# Electrical feeder, sizing the neutral



## Bryant (Aug 27, 2019)

Question pertains to a 200 amp service (second panel) at another building off a 400 (380) amp meter can at the existing home.There was some discussion among us that an old rule of thumb was that the neutral could be two sizes smaller than the ungrounded conductors for the feeder from a 380 (400) meter can, other 200 amp panel unaffected.. Can't remember where the section was that made an allowance to drop two sizes. Had something to do with the balance of the 2 unground conductors in harmony with the neutral. any thoughts?
Thanks in advance


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## jar546 (Aug 27, 2019)

I'd like to clarify a few things.  First is the 200A in question a feeder from the service panel/disconnect or is the 200A a separate service.  You can't have a service after a service.  Rule of thumbs are typically inaccurate and outdated.  Next, a lot of what you are asking is completely dependent on the load calc for the service and panel in question.  Without knowing the loads there is no rule of thumb.


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## Bryant (Aug 28, 2019)

A 400 amp service with 2 200 amp panels, first one which is not the scope of this question is back to back, The one in question is a 200 amp disconnect running to a sub panel of which is 200 amps. Some years ago when this was installed, individual conductors were ran in conduit, forget the wire type, but suffice to say rated for a service feeder with a 4 wire setup with two ungrounded, one grounded, and  a equipment ground.
My question  concerned the neutral and the allowance to go to sizes smaller than the ungrounded conductors. There was an exemption to this rule allowing it to be two sizes smaller, but I can't seem to locate it. It may be that it was several code cycles back in the NEC and now is  no longer applicable. At any rate the premises was that the neutral could be sized based on the phases (perhaps amperes  is the correct term) of the two full current carrying ungrounded conductors.
I'm not going to lose any sleep over it, but the electrical inspector at the time, said no way, ain't happening.
Typical home load calculations for SFD.
Not a electrical inspector here, just asking if this is allowed and a section to go with it.
Thanks in advance


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## jar546 (Aug 28, 2019)

Sounds like you have a 400A service meter base that goes directly to two 200A service disconnects that I assume are grouped next to each other.  Your answer is based on the unbalanced load of the ungrounded conductors.  You have to start with 220.61 first and take it from there.  You may want to look at 310.15(B)(7).  If I am wrong, and this is a feeder then 215.2 is where you would get information.  There are more sections but you can follow them from where I started you.  So the answer is that there is no general rule of thumb except by those that learn by watching others and not knowing why they do what they do or if they know what they do.  You are doing it the right way by asking and looking it up and reading it yourself.  Thanks for participating.


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## Paul Sweet (Aug 28, 2019)

The neutral may be smaller if there are several 240V loads (HVAC, water heater, range, etc.) because these loads only use the hot conductors and not the grounded (neutral) conductor.  If one of the panels is predominantly 120V loads (lights & receptacles) then it's neutral should be full size.


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