Watt's up with this?

November 17, 2009 4:06 PM

I bought a new lamp today and the sticker by the bulb receptacle says "40 Watts max", so I think fine...

I'll just buy a CFL and get more light buy buying a 60 watt "replacement" bulb. I did that, got a 60 watt equivalent, which uses 15 watts according to it's packaging.
Then I notice the fine print on a tag on the lamp that says: "Requires one 40-watt maximum standard bulb or 11-watt maximum CFL bulb."
Will my 15-watt bulb really do any harm? Am I about to burn down my office?
I did search previous AskMe's, but couldn't find this particular answer (or anything close enough to help me feel more comfortable with this).
Thanks!

posted by dbmcd to Technology (9 answers total)

Must say I am surprised to hear of the "or 11-watt maximum CFL bulb." part. I have always assumed that max Watt is about heat-generated, and resistance in the wires.

I have used many a 60W equivalent CFLs in 40W fixtures, now I will have to research it, like you are.

I did recently switch to LED bulbs, since CFLs have terrible light, but that is not relevant here.
posted by lundman at 4:25 PM on November 17, 2009

The only plausible explanation I can think of for their warning is that the inrush current of a CFL is much, much higher than its steady-burning current, and might, just momentarily, exceed 40W of power. But because this is just a momentary current surge, it doesn't have a chance to really heat up wiring and cause fires.

See: Watt's up with these CFL bulbs

Their warning notwithstanding, I can't think of any plausible reason that a 15 W CFL would be more hazardous than a 40W incandescent. Maybe they just want you to buy their larger, more expensive lamp. Or maybe the label is a requirement for UL listing.
posted by brain at 4:28 PM on November 17, 2009

This blog posting lists some of the inconsistencies in the stated "conversion rates" between incandescent and CFL bulbs. It would appear that the standards (particularly in Europe) are a bit confused. The writer's final conclusion:

Consumers therefore need to be advised to choose a higher watt CFL than recommended to get as much light as from the original bulb and to compensate for the eventual light degradation and poorer quality of the CFL replacement. And the EU standardisation directive needs to be adjusted to reflect reality.

However unreasonable the conversion standard might be I would be hesitant to install workplace lights that contravene a safety warning. You could expect problems with safety inspections and big problems in the event of one of the fittings actually causing a fire. Easier to change the fitting if you want more light perhaps?
posted by rongorongo at 4:59 PM on November 17, 2009

Will my 15-watt bulb really do any harm?

No. That label is wrong. It is all about heat. Place your hand next to a 60 watt incandescent and then next to an equivalent CFl. The CFl is much cooler.
posted by caddis at 5:03 PM on November 17, 2009

See: Watt's up with these CFL bulbs

That is only relevant to generators which have a very limited peak power. Motors have similar issues such that a generator sufficient to power your sump pump is a honking big generator. The start-up current lasts for a very brief moment and won't affect the wiring or shade of a lamp. Unless your circuit is already maxed out (which is most dangerous) then the extra hundred or so watts the lamp draws in the first second or so means nothing. I agree with you that the label is probably about selling bigger lamps or just some non-tech person thought that lumens were relevant.
posted by caddis at 5:11 PM on November 17, 2009

The electronics in compact fluorescent bulbs are actually very heat sensitive. For some enclosed fixtures, you will find that CF bulbs burn out very quickly even though the fixture is rated at much higher power level. If this lamp has an enclosed fixture, they are telling you that your CF bulbs will fail too quickly. If this lamp is open...... I don't know.
posted by Chuckles at 5:44 PM on November 17, 2009

Previously:
Do compact florescents change maximum wattage allowed?
posted by Chuckles at 5:47 PM on November 17, 2009

Best answer: If a fixture is rated at 40 watts maximum, that's a pretty solid indication that its ventilation is piss-poor, which in turn means that if you plug a CFL in there it will run hotter than it normally would. Try your 15 watt CFL by all means (it will still run much cooler than a 40 watt incandescent, so you won't set anything on fire) but don't be surprised when you get less than half its rated service life out of it.
posted by flabdablet at 7:27 PM on November 17, 2009

Response by poster: Thanks all, I'm reassured. Chuckles, I did read that the question you linked to before asking, but it didn't quite answer my question - thanks for linking anyway.
The lamp is open, so I don't *think* ventilation is an issue - but I'll watch it carefully.
posted by dbmcd at 8:50 PM on November 17, 2009