Why another fuse?

[darthandy]

As I get things together to do my fuse block install, various relay bypasses and, hopefully, repair of my ignition switch, I noticed something in a couple of other threads on the subject. When someone talked of installing a fuse block next to the battery so that accessories can be run off that rather than off the battery terminal, there were a couple of suggestions to put an in line fuse between the battery and new fuse block. Why? I mean, it is a fuse block - don't those fuses give you the protection you need? Why have a fuse between the battery and fuse block? There may be a good reason, but I just don't see it. Anyone?

 

Andy

[V7Goose]

You will notice that the stock bike has a single main fuse between the battery and the fuse block also. There are two reasons for this design. First, if there is a fault in the fuse block or a feed wire to it becomes chafed and shorted, having another fuse closer to the battery provides some additional protection (although the risk of that problem is very small). Second, there is the question of the Total possible current draw by all circuits and the size of the wire feeding the block. The main fuse is 30 amps. Each sub-circuit needs to have the wires sufficiently large enough to safely handle the total current allowed by the individual fuse in the circuit, and the feed wire to the block needs to be large enough to handle the maximum total combined current from all sub-circuits. For example, if there is one 20A fuse, four 10A fuses, and three 5A fuses, then the total possible current draw would be 75 amps, which would require a welding cable to feed the fuse block.

 

The reality is that it is almost impossible for all the sub-circuits to be drawing max current at any one point in time, so the design calls for a more reasonable wire size to support the EXPECTED total current draw, but then that smaller wire must be protected by the appropriate size fuse. That is the real reason why there is a main fuse.

Goose

[darthandy]

OK, now I get it. I plan on using 10 gauge wire from the battery to the fuse block and then a separate circuit for the driving lights, headlight and ignition along with an appropriate fuse size for each in the fuse block. I'll have to calculate what the potential maximum draw to the block would be and go with a fuse that will handle it plus a reasonable safety margin.

 

As always, thanks for the great info.

 

Andy

[V7Goose]

You won't need a new fuse if you use 10 gauge wire. 10 gauge can handle 100 amps up to a 10' run, and 200 amps up to a 3' run.

 

If you want to make the wires a little easier to mess with, a 14 gauge feed will handle 40 amps up to a 7' run and still allow longer secondary 20 amp runs up to 15'.

 

I only mention the secondary run because I have a mental block about using a smaller gauge feed wire to the fuse block than any of the secondary run sizes. Technically there would be no problem powering the fuse block with a 3' 14 AWG feed, then running secondary 13A 15' circuit which would require a 10 AWG wire, but I can't do it. I just feel better if the feed line is at least as big as the maximum secondary line.

 

Here is an easy chart to help you choose wire sizes - it doesn't tell the whole story but it is plenty good enough for what you are doing:

http://www.tessco.com/yts/industry/products/itm/automotive/get_wired.html

 

The missing part is that wire size selection is done for both safety (heat) and voltage drop (resistance, which causes heat). That chart completely covers the safety issue, but does not talk about the circuits where you want to keep the voltage drop to 3%. When in doubt, it is always great to use the next larger size wire for any circuit, just do not ever use a smaller size than called for. This is probably overkill, but finally, a comment on AWG (American Wire Gauge) vs SAE (Society of Automotive Engineers) wire sizes - look at the marking on the insulation to know which one you have - SAE wires are up to 12% smaller than AWG in the same gauge, so consider going to the next larger size if you are near the amp or length limits of a specific circuit. That chart I linked to above does not say if it is for AWG or SAE wires, so for safety, best to assume it is AWG.

 

Goose

Edited November 5, 2011 by V7Goose

[darthandy]

Thanks again Goose. The wire feed to the fuse block is less than a foot long with a ring type connector at each end, so I should be OK. As mentioned, I will be calculating the requirements per circuit, but since I have plenty of 14 gauge gauge wire (All my wire is AWG, by the way) I'll probably use that. It may be overkill, but I prefer having more margin rather than less. Why am I touchy on this subject? Well, I have to chuckle when forum members complain about the marginal or at least low capacity of the electrical system on our Ventures. They should check out my 1994 Triumph. Plugging in a GPS unit is enough to dim the lights! A big problem is the gauge wire they used. On these early (Pre-2000) bikes, every circuit seems to run wire gauges about one size down from what you would expect. I believe they have improved this on the newer models but we have to live with it on the older ones. A popular mod on these bikes is to replace the wires from the battery to the starter which immediately gets your starter spinning faster. That makes an important difference on these bikes since they also have weak sprague clutch springs - replacing a destroyed sprague clutch unit is a frequent happening if you don't install a stronger spring and heavier gauge starter wiring. Guess what I get to do this winter? Oh - guess what other problem we have...too much current going through the ignition switch which eventually fries it - and this is without adding any accessories! Sound familiar?

 

And thanks again for the chart. Electrical work is not one of my strong points so I appreciate all the help I can get. Another member has offered to talk me through fixing the ignition switch - at least once I get it off. Thanks to this site I've found some nice write-ups on getting it out. Now if my body will just cooperate and quit messing me up so that I can get out there and start working!

 

Andy