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Minimum 90C Supply Conductors

Posted by globe199 (My Page) on
Thu, Dec 31, 09 at 15:25

I bought a bathroom light fixture with a "warning" that it must be attached to 90C wiring. The wiring I have is type NM, which I believe is only rated to 60C.

How big a deal is this? The location is such that it would not be totally difficult to replace if necessary, but is there any easier fix? A guy at Menard's said it's a big deal, but that I could use heat-shrinkable insulation. What about 90C electrical tape?

Any thoughts?


Follow-Up Postings:

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RE: Minimum 90C Supply Conductors

It is a big deal. Find a different fixture or change the wiring. The fixture produces too much heat for the wiring. The insulation will cook, get brittle and fall off.

If there is an attic above the fix is quite simple. Remove the cable from the existing box. Install it into a new box about a foot away. Join the new box to the old box with section NM-B.


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RE: Minimum 90C Supply Conductors

Are you sure that your cable is only NM? If it is in fact NM-B you can use it for this application per NEC 334.112. It reads as follows 334.112 Insulation. The insulated power conductors shall be one of the types listed in Table 310.13(A) that are 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.

If you do not have NM-B then do as joed has suggested.


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RE: Minimum 90�C Supply Conductors

I have seen this warning on literature for fixtures that did not have especially heavy internal wiring, and have always wondered what the problem was. Thanks for the explanations. The heat produced by the bulb may not peak near the internal wiring of the fixture.

It is good to see that the world's joed people agree with one another:

Here is a link that might be useful: 90c Supply Conductor v. 60c Supply Conductor


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RE: Minimum 90C Supply Conductors

"I have seen this warning on literature for fixtures that did not have especially heavy internal wiring, and have always wondered what the problem was. "

It has nothing to do with the internal fixture wiring.

Since only short sections are used, higher temperature rated insulation is not a significant cost.

The problem comes about with compact fixtures and enclosed bulbs.

You will often notice foil faced insulation on the back of the fixture mounting plate also.

Older NM cable used PVC insulation only rated for 60 C.

If the fixture generates heat it has to go somewhere, and it will cook the old PVC insulation in short order, causing it to become brittle and crack (sometimes without any movement of the wires).

You do not have to rewire an entire house, just make sure that the permanent wiring entering the fixture box is 90 C rated.
Newer NM wire uses a different insulation (THWN even though it is not marked that way).

Some of the comments on the Bob Villa site are wrong and stupid.

Unless you want to have overheated wiring and the possibility of arcs and resulting fires, change the wiring to the fixture box to 90 C or do not use the fixture.


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RE: Minimum 90C Supply Conductors

Thank you.

Incredibly, I have found a lamp that has no warning about the temperature rating for the branch circuit conductors on the lamp itself, the product literature, the product specifications, or the installation instructions. The warning is a paper sticker attached to a disposable box containing the internal reflector, which is shipped separately from the lamp, because different models have different porcelain sockets for different types of bulbs.

It amazes me that a company would take the trouble to use a porcelain socket (pulse rated to 5 KV) and then put the branch circuit conductor warning on a shipping box.


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RE: Minimum 90C Supply Conductors

"...put the branch circuit conductor warning on a shipping box."

They usually want it clearly visible on the exterior packaging so folks with older 60 C wiring know not to purchase and use it.

The reflector could be the device that pushes the fixture to 90 C conductors.

Porcelain sockets are not used for their high voltage performance, but for the high temperature performance.

Unlike plastic sockets, they are normally totally unaffected by heat, since their firing temperature is far hotter than any light fixture will achieve.

Porcelain holders are used for the carbon electrode holders in actual arc lights.

No temperature issue, no UV issue, and the leads remain asbestos covered.


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RE: Minimum 90C Supply Conductors (NEMA Bulletin)

Here is a direct download link for a NEMA pdf file about this subject:

Here is a link that might be useful: Use of Type NM-B Cable for Wiring of Residential Lighting Fixtures


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RE: Minimum 90�C Supply Conductors

brickeye,

Thanks for the additional details.

The manufacturer is looking into whether just my ceiling light or an entire production run is lacking the required UL sticker.

The workaround suggested my NEMA is not difficult, time consuming, or expensive. It does raise issues of where to install the new splice boxes (one for each cable entering and leaving the octagonal ceiling box) when the joists in the attic are covered to their tops in blown cellulose.

I suppose it would be better to attach the new boxes to the sides of the floor joists just below their tops, so that the tops of the joists have nothing that obstructs laying pieces of plywood on the joists, the only method of navigation up there. It would be really nice to have a box with a clear cover. Right now, the original NM cable from 1950 runs under the joists in the small gap between their bottoms and a cream-color material similar to masonite that is above the ceiling itself, which is 1/2 inch thick cement board.


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RE: Minimum 90C Supply Conductors

"The workaround suggested my NEMA is not difficult, time consuming, or expensive. It does raise issues of where to install the new splice boxes (one for each cable entering and leaving the octagonal ceiling box) when the joists in the attic are covered to their tops in blown cellulose."

NEMA actually has no authority over permanent structure wiring, they only cover the wiring inside equipment.

The NEC acceptable fix is what they are parroting.

You do not need two boxes, just use a single box of adequate size.

Boxes may also be buried in insulation, so just stick it on the side of the nearest joist to the fixture.


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RE: Minimum 90C Supply Conductors

brickeye,

Thanks again.

The present 4 inch octagonal ceiling box has one NM cable (from 1950) entering it and two leaving it, one for the light over the kitchen sink and one for the outdoor light. I had not thought of combining the two outgoing wires into a single box.

I now regret not having taken photos of all of the electrical wiring in the attic while I was removing failed rockwool insulation. I spent over thirty hours lying on two sheets of plywood placed on top of the floor joists, and barely gave the wiring a glance.


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RE: Minimum 90�C Supply Conductors

minimum supply conductors- I learned from material I read along with my electrician that a fixture built in any home built before 1985 needs a 'minimum of 65 degrees supply conductors'. Perhaps from what I am gathering it would be more accurate to state that it should be called a MAXIMUM of 65 degrees. Please explain if I am misinterpreting this. (My home was built in 1925.)


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RE: Minimum 90�C Supply Conductors

"minimum supply conductors- I learned from material I read along with my electrician that a fixture built in any home built before 1985 needs a 'minimum of 65 degrees supply conductors'. Perhaps from what I am gathering it would be more accurate to state that it should be called a MAXIMUM of 65 degrees. Please explain if I am misinterpreting this. (My home was built in 1925.)"

More like 'may have the minimum 65C conductors.'

The requirement is that the conductor insulation rating be a minimum of 65C (though it could be higher).

Your interpretation would not allow the use of 90C insulation.

The use of 65C rated insulation goes back a long way.
It was the minimum rating allowed.

Until NM-B was required, NM used 65C conductors.
NM-B uses 90C conductors.
A manufacturer could use 100C conductors and still label the product as NM or NM-B (no one ever would since higher rated insulation almost always costs more).

A 1925 house should have insulation rated a minimum of 65C.
Newer work may have used higher rated insulation, but even the oldest stuff is most likely 65C or higher since that was the lowest allowed value for many years.


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