In article .com, Pete
C. wrote:

HerHusband wrote:

Hi Cathy,

Say you've got an old circa 1954 ceiling light fixture.
Spec'd for 2 max 40 watt incandescent bulbs.
My Sylvania 100w-output CFL's take 23w input.
100% safe to use 2 of these CFLs instead of 2 40w incandescents??

There are two concerns with light bulb fixture ratings:

1. The current draw. If the wiring is rated for 80 watts (two 40 watts),
you could potentially overload the wiring with larger bulbs (though
unlikely unless the wires are very tiny).

I don't think you're going to find anything less than 18ga wire in a
light fixture, and the allowed ampacity for 18ga in a fixture is 6A (NEC
table 402.5) which equates to 720W at 120V.


2. Heat Output. A larger incandescent bulb will put out more heat, which
can degrade the insulation on the fixture wiring. Especially with older
fixtures where the insulation isn't rated for higher temps. I've seen many
ceiling fixtures where the bulbs had heated up the insulation enough for it
to crumble away, exposing the bare wires.

In the case of the CFL's, you would be drawing LESS current (23 watt vs 40
watt), and CFL's put out much less heat than incandescents. So you won't
harm the fixture or wiring in any way by using the larger CFL bulbs.

Yes, due to the much lower percentage of input Wattage that is output as
heat, you can use CFLs with input Wattages well about the incandescent
Wattage rating on a fixture without issues.

Actually, CFLs are more efficient at producing non-radiant heat than
incandescents, despite being more efficient at producing visible light.
What CFLs produce less iof is infrared.

I did actually heat a fixture very slightly more with a 42 watt CFL than
with a 60 watt incandescent. I have a non-contact thermometer.

However, if the fixture is enclosed, you "may" have problems with the CFL's
themselves. Some CFL's aren't designed for even the minimal heat build up
in an enclosed fixture and will burn out quickly. When you shop for the
CFL, try to find ones that are specifically rated for enclosed fixtures
(though many don't specify either way).

Yes, some of the cheapest CFLs are pretty heat sensitive themselves and
if they don't get good ventilation they have fairly short life spans.


I have several 75 watt CFL's installed in enclosed ceiling fixtures around
the house originally rated for 60 watt incandescents. They've been running
for years now with no problems. A huge plus with our high ceilings that
require ladders to reach the fixtures.

75W equivalence rating I presume? That would be well under the max you
could install, and if the area could use more light the 40+ input Watt
CFLs would work well and be safe.

I would expect the maximum common CFL wattage to not produce more
fixture heating than a 40 watt incandescent to be 26 watts.

- Don Klipstein )