Wire sizing question

deegore

Hi, I'm planning on running a phone charger from my 12v batteries. It will take 40ft of wire to reach.
The phone charger is 2 amps at 5 volts. Does this mean the wire will be supplying 10 watts? 
If so, 14 guage wire should be fine. Does this sound right?

BB.

Yep... That is pretty much correct... The actual current may be:

• 10 Watts * 1/0.85 phone charger eff * 1/12 volts = 0.98 Amps @ 12 VDC

Use a generic voltage drop calculator to see how heavy of wiring you will need... I would suggest a maximum of 0.5 volts drop:

http://www.calculator.net/voltage-drop-calculator.html

So:

• 40 feet of wire (one way run, some calculators use 2 way distance), 1.0 amps, 0.5 volt drop on a 12 volt battery bus:

14 AWG:
http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=8.286&voltage=12&phase=dc&noofconductor=1&distance=40&distanceunit=feet&amperes=1&x=58&y=11
Voltage drop: 0.20
Voltage drop percentage: 1.67%
Voltage at the end: 11.8

So, that looks about right. You should have a ~2 to 5 amps fuse on the 14 AWG wire near the battery + bus connection (yes, you can put as high as 15 amps for 14 AWG, but that wire length is not going to carry more than a couple of amps before voltage drop goes too low).

-Bill

deegore

Thanks, I have a couple more questions.
I have another 40 foot run to an outlet that I want to power up to 100 watts at 12 volts.
It looks like 8 gauge wire will work with a 10 amp fuse. 
Can I safely use an inline fuse that comes with 12 gauge wire at the battery, and connect 8 gauge wire to that?

The other question is, can I run an 8 gauge wire from my battery and connect it to two 14 gauge wires for a couple lights?
Both lights on at the same time would be well under the amp rating for the length of 8 gauge wire.

BB.

In general, for US/North American fuses+breakers, if you want to run a continuous 10 amps, you should increase the value by 1.25x (10 amp load * 1.25 = 12.5 amp fuse, round up to next standard size).

An 8 awg wire can support much more current than 14 awg wiring. If you have a fuse/breaker on the 8 awg wire rated at 15 amps--Then you will not have any problem.

However, if you have a fuse/breaker rated for 40-55 amps (NEC rating for 8 awg wire in conduit), then connected your 14 AWG circuits--That would be unsafe. If there was a short on one of the 14 awg branch circuits, then the 14 awg wire could overheat and cause a fire.

-Bill

deegore

Thank you, I think I understand.
To put it another way, if a 40 foot run calls for 8 gauge wire to limit the voltage drop, one foot of 14 gauge wire in the circuit won't cause problems as long as the amps aren't too high. Is that correct?

BB.

As long as you do not exceed the capacity/rating of the "weakest link" (the 14 awg wire in your case), then all will be fine.

It is a bit unusual to mix gauges of wiring like that in standard AC household wiring--And it could cause problems down the road (for example) if somebody else services the wiring and puts a too large breaker in the circuit (see the 8 awg, and do not know of the 14 awg elsewhere in the system).

But--For low voltage systems with longer wire runs, using larger gauge wiring to obtain lower voltage drop is common. So having undersized breakers/fuses on those wires is not unusual or unsafe.

-Bill