Wire sizing question

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?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,651 admin
    edited October 2016 #2
    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
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • deegoredeegore Registered Users Posts: 8 ✭✭
    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.BB. Super Moderators, Administrators Posts: 30,651 admin
    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
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • deegoredeegore Registered Users Posts: 8 ✭✭
    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.BB. Super Moderators, Administrators Posts: 30,651 admin
    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
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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