6/3 w-ground

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Well, I've drug up some interesting info with what little I have (2002 ROP/ROCs, 2005 ROP).

The 90/60 NM cable issue was proposed by NEMA and accepted in the 1987 cycle. Interestingly, SE cable was the actual cable used in the tests, apparently.

Since I don't have access to the 1987 ROP, someone else will have to tell us why NM was chosen instead of SE, and why it stuck that way. Now, the only answer we're likely to see (based on my read of the last two cycle's ROPs) is "insufficient technical substantiation."

2002 7-188 Log #3173 NEC-P07 (338-10(B)(4)(a))
Final Action: Reject
Submitter: Frederic P. Hartwell, Hartwell Electrical Services, Inc. / Rep. Mass Electrical Code Adv. Committee​

Recommendation:
Delete the phrase "excluding 334.80".​

Substantiation:
This is a companion proposal to the one submitted on 334.80. If that proposal is accepted, then the exclusion of 334.80 serves no purpose, since the wording proposed for Type NM cable is technically correct for any cabled wiring method from type AC cable (where it is already in place) to this method and others. Furthermore, the current NEC blanket exclusion of applicability of 334.80 means the only code rule standing between this product and thermal insulation is 310.10. That section is so broadly written that few know how to apply it in this case.​

It is important to recognize Type SE cable was the very wiring method that failed in the tests run to substantiate the merits of 334.80. Those tests, for example, resulted in the literal incineration of the conductor insulation and cable jacket of 2 AWG AL Type SE cable embedded in cellulose thermal insulation while drawing current at its nominal Table 310.16 ampacity. The problem is compounded in large cables because people run large cables in the expectation of drawing large amounts of current. Due to the I2R relationship, high current values (taken to the second power) quickly overwhelm the favorable effects of lower cable resistance (a first power factor). Running this type of cable embedded in thermal insulation without massive derating (well beyond 90oC to 60oC) is an extreme safety hazard.​

Panel Meeting Action: Reject
Panel Statement:
Inadequate technical substantiation was provided to justify this change.​

Number Eligible to Vote: 15
Ballot Results: Affirmative: 14 Negative: 1
Explanation of Negative:
STRANIERO: The panel rejected this proposal citing that inadequate technical substantiation was provided. The substantiation submitted with the proposal simply states that thermal degradation of electrical cable results from the installation of cables where they will be subject to installation in thermal insulation. The panel statement does not address the submitter's proposal or substantiation. The same thermal degradation that applies to NM, UF, and AC cable when installed in thermal insulation applies to any other wiring method installed in thermal insulation where heat dissipation is impeded. The panel should provide technical substantiation on why Type SE cable should be exempted from the ampacity requirements of Types NM, UF, and AC cables when SE cable is installed under the identical condition of in thermal insulation.​
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Yea...Thanks George.
That clears up a lot of questions that I had in my mind about the 60C requirement on NM-B.
steve
 
georgestolz said:
Really? I'm still curious as ever! :D
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It's Curious George!


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