TW wire was 'the norm' from when to when?

fixizinMarch 14, 2011

Just doing a little "forensic" analysis on an older building, before pros show up to bid on the big re-wire, re-PANEL, conversion of overhead service drop to buried, etc.

Anyway, it's all EMT, metal boxes, and TW copper conductors... SIX different brands of TW, obviously from different eras, judging by the "age" of the rubber, the style of fonts/graphics printed on the rubber, etc... figured the original builder used only 1 or 2 brands of TW back in the early 1950s, and the other brands came from the increasingly bad "work" since then. At some point the "upgrades" stopped, because there's no 90C wire (e.g. THHN) to be found.

Just curious as to what years TW (60C, rubber insulation) was "standard issue", and when the change to 90C wire (thermoplastic insul.) occurred?

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TW is not rubber. It is thermoplastic.

    Bookmark   March 14, 2011 at 9:12AM
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TW is PVC, poly vinyl chloride.

It showed up after WWII until recently replaced in NM by 90 C THxx type.

It is normally rated at 60C and is visibly thicker than THxx type insulation.

TW in NM appears about 1/16 inch thick, while THxx is closer to 1/32 thick (thickness is over the wire).

    Bookmark   March 14, 2011 at 9:26AM
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I have not attempted to determine if any manufacturer still makes TW. But the NEC still includes it and if an installer has some, it can be installed in accordance with the NEC and local codes, if any. I still see some that I installed in 1958 that has been, and still is, completely trouble free.

    Bookmark   March 14, 2011 at 12:56PM
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Remember that you can't necessarily determine when the upgrades stopped based solely on the absence of 90� wire because there's no way to determine how much TW wire the electrican/owner had in stock. Let's, for sake of discussion, say that TW was phased out in 1970. If the building owner got a good deal on TW wire, he could have been using it in the 1980's. Especially in years past folks weren't too quick to throw stuff out.

I can get you a good deal on some brand new Armstrong vinyl floor tiles from the 1960's.

    Bookmark   March 14, 2011 at 3:44PM
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I was still installing tw in the late 70's.

    Bookmark   March 26, 2011 at 4:10AM
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I know this is a dated thread, but today I pigtailed a strand of THHN to TW wire.... TW looked much heartier than the new THHN... both 14g.

Is there any harm in joining the two under a wirenut?

    Bookmark   October 1, 2013 at 12:46PM
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No harm whatsoever.

    Bookmark   October 1, 2013 at 3:16PM
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So I am assuming the issue with TW is the 60C rating. What precautions need to be considered? In regards to light fixtures esp?

I can imagine wrong wattage bulbs are an issue. When a fixture calls for a specific wattage these days, does it take into acount 60C wiring?

    Bookmark   October 2, 2013 at 2:57PM
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Ron Natalie

Wattage doesn't mean anything really for insulation temperature. A higher temperature rating will mean you can have slightly higher ampacity on the wiring but for most branch circuits it's immaterial, you'll use 14 for 15A, 12 for 20A.

Certain fixtures might require 90C wiring for the fixture wiring for heat issues. You can splice in a length of 90 to your 60c branch circuit wiring at some convenient point (next box in the chain). The whole circuit doesn't need to be 90c wiring.

    Bookmark   October 2, 2013 at 4:15PM
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