# DC current loss over a lenght of wire

• posted

I can't seem to find an answer to this question anywhere, So I've come to the experts. I need to power a device that requires 500ma of 12v dc current, but it has to be 200 feet away from the transformer. Will a

1amp power supply provide adequate current over a 14 gauge wire? Is there a formula to figure this out? Thanks in advance
• posted

Resistance of 14 AWG wire is 8.9 ohms per 1000 meter.

200 feet times 2 (for + and - wires) = 400 feet = about 125 meters =>

about 1.1 ohms. Voltage drop over the 400 feet wires will be 0.5 amp (500 ma) x 1.1 ohm = 0.55 volts. Therefore, if you start off with 12 volts power supply and the load device drawing 500 ma, you will lose 0.55 volts over the 400 feet of wires and get 11.45 volts at the device. If your device operates fine at 11.45 volts, it will work fine.

• posted

Hi Servozoom,

At this site there is a nice calculator to find resistance of wire

• posted

Note: Many "12V" transformers are actually specified to give ~11.5V and then your input voltage can vary....

RickR

• posted

And just to clarify on your subject/question, there is essentially no loss of current. The current going into the supply end of a wire will equal that coming out at the load end.

• posted

That is correct (you knew that) and a good point. However, the OP should know that the current will be lower with 400 feet of wire between the source and the device than if the device was connected directly to the power supply. The length of wire does reduce the current by ~22 mA (see next paragraph), but that current is not "lost" - it simply was never pulled from the supply in the first place.

The device is rated 500 mA at 12 volts, which is the equivalent of 24 ohms. Add the ~1.1 ohms resistance of the wire, and the total resistance is 25.1 ohms. 25.1 ohms will draw ~478 mA instead of the nominal 500 mA.

Ed

• posted

Thanks so much to everyone for the info, I've learned quite a bit.

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