My truck is 6 volts, positive ground. The lighting circuits are protected by the 15 amp and 30 amp circuit breakers attached to the back of the instrument cluster. I am wiring in fuse holders for the stuff that's not protected by breakers. How do I determine the appropriate amperage for the fuses? This is what I've got: 1) Wiper motor, 2) Heater blower fan motor, 3) Flasher for turn signals, 4) Cigarette lighter, 5) Future power to inverter for tunes.
I know that item #5 will have to wait until I figure out what I'm doing for radio/cd/powered speakers,etc. The wiring diagram that I found on this forum for item #3 (turn signal flasher) says 7.5 amps, but I do not know if it's meant for a six or twelve volt system. I don't even know if that makes a difference. If no one has direct knowledge of proper fuse sizes, then what's involved in figuring it out if I can learn the draw? I see in the 1953 Service Manual that the heater motor normal draw is 10 amps at 6.8 volts. I also see that the draw for the horns is 12 to 14 amps at 6.2 to 7.2 volts. I couldn't find anything in the manual or on the motor itself for the wipers. And, as far as the horn goes, I hadn't considered a fuse here. Should I? I have a cheap ammeter/voltmeter but have never been trained in how to use it. Maybe it's time....

 

For what it's worth, I found some old posts on the subject, but none seemed to be for a truck running six volts. But, for a truck running I assume)12 volts, the cigarette lighter was 20 amps and the wiper motor was 15. Does that sound right?

 

You are wise to consider the draws before you fuse the circuits. A lot of forum members will have more knowledge of the draws from experience than I do. Look at it as an opportunity to get a new tool. You should be able to get a decent amp clamp/meter for $100.00 or less if you shop around, you do need to get one that will read DC amps, a lot of the ones in the hardware stores are AC only so make sure the one you get has a DC amps setting. Then providing the device you are checking the load on is in good shape you will have a good idea of the draw. Perhaps others here will have a formula or rule of thumb for how far beyond the load do you fuse the circuit.
The main thing is you do not want the load to exceed the capacity of the wire. Rule of thumb there 14 gauge wire 15 amps, 12 gauge wire 20 amps, 10 gauge wire 30 amps. At least that is what I have used successfully over the years.
Good luck and enjoy that new amp clamp.


B

 

On second thought I gave you bad advice, although I have not worked with 6 volt systems I have worked with 24, and I should have remembered that the higher the voltage the more amps you can put through a wire. So my rule of thumb for amperage for wire size although good for 12 volts may be way off for 6. Curious now, I will look into that and get back to you.


B

 

Ok, third time is the charm. I should check before I give bad advice which I did and very sorry about that. I did not find a chart for 6 volt, but did find a chart for 12 http://www.engineeringtoolbox.com/am...auge-d_730.htm so I was way off on my rule of thumb for wire capacity, thinking home wiring, bad advice. Again sorry.
I did find on an old tractor site an article on converting 6 to 12 volt which stated that 6 volt wire needed to be 2x larger than 12 volt wire for the same amp load. Not sure if that is accurate or not but worth considering. I think with enough searching there should be some information out there. That would be good to know before you start running wire.

B

 

Well this just gets more embarrassing with every post. Now I see the link does not work. Try this one Wire Size Calculator | GTSparkplugs

 

LOL, you're getting there, Oldb!

"...6 volt wire needed to be 2x larger than 12 volt wire for the same amp load." That isn't really correct; what it should say is that for the same Wattage or HP load, the current at 6v will be double that of a 12v. So you need a larger wire, but not double the size. For instance, headlights are the same wattage whether 6v or 12v, so on 6v they need heavier wires.

You are absolutely correct that fuses (or circuit breakers) are sized to protect the wire, not based on the loads. The Wire Size Calculators are generally set up to show what size wire to use to have minimal voltage drop to the load. They don't show the currents at which the line starts smoking. So they are pretty conservative values.

The original CB's protected everything in the truck. As stated, they were 15 and 30 amps for a 6v system. They seemed to work OK!

 

So far I have found two sources for wire sizes for my truck. The first is the factory service manual, which does not give a complete list, but does describe about half of those wire sizes behind the dash. The other is what remains of my original wiring. Though any original has badly cracked rubber and frayed cloth, the gauge can still be determined. Based on all this, the wiring for the ignition circuit and lighting circuits has been installed. The wiper motor and the heater blower fan motor each have their original wiring, so I can figure out the wire gauge there. I was just going to run a #10 to the cigarette lighter. That's what I have running to the horns and to the charging circuit.
The various wiring diagrams that I have for this truck show that only the lighting circuits (except the courtesy light) tap off the circuit breakers. Headlights tap off the 30 amp. Tail, parking and brake lights from the 15 amp. The brake light circuit is interrupted by the turn signal, and the brake light filament gets its juice from the flasher, which taps off the accessory post on the ignition switch. This is a circuit that I want to protect. I have a diagram that shows a 7.5 amp fuse here, but the diagram may be assuming that the whole system is 12-volts. If so, then I guess I'd want 15 amp fuse here for my 6-volt system? That would be sort of consistent with the doubling of amperage with a 6-volt system. I do not see any other electricity using components tapping off the circuit breakers. I found an older post from AxRacer that said that it wasn't until around 1960 that fuses started to be used. What is puzzling then is that this means that wiring diagrams that I have showing fuses would be no earlier than that. And by that time, 6-volt systems were pretty much gone.
So, now I'm thinking that if the fuse is there to protect the wire, and I have determined the wire size, and I know that the volts max out at about 6.5, then I should be able to size the fuses based on wire size?

 

Sometimes it helps to understand that many items are determined by watts. Volts x amps = watts (you can twist this equation around to determine each.) So a 24w load @ 12v = 2 amps, the same at 6v = 4 amps. The tricky part is many of us are used to common wire gauges recommended for 120v home loads. Automotive wiring is a bit different. Different insulation, duration of load, type of load, is it a branched circuit and the likelihood that all devices on circuit will be powered together, etc. It's not so much about how much load the conductor can handle as it is how much heat the insulation can protect/isolate. I had a Ford class where the instructor had a 2.3 mounted on a cart. He had a single 14 ga. wire connecting the starter to the battery. Yeah, it started the engine just fine but that wire was smoking hot! LOL He was just making a point. I used to have a MAC Tools business and one night I had a long drive. I began smelling hot wiring and once home I found the headlamp switch smoking. Investigation revealed there was 32 bulbs on the running light circuit without using a relay! (Not to mention it was wired with 16 ga. Thanks, GM *rollseyes*)

Check this chart and come back with questions: Wire Size Calculator

BTW, here's a heads-up: Many OEMs have issued TSBs about peeps buying/using Storehouse fuses from Harbor Freight. Their fuses have caused a number of vehicle fires and OEM testing has revealed high inconsistency in the fuse ratings vs. actual current required to cause the fuse to open. Some were found to go 4x-5x their rating before blowing! Buyer beware. I'd also be kinda wary about using their wire.

 

1948/55: 6 volt positive ground / 1956 and later: 12 volt negative ground.

14526 glass fuses & circuit breakers:

 

See the diagram below. It is for a '53, so 6v. Should cover everything you need as far as gauges.

As a side note, the EXTREMELY valuable "Earl's Reference Data" pages on "Club FTE" appear to have been deleted?!?!

 

I have a question hope not real stupid but could you not use 10-12 guage wire for all as I have been told that the current runs on the outside of the wire strand and the bigger the better so to speak as the current has more room and does not or maybe not overheat. You could still use what size fuse you need. By the way I don't know a lot about wiring. Just a thought and no I have not done my truck that way (yet)

 

Sure, but it's unnecessary and could cause some issues with certain connections, grommets, etc. Look at how long the original wiring lasted. It's just the age and condition of the insulation and connections that are worrisome today.