# Quick question, phone line can carry how much current?



## Tater Rocket (Oct 27, 2002)

Can standard phone line carry 300-400 mA? I bet by those numbers you can guess why.... 

I am wondering because phone line is the smallest wire I have, and I need it small so it fits in my mod. The copper in the phone line IS solid, so what do you all think, will it carry that much current?

Also a quick resistor question....

The wattage rating on resistors, is that the wattage going through it, or the wattage being DISSIPATED in heat?

Thanks,
Spud


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## dano (Oct 27, 2002)

Phone line wire carries 48V, but not sure about the current...

--dan


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## Jonathan (Oct 27, 2002)

Standard phone line is usually 22 or 24 gage wire. If we presume 24 gage, you have an area of 404 circular mil, and a resistance of 0.027 ohm/foot. At 400mA you are producing about 4mW of heat per foot, which should not be a problem at all.

The wattage rating on a resistor is the power being dissipated as heat in the resistor, the voltage drop _across_ the resistor times the current _through_ the resistor. The wattage rating will depend upon the ambient temperature and the heat sinking on the resistor.

-Jon


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## Tater Rocket (Oct 27, 2002)

Ok, thank you very much! That answer was very informative and helpful. 

Spud


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## camisdad (Oct 27, 2002)

About 15 years ago I built a controller plugged into a phone system. The satndard phone line then was specified as a current loop which was carrying either 20 mA or 40 mA, I can't quite remember. 
Based on that, and given standard engineering practices, I would say that it would be quite unlikely that such a wire would carry 400mA.


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## Saaby (Oct 27, 2002)

You know, if the power went out but the phone didn't I wonder if you could power a tiny, low low power consumption ligt (Read: LED...not Luxeon)

It would be useful in most places I would think. Don't know how useful it'd be in my hosue (Fiber Optic at street so the power comes from my mains and there's actually an UPS in the basement for the phones) but I suppose it could be an interesting homebrew light...

Guess it's time to start looking at 2600 stuff agian...they should know...


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## James S (Oct 28, 2002)

It must be OK since the telephone company used to do exactly that. If people here are old enough (you don't have to be older than maybe 25) to remember that phones used to have incandescent lamps in them for lighting up the buttons. These were powered from a little wall wort style transformer that was just connected to the second pair of wires. These lamps were small, but certainly drew at least as much power as you're talking about, and you never burned out the wiring by picking up 5 or 6 extensions at once and powering them all.

At one point as a kid I actually connected my dads stereo to 2 extra pairs and in my bedroom connected up full power speakers. I never really cranked it up, but it worked just fine and I know I was pumping many watts at times through it

-James


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