View Full Version : Light Fixture w/Fiberglass in Junction Box

05-31-2009, 07:50 PM
Does this create a fire hazard? The fiberglass on the top of the fixture actually sticks into the junction box.

05-31-2009, 08:08 PM
As far as the fixture I'm assuming you're talking about insulation. Some fixtures are rated for zero clearance with insulation, others are not. Do you know the make/model? Could you post a picture of the junction box? Why is insulation sticking into it?

Speedy Petey
05-31-2009, 08:53 PM
The fiberglass is fine. Nothing to worry about.

06-01-2009, 04:59 AM
Does this create a fire hazard?

Actually quite the opposite... The insulation on a light fixture can PREVENT a fire!

This is because light fixtures can generate a LOT of heat. The insulation protects the building as well as the wiring in the electrical box from getting too hot.

With older wiring above ceiling light fixtures (and even behind wall mounted fixtures), the insulation on the wiring can become brittle and crumble off from exposure to this heat - leaving bare wires. New higher temperature rated wiring is not supposed to do this.

This is why it may say to use a certain temperature rated wiring in the light fixture installation instructions. Also why many light fixtures have a sticker which says Max Wattage Bulb [whatever].

06-01-2009, 05:25 AM
The fixture itself states 100 watts (it is part of the molded metal fixture), but then there is a sticker over that that says 60watts. It is a bit confusing.

06-01-2009, 11:37 AM
I wouldn't exeed the 60W on it, then.

Engineer Ben
06-01-2009, 11:44 AM
The heat comments from the fixture hold true. Not sure what insulation we're talking about. Insulation project into a jbox would be a possible sparking concern, but in general insulation is not that combustible from sparking if the wiring were bad. Insulation would go up in an open flame of wood and other materials.

06-03-2009, 08:18 AM
...Not sure what insulation we're talking about...

The round yellow stuff in the following picture...


06-03-2009, 01:16 PM
Here's the data, you decide the fire hazard.
If the insulation has the same R value as dead air, it shouldn't cause add'tl heating inside the junction box.

If the 60w label is for 90C wire that is one hot lamp. Probably it's for pre-1985 60C wire.
The wire temp depends on the thermal resistance of the lamp, and this issue doesn't come up with wall mounted fixtures, I guess because the heat goes up instead of into the junction box.

I've put 200w 90C lamp fixtures in locations that had 60C wire and relabelled them for 100w max, but UL wouldn't address this thermal resistance derating issue (I asked them).

So if 200w gives you 90-30 = 60 C rise above ambient temp. then the thermal resistance is 200/60 = 3.3 degrees C per watt.
For 60 C wire and the same fixture you need a 30 C rise above amb., so 3.3 x 30 = 100w, max.
Or just go with a CFL.