I just installed a electric tongue on my cargo trailer. It has a three foot whip with a 30 amp fuse and is 10 gauge wire. I was going to wire up a hot lead from the battery to the rear of my truck for power. Then I remembered that I have a wire already ran to the back of the truck for the extra lights on my salt spreader and could use that. The only thing is I think it is 12 gauge wire. It is fused also with a 15 amp fuse. The jack is only run for maybe 30-45 seconds to lift the trailer. Do you guys think this will be okay or should I run like a 8 gauge wire to the back? I just don't want to burn up the motor in the jack. But I figured the short run time might make it okay. My truck is a extended cab so it is like a 19 ft run from my battery to the bumper. Am I right or wrong? Suggestions??
12 AWG is rated for 25 amps - if properly terminated. So it would be a touch small for sustained usage at maximum power.
BUT, electric motors pull max current only when under max load - which happens when lifting a very heavy tongue. It will pull less when lowering or when lifting less than it's maximum capacity. So unless it it really straining, it is pulling less. And the tongue jack should only be running for very short periods of time. All those factors work in your favor.
If it were me, I would try it. The risk? If you try to pull more current than the wire can handle, two things happen:
1) Voltage drop. And over such a long run, the V drop may be a full volt or more. The jack will run slower and not make as much power as it is capable. If jack can't lift, then you should install larger wire.
2) The wire will heat up. If a LOT more than it can handle, or running for a long time, it will heat up enough to melt insulation and short out.
Also key is the quality of the terminations. I am assuming your 12 AWG wire is terminated directly to battery? Make sure the termination is a very high quality crimp or better yet, soldered.
Put in a 25 amp fuse in the wire (at the BATTERY end, I hope - so it shuts off power if the insulation melts and shorts to truck frame!), plug it in to the jack and give it a try. I bet it will do it. If it pops the 25 amp fuse, try a 30 amp, but then monitor the wire temp. Keep a hand on it whenever lifting. If it gets HOT, shut down the jack and rewire.