EDIT: Hit the "REPLY" button to see the charts with all the columns properly spaced.
I like TW2007's clean set-up under the right side cover. Very well done, and storage for spare fuses included. Unfortunately, the install isn't waterproof, and I ride through water that would drown those fuse blocks, so I'd choose waterproof inline fuse holders instead of fuse blocks. If you don't use your TW as a submarine, though, the fuse blocks will work just fine--a little rain won't hurt a thing.
All circuits should be fused. Fuses protect the vehicle, not the accessory, so fuses need to be as close to the battery as possible. Below is a 12 Volt DC chart for insulated, braided wire in free air. The wire gauge is on top. The left column is the amps the wire is carrying. The second from left column is the watts. Going down and across provides the maximum length of wire from source to accessory that can be run without exceeding a 5% loss to resistance, which is generally an acceptable amount. Where there are xxx, the ampacity of the wire is exceeded.
Amps Watts Gauge of wire
Wire 12V #14 #12 #10 #8 #6 #4 #2 1/0 2/0 3/0
1 12 84 131 206 337 532
2 24 42 66 103 168 266 432 675
4 48 18 33 52 84 133 216 337 543 675
6 72 14 22 33 56 89 141 225 360 450 570
8 96 10 16 27 42 66 108 168 272 338 427
10 120 8.5 13 22 33 53 84 135 218 270 342
15 180 6 8.5 13 22 35 56 90 144 180 228
20 240 xxx 6.6 10 16 27 42 67 108 135 171
25 300 xxx xxx 8 13 22 33 54 86 108 137
30 360 xxx xxx 6.6 11 18 28 45 72 90 114
40 480 xxx xxx xxx 8 13 21 33 54 67 85
To determine the proper gauge wire for an accessory, check the paperwork or device itself for how many amps it draws. Sometimes the amperage isn't listed, but wattage is. Divide wattage by 12 to get amperage. For instance, the H4 headlight bulb on 2001+ TWs is listed as 55/60w. 55 watts on low beam, 60 watts on high beam. 60w/12v=5amps. The headlight circuit would require a 5 amp supply. Going to the chart, I see that a #14 wire will carry 6 amps at 14 feet, so I choose #14 wire for the headlight circuit because the total length of wires will be way under 14 feet.
Now I have to choose a fuse. A fuse needs to exceed the amperage of the circuit, but be under the ampacity of the wire. I see in the chart that #14 wire can carry 8 amps at a length of 10 feet, and since the headlight circuit wire length is under 10 feet, I can use any fuse over 5 amp and under 8 and adequately protect the vehicle.
Since in reality, the total length of wires feeding the headlight circuit is under 8.5 feet, I could get away with a 10 amp fuse and still adequately protect the vehicle. Personally, I would choose a 7.5 amp fuse because the size is readily available, provides 50% excess capacity to the accessory, and a 33.3% safety margin on the wire itself.
Next step in the wiring project would be determining the ampacity of the switch. I'd make sure the switch had at least a 10 amp capacity because a lower capacity switch could possibly confuse its role with that of the fuse, overheat, and burn itself out. A 7.5 amp fuse would then protect the accessory (the bulb), the wiring, and the switch in case of a short. Keep in mind the "chain of protection"--the fuse is designed to be the weakest link in the chain and is expected to blow and break the circuit in event of excess current flow. A fuse is nothing more than a switch that can only be turned off once.
In the real world, whether wiring a house or a vehicle, I always design "Code Plus" installations. I like a 100% safety margin on all things electrical. Therefore, for a 5 amp accessory, such as an H4 headlight, I'd use a minimum of 10 amp circuit hardware. That means a 10 amp fuse holder, 10 amp switch, and 10 amp wire, minimum. I also like to size the switch capacity to fall somewhere higher than the fuse, but lower than the wire--it is generally much easier to replace a switch than a wire. I also want the brightest light possible, so I want to eleminate any voltage drop due to resistance in the wire. To limit voltage drop simply go up one size wire over what you can get away with. Therefore, For a 5 amp headlight circuit, I'll use a minimum 15 amp fuse holder and #12 wire (which will carry 15 amps at 8.5 feet) a 10 amp switch (which will protect the wire should the fuse short instead of open for some reason), and a 7.5 amp fuse. In fact, that is exactly how I wired the H4 conversion headlights on my mini-van to the relay. Power to the 40 amp relay is #10 wire (30 amp capacity at this short a run) protected by a 15 amp fuse in a 30 amp fuse holder. People often comment on how white and bright my headlights are, asking how powerful. They are simply Silverstar Ultra 55/60w, but with no drop from resistance in the wiring.
Next question is, how much does insufficient wiring affect luminosity of a headlight? Check this chart out:
10.5V : 510 lumens
11.0V : 597 lumens
11.5V : 695 lumens
12.0V : 803 lumens
12.5V : 923 lumens
12.8V : 1000 lumens ←Rated output voltage
13.0V : 1054 lumens
13.5V : 1198 lumens
14.0V : 1356 lumens ←Rated life voltage
14.5V : 1528 lumens
Alternator ouput is about 14.5 volts at highway rpm. Even a 5% loss brings volts at the light down to 13.775, for a drop in headlight luminosity of 1528 lumens down to about 1250 lumens, nearly a 20% drop in brightness. One would have to switch to 80 watt headlights to match the brightness, but odds are the voltage loss would make the 80 watters less bright than the 60 watters with good wiring. Fairly significant, no?
On to another project, installing grip heaters on Tdub. The new grips arrived yesterday and I now have everything I need to do the install. Let's look at engineering the wiring.
According to internet research, the grip heaters I bought draw 40 watts on high. 40 watts divided by 12 volts is 3.3 amps. I already have #14 wire, which can carry 15 amps for 6 feet so that's good. I'll be using a 10 amp SPDT switch, so I'll limit my fuse to 7.5 amps, which is 127% of the maximum current draw. Good there. I have a waterproof 20 amp inline fuse holder. Overkill. The weakest link is the fuse at 7.5 amps. The second weakest link is the switch at 10 amps. Either or both will blow before the fuse holder, the third weakest component melts. It's all good.
Here's a simplified chart of what wire to use when.
Wire Gauge Stranding Ampacity Corrected Standard Usage
10 19x23 30 n/a 30
12 19x25 25 20 20
14 19x27 18 14.4 15
16 19x29 13 10.4 10
18 not listed 10 8 8
At the short lengths on a TW, don't worry about distance.
Here is a chart of typical loads of accessories:
Instrument/Indicator 0.12 amp 1.5 watts
Rear Turn 2.1 amps 27 watts
Stop/Tail and Front Marker/Turn Signal Lamps 2.1 amps 27 watts (Stop/Turn) 0.6 amps 3.5 watts (Tail/Marker)
Early Headlight 3.0 amps 35 watts
Late Headlight 5.0 amps 60 watts
HID conversion headlights 3.3 amps 40 watts (higher initial fire current required)
Wolo Bad Boy or Stebel Nautilus Horn 10 amps 120 watts
Remeber that you will want to size hardware for the sum of the draw of every accessory on a circuit. Add those current draws up!