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Thread: wire temperature rating

  1. #1

    Default wire temperature rating

    During my rough in inspection, the inspector noted some older wires that I had installed in my ceiling canister lights. Some of the older ones are not rated for 90 degrees C. He stated I can determine if they are rated by looking at the coating of the stripped wires. I contacted the mfg’er of some wires that I new were <10 yrs old and this Tech guy said as long as the wire is a “type NM-B” that they would comply.
    I contacted the inspector on this and he said “not necessarily”. Can anyone advise on this? Is there any way of determining for sure? Some wires are for sure going to get replaced, but 1 wire that I’ve used quite a bit of is ID’ed as “Essex E18679 Type NM-B 12 CU 2 CDR with Awg.”

  2. #2

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    The -B in "NM-B" signifies the 90 degree rating. Wait for others to chime in though on why the inspector said what he said.

  3. #3
    Join Date
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    If the outer jacket is marked NM-B (non-metallic class B) then it is a 90C rated cable insulation. One of the characteristics of the insulation is a very thin clear membrane over the pvc coating. A 60C NM cable doesnt have this and the insulation generally is applied in a thicker coating making the overall wire diameter greater than modern NM-B thhn wires. However this is no way to determine temperature rating... if it isnt marked NM-B on the outer nylon jacket that encompasses all the wires then consider it a 60C cable which will just be marked NM and no "B".

    Roger

  4. #4
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    Send your inspector back to school:

    334.112 Insulation. The insulated power conductors shall be one of the types listed in Table 310.13 that is suitable for branch circuit wiring or one that is identified for use in these cables. Conductor insulation shall be rated at 90°C
    (194°F).
    FPN: Types NM, NMC, and NMS cable identified by the markings NM-B, NMC-B, and NMS-B meet this requirement.
    Mark
    Kent, WA

  5. #5

    Default Temperature rating

    When the inspector told me that NM-B doesn’t necessarily mean it meets the temp rating, he told me to look for that glossy/waxy coating over the stripped wires. One of my newer wires (NM-B) has this, but some of the other NM-B type wires don’t seem to (or at least not as much). I would still think an identifying number (per industry standard) would be a better way of determining over a visual for some sort of coating.

  6. #6

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    Quote Originally Posted by geechj View Post
    One of my newer wires (NM-B) has this, but some of the other NM-B type wires don’t seem to (or at least not as much). I would still think an identifying number (per industry standard) would be a better way of determining over a visual for some sort of coating.
    The identifying "number" is the B. That means rated for 90 degrees celsius. Some of the older stuff without the thin thermoplastic sheath over the individual conductors is probably not labeled NM-B, it's just NM or it just says "non metallic sheathed cable."

  7. #7
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    My comment about the coating was just an explaination of a physical characteristic. In no way would you use that as a means to determine temperature rating. The inspector most likely is just not elaborating or communicating correctly. I doubt it is due to lack of knowledge. Give him the benefit of the doubt. As for your wiring if you cant prove 90C by a label or embossing on the outer jacket you must consider it 60C wire. One fix is to install a short piece of NM-B to the 60C wire then run it to the fixture but I would prefer to avoid these splicing areas and just replace the wire if possible.

    Roger

  8. #8

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    Thanks for the input. Is it worth arguing with an inspector if my wires clearly ID “NM-B” on them, contrary to what he informed me that that doesn’t necessarily mean its 90 degrees? According to above replies NM-B does in fact mean 90 degrees. Is there a specific reference in the code that I could direct him to, or question him on? If it was just a few chunks to replace I would, but its my whole living room lighting.

  9. #9
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    Mark gave you the reference.... he will have considerable trouble arguing with that code article specifically targeting the wire temperature rating of non-metallic sheathed cables.

    Roger

  10. #10

    Default NM and NM-B

    ENGINEERING DEPARTMENT
    BULLETIN
    No. 92
    December 7, 2005
    Use of Type NM-B Cable for Wiring of Residential Lighting Fixtures
    It is common for residential recessed lighting fixtures to require supply wire insulation rated for
    at least 90ºC. This is because recessed lighting fixtures are designed with a target temperature of
    90ºC in the junction box for the supply conductors. It should be noted that currently, lighting
    fixtures rated over 90ºC are marked "not for use in dwellings".
    Three common questions on the use of Type NM-B Cable for wiring lighting fixtures are
    commonly asked. The official NEMA position to each follows:
    1) Can Type NM-B cable be used with recessed lighting requiring supply wire insulation rated
    90ºC?
    Yes, NM-B cable can be used with these fixtures. The 1984 NEC required the temperature
    rating of Type NM Cable to be increased from 60ºC to 90ºC, to take into account the increased
    use of thermal insulation in dwellings. The 90ºC cable is identified as Type NM-B. NM cable
    manufactured prior to the 1984 NEC is rated at 60ºC and is identified as Type NM.
    2) What if the dwelling is wired with 60ºC rated Type NM cable?
    When the dwelling’s existing wiring is rated 60ºC, install a junction box approximately 18 inches
    away from the newly installed lighting fixture and then run Type NM-B from the junction box to
    the fixture.
    3) Can NM-B cable be used at the 90ºC ampacity?
    No, the 90ºC ampacity can only be used for conductor derating due to bundling or elevated
    ambient temperatures provided the final derated ampacity does not exceed that for a 60ºC rated
    conductor. For the purpose of ampacity calculations, the 2005 National Electrical Code states
    that the “ampacity applied shall be in accordance with the 60ºC (140ºF) conductor temperature
    rating.”
    Distribution List:
    Codes and Standards Committee
    NEMA Technical Services Department

    Hey mark maybe you should not jump to conclusions especially with AHJ. HE HAS ALREADY BEEN TO SCHOOL
    Rasarx Tacoma wa

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