400 amp residential service feeders

mcpheeters33

New User
Location
st louis
Occupation
project manager
I have never installed tis size service in a residential home. If I used aluminum feeders, what size wire would be sufficient?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
(2) 200 amp panels
200 x 83 percent =166 amp wire
310.15 (B)(7)

4/0 Al wire rated at 180 amps

I would argue with what you are saying but 4/0 can be used since it is 180 amps and we can go to the next size breaker as long as the load does not exceed 180 amps.

Using 83% is allowed if the panel controlled the entire load of the dwelling. With 2 panels neither panel will be allowed to use the 83% rule. I know many areas allow it but it is not compliant, IMO
 

frofro19

Member
I would argue with what you are saying but 4/0 can be used since it is 180 amps and we can go to the next size breaker as long as the load does not exceed 180 amps.

Using 83% is allowed if the panel controlled the entire load of the dwelling. With 2 panels neither panel will be allowed to use the 83% rule. I know many areas allow it but it is not compliant, IMO
Wow, I've never thought of it that way. 4/0 AL has always been the norm around my area for (2) 200 amp panels. After reading 310.15 (B)(7) (1) and (2) numerous times, it says that the conductors supplying the entire load associated with one family dwelling shall be permitted to use the 83 percent rule. Wouldn't the parallel feeders from the meter base be feeding the entire load? I'm not seeing where it says not use parallel feeders or a single set of conductors. Maybe I'm reading more into it that I need to.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
Here is what it says. You have 2 sets of service conductors and neither one carries the entire load. If the nec accepted parallel then there would be no need for this sentence, IMO. The reduction in ampacity is because of the diversity in the load in a dwelling. If you have 2 panels then the diversity is gone. You can have all the heating and a/c loads in one panel and hardly any loads in the other.

As I said, many inspectors allow it but it is not compliant, IMO

For a service rated 100 through 400 amperes, the service
conductors
supplying the entire load associated with a
one-family dwelling,
 

frofro19

Member
Here is what it says. You have 2 sets of service conductors and neither one carries the entire load. If the nec accepted parallel then there would be no need for this sentence, IMO. The reduction in ampacity is because of the diversity in the load in a dwelling. If you have 2 panels then the diversity is gone. You can have all the heating and a/c loads in one panel and hardly any loads in the other.

As I said, many inspectors allow it but it is not compliant, IMO
I see what you are saying. On the other hand, you could have only half of the load on each panel and be well below the rated amperage of the the wire and panel. Regardless 4/0 should be good for the service, maybe just use a different rule, such as next size up. Lol.
 

mwm1752

Senior Member
Location
Aspen, Colo
Here is what it says. You have 2 sets of service conductors and neither one carries the entire load. If the nec accepted parallel then there would be no need for this sentence, IMO. The reduction in ampacity is because of the diversity in the load in a dwelling. If you have 2 panels then the diversity is gone. You can have all the heating and a/c loads in one panel and hardly any loads in the other.

As I said, many inspectors allow it but it is not compliant, IMO
Per his opening comments true but he also mentioned feeder which is not part of the service conductors. so maybe we clarify service lateral --
as long as he runs parallel 4/0 AL he could tap to each 200 amp OCPD- there would be only one set of service conductors - to have 2 sets of service conductors is special permission, is required by AHJ ( possible, maybe not feasible in most cases) but still required to carry the load calculations for each panel
 
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