bugs1961

This is transferred from this thread: https://www.yotatech.com/forums/f116...d-help-181902/ because it might interest folks in the electrical area of the forum. The attachments are in the link.


If you want to know what size wire you need for any circuit, it is possible to calculate the required size and have a safe and efficient circuit. Some basics to work with:

1. Wire has a constant resistance based on size and temperature. The chart attached shows resistance values for standard automotive wiring at appx. room temperature in Ohm's per 1000'. Divide the number in the chart by 1000 to get Ohm's per foot. Multiply the Ohm's per foot by the length of the wire to get total resistance.

2. The steel body of the vehicle has a resistance of the equivalent to 2ga wire and should be used as the return to ground unless you're using larger than 2ga wire. The attached writeup of a test I performed explains how I came up the the equivalent resistance of the body. (Toyota also uses the body as the return to ground for all chassis electrical equipment.)

3. Keep the ground wire for any electrical equipment as short as possible by attaching the wire to the closest grounded bolt or nut.

4. Determine how long the power wire will be and use Watt's Law and Ohm's Law, along with the wire resistance of the wire size you intend to use to determine how much voltage drop you will have in your circuit (see attached writeup). If you have .5v drop or less, the circuit will be safe and efficient.

5. Place a fuse as close to the battery as possible to protect as much of the wiring as you can. The fuse needs to be large enough to allow the maximum current your electrical equipment will use but still protect the wiring. The current that would cause a .75v drop in the circuit should be the maximum allowed by the fuse. This fuse would blow well before there was damage or a fire.

6. Bookstores have great books on automotive electricity that will take the mystery out of wiring. The one I reference is still available in stores and is great. Keep in mind that rules of thumb for household 110v wiring do not apply 12v automotive wiring--use the calculation or the wire size recommendation charts in automotive electricity books!

7. The wiring in the vehicle was sized to handle the loads Toyota installed and may well not be big enough to handle any added load. If you want to add electrical equipment to your vehicle, you should run a fused supply wire from the battery to a new fuse block you install under the dash. Painless Wiring makes great fuse blocks that can form the foundation for adding electrical equipment. Use the above method to determine the size of the supply wire.

Take care and have a great day,
Bugs