# direct current loss per foot of wire

Discussion in 'Electronic Basics' started by servozoom, Nov 28, 2005.

1. ### servozoomGuest

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

Nick (servozoom)

2. ### John PopelishGuest

You can find many tables of wire data through Google, like this one:
http://www.pupman.com/listarchives/1998/April/msg00189.html

It lists the ohms per 1000 feet (ohms /Kft) and feet feet per ohm for
many wire gauges. You should be able to apply Ohm's law to some
possibilities and figure out how much of your 12 volts will get lost
in the wire, out and back.

3. ### FigaroGuest

|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
|
| Nick (servozoom)
|

---------------------------------------------------------------------------

See http://www.epanorama.net/documents/wiring/wire_resistance.html section
on Current Ratings

V = DIR/1000

D = 400 feet (200 feet x 2 round trip)
I = 0.5 amps
R = 2.575 ohms per 1000 feet

then V = 0.52 volts = voltage drop

your power supply must be 12.52 volts so that the voltage at the far end
device will be 12.0 volts

4. ### purplefloydGuest

Thanks so much to everyone, outstanding info, John's chart is a keeper,
and Figaro's calculation made my job much less painless. You guys are
the best.

5. ### meGuest

Oh boy, where do we even begin:

This question is impossible to answer because you don't provide useful
information. First there's a question of your power source and then

You claim you are 200 feet away from the transformer and then talk about
a 1 amp power supply. So do you mean a 1 amp wallwart? You can't put DC
into a transformer so you'd have to rectify it on the output of the
transformer, it would also be a good idea to regulate it. You also don't
state what voltage is acceptable at the load. 12V +/- what? Bare in mind
that wallwart voltage tends to go all over the place depending on the

14 gauge wire has a resistance of .00297 ohms per foot (assuming copper
wire, and room temperature, resistance goes up with temperature), so 200
feet of it * 500mA gives you about .3V of drop. That means the voltage at
the output of the transformer must be at least .3V above what you need at  