310.15(B)(6) for dummies

[chris kennedy]

First please excuse me for not following the numerous threads I have seen posted about this, and excuse me for not searching past threads.

I have an issue on going with my shop and our dwelling guy. I did a 400A service a while back and ran 3/0 from my two 200A mains to the panels. Had our dwelling guy there to land my PNL's and says in this situation he has been running 2/0 to the two PNL's for years and has never had an inspector question it.

I have another this Wed, need to order the feeder wire. Am I missing something that would allow 2/0 to be used for two 200A panels??? I read that 2/0 would be allowed for 100% of the load.

Thanks

 

[Dennis Alwon]

You are correct- you cannot use (B)(6)

 

[augie47]

You are correct, as Dennis states.
It is a violation that slips by fairly often in these parts.
One reason being, provided the load calculation allows a 4/0 AL could be used and many inspectors would rather let the 2/0 cu slide than have a 4/0 AL.

 

[chris kennedy]

Thanks gentlemen.

 

[ceb58]

Show him this

For application of this section,
the main power feeder shall be the feeder between the
main disconnect and the panelboard that supplies, either by
branch circuits or by feeders, or both, all loads that are part
or associated with the dwelling unit.

It would apply if there were one panel feeding all loads.

 

[Ponchik]

Show him this It would apply if there were one panel feeding all loads.

So, if there is a meter main combo panel and another sub panel in the house, then 310.15(b)(6) does not apply?

 

[Dennis Alwon]

So, if there is a meter main combo panel and another sub panel in the house, then 310.15(b)(6) does not apply?

It would depend whether or not there were other loads at the meter main. If no other loads were taken from the meter main then you could use (B)(6) to the interior panel.

Remember in Chris' situation there are 2 main service panels so neither one carries the entire load of the dwelling so (B)(6) cannot be used.

 

[ceb58]

So, if there is a meter main combo panel and another sub panel in the house, then 310.15(b)(6) does not apply?

It would depend whether or not there were other loads at the meter main. If no other loads were taken from the meter main then you could use (B)(6) to the interior panel.

Remember in Chris' situation there are 2 main service panels so neither one carries the entire load of the dwelling so (B)(6) cannot be used.

What Dennis is saying is correct. If you installed a meter combo with say an 8 circuit panel built in. If you feed something like an outside A/C or well pump or whatever then you could not use table 310.15 (B)(6) for the feeders I.e.: 2/0 Cu. But if you brought those same loads back out of the main panel then you could use 2/0. So by the way it is written in the case of a meter combo with branch circuits you must increase the wire size for lowering the amp load on the wire. :?:?:?

 

[chris kennedy]

So by the way it is written in the case of a meter combo with branch circuits you must increase the wire size for lowering the amp load on the wire. :?:?:?

That in a nut shell is what they can't seem to wrap their heads around.

 

[jumper]

That in a nut shell is what they can't seem to wrap their heads around.

That's cause the rule is so screwed up.

MH has proposed more than once to get this changed, but got rejected.

 

[augie47]

To muddy the water a bit more add 215.2(A)(4):
(4) Individual Dwelling Unit or Mobile Home Conductors.
Feeder conductors for individual dwelling units or
mobile homes need not be larger than service conductors.
Paragraph 310.15(B)(6) shall be permitted to be used for
conductor size.

 

[jumper]

To muddy the water a bit more add 215.2(A)(4):
(4) Individual Dwelling Unit or Mobile Home Conductors.
Feeder conductors for individual dwelling units or
mobile homes need not be larger than service conductors.
Paragraph 310.15(B)(6) shall be permitted to be used for
conductor size.

Thanks Gus, we can now argue this for 300+ posts and never agree on any one answer.....

 

[chris kennedy]

FWIW, the 400A Siemens combo with the two 200A mains is factory wired with 3/0 from the meter side to the mains.

 

[Dennis Alwon]

To muddy the water a bit more add 215.2(A)(4):
(4) Individual Dwelling Unit or Mobile Home Conductors.
Feeder conductors for individual dwelling units or
mobile homes need not be larger than service conductors.
Paragraph 310.15(B)(6) shall be permitted to be used for
conductor size.

And I believe that will be gone in 2014.

 

[jumper]

FWIW, the 400A Siemens combo with the two 200A mains is factory wired with 3/0 from the meter side to the mains.

Yep, and while you are correct-neither panel will probably ever have a load greater than 100A for a standard resi.

Eh... such is life and the NEC. But code is code and we have to be compliant... well most of the time at least.

 

[Dennis Alwon]

FWIW, the 400A Siemens combo with the two 200A mains is factory wired with 3/0 from the meter side to the mains.

Not sure if you meant this but Cutler Hammer has one set of 3/0 copper conductors from the meter to the panel section. They are compact conductors and not running thru any raceway so I guess it is similar to a free air install.

I questioned this and our state AHJ called someone and found out it was tested to withstand the load and it did.

 

[chris kennedy]

Mine has 2 sets of 3/0, one to each main.

 

[Dennis Alwon]

Mine has 2 sets of 3/0, one to each main.

You have to wonder why Siemens doesn't follow suit and save some money. I tried to convince the inspector if there is only one set of 3/0 copper as service conductors then I only needed a #4 grounding electrode conductor. He didn't go for it. In his opinion and others that is part of the factory equipment and thus there are no service conductors so you have to use note 2.

 

[kwired]

To muddy the water a bit more add 215.2(A)(4):
(4) Individual Dwelling Unit or Mobile Home Conductors.
Feeder conductors for individual dwelling units or
mobile homes need not be larger than service conductors.
Paragraph 310.15(B)(6) shall be permitted to be used for
conductor size.

But a 400 amp service conductor supplying two 200 amp feeders is not the same as a 400 amp service conductor supplying a 400 amp feeder either.

200 + 200 does not always equal 400 in the electrical world:happyyes:

 

[Dennis Alwon]

But a 400 amp service conductor supplying two 200 amp feeders is not the same as a 400 amp service conductor supplying a 400 amp feeder either.

200 + 200 does not always equal 400 in the electrical world:happyyes:

It is to me in the residential world...... But I am not sure what that has to do with what Gus stated.