Heat shrink tubing to repair or improve wire insulation

Okay, so this is less of a "how do I" and more of a "why does this work."

I was reading about the problem of light fixtures that are marked "use with 90C conductors only," but the house has 60C or 75C rated conductors. Various solutions were proposed, including using heat shrink tubing to cover the old conductors.

So I understand the problem to be that the heat buildup in the ceiling box exceeds the 60C or 75C that the conductor's insulation is rated for, which can cause insulation failure and arcing. It's the insulation and not the conductor itself that has the degradation problem.

So, if one takes a heat shrink that's UL-listed for insulating, 600V, up to 135C, and applies it to the old 60C wires, it should then protect up to 135C.

But heat shrink has to get hot (90C+) to shrink. So, to apply the heat shrink, we are actually temporarily creating the very situation we're trying to address.

Is that acceptable, basically because even if the old insulation does degrade during the heat shrink repair process, the new heat shrink is repairing the damage at the same time? And the copper conductor should be unaffected at 90C or even 135C, since its capable of withstanding much higher temps before melting?

 

It seems like a reasonable approach, probably better than nothing. My concern would be more with the conductors extending up in the box clamp and just outside the box that you can't get shrink tube onto.

I don't think an inspector could accept this given that the light fixture manufacturer states that 90C conductors are required; and 60C light fixtures exist. Granted most aren't fashionable due to being more of an older pendant style design, but legal options do exist. Rewiring from the switch to the box with 90C cable is also a possibility.

BTW, the ratings on the insulation aren't where it immediately melts. Those are the temps that the insulation is expected to endure regularly over very long time periods (+50yrs) we expect premises wiring to be safe and functional. A couple seconds of high heat to activate the shrink is no big deal.

 

That was kind of my thought as well -- better than nothing, but not "right." Though from my reading, a fair amount of folks are doing it.

I didn't know that the insulation rating is long-term vs. immediate failure. That makes things a little clearer, thanks.

It's mostly academic, as I came across this situation in some reading and the discussion ran the gamut from everything from heat shrink, to rewire, to take the insulation off of new wire and slide it over the old conductor, to ignore the 90c requirement because it doesn't mean what it says. When you see folks talking about ignoring labels and extending ungrounded branch circuits and the like, it makes you begin to question things.

 

If you have an accessible attic above you can all add a junction box in the attic. Reroute the 60° cable to it and run new 90° cable from the junction box to the ceiling box.