# 400 amp service wire sizing



## sls7474

My buddy installed a 400 amp meter with 2-200 amp breakers in his pump house. 1 is going to a detached garage and the other to a new home. I believe he can use table 310.15(b)(6) for the wires through the weatherhead, but what table should be used to size the wires to the detached garage and the home. Because each breaker will carry the entire load for each structure can he use the same table? It is already installed with 500kcmil copper through riser and 3/0 to garage and house but I think he could have used 400kcmil and maybe 2/0 to garage and house. Any thoughts?


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## wildleg

> 310.15 B 1 7
> 1201240-Volt, Single-Phase Dwelling Services and
> Feeders. For one-family dwellings and the individual
> dwe11ing units of two-family and multifamily dwellings,
> service and feeder conductors supplied by a single-phase,
> 120/240-volt system shall be permitted be sized in accordance
> with 31 0.15(B)(7)(l) through (4).
> (l) For a service rated 100 through 400 A, the service
> conductors supplying the entire load associated with a
> one-family dwelling. or the service conductors supplying
> the entire load associated with an individual dwelling
> unit in a two-family or multifamily dwelling, shall
> be permitted to have an ampacity not less than 83 percent
> of the service rating.
> (2) For a feeder rated 100 through 400 A, the feeder conductors
> supplying the entire load associated with a onefamily
> dwelling, or the feeder conductors supplying the
> entire load associated with an individual dwelling, unit
> in a two-family or multifamily dwelling, shall be permitted
> to have an ampacity not less than 83 percent of
> the feeder rating.
> (3) In no case shall a feeder for an individual dwelling unit
> be required to have an ampacity greater than that specified
> in 310.15(8)(7)(1) or (2).
> (4) Grounded conductors shall be permitted to be sized
> smaller than the ungrounded conductors, provided that
> the requirements of 220.61 and 230.42 for service conductors
> or the requirements of 215.2 and 220.61 for
> feeder conductors are met....


for the service
.83 * 400 = 332 -> #400 @ 75

for the feeder to the house

assuming you are using the whole 200 amps 
.83 * 200 = 166 -> #2/0

I guess the same applies to the other feeder (?)


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## Dennis Alwon

IMO, you cannot use the reduction for the service as it does not feed the entire load. Secondly the garage is not a dwelling so that cannot use the reduction however the feeder to the house can benefit from the residential table reduction.


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## sls7474

If the garage is not a dwelling, and fed from different breaker from same panel and the house has only one source of electrical why wouldn't that source be serving the entire load for the house? Also the 400 amp service feeds everything on the property. So maybe reduction for house and Service wires but not garage?


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## wildleg

Dennis Alwon said:


> IMO, you cannot use the reduction for the service as it does not feed the entire load. Secondly the garage is not a dwelling so that cannot use the reduction however the feeder to the house can benefit from the residential table reduction.


so the pump house is not _associated with the dwelling_ ? I disagree. of course, it doesn't matter what I think - I guess he should ask the ahj



> (l) For a service rated 100 through 400 A, the service
> conductors supplying the entire load associated with a
> one-family dwelling. or the service conductors supplying
> the entire load associated with an individual dwelling
> unit in a two-family or multifamily dwelling, shall
> be permitted to have an ampacity not less than 83 percent
> of the service rating.


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## Dennis Alwon

wildleg said:


> so the pump house is not _associated with the dwelling_ ? I disagree. of course, it doesn't matter what I think - I guess he should ask the ahj



If the garage is part of the dwelling then you could not use the reduction table for either feed. You may be correct- on the interpretation but I cannot see how you could use the dwelling table


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## sls7474

Here in California probably 99% of the houses have a garage attached. So your interpretation is they cant use the dwelling table for any of those?


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## Dennis Alwon

sls7474 said:


> Here in California probably 99% of the houses have a garage attached. So your interpretation is they cant use the dwelling table for any of those?


I meant a detached garage. When I said a part of the dwelling I was responding to the post that was mentioning associated with the dwelling.

Sure you have an attached garage and run a feeder to it then you cannot use the reduction table for dwellings unless that panel carries the entire load of the dwelling


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## shortcircuit2

sls7474 said:


> My buddy installed a 400 amp meter with 2-200 amp breakers in his pump house. 1 is going to a detached garage and the other to a new home. I believe he can use table 310.15(b)(6) for the wires through the weatherhead, but what table should be used to size the wires to the detached garage and the home. Because each breaker will carry the entire load for each structure can he use the same table? It is already installed with 500kcmil copper through riser and 3/0 to garage and house but I think he could have used 400kcmil and maybe 2/0 to garage and house. Any thoughts?


Because the service consists of 2 switches the SE riser can be sized to the calculated load on the service. So for example if the load were 245 amps...he could have used 250MCM copper for the riser to the weatherhead.


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