You do realize certain lamp types do not have a filament at all - such as fluorescent lamps, and HID lamps.120414-1935 EDT
Most incandescent lamps have a tungsten filament. Radio vacuum tube heaters or filaments are also tungsten. These have a positive temperature coefficient of resistance. Thus, large initial current. In contrast the Edison lamp had a carbon filament and this has a negative temperature coefficient of resistance.
You can see an Edison lamp in photo P13 at my web site http://beta-a2.com/EE-photos.html
Thank you for the explanation and i do agree with it. But doesn't the switch have to have the "T" rating on it?120414-2331 EDT
A standard normal life 120 V 100 W tungsten filament lamp has a room temperature resistance of about 10 ohms (I just measured one). Its operating resistance at rated voltage should be close to 120[SUP]2[/SUP] / 100 = 14400 /100 = 144 ohms.
For said 100 W bulb the RMS current at 120 V is about 0.833 A once at steady state operating temperature. Peak turn on current for a room temperature bulb at a voltage peak is about 170 / 10 = 17 A. See my P1 waveform at http://beta-a2.com/EE-photos.html .
If you had 18 of these bulbs on one switch your peak current some of the time might approach 18 * 17 = 306 A.
In halogen bulbs the filament is run somewhat hotter so the hot to cold resistance ratio maybe somewhat more than 14.4 . I do not have one to measure.
T rating is required for DC. AC only you don't need it.Thank you both for the replies.
So if most incandescent bulbs including the Edison base have tungsten filament, then i suppose the switches that are installed to control these bulbs must have the "T" rating. But almost non of the switches are "T" rated unless you ask for that specific switch, and non of the dimmers (at least i don't know of any) are "T" rated. Further more i haven't had any inspector ask me if the installed switch is "T" rated.
i am confused. :?
i looked up the white book and it said if a switch is to control a tungsten load then the switch must be "T" rated.
Thanks for making the correction. I did not read the UL Guide Information for Snap Switches (WJQR) far enough.T rating is required for DC. AC only you don't need it.
Just about every incandescent and fluorescent lamps.Which bulbs have a tungsten filament? All bulbs or just halogens?