Few lampholder sockets, except for outdoor fixtures or three-position floor lamps, are rated to accommodate the all-too-common 100W incandescent light bulb (lamp).
This photo shows what happens over time, if you use 100W incandescent lamps in the wrong fixtures (most of which are rated 60W or 75W maximum). Incandescent lamps are being replaced by CFL’s and LED’s because 90% of the energy they consume is invisible heat radiation. The heat destroys lamp holder screw-shell sockets and the wires leading to them.
In the photo, the insulation around the screw-shell has become brittle from baking by the 100W lamp. The screw-shell itself makes electrical contact via rivets at the base, which is cheap aluminum and has also baked its way loose of the rivets. The result is a familiar problem in old houses with old fixtures: a slowly-worsening problem of lamp flicker and intermittent lamp failure. The intermittent, poor contact at the loose rivets can generate heat from arcing at the connection, and could ignite wires and start a fire.
Lampholders, such as the E26 (aka ‘Edison-base’) shown, have a Code-required ‘listing’ (underwriter insurance for the manufacturer) up to a maximum wattage for the particular application ‘listed’. The listing is based on actual testing of the finished luminaire product and/or takes into account factors including the characteristics of the lampholder and other parts (Ceramic- or plastic-insulated? What type of plastic? Guage and type of metal used for the outer screw-shell?), and of the luminaire where it is installed (wall- or ceiling-mount? enclosed or open for ventilation? other lamps nearby?). Most luminaires have a listing for maximum lamp power of 75W per lampholder or less. Candelabra lampholders often max out at 40W. Recessed fixtures typically accommodate 75W. Some lampholders may accommodate 150W while others may be rated less than 25W.
The problem of heat-damaged lampholder insulation becomes an electrocution hazard, especially in old houses where wire polarity is not marked by color, and wiring polarity has become reversed on some branch circuits. Normally, the outer screw-shell seen in the photo has the same voltage as earth, your plumbing pipes, and most anything conductive in your house that you are able to touch without getting out a tool-box. When the house wiring has reverse polarity (due to amateurs modifying old, unmarked wiring on the assumption ‘there is no polarity in AC’), the exposed-screw shell in the photo would be energized 120V to ground, so that if you touched the screw-shell and something grounded at the same time, your body would carry current from the screw-shell to the grounded something. This might cause a heart attack or knock you off a ladder, etc. At best, it feels very unpleasant and can cause health problems months or years later that you’ll never connect to a seemingly minor electric shock incident.
The best solution to all of this is to use new high-efficiency lamps like CFL’s or LED’s, which max out below 75W of actual energy consumption, while producing as much as 150W equivalent light output, compared to incandescent lamps. And since they’re turning more energy into light and less into heat, even the actual energy consumption produces less heat than the equivalent-wattage incandescent lamp would do (a 15W CFL will operate cooler than a 15W incandescent lamp, and produce as much light as a 60W incandescent lamp).
If you must use incandescent lamps, DO NOT EXCEED 75W unless you can positively identify the fixture as being rated for more than 75W. Assume the fixture is 60W-rated per lamp socket, unless you can tell otherwise, and do not exceed whatever rating you can find (usually a label on the screw-shell insulation, or somewhere inside a diffuser dome or globe that houses multiple lamp sockets).
- Overlamping low-voltage (12V) track or cable track lighting systems
- Have LED’s come of age?
- Discussion of overload in Learning to live with multi-generational wiring in old houses
- Polarity matters in AC wiring
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