Wiring a switch to a portable emergency pack

Discussion in 'The Projects Forum' started by spartan77777, Nov 28, 2012.

  1. spartan77777

    Thread Starter New Member

    Nov 28, 2012
    being an almost total newbie, attempting to build a portable emergency power pack using a 12V 35AH AMG battery in a plastic battery box. Will be installing a dual USB socket, a 12V DC car cigarette lighter style socket, set of terminal posts for external units. Also would like to install a master switch and individual switches for each component. Have a few questions.
    - What size fuse should I use for this type of installation and should I use a single fuse or should I fuse each segment seperately?
    - I will be hooking up a Wagan EL2003-5 400 Watt Continuous Power Inverter that has a peak 1000W power, what kind of fuse should I use for this.
    - I am considering switches available from amazon 10 for $9.99, 12V 16A switches. IS 16A sufficient to drive typical items I would drive through these or should I consider higher amp switches.
    - Switches have 3 legs. Being a portable unit, I don't really have a ground. How would I handle ground in this type of an installation.

    Thanks in advance for your explanations/recommendations.
  2. #12


    Nov 30, 2010
    The fuses will be rated according to each load. For instance, the 400 watt inverter would use more than 33 amps if it were 100% efficient, and it isn't. That gives you less than an hour on that battery and connectors MUCH larger than a cigarette lighter socket. You need to use Watts Law to figure (P=IE) so 1000 watts (power) = 83.333 amps at 12 volts (again assuming 100% efficiency). This requires size 3 AWG wire according to the NEC (National Electrical Code).

    The "ground" will be one terminal of the battery, whichever you choose.
    The switches will only need 2 legs.
    You are in danger of melting the battery with loads this size.
    That's a start.
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    For efficiency, adding 20% will often get you close to the right size, so for 400W continual, calculate for 480W, which results in 33A or a 35 Amp fuse/breaker. If your inverter has a smaller value fuse in it, such as a 30A fuse, use the lower value instead of a 35A. (inverter output ratings are usually over-stated, These numbers are only educated guesses, there aren't full specifications on the Amazon link). Most vehicle power sockets are limited to 20A.

    FIRST: Fuse should be as close to battery as possible.

    For a 35A Draw, 4 feet or less apart with 10 ga wire (minimum suggested size).

    I'd suggest acquiring a fuse or a circuit breaker from a car audio shop local to you, as they will sell both the wire, switches, and fuse blocks capable of handling high current. Blade style fuses aren't designed to carry that amount of current for any time, and many will blow after a dozen minutes run at near rating (if the small battery doesn't fail first).

    The container will also need adequate ventilation to keep the inverter and battery at safe temperatures, the comments at Amazon state that the fan seems to be quite loud, indicating high airflow.
    Last edited: Nov 28, 2012