Interpretation of 338.10 new requirements

[cdAtlanta]

The 2011 NEC revised article 338 in a way that I am having a hard time understanding and AHJs seem to be having as well. I am looking at this from the multifamily point of view where each unit is fed with an SE type of cable from the meter locations to the load centers and that cable is in touch with insulation for most of the run. I am referring to the second paragraph of 338.10(B)(4)(a). This new requirement of using the 60 deg column when installed in insulation is becoming a bit confusing. Here are the main points of concern:
1 - The phrase "installed in thermal insulation" has been interpreted thus far in two different ways: some argue that insulation on one side only would not apply and some argue that in touch with insulation is enough for the rule to kick in.
2 - If an apartment is fed with 120/240 we can go by table 310.15(B)(7) and let's say a 125 Amp service would be fed by a 1/0 AL SE cable (regardless of insulation being present or not according to informational note 2 of article 338.10(B)(4)(a)). If the same apartment is fed with 120/208 we have to go by table 310.15(B)(16) as usual but now in presence of insulation we might have to upgrade to the 60 degree column so the 125 Amp service would need a 3/0 AL SE cable. I cannot make sense of the same amps traveling in the conductors yet one having to use a 3/0 cable whereas the other one is ok with a 1/0 cable (with the same type of cable installed under the same circumstances). Furthermore, most manufacturers limit the typical tenant breakers to accept up to 2/0 conductors so we would need to increase the size of the breakers and the meter stacks to accommodate this under 120/208 but not under 120/240.
So far I have consulted a manufacturer, an engineer, and two AHJs, not two answers have been the same. Has anyone come across it?

 

[tom baker]

Yes and no. I am reading thru the 2014 NEC ROC and there is a lot of discussion on this topic yet. In washington we are not allowed to use SE cable, so don't see much application.

 

[MarineTech]

#1

I'ts really up to the interpretation of the AHJ as to what "installed in thermal insulation" describes per SE.

My opinion would be touching insulation on one side would effect heat dissipation. So yes, some level of derate is in order.

#2

To me, Table 310.15(B)(7) is based on the definition of a service for dwellings, and how the service amperage is calculated in regards to dwelling type and load diversity.

For a Dwelling Occupancy service of 120/240v, the service ampacity is total calculated VA divided by 240 VAC.

For 120/208v, if allowed in Table 310.15(B)(7), would be VA divided by 208 VAC. Which would potentially bump up the conductor size in the table.

Other factors include the 3 phase 120/208v approach in comparison to single phase.

 

[Gregg Harris]

As Mr. Baker has stated in post # 2 there is still conversation for 2014 change to the verbiage and requirements. The only positive that I can see so far is that CMP #7 is rejecting on the bases that they want to see testing data on SE cable "imbedded in thermal insulation". My interpretation of the verbiage would be if the conductors are surrounded by thermal insulation preventing heat dissipation, as apposed to the cable being on either side of insulation in a wall cavity.
338.10 (B)(4)(a)
(a)

Interior Installations. In addition to the provisions of this article, Type SE service-entrance cable used for interior wiring shall comply with the installation requirements of Part II of Article 334, excluding 334.80.
Where installed in thermal insulation, the ampacity shall be in accordance with the 60?C (140?F) conductor temperature rating. The maximum conductor temperature rating shall be permitted to be used for ampacity adjustment and correction purposes, if the final derated ampacity does not exceed that for a 60?C (140?F) rated conductor.