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Can I run a extension cord 50' from my batteries to my 12v light with out a lot of voltage line loss? Answered

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Yes,
If you're running just 1 light
16 ga wire should be fine.

You can calculate what the voltage drop, and the amount of power lost, provided you know the resistance of your 50 foot extension cord, and you can calculate that using this table:
http://en.wikipedia.org/wiki/American_wire_gauge#Table_of_AWG_wire_sizes

E.g. the resistance of a length of 100 feet of 14 AWG copper is:

R = (100 ft)*(2.525 Ω/1000 ft) = 0.2525 Ω

Notice I multiplied by 2, because  total length of wire in your 50-foot extenstion is 100 feet.

Next : multiply R by I2, and there's your power loss.

You would probably need heavy copper cable, and that would be expensive. But it would depend upon the light - what sort is it?

L

If your light uses very little AMPERES... then you will be ok.. only a little voltage will be lost in the very long cord.  If you are trying to run some high amperage lights you will loose voltage across the long cord.  It kind of depends on how thick the wires are and how many amperes you are trying to use at the end of that cord.  Here's an example.  once i tried to run an air compressor using a LONG extension cord.  The compressor motor would not turn very well.  So i figured out i had to have a short power cord, and a LONG airhose.   One problem with trying to run LOW VOLTAGE is....  most low voltage items are HIGH AMPERAGE.

Seconded.  Even copper at long distance has a resistance, and low voltage with ohms law kicks your butt for efficiency...

V=IR

Big V means low I has to run through that same resistance, compared to small V needs big I.