Fuse principle question

[Chuey]

I'm about to make a wire loom for my 1983 R100 and its new electrical outlet for my heated jacket. I plan to use 12 gauge wire and put a fuse in the positive line, right so far? What size fuse should I use for this type of circuit?

Thank you.

Chuey

[Major Softie]

I'm sure 12g is big enough - might even be overkill.

As far as the fuse, what does the jacket draw? I would think something like 20% higher than the jacket's rating would be fine. If it blows too easily, go up another 10 - 20%. 12g wire is good for 20A in much longer runs, probably more like 30A in a run of only 20" or so. I very much doubt that the jacket draws 10.

Just checked the Tourmaster site. I don't know what jacket you have, but the Tourmaster jacket is 6.7A, so a 10A fuse would be fine. Tourmaster puts a 15A fuse in their harness, which is expected to power as much as a jacket, chaps, and gloves.

[dwerbil]

In the AC world, number 12 would handle 20 amps. I assume the same in the DC side?

[Major Softie]

In the AC world, number 12 would handle 20 amps. I assume the same in the DC side?

But, 12g is rated at 20A in the AC world for much longer runs. That's why I presume it can handle much more when less than 3 feet long. It's not the AC that's the issue in these kind of distances, it's the voltage. Wire ratings in The Code are not about the wire's actual capacity, but voltage drop over distance. Charts can vary a bit, but from one example: 12g can handle 20A at 120v in a 54 ft. run with a 3% drop. In a 12v system, it's MUCH shorter, but still, 12g can handle 20A for 5 ft. with a 3% drop.

Most charts show 20g's actual max amp capacity as 30A, but that also varies with temperature.

[Duane Ausherman]

That 12 g will not melt until it has over 200 Amps through it. That is from memory. You don't want to consider the current that it takes to melt the wire, but it is just to show you just how much over design you have.

You will be OK.

[Seth]

200 amps may not melt the wire, but it surely will destroy most (plastic) insulation.
200 volts would not be an issue, but it's amps that heats things up.

[Chuey]

So I'm using big enough wahr that it should have plenty of room for the smoke so it doesn't need to get out?

Chuey

[DaveBBR]

You'll be fine. Set the fuse 20-25% above the estimated load and keep the length of the run under 10 feet. 12 ga bare wire is rated for 80A in open air, 20A when insulated, 15-18A when overstuffed into a too-small conduit. You'll need to run a separate circuit for your on-bike coffee maker and microwave.

[Major Softie]

You'll be fine. Set the fuse 20-25% above the estimated load and keep the length of the run under 10 feet. 12 ga bare wire is rated for 80A in open air, 20A when insulated, 15-18A when overstuffed into a too-small conduit. You'll need to run a separate circuit for your on-bike coffee maker and microwave.

That's another good point. When I added my wiring loom, I was thinking ahead, so I made it big enough that I could add my on-bike coffee-maker and microwave on the same loom. Thus, I used 4g. 6g would have given me a 3% drop, but I wanted to be sure...

Actually, that's not that far from the truth. When I put Fiaams on my Oilhead, I made a loom of 10g wire, just so I'd have extra power for later. When I added an aux. light, I put it on that same loom, and it still has capacity for more . . . like if I add that coffee-maker and microwave...

[Chuey]

Microwave; that's what I give other riders going the other way. Mine is off the grid.

Chuey