SER for multifamily unit feeders

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Ksparks

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I have a meter center feeding multifamily units with a 125A breaker with 2/0 AL SER cable. The corridors and units get blown in insulation above ceiling full to deck. 338.10(B)4a says where installed in thermal insulation the cable ampacity must comply with the 60 degree column of 310.15(B)(16) table, which would increase size of feeder to 3/0. would it be correct to say this does not apply because its the main feeder from meter to unit panel and based on the informational note 2 referring to 310.15(B)(7)2 the feeder conductor shall be permitted to have an ampacity not less than 83%? followed by 310.15(B)(7)3 In no case shall feeder for individual dwelling unit be required to be greater than that specified in 310.15(B)(7)(1)or(2).
 
Ksparks said:
I have a meter center feeding multifamily units with a 125A breaker with 2/0 AL SER cable. The corridors and units get blown in insulation above ceiling full to deck. 338.10(B)4a says where installed in thermal insulation the cable ampacity must comply with the 60 degree column of 310.15(B)(16) table, which would increase size of feeder to 3/0. would it be correct to say this does not apply because its the main feeder from meter to unit panel and based on the informational note 2 referring to 310.15(B)(7)2 the feeder conductor shall be permitted to have an ampacity not less than 83%? followed by 310.15(B)(7)3 In no case shall feeder for individual dwelling unit be required to be greater than that specified in 310.15(B)(7)(1)or(2).
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Greetings Ksparks,

First I think it is important to note that this specific issue was part of a huge change in the 2017 NEC edition where the thermal insulation component only applies to Type SE Cables sizes 10 AWG and smaller where installed in thermal insulation. The typical 75C ratings are maintained depending on the manufactures listings and so on and terminal limitations as well. That being said, it was not like this in the 2014 NEC and could be interpreted as you have stated and justly so based on the language. So the slight reprieve lies within the 2017 NEC only.


Now, one could ask a local AHJ for a code modification to a portion of an installation that allows the use of the 2017 NEC in spite of the 2014 NEC with this regard but it would be hit and miss on acceptance.


Now, with that said based on the 2014 NEC, you can use 310.15(B)(7) and since the cable is in thermal insulation you do the math per 310.15(B)(7) and so on with the understanding that your final selection choice will be from the 60C column and not the 75C column....at least until you all adopt the 2017 NEC.
 
Thanks for response, My math is 125A x 83%=103.75A, the 2/0 AL SER at 60degree is at 115A. so I think I have enough wire size even after de rating for running thru insulation. would this be correct assumption?
 
Ksparks said:
Thanks for response, My math is 125A x 83%=103.75A, the 2/0 AL SER at 60degree is at 115A. so I think I have enough wire size even after de rating for running thru insulation. would this be correct assumption?
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Well the running through insulation is not derating per se' but has the same effect I guess. The point being that as long as none of the issues covered in 310.15(B)(2)(a) or (B)(3)(a) are in play then yes your math is relatively simply once you do your 83% (minimum) and then select from the 60C column as demanded by 338.10(B)(4).

So yes, 125% x.83 = 103.75 a 2/0 AL SER would be adequate under the scenario you have presented (minus any adjustments or corrections to be considered)

Of course that is assuming you have done all the load calculations correctly to know that 125A is adequate...;) (just kidding).....:angel:
 
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