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I had a lamp that broke recently after getting smashed against a wall by accident (there are now numerous exposed conductors, making it unsafe to use), so I went to the store to get a new one. I intend to use the same very nice LED bulb in the new lamp. I have measured the lamp's power consumption with the LED bulb as 15W before it broke, using a kill-a-watt meter. I was expecting to see a single power rating on each lamp, measured in watts. I was also expecting that that rating would be around 60W to handle old inefficient bulbs, hence making (nearly) every lamp usable with my LED bulb. Unfortunately, this is not what I saw. Most of the lamps had a separate rating for an LED an incandescent bulb, with < 10W for the former and around 60W for the latter. Some also had ratings with a similar < 10W wange for self-ballasted compact fluorescent bulbs instead of LED ratings.

What is the reason for these different ratings depending on how the electricity is used? Surely, if the lamp can deliver 60W to an incandescent it can deliver much less to an LED without problem? Can I use my 15W LED bulb in a lamp that is rated for an 8W led or a 60W incandescent?

EDIT: I think that this question is on topic as, although I am not a professional electrical engineer myself, this is important knowledge for anyone looking to design a lamp or other light fixture.

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  • I'm confused by the term " but a 60W incandescent". Are you saying the lamp is rated safe for use with either an 8 W LED bulb and a 60 Watt incandescent but not a 15W? – Misunderstood Feb 11 at 7:42
  • @Misunderstood The label says something like "Don't use with greater than 60W incandescent bulb or greater than 8W led bulb" – john01dav Feb 11 at 9:21
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Different bulbs dissipate heat differently

The reason why you see different power ratings for incandescent vs. self-ballasted (LED, CFL) lamps for a given fixture is because the different kinds of lamps dissipate heat differently. Incandescent bulbs give out large quantities of heat, but most of it is dissipated from the bulb as infrared radiation -- this escapes the fixture as long as it is not blocked/absorbed by parts of the fixture.

However, while self-ballasted lamps generate far less total heat than incandescents, most of that heat is generated in the lamp base area and must be conducted out. This leads to "heat trapping" in some fixtures that are not able to conduct heat away from the lamp socket area well, and limits the ability of the fixture to handle higher power LED/CFL bulbs.

The answer is to find fixtures that aren't so stringently limited -- I, personally, would be inclined to integral LED fixtures in this day and age vs. Edison-base designs to begin with, as using Edison bases for LEDs inflicts many compromises on the both fixture and bulb designers, vs. the design freedom and performance capacity possible with fixtures designed around LED technology.

  • Looks like we were writing answers at the same time - if I saw yours first I might not have bothered. But one reason still for Edison base lamps is aesthetics. Personally, I prefer ceiling fixtures, but for those who like table or floor-standing lamps instead of permanently mounted fixtures, there is a lot more variety of artsy/interesting/etc. lamps where you supply your own bulbs. – manassehkatz Feb 10 at 2:44
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    @manassehkatz -- yeah, table/floor lamps have been slower to adapt to the changes in design constraints possible with LED tech than ceiling/wall fixtures have -- most of the evolution has been driven by the commercial side, even. – ThreePhaseEel Feb 10 at 2:46
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    More than that, a table/floor lamp without an integrated light source, can be designed and manufactured by pretty much any type of company (e.g., furniture or general household goods) with virtually no electronics - just wires & a switch. That also avoids the perception by many consumers that buying a lamp without a replaceable bulb makes it a "throwaway" lamp. That really isn't the case with a quality LED fixture that will last many years, but that's the perception. – manassehkatz Feb 10 at 2:50
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    @Janka Yes, but the incandescent will be fine with the base at 200C. The driver for a LED or CFL will burn up at 60ish ambient. It's more about protecting the bulb than the fitting. – Someone Somewhere Feb 10 at 10:14
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    @Misunderstood The catch is that with an Edison base LED replacement bulb the LEDs are often on the base and therefore the heat is generated at the base (either in it by drivers or on it by LEDs). – manassehkatz Feb 11 at 15:19
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Heat

Incandescent bulbs generate heat via a filament (along with light) and radiate heat in largely the same pattern as light - the entire glass bulb.

LED & CFL bulbs generate heat primarily in the driver circuits in the base and in the light-producing components. In the case of CFLs, the light is produced in a big twisty (to make it fit in an Edison form factor) tube, so heat is dissipated relatively well - though there are still concerns about the base. In the case of LED bulbs, the light-producing components (the actual LEDs) can be anywhere. However, with a typical Edison base LED bulb, the LEDs are attached to the base. The result is that for Edison base LEDs (unlike "tube" or other form factors), all the heat radiates from the base. With an Edison base LED bulb, the "bulb" part primarily functions as a diffuser for the light but is not an active component (like the twisty CFL tube) or vital to functioning of the bulb (like an incandescent bulb where if the filament were exposed to air it would burn up very quickly).

While the total amount of heat is far less than in the less efficient incandescent bulb, it is much more concentrated. So much so, that some relatively high-powered LED bulbs even have little fans that turn on automatically to dissipate heat (I'm talking about equivalent of 200W incandescent or larger, not the normal residential 25W to 100W incandescent replacements).

This heat problem can be managed much better if the LEDs are spread across a larger area. So a "LED tube" or a fixture that looks like a traditional fluorescent tube fixture (but in reality has LEDs affixed to the fixture directly, hidden by the diffuser).

As a result, a lamp may be designed and tested based on an LED bulb that is typically used as a "60W replacement". The lamp may function fine with a larger bulb, and could certainly provide the necessary power to the bulb, but not tested for safety with that higher power bulb.

I have seen one very specific lamp, the KosherLamp, which only takes (depending on the model) particular relatively small LED or CFL bulbs. These lamps are designed to be on for 25 hours at a time (for Orthodox Jews this makes perfect sense, for everyone else "why would you do that?") with an adjustable shade to provide full light --> dim --> near-blackout-as-if-off modes. In this particular case, the maximum power is an absolute max because the lamp is not open like normal lamps. In fact, designing such a device at a reasonable price/size pretty much couldn't happen until CFLs came along.

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    This is not true: generate heat primarily in the driver circuits. By far, the heat is generated by the LEDs. – Misunderstood Feb 11 at 6:45
  • @Misunderstood You're right - I didn't get that correct. I'll fix it. – manassehkatz Feb 11 at 15:10
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It's thermal

Incandescent lights love heat. They work great as oven lights. The only thermal limitation with incandescents is the bulb heat melting the fixture or setting the building on fire.

CFLs have tubes that make heat, and electronic driver circuits that are relatively sensitive to heat. If a fixture is too enclosed, a CFL's driver will soon burn out. CFLs also dislike having the driver above the tube, because heat rises and mercury falls, so a lamp-down orientation limits output further.

LED's also have electronic driver circuits whose dislike of heat is similar. However, their LED emitters are even more sensitive to heat - they must be cooled with big heatsinks and kept under 85C. (185F). Again, a fixture with poor circulation may have problems.

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Some LED bulbs don't draw power continously but only at the peak of the voltage. This is because of the smoothing cap behind the rectifier. So the current distribution isn't sine as for incandescent bulbs but alternating spikes to plus and minus.

This makes such LED bulbs drawing higher peak currents for the same electrical power rating (which is the area covered by the product of voltage and current.) Those peak currents may overload the wires and fuses inside a lamp fixture with thin wiring.

Use "filament LED bulbs", they don't feature a smoothing cap and don't have this problem. The downside is a heavy stroboscope effect.

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    I seriously doubt this -- the ampacity margins on fixture wire are quite high (and the ratings quite conservative) – ThreePhaseEel Feb 10 at 2:28

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