[design store]

I have a tri-light table lamp that has a sticker on the bulb socket that warns of fire hazard over 100watts. BUT a stamp on the socket reads 250V 250W.
Is there truly a danger in using a 150watt tri-light bulb?

[BIG-AL2000]

All it means is that the heat generated from a bulb higher than 100w could be a risk. I would say that, unless the lamp is going to be on for long periods, a 150w bulb should be fine.
To be safe, don't leave it on if you are not in the house.
It is only the manufacturers being cautious.

[bhayne]

Unfortunately, watts is a bad measurement. The lamp is rated for 100W/ 110V = 0.83A
The socket is rated for 250W/ 250V= 1.0A

Your 150W bulb draws 150W/ 110V =1.36A.

So your lamp and socket are both underated for the 150W bulb. Since those lamps only have a 50W element and a 100W element, you may still use the bulb, just don't use the brightest setting (dim = 50W, normal = 100W, bright = 150W).

[omzig]

Wattage restrictions on light fixtures don't just take into account the amount of current that it can handle, but also the heat that is produced by the bulbs themselves.

For example, many recessed fixtures are limited to a 65W bulb because they are enclosed and heat can build up. The socket and fixture wires could certainly carry more than the half of an amp that a 65W bulb would draw.

The stamp on design store's lamp is probably its electrical rating and the sticker is the entire fixture's rating, taking into account the heat produced.

[bhayne]

If wattage was the only concern, why would someone spend the money to put a 250W socket in a 100W fixture. Just doesn't make sense.

Now excuse me, I think I'll buy a Lamborgini engine to stick in my ford pick-up!

[omzig]

If wattage was the only concern, why would someone spend the money to put a 250W socket in a 100W fixture. Just doesn't make sense.

Now excuse me, I think I'll buy a Lamborgini engine to stick in my ford pick-up!!

My uncle owns a lighting store and he gave me a parts catalog that has lamp sockets in it. In one style that I'm looking at, they have a 250W version for $1.27 and a 125W version for $1.25. I think I'd use the 250W for 2 cents more. Many of the styles aren't available in anything less than 250W.

[bhayne]

Tell that to the poor soul that you laid off after the 1 millionth sell because you lost $20,000.

[omzig]

Tell that to the poor soul that you laid off after the 1 millionth sell because you lost $20,000.

Touchè

I have a dehumidifier that uses 6.5A but has a 14-guage cord... go figure.

[tkrussell]

Design Store, the lamp you have most likely has a label on it somewhere by Underwriters' Laboratories, indicating that it is "Listed". This means the manufacturer has brought his product to UL for testing and listing, and brought it as that it will only be used with a 100 watt lamp, for what ever reason they chose.

I suspect they chose 100 watt to avoid any product liablility law suits against them, in their belief that 100 watt is safer than 150 watt, or the max of 250 of the socket.

Why the socket for 250W? A combination of purchasing the most popular socket available at a very cheap price for the hundreds or thousands they need for their product, and a bit of over engineering, their thinking to use a higher rated socket, and derate it's use.

Also, keep in mind the more availabity of halogen lamps, which are notorious for being used improperly and have been causes of fires, due to the high surface temperature of the bulb.

Make sense?

[tkrussell]

Omzig, if you are wondering why they used #14 wire for a unit with a 6.5 amp draw, while they could have used #16 wire, a de-humidifier is a small refrigerator, and refrigeration compressors draw much more to start than the running load amps.

And these units tend to be used in areas that a #16 cord could not withstand the physical abuse.

[bhayne]

It is probably made in US and not in China (in response to previous answer). Ever wonder why Jobs are moving to China?

Back to the original question. The wattage is more relevant if the position of the fixture is inverted, as per pot lights. However, I believe extra protection is required for these fixtures to protect them from high temperature.

You should also know that temperature affects resistance in conductors. The higher the temperature, the more resistance in the conductor. Current flowing in a resistive conductor also produces heat. So a conductor that is sized for 1A will increase in temperature if you make it draw 2A (the temperature will increase 4X). The heat from the bulb will also make the conductor more resistive and the current flowing through the conductor will heat it even more.
This is a cascading to catastrophy feature of electricity that results in many fires also kwown as 'I squared R' (I2R) relationship. The voltage is fixed but the heat created in a conductor increases as the square of the current.

[omzig]

Omzig, if you are wondering why they used #14 wire for a unit with a 6.5 amp draw, while they could have used #16 wire, a de-humidifier is a small refrigerator, and refrigeration compressors draw much more to start than the running load amps.

And these units tend to be used in areas that a #16 cord could not withstand the physical abuse.

This is what I figured too, but I have another dehumidifier that is rated at 8.3A and it has a #16 cord.

[tkrussell]

Bhane, please consider,

The temperature effect on the resistance of wiring in this case of fixture wiring is negligible, due to the low temperature coefficient of copper, using reasonable everyday values and irrelevant to the topic of wiring to a light fixture.

For example,a length of 10 feet of #14 copper wire, in direct contact with a heat source producing 260D C(500D F), heating the entire length, will increase the resistance .393% for each 1.0Deg C rise above ambient 68DegF(20 Deg C).

The resistance of this 10 foot of wire increases from .0506 ohm to .1 ohms, resulting in an I2R loss of .4 (4 tenths) of heating watts, assuming a 2 amp resistive load 120 VAC @1.0Pf, an increase of only .1976 watts of heat.

An increased resistance of .05 ohms is not enough to affect the load's current draw, and an increase of .1976 heating watts is not enough heat created to certainly overcome the 500 Deg heat source.

Although,

cascading to catastrophy feature of electricity that results in many fires also known as 'I squared R' (I2R) relationship.

is directly related to hot contact spots, otherwise known as loose or poor connections.

A loose connection creates resistance, then large I2R loss heat, damages the material, increases the resistance thus creating more heat, etc. etc. until, with small wire as #14, invariably reaching melting point of copper, all in a small area. Once the metal melts, the circuit is opened and current stops.

In larger wire with larger contact spots, the heat is usually very destructive, hopefully in a approved enclosure to contain the heat, and possible shorting through conductive carbon material, and creating a spark shower.

I make a great deal of money due to this effect. If I could only convince owners of large electrical equipment to understand the importance of preventive maintenance.

A breaker that has burnt at the plugin busbar is a good typical example of this heating effect. Or those ever popular Whiz Wire'd outlets.

Just interested in being sure the readers are informed accurately and completely.

You get the feeling your not just dealing with a dirty finger nailed, a** crack showing sparky?

[bhayne]

Since the lamp is rated at 1A, I am considering that the lamp wiring is also rated at 1A. If I was designing a 1A rated lamp I surely wouldn't use 14AWG wiring! Also, what about the rating of the multi-position swich?

The main concern for increasing the wattage of the bulb is not the heat (since the socket is rated for a 250W bulb). The main concern is that the parts (internal wiring, switches) are not designed to handle increased current.

After all, these desk lamps are moved quite often and who hasn't had one fail because of a loose connection?

[tkrussell]

OK I see your point, however, do you think it is practical to use a wire only rated for 1 amp in a table lamp, simply because the manufacturer places a label not to use a lamp larger than 100 watt?

The minimum size wire allowed in the US, in a building, for line voltage branch circuits is #14. The minimum allowed for portable appliances and light fixtures is #16.

So if your explanation is based on the theoretical, fine, but, in a real world application, I think it is best to use something that is standard, reasonable, and customary.