House Service Wire size

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gigawatt

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Our local inspector has been recently failing service inspections based on the fact that people are using 4/0 SER for 200 amp parallel runs to 400 and 600 amp residential services. In the instance of having separate 3 parallel runs of URD from the transformer, through a CT Can to individual 200 amp breaker disconnects and 4/0 SER to feed 200 amp individual panels, is he correct in his analysis?
He says that we need to run 300 MCM instead of 4/0 for parallel runs even if the go to individual panels from 200 amp breakers. I thought the exception for residential service size wire included parallel runs.

Thanks
 

mcclary's electrical

Senior Member
Location
VA
Our local inspector has been recently failing service inspections based on the fact that people are using 4/0 SER for 200 amp parallel runs to 400 and 600 amp residential services. In the instance of having separate 3 parallel runs of URD from the transformer, through a CT Can to individual 200 amp breaker disconnects and 4/0 SER to feed 200 amp individual panels, is he correct in his analysis?
He says that we need to run 300 MCM instead of 4/0 for parallel runs even if the go to individual panels from 200 amp breakers. I thought the exception for residential service size wire included parallel runs.

Thanks



What code cycle are you under? under 08' he right
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I agree with the others. Only if the feeder serve the entire load of the unit would 310.15(B)(6) come into play. Here read it. :)

310.15(B)(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For individual dwelling units of one-family, two-family, and multifamily dwellings, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service-lateral conductors, and feeder conductors that serve as the main power feeder to each dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. 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. The feeder conductors to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.61, and 230.42 are met.

In the case of 2 or 3 200 amp panels none of the panels will carry the entire load of the dwelling.

Also- 300 kcm is only rated 190amps. This would be fine as long as the calculated load for the delling is not greater than 380 amps for 400 amp service or 570 for a 600 amp service.

This changed has made many EC's go to using copper where 3/0 is required and the wire will fit under the lugs in the panel.
 

Dave58er

Senior Member
Location
Dearborn, MI
Our local inspector has been recently failing service inspections based on the fact that people are using 4/0 SER for 200 amp parallel runs to 400 and 600 amp residential services. In the instance of having separate 3 parallel runs of URD from the transformer, through a CT Can to individual 200 amp breaker disconnects and 4/0 SER to feed 200 amp individual panels, is he correct in his analysis?
He says that we need to run 300 MCM instead of 4/0 for parallel runs even if the go to individual panels from 200 amp breakers.

FWIW these are not parallel conductors. ;)
Parallel runs are joined at both ends. (Landed on one feed and one load)

I thought the exception for residential service size wire included parallel runs.

I don't have my code book with me to check into this part but, again, even if it does in this case it wouldn't apply.
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
[B said:
gigawatt[/B]]http://forums.mikeholt.com/showthread.php?p=1187541#post1187541
I thought the exception for residential service size wire included parallel runs.

No where in T. 310.15(B)(6) does it include parallel runs ( I agree these are not parallel runs). In fact, if you look at the table for 400 amps it states 400kcm copper or 600 kcm alum. Those are your options. 600 amps does not even enter into the equation with this Table.
 

augie47

Moderator
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Location
Tennessee
Occupation
State Electrical Inspector (Retired)
(Dennis, you knew I would have to jump in with my 338.10 :) )
gigawatt, you have somewhat hit on a "moving target" and controversial subject.
Prior to '08 Code there was controversy about a residential service with more than 1 feeder to more than 1 panel and the application of 310.15(B)(6). Some said "yes", some, "no". In '08, I think most folks agree the controversy was settled by clarification that 310.15(B)(6) applied only to a feeder that carried the all loads associated with the dwelling. So, depending on what Code cycle you are in and your AHJ's outlook, 310.15(B)(6) would most likely not be accepted with multiple feeders. ( a possible change upcoming). That said, if one is allowed to use the 75? rating of 4/0 AL SE (180 amps) it can be argued that 240.4 would allow you to still use the 200 amp breaker (next largest size) without applyimng 310.15(B)(6).
However, another change in the '08 Code (338.10) restricts SE in interior applications to the 60? rating of 150 amps and with the re-write of 310.15(B)(6) that Table can not be used with multiple feeders.
Bottom lime, IMHO, if you are under the '05 Code and your AHJ interprets 310.15(B)(6) as allowing multiple feeders or allows you to apply 240.4 you would pass with 4/0 AL SE, but under '08 the 60? rating will probably block you.
 

augie47

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Location
Tennessee
Occupation
State Electrical Inspector (Retired)
we all have at least one burden to bear even if we foolishly elect to do so :)
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
However, another change in the '08 Code (338.10) restricts SE in interior applications to the 60? rating of 150 amps and with the re-write of 310.15(B)(6) that Table can not be used with multiple feeders.
Sounds like a resurgence of 150a panels is in the making.
 

gigawatt

Member
house service wire size

house service wire size

The thing that continues to stick in my craw is that in 310.15 it states "feeder conductors" and never disallows multiple runs. I agree that if the feeders are going to a 600 amp main they should be required to support the whole load, but if they are only going to a 200 amp breaker there would be no reason for them to have to carry more than 200 amps as a fault current.
 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
The thing that continues to stick in my craw is that in 310.15 it states "feeder conductors" and never disallows multiple runs. I agree that if the feeders are going to a 600 amp main they should be required to support the whole load, but if they are only going to a 200 amp breaker there would be no reason for them to have to carry more than 200 amps as a fault current.

I don't think there is an argument there. As you say, if they are connected to a 200 amp breaker, there is no reason for them to carry more than 200 amps (drop the "term" fault")..
The situation gets complicated in that the provisons of 310.15(B)(6) allow a conductor which does not have a 310.16 ampacity of 200 amps to "carry 200 amps" and '08 clarified that this exception is only allowed on feedres that carry the entire load of the dwelling and thus are subject to the load diversity of the entire house.
Take for example a feeder going to an air conditioning subpanel. The load of that feeder may well be constant for a long period of time. Whereas the "whole house" feeder in most every time frame will see consistantly changing loads.
 

iwire

Moderator
Staff member
Location
Massachusetts
If they are under the 2008 NEC I see no room for misinterpretation.

If the there are two 200 amp panels neither of the feeders carry all the load of the dwelling unit so you cannot use Table 310.15 for those feeders.
 
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