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Old 12-10-2016, 08:53 AM   #1
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Default LED bulb surface temperature

Had a customer ask an interesting question.

If the fixture is rated for, lets say, a 60 watt bulb, could he use a 13 watt LED which is the equivalent of a 100 watt incandescent?

We know the restriction on the wattage is due to the heat radiating off the incandescent. And the heat is primarily centered around the glass.
Whereas, the heat off the LED is centered on the heat sink, not as large of an area as the glass.

Anyone have any information as to whether installing a larger LED would be a fire hazard?
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Old 12-10-2016, 10:12 AM   #2
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Quote:
Originally Posted by wcord View Post
Had a customer ask an interesting question.

If the fixture is rated for, lets say, a 60 watt bulb, could he use a 13 watt LED which is the equivalent of a 100 watt incandescent?

We know the restriction on the wattage is due to the heat radiating off the incandescent. And the heat is primarily centered around the glass.
Whereas, the heat off the LED is centered on the heat sink, not as large of an area as the glass.

Anyone have any information as to whether installing a larger LED would be a fire hazard?
I'd say the fixture rating is for real watts (ie 13W) not equivalent watts
(ie 100W). So they could put a 200W equivalent in there if they wanted
to as it's only about 36 real watts......if it fit.
P&L
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Old 12-10-2016, 10:24 AM   #3
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I'm still looking for a LED bulb that can be put in an enclosed fixture.
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Old 12-10-2016, 10:34 AM   #4
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I'd say the fixture rating is for real watts (ie 13W) not equivalent watts
(ie 100W). So they could put a 200W equivalent in there if they wanted
to as it's only about 36 real watts......if it fit.
P&L

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Old 12-10-2016, 10:45 AM   #5
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Heat is measured in power absorbed over time. Watts is a measurement of power, so the amount of wattage something consumes is directly related to how much heat it generates.

It's literally impossible for something using 13Wh of energy to put out more heat, or even the same heat, as something using 60Wh of energy.
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Old 12-10-2016, 10:49 AM   #6
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Heat is measured in power absorbed over time. Watts is a measurement of power, so the amount of wattage something consumes is directly related to how much heat it generates.

It's literally impossible for something using 13Wh of energy to put out more heat, or even the same heat, as something using 60Wh of energy.
That's terrific. Now please explain scientifically how 50 watt halogen puck lights wont burn down a kitchen cabinet because after all it's only a 50 watt lamp.
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Old 12-10-2016, 01:13 PM   #7
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Halogen lights are hot because FEMA fills them with chemtrails.
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Old 12-10-2016, 01:40 PM   #8
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Quote:
Originally Posted by Big John View Post
Heat is measured in power absorbed over time. Watts is a measurement of power, so the amount of wattage something consumes is directly related to how much heat it generates.

It's literally impossible for something using 13Wh of energy to put out more heat, or even the same heat, as something using 60Wh of energy.
What if the 13w device is much, much less efficient than the 60w device?

What if, for example, the 13w device is putting only 1w towards it's intended purpose (let's say light) and 12 watt to heat, while the 60w device is putting 55w towards it's intended purpose and only 5w to heat?
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Old 12-10-2016, 01:47 PM   #9
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What if the 13w device is much, much less efficient than the 60w device...?
The ratings are based on expected heat load from an incandescent bulb, and incandescents are fantastically inefficient: The average incandescent converts <5% of the power to light; they are space heaters that also happen to glow in the dark. So it's practically impossible to exceed the fixture thermal rating by accident.
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Old 12-10-2016, 01:51 PM   #10
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Quote:
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The ratings are based on expected heat load from an incandescent bulb, and incandescents are fantastically inefficient: The average incandescent converts <5% of the power to light; they are space heaters that also happen to glow in the dark. So it's practically impossible to exceed the fixture thermal rating by accident.
I have to admit, this is above my head.

Quote:
The ratings are based on expected heat load from an incandescent bulb, and incandescents are fantastically inefficient
The wattage ratings on bulbs (incan, CFL, or LED) are how much power they use, no? So I don't understand your above statement.

I'm still not understanding how a very efficient 13w device can't put out more heat than a super efficient 60w device.
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Old 12-10-2016, 02:11 PM   #11
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...I'm still not understanding how a very efficient 13w device can't put out more heat than a super efficient 60w device.
In theory it could. But incandescent bulbs are so inefficient that to make things simple we can pretend that all of their power rating gets turned directly into heat.

That means if you've got a fixture rated at 60W, then it is designed to tolerate 60Wh of heat energy continuously. So any other bulb that either
A) uses less power or
B) uses the same power more efficiently
will expose that fixture to less heat.
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Old 12-10-2016, 03:18 PM   #12
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efficiency is an actual quantity, even though it is used in everyday language.


the efficiency of a mechanical/electrical device is usually defined as the percentage of work performed divided by the total energy used.

in the case of the incandescent bulb, the efficiency would be:

Watts of light energy/watts supplied on the circuit.

now, one could find the light energy by measuring, or more easily measure the amount of heat output along with the heat lost in the conductors (voltage drop).

so the Watts of light energy = Watts supplied - Heat loss of bulb - heat loss of conductor resistance.

and the efficiency would end up being something along the lines of what John said, or in other words, you would lose 80 to 95 percent of the energy supplied doing work other than that which was intended (ie. lighting).


hope that helps.
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Old 12-10-2016, 04:35 PM   #13
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The numbers vary by brand etc but they're roughly like this:

Incandesent -> 2.5% eff -> 60W@2.5% -> 1.5W light and 58.5W heat
LED -> 10% eff -> 13W@10% -> 1.3W light and 11.7W heat

While the location of the heat will be different between the two, 11.7W
of heat is very unlikely to cook the wires etc the way 58.5W will.
@daveEM made a good point in post #3 re; enclosed fixtures. AFAIK, this
is related to lamp life only, not overheating of wires etc..
P&L
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Old 12-10-2016, 04:53 PM   #14
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Quote:
Originally Posted by PlugsAndLights View Post
I'd say the fixture rating is for real watts (ie 13W) not equivalent watts
(ie 100W). So they could put a 200W equivalent in there if they wanted
to as it's only about 36 real watts......if it fit.
P&L
Lol..... sometimes my motto is "if it fits, I sticks".
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Old 12-10-2016, 05:11 PM   #15
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Quote:
Originally Posted by Big John View Post
Heat is measured in power absorbed over time. Watts is a measurement of power, so the amount of wattage something consumes is directly related to how much heat it generates.

It's literally impossible for something using 13Wh of energy to put out more heat, or even the same heat, as something using 60Wh of energy.
Very much so, if not we'd have seen LED heater infomercials coming out the ears.
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Old 12-13-2016, 04:45 PM   #16
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Quote:
Originally Posted by wcord View Post
Had a customer ask an interesting question.

If the fixture is rated for, lets say, a 60 watt bulb, could he use a 13 watt LED which is the equivalent of a 100 watt incandescent?

We know the restriction on the wattage is due to the heat radiating off the incandescent. And the heat is primarily centered around the glass.
Whereas, the heat off the LED is centered on the heat sink, not as large of an area as the glass.

Anyone have any information as to whether installing a larger LED would be a fire hazard?
Isn't that comparison of saying a 13W LED being the same as a 100W incandescent only equivalent in the amount of lumens it produces, not the amount of energy.
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Old 12-13-2016, 04:55 PM   #17
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Isn't that comparison of saying a 13W LED being the same as a 100W incandescent only equivalent in the amount of lumens it produces, not the amount of energy.
Yes, and unlike the case of an incandescent bulb the color temperature of an LED is not tied to its thermal temperature.

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Old 12-13-2016, 05:00 PM   #18
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Yes, and unlike the case of an incandescent bulb the color temperature of an LED is not tied to its thermal temperature.

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You mean you can't switch to cool lighting during the summer to beat the heat?
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Old 12-13-2016, 05:04 PM   #19
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You mean you can't switch to cool lighting during the summer to beat the heat?
It might be subjectively more comfortable

BTW, I have one bathroom in my house that is not tied to the HVAC system.
I put in a heat lamp bulb in the can next to the shower for winter and replace with an LED trim each summer. Does that count?

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Old 12-13-2016, 05:09 PM   #20
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It might be subjectively more comfortable

BTW, I have one bathroom in my house that is not tied to the HVAC system.
I put in a heat lamp bulb in the can next to the shower for winter and replace with an LED trim each summer. Does that count?

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Just use a red LED, same redness, less wattage.
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