Why you pay attention to the wattage a fixture is rated for:

Status
Not open for further replies.

480sparky

Senior Member
Location
Iowegia
Service call a while back, customer said bulbs kept burning out and flickering. Arrived to find 100w lamp installed in an enclosed fixture (typically, that means 60w max).

20130926_095308.jpg


20130926_094548.jpg
 
Service call a while back, customer said bulbs kept burning out and flickering. Arrived to find 100w lamp installed in an enclosed fixture (typically, that means 60w max).
Clearly they should have used higher temperature rated fixture wire.
At least that would have helped with the flickering.
:)
Solution is to drill some vent holes in the fixture shell and the glass globe. :angel:
 
Last edited:
Could be worse. Seems like when I run across that problem the wires are crispy all the way to the cable feeding the box. Then you have to go into the attic and splice on new section of wire.....or at least you should.
 
Clearly they should have used higher temperature rated fixture wire.
At least that would have helped with the flickering.
:)
Solution is to drill some vent holes in the fixture shell and the glass globe. :angel:
Those are good ideas but copper is not only a good conductor of electricity but an excellent conductor of heat. As such the heat will follow the copper conductor regardless.
 
Those are good ideas but copper is not only a good conductor of electricity but an excellent conductor of heat. As such the heat will follow the copper conductor regardless.
True, but it will also be conducted through the insulation and then convected from there to the air in the fixture or raceway.
End result will be that using higher temp wire (and maybe larger gauge to conduct away the heat better) will improve things for both the wiring and the socket.
Those pesky bulb will continue to burn out though.
A nice 60W halogen will give as much useful light as the 100W tungsten, and will also tolerate the higher ambient temperature in the fixture better.

However, unlike the above, all of those earlier suggestions were tongue firmly in cheek. (Particularly the one about drilling the glass.)
 
True, but it will also be conducted through the insulation and then convected from there to the air in the fixture or raceway.
End result will be that using higher temp wire (and maybe larger gauge to conduct away the heat better) will improve things for both the wiring and the socket.
I have a problem understanding your logic the "conducted through the insulation" because if it is 60, 75, 90, 105 deg there wouldn't be any different and is a mute point. That is a new slant to this issue that I have never heard before, convection through the insulation to the air. Its the insulation life, that it will not degrade at its rating. A given size wire at any temperating will all act as a heat sink conducting heat away from the heat source. Its the heat the degrades the insulation. As such 90 and 105degC insulation has the ability to withstand the higher temperature without degrading. The heat will still be there regardless. Yes, a larger wire presents a larger heat sink spreading out the heat but I question how effective that would be and not just based on someones theory. If it were to be that effective that very simple solution would have been used decades ago.
If you could come up with some studies that would support your theories I would welcome the opportunity to learn more about these applications.
I have seen 60degC insulated wires, the old TW, where the insulation was so severely damaged not just in the box but beyond the box to the point if you move the wires the insulation will crumble and fall/ of the wire. By using 90 deg and more wire that's much less likely to happen, 105degC from the lamp holder itself and THHN 90degC rated wire into the box.
 
Last edited:
I have a problem understanding your logic the "conducted through the insulation" because if it is 60, 75, 90, 105 deg there wouldn't be any different and is a mute point.

It is not moot. The insulation on wire is designed to insulate against the flow of electrons from one side to the other. It does not insulate particularly well against heat, although it is still a lot poorer conductor of heat than the copper metal.
But the heat that goes out through the copper will end up somewhere, and that somewhere, for a long wire at least, will be conducted out through the insulation. The reason that when multiple wires are in the same raceway the same current (and therefore I2R power) will cause those wires to get hotter. That is because the heat coming from the wires through the insulation is trapped inside the raceway until it is conducted out through the walls of the raceway. That will happen more slowly than if the same insulated wire were in free (convecting) air.

Now, as in pretty much all things thermodynamic, the amount of heat moving from one body to another will be proportional to the temperature difference between them (although not necessarily linearly when convection comes into play.)
The higher the wire temperature, the faster it loses heat through the insulation and the higher the temperature of the outside of the wire, the higher the conduction and convection of heat away from the insulation.
So a wire whose insulation can tolerate a higher temperature will be able to give up heat to its surroundings better.

Fixture wire that can tolerate 100C will be able to carry more current without damage than that same copper limited to 60C. And that wire will be losing heat through the insulation to a 30C ambient very roughly 70/30 as rapidly per unit length.

(At any given amperage, the wire will heat up until the point where the heat losses exactly match the I2R heat creation. The hotter that temperature can go, the more current the wire can safely carry. That is why 18AWG 100C wire can safely carry more current than 18AWG 60C wire.)

Executive summary: When you overheat the fixture by putting in too large a bulb, you are better off with higher temperature fixture wire, up to and somewhat beyond the rated termination temperature of the socket, etc.
 
Status
Not open for further replies.
Top