Conductor Size for 320A/ 400A Service

[tronic]

Hello All,

It's been awhile since I last posted, but I am having a complete mental block on this question.

Here goes:
What size conductors should be installed on the line side of an all-in-one 320A/ 400A Meter Socket that feeds Two separate 200A Breakers.
I do not have a load calc. One 200A will feed the residence and the other a "shop".
We will often install 4/0Al for a residential 200A Service and call it good, with out a load calc. So, I am trying to do the same thinking here, as the customer does not have a clue as to what equipment his "shop" will have.

 

[Dennis Alwon]

If you are installing a 400 amp service then you can use conductors based on the table in art. 310

Not sure but if I were doing a calc I would use the 83% for the 200 amp service for the house and 200 amps for the shop which means you could use a wire rated for 366 amps

 

[electrofelon]

If you are installing a 400 amp service then you can use conductors based on the table in art. 310

Not sure but if I were doing a calc I would use the 83% for the 200 amp service for the house and 200 amps for the shop which means you could use a wire rated for 366 amps

I would have to look through the wording again, but my 1st thought is you cannot do that. Isn't the load calc for the residence still 200 amps? The 83% rule modifies how you size the conductors, And once they're not serving the entire load of dwelling unit you cannot use it.

 

[petersonra]

I would have to look through the wording again, but my 1st thought is you cannot do that. Isn't the load calc for the residence still 200 amps? The 83% rule modifies how you size the conductors, And once they're not serving the entire load of dwelling unit you cannot use it.

I think you have to have a load calc to do the 83% deal indirectly since you can't size the service without one. But once you get to a specific service size it does not seem to matter, so I think you can use 83% of 400 A as the required conductor ampacity for the service conductors carrying all of the load.

(1) For a service rated 100 through 400 amperes, 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.

 

[electrofelon]

I think you have to have a load calc to do the 83% deal indirectly since you can't size the service without one. But once you get to a specific service size it does not seem to matter, so I think you can use 83% of 400 A as the required conductor ampacity for the service conductors carrying all of the load.

I dont think you can use the 83 rule for the whole service. OP didnt specifically say, but it sounds like there is a detached "shop"

So lets say you have a house with a load calc of 200 amps, and a detached shop with a load calc of 200 amps. Ignoring any possible demand factors you could apply for the service calc, what is the load calc for the service then? I contend it is 400 amps (and no 83 rule allowed).

 

[petersonra]

I dont think you can use the 83 rule for the whole service. OP didnt specifically say, but it sounds like there is a detached "shop"

So lets say you have a house with a load calc of 200 amps, and a detached shop with a load calc of 200 amps. Ignoring any possible demand factors you could apply for the service calc, what is the load calc for the service then? I contend it is 400 amps (and no 83 rule allowed).

Look at what I quoted above. It does not appear to matter what the load calc says once you have a 400 amp service. You can size those conductors at 83% of the service rating.

 

[suemarkp]

How is the shop any different than a 200A Service with a 100A feeder to a shop. The service entrance conductors still get the 83% discount whether they feed a shop or not. The only difference here are the conductors to the panel feeding the shop. That one wont get a discount and need to be sized at 100% of the load.

 

[tronic]

Thank you all for your help with this. As it turned out I was not given the correct info about the lug sizes of the meter socket. EC was certain he could only go up to 4/0. Then, this afternoon he advised me that the lugs were indeed capable of parallel 250's. So that is what we decided to move forward with.

I have missed this forum and the different view you all give me. Think its time I come back and start asking all my nonsense again. The "new" system doesn't show all my previous bone headed questions but I have been a "member" since 2015... Anyway, thank you again...

PS yes, the shop is a separate out building...

 

[augie47]

You might be interseted in 230.40 Ex #3. If allowed locally, the NEC allows you to run to the detached structure from the meter without a disconnect.

 

[electrofelon]

You might be interseted in 230.40 Ex #3. If allowed locally, the NEC allows you to run to the detached structure from the meter without a disconnect.

I love that rule and use it all the time. Cheaper equipment, less/cheaper wire, less conduit fill and it even comes with a free milkshake