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I bought a 15W led bulb which is the equivalent of a 120W incandescent. But, I can't find a cord grip ceiling bulb holder rated higher than 100W.

Is it safe to put the 120W equivalent bulb in a 100W ceiling socket?

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Yes, that is fine

"like" /"equivalent", etc. are based on light output.

Limitations on fixtures is based on heat output.

Some power (Watts) becomes light, the rest becomes heat. LEDs are more efficient and produce more light, and therfore less heat, than incandescent bulbs. Fixtures are rated on worst case scenarios. But a 15 W LED bulb can only put out 15 W heat. No problem.

It may be that fixture power rating is based on efficiency and expected heat output of standard incandescent bulbs, rather than total power output. But that would make very little difference. A quick search finds incandescent efficiency in the range, at best, of 5% to 10%. If a 100W incandescent bulb is 10% efficient (very optimistic) then 90% of the power - 90W - is lost as heat. If the 15W LED produced entirely heat and no light at all (which wouldn't be too useful), it would only produce 15W of heat. No contest.

All that being said, there can be heat issues with LED lighting. Generally this is with new fixtures designed specifically for LEDs, not putting LEDs into standard Edison base fixtures. In fact, some high-power fixtures include fans to dissipate the excess heat. This is because (a) an LED can be extremely compact as there is no functional need for a large "globe" around a filament (when there is a "filament" in an LED bulb, that's just for show) and (b) the driver circuitry between the incoming AC power and the actual LEDs produces heat. This is also the reason why some LED bulbs will specify "don't use upside down" - heat rises and if the heat is all generated at the bottom of the bulb and you install it upside down so that the bottom (as designed) is at the top, heat may not dissipate properly.

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  • Is that 90% of the power in the bulb, or does it heat the room? – user253751 Sep 3 '20 at 20:42
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    Using that example, 10W becomes light, 90W becomes heat. The heat will end up heating the room. Which is another reason why more efficient lighting is a big deal - if you are in a hot area and using air conditioning, every bit of heat you add inside a room requires additional power to remove the heat. In heating season it isn't so bad, but electric resistance heat (which is just like incandescent bulbs, just doesn't glow as bright) is generally more expensive than heat pumps, natural gas or other forms of heating. So incandescent bulbs are still not a good idea. – manassehkatz-Moving 2 Codidact Sep 3 '20 at 21:19
  • As a slightly famous Dutch blind environmental activist used to say: "When I'm cold, I switch on the light". – gerrit Sep 4 '20 at 8:35
  • @manassehkatz-Moving2Codidact But at the same time in moderate or cold climates the savings are much less than you would nominally assume because in 8 months out of 12 you actually like the heat and save the corresponding amount of explicit heating. Modern power plants combining electricity and heat production actually are environmentally more friendly than your typical oil home heating. – Peter - Reinstate Monica Sep 4 '20 at 11:17
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    @Peter-ReinstateMonica And using the heat part of the power plant for heat production is more efficient than turning the electricity part into heat. – user253751 Sep 4 '20 at 12:32
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You can.

The lamp holders are rated for how much heat the lamp produces, and a 15W lamp produces much less heat than a 100W lamp.

It's only like a 120W lamp in the amount of glow it produces, that dosn't bother the lamp holder at all.

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Maybe.

Two things to consider:

  1. Overheating of the fixture

and

  1. overheating of the bulb.

(1) is not going to happen. 100W incandescent bulb is ~95W heat source. 15W LED lamp is ~11W heat source. No problem here.

(2) may be a problem, or maybe not. As the incandescent lamp generates 8x the heat, it will rise its own temperature above the environment roughly 8 times more than the LED lamp. But, LED lamps are also much less tolerant to high temperatures.

If we consider the incandescent lamp glass heating up to 140C in a hot day (40C in the room, lamp 100C above the environment), it will be OK. A LED in the same conditions will heat 100/8 = 12.5C above the environment, e.g. 52.5C . Not a big deal, but probably not an optimal temperature for a LED either.

In the worst case, you'll get some less lifetime from the LED lamp.

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  • This is a pendant lamp (cord grip holder) so the lamp should have good ventilation. – Jasen Sep 4 '20 at 4:36

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