Datsun Roadster Forum-311s.org

ok as most of you know by my past posts I have a 67.5 i am working on and all your help has been great. But this one has me really stumped. due to a wireing harness meltdown before i got the car, all the wire had to be taken out. I have set up 2 new 6 fuse, fuse boxes and am running all new wire my self. Not a problem for me. What the problem now is I want to divide up several things like the headlights then the fog lights / heater/ rear lights/ dash lights but i have no clue how to determine what fuse to use for each. I was thinkinng their was some calculation that involves the amps, wire gauge or something. Some way to find out what amp fuse I need to install so I wont have any other wire issues. I have installed the autolight single wire alternator and was going to put a 60 amp in line fuse going to the fuse block if you hade any thoughts about this?

Once again you guys are very helpful. Thanks. One day I will have a web site so you can see the mess I got my self into.

Someone in our forum did it just recently...

They had 2 fuse boxes. One hot wired from the ignition. The other direct draw from the battery.

I hope he reads this, cuz he might be the very person to talk to.

BTW: I don't if the 67.5 is the same as the 68-70 but there is a used set on sale. With a little work you can rewire to new and duplicate more for sale as refabricated ones.
http://cgi.ebay.com/ebaymotors/ws/eBayI ... 2441396052

I'm the guilty one Minh is speaking of...I did exactly what you are in the process of doing. I split up my loads between two 6 fuse blocks. I decided to make one block hot all the time so I can do things like listen the the radio with the car parked, have an alarm, map light, cigar lighter (cell phone charger), etc. Everything else gets run off the block that is hot when the ignition is on.

First off, the 60 amp inline fuse off your alternator is a great idea. 60 sounds low though. If that's all your alternator puts out then you should be OK. One suggestion, run a circuit breaker instead of a fuse. That way if you get a momentary short that bumps you up over 60 amps, you can fix the problem and reset the breaker. You'll be on your way instead of stuck on the side of the road trying to jumper your blown 60 amp fuse with a paperclip and chewing gum.

As for the size of the fuse to use, take a look in your local junk yard or under the hood of your "other" vehicle. It's a good place to start to get ideas for what size fuse to run. A rule of thumb that I use is 3 amps per bulb on non headlight/fog light type bulbs. This would apply to turn signals, marker lights, brake lights, dash lights, etc. Remember, some of your bulbs like brake/tail lights may be dual filament. I count these as 6 amp bulbs since both filaments can be lit up under braking when the headlamps are on.

For headlights I simply run a 30 amp fuse. Seems to be an industry standard. Same for fog lights. For the other loads, you can get your volt meter out, connect it inline with the load, and measure the current draw of the particular function. Just make sure your meter is fuse protected.

More important than fuse selection is wire selection. It's no big deal to guess wrong on the fuse and blow it out. It's another thing entirely to guess wrong on wire size and burn your car down. Take a look at the attached link:

http://www.powerstream.com/Wire_Size.htm

Here you will find a very handy chart for sizing wire appropriately to the loads you want to carry over it. Factor in a little extra just to be safe. For example, the chart says a 18 gauge wire will carry 16 amps max. If you calculate your tail lights will draw 16 amps, I'd bump the wire size up to 16 gauge, just to be safe. Especially when it's something that can be on for hours at a time.

Good luck! Hope this helps.

I'm not much of an electrician but you can figure current draw if you know the wattage of the draw. Wattage = amperage x voltage or Wattage / voltage = amperage. So a 55w headlamp bulb on a 12v system has a nominal draw of 4.6 amps. That doesn't take into account anything other than what the bulb draws - the wiring and splices/connectors will absorb some current as well. If you really want the exact number you need to measure the resistance of the circuit. Voltage = Current x resistance, so Voltage / resistance (in ohms) = current
From that point you want enough excess current capacity that a small increase in draw won't burn the fuse but not so much that the fuse will allow more current to flow than the smallest wire in the circiut can carry - otherwise, it'll melt or burn.