[Steve Burke]

I plan to run 2/3 SE (copper) from the main disconnect in a front unit, through the attic and over a 5 foot covered breezeway (along side a 4x6), into the attic of another unit and down the wall cavity to the 125amp lug-only subpanel. To protect the SE cable, i plan to sleeve with 1.5" EMT over the 5' breezeway, and then again from the top plate down to the top of the sub panel (~30 inches). Is the sleeving arrangement OK and do you think i will be able to achieve 125amps with the copper SE cable?
Thanks!

 

[Stubbie]

I'm not sure what you mean by 2/3 Se but whatever the feeder is needs to be 4 wire. 2 hots a neutral and ground. The sub panel will need neutral and ground separate. So main bonding means should be removed.. this varies by panel maker. It is commonly a green bonding screw.

#2 copper 3 conductor with ground SER if connected between panels would give you 115 amps you would need #1 to get 125 amps.

I see no problem with your plan be sure to fasten the cable and protective conduit by an approved means and proper spacing. You do not have to sleeve inside the wall if being the SER is fished in an existing finished wall cavity.

I would like a bit better explanation of what you mean by running from a main disconnect. If the disconnect is only for the use of the additional unit and to protect its feeder then fine. But you cannot use the same disconnect (double lugging) Each unit must have its own main.

 

[Steve Burke]

Stubbie,
thanks for the clarification for running cable in the wall, and the breezeway. i will plan to do as you suggest.

I'm not sure what you mean by 2/3 SE but whatever the feeder is needs to be 4 wire.

Oops, I meant using 3 (#2awg) conductors and 1 (#8) ground. the supplier here calls this "2/3."

I would like a bit better explanation of what you mean by running from a main disconnect. If the disconnect is only for the use of the additional unit and to protect its feeder then fine. But you cannot use the same disconnect (double lugging) Each unit must have its own main.

The disconnect is only for the additional unit. The front unit has a separate meeter and seperate disconnect.

The problem I just ran into though is the electrical supplier told me that this 4 wire cable was NM-B not SE (service entrance) -- not sure if this is only labling or if there is a rating associated with SE. Can you help me understand what problem arises with this NM-B feeder cable as opposed to being SE. I do understand that the ampacity will be reduced to the 60 degree column for #2 awg (although the conductors are THHN) so i will not achieve the full capacity of my 125amp panel. This unit has gas heat, gas H2O heater, gas stove, gas dryer and is only ~900 square feet. So I guess the reduction is not that critical. any thoughts here?

 

[Stubbie]

Ok that is tells me what I wanted to hear. If you use nm-b you will have to use the ampacity of 310.16 60C column. If you use SER and the feeder being a main power feeder you can use table 310.15 (B)(6) and #2 copper will be good for 125 amps. The feeder only has to be good for the ampacity of the calculated load but in your case using ser will give you the full panel rating. You can feed it anything less than what it is rated if you like. Ser is a service entrance rated cable and as such has thhn and thwn-2 rated insultation on eh copper coducters. This allows you to use the dwelling feeder table mentioned previously.
NM-b is rated for 90C but its conductors are not identified as being any of those insulators needed for the cable to qualify for the dwelling feeder table or the 75 C column of 310.16, it is restricted to 60C ampacity. SER is also restricted to 60C for interior installation for other than a main feeder.