are treated the same.
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I disagree. I do agree with the CMP's intent (as posted in Ryan's message from the other thread referenced) for removing the (s) from panelboard(s)from the wording in 310.15 (b)(6) in that the reason for the smaller conductors being allowed for service conductors is that the service will see more "load diversity" than downstream feeders to subpanels.

I know, load diversity is not an NEC word...I'll give myself 20 lashes with the NEC whip tonight. ;)
 
The load may be diverse or it may not be. If most of the house breakers were in the subpanel, then its probably diverse. If the feeder only has only a heat pump and a convenience receptacle, then you need to watch the sizing. My 400A service is that large because the house is sort of like two houses (two kitchens, 3 floors) and a lot of smaller loads (pool, tools, outbuildings). Each of my 200A panels would be similar load and diversity wise to a normal house with a single 200A panel. But someone could choose to put 120 amps of heat pump and 80 amps worth of endless hot water on a 200A panel and that may not be diverse enough.

I'm not sure how they can solve this. Mandate a minimum number of circuits in the destination panelboard, especially 15 and 20 amp circuits, and even better if those have a calculated load associated with them. Put a limit that no branch circuit breaker can be larger than 40% of the feeder rating. Raise the floor for 310.15(B)(6) subpanel feeders to 150A feeders or larger, since a single 100A load is difficult to come by in a dwelling. Or perhaps redefining a lighting and appliance panelboard to have a much higher threshold of smaller neutral equipped circuits, perhaps 50% instead of 10%.
 
wirenut1980 said:
I disagree. I do agree with the CMP's intent (as posted in Ryan's message from the other thread referenced) for removing the (s) from panelboard(s)from the wording in 310.15 (b)(6) in that the reason for the smaller conductors being allowed for service conductors is that the service will see more "load diversity" than downstream feeders to subpanels.
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Not sure what you disagree with. What I said agrees with this.
 
OK, I may be a little late in getting into this discussion, but I think the basic question was if 310.15(B)(6) can be applied to a feeder to a subpanel.

Doesn't 215.2(A)(3) settle this?

"Feeder conductors for individual dwelling units or mobile homes need not be larger than service conductors. Paragraph 210.15(B)(6) shall be permitted to be used for conductor size."

IMO, that says yes, you can use #4 copper or #2 Al for a 100A subpanel.
 
I take that section to mean if you had 4AWG copper service conductors for a 100A service you don't need more than 4AWG for 100A feeder. If the 100A feeder was off a 200A service, then you couldn't use 4AWG for the feeder. In other words, why would you require a 3AWG feeder off a service with 4AWG service conductors.
 
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