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Discussion Starter · #1 ·
I have a customer that has installed a whole lot of new electrical equipment, mainly electric heating (floor heat, individual heating units in all bathrooms, another range, cooling, etc.) I spoke with her about upgrading the service and that is what she wants to do. But, I have a question. What size conductors are normally used for a service using a meter socket rated at 400 amp (non-continuous) 320 amp (continuous) that is going to feed two separate 200A (main circuit breaker) panels. (The panels will be right next to each other.)
 

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I have a customer that has installed a whole lot of new electrical equipment, mainly electric heating (floor heat, individual heating units in all bathrooms, another range, cooling, etc.) I spoke with her about upgrading the service and that is what she wants to do. But, I have a question. What size conductors are normally used for a service using a meter socket rated at 400 amp (non-continuous) 320 amp (continuous) that is going to feed two separate 200A (main circuit breaker) panels. (The panels will be right next to each other.)
The class 320 socket is rated by either way depending on lugs but typically 2 sets of 4/0 is common for 400 amp resdentail service.

but for POCO uility side that depending on overhead or UG but with OH service typically 500 Kcm alum but UG that is up to POCO call on what they run on the lateral runs can be either 350 or 500 Kcm alum depending on the load caluations and what POCO can provided on their side.

Before you order the 320 class socket make sure you read the POCO specs carefully due there are some specific model they are allowed. so be aware of that. ( either horn bypass or lever bypass or none )
 

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Discussion Starter · #3 ·
Honestly, that's what I was thinking of using and probably will use. I read somewhere that 3/O was acceptable, and then 350mcm... but, neither of those options seem to satisfy the requirement. At least, in this case. This is definitely not something I do all the time, in fact, I don't think I've installed a 320 amp service in a very long time. - Thank you for the advice on the power company requirements! I will definitely have to look into that.
 

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Honestly, that's what I was thinking of using and probably will use. I read somewhere that 3/O was acceptable, and then 350mcm... but, neither of those options seem to satisfy the requirement. At least, in this case. This is definitely not something I do all the time, in fact, I don't think I've installed a 320 amp service in a very long time. - Thank you for the advice on the power company requirements! I will definitely have to look into that.
I think some location can take 3/0 but for my SOP I always run 4/0 Al for 200 amp resdentail service ., but commercial service that do change a bit depending on what state ya in on the code requirement but typically 250 Kcm Al or 4/0 CU I dont run into 350 not too often unless oddball caluation come up or long distance to comprised the voltage drop.

And a tip for ya with twinner 200 amp services I try to put one panel with crictial circuits on one and other panel non crictail circuit due some case add a transfer switch or generator connection so it is set up for it. ( I would get the ground and netural bar separated bar if possible and be ready for conversion if the situation called for it)

Oh yuh I forgot one more tibbit in the mix.,, for overhead service expect a 3 or 4 inch riser conduit so prepared to secure it good. especially if over the roof POA ( point of attachment )
 

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Discussion Starter · #5 · (Edited)
I think some location can take 3/0 but for my SOP I always run 4/0 Al for 200 amp resdentail service ., but commercial service that do change a bit depending on what state ya in on the code requirement but typically 250 Kcm Al or 4/0 CU I dont run into 350 not too often unless oddball caluation come up or long distance to comprised the voltage drop.

And a tip for ya with twinner 200 amp services I try to put one panel with crictial circuits on one and other panel non crictail circuit due some case add a transfer switch or generator connection so it is set up for it. ( I would get the ground and netural bar separated bar if possible and be ready for conversion if the situation called for it)

Oh yuh I forgot one more tibbit in the mix.,, for overhead service expect a 3 or 4 inch riser conduit so prepared to secure it good. especially if over the roof POA ( point of attachment )
Thanks for the help and the tips; they are appreciated! This is going to be an overhead service. I already have one new mast, (through the roof), with 4/O aluminum conductors with a 2/O neutral. I was just going to run another 2" mast through the roof and install another set of 4/O 4/O 2/O aluminum conductors in parallel. I am pretty sure that will satisfy the requirements. - Please, if you have the time, let me know if you think otherwise.
4/O aluminum is what I normally run for a standard 200A rated service around here. I am thinking 2 4/O's should have an ampacity of 360 amps. If I read the code right it doesn't require your conductors to be rated more than 83% of your service rating for a residential service.
As for dividing the circuits; that is a great idea. - However, in this case, I have already built half of the service. I actually replaced the existing service before I started pulling wire on this place. But now, because they have added so much stuff, I think they really ought to have a larger service and they are willing to pay for it. I'm going to be removing a meter base I just put in about 3 or 4 months ago. And yes, they have added that much stuff since I began the work. I had no idea they would be adding so much.
 

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Since there are 2 panels 4/0 aluminum will work but the service will technically be 360 amps and not 400. 4/0 aluminum is only good for 180 amps at 75C so depending on the situation you may be good with 4/0. What code are you under?
 

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Discussion Starter · #10 ·
Since there are 2 panels 4/0 aluminum will work but the service will technically be 360 amps and not 400. 4/0 aluminum is only good for 180 amps at 75C so depending on the situation you may be good with 4/0. What code are you under?
In this case, I think I would be good with 4/O, but I may use 3/O copper or 250mcm aluminum just to make sure I am good for 400A. (Maybe she will get two vehicle chargers and a hot tub in the future... who knows.) I'm using the 2017 NEC. As for the service rating, using NEC 310.15(7)(1)(2) and (3), would it not technically still be rated at 400A with 2 each 4/O's aluminum run in parallel? I have always questioned this and I think it has even come up recently in another post I took part in. (I'm always learning.)
 

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You cannot use 2/0 copper for 2- 200 amp panels.

Matt- show me a code section that says 2/0 copper can be used on 2-200 amp panels.

You can't. The same rule for feeders applies for service conductors

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.
 
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You cannot use 2/0 copper for 2- 200 amp panels.

Matt- show me a code section that says 2/0 copper can be used on 2-200 amp panels.

You can't. The same rule for feeders applies for service conductors
Service equipment overload protection

230.90
Exception 3

Two to six circuit breakers or sets of fuses shall be permitted as the overcurrent device to provide the overload protection. The sum of the ratings of the circuit breakers or fuses shall be permitted to exceed the ampacity of the service conductors, provided the calculated load does not exceed the ampacity of the service conductors.
 

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Service equipment overload protection

230.90
Exception 3

Two to six circuit breakers or sets of fuses shall be permitted as the overcurrent device to provide the overload protection. The sum of the ratings of the circuit breakers or fuses shall be permitted to exceed the ampacity of the service conductors, provided the calculated load does not exceed the ampacity of the service conductors.
Matt I have a question ., what year code cycle you are on now ?

and this part is specifically for resdentail side or it cover both redsdientail and commercial ?
 

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Fair enough to know and I am on modified 14 verison .,

but I went thru the NEC and I cant really recall there used to be a small table that listed the common resdentail conductor size there. I know older edition did have it.
The table is gone and it has to be 83% or more like Dennis posted. POCO here always wants underground that they size and pull, but I’d imagine the table or 83% would apply to the riser conductors coming in line side . The city lets us come off the load side with 2/0 copper to 2-200 MCB panels because of the exception.
 
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