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post #6 of (permalink) Old 04-19-2002, 07:12 AM
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Re: Whats a good donor fuse block for my 3a

Hey Dave, I wasn't keeping it a secret, I PROMISE. I think I've posted that a couple of times between here, JU and POR. Heck I figured people would be getting sick of it by now.

Figuring the load is pretty easy. Start with P=IE.
Where P= Power (in watts)
Where I= Current (in amps)
Where E= Electromitive force (in Volts)

Figure that your "average" running voltage is 12-14 volts, then use 12 for safety's sake. Most of the stuff you'll probably be using is light-related, so just add the wattages for all the bulbs on a run together and solve for I.

Example: headlights at 100W each: 200W=I*12. Switch it around and you have I=200/12 which is 17amps. I generally add a safety margin of 10-20% and you come up with about a 20-25 amp fuse. The more delicate the electronics you are running, the less you want that safety margin to be.

If that doesn't work, you can V=IR
Volts=Current (in amps)*Resistance(in ohms)

Again, we know voltage to be 12-14
Hook a multimeter to the power lead for the run and measure resistance to a GOOD ground (note if you are hooking things in PARALLEL, this will make the resistance go DOWN, so measure it the way you are going to run it).

Example: Tail lights, the circuit has a total resistance of 10 ohms. Plug that in and we have 12=I*10. Solve for "I" and you get I=12/10 or 1.2 Amps.

Factory supplies for the fuse panel are 8-10 ga. wire, but I'm a belt and suspenders man, so I say use 4 Ga welding cable for power and ground supplies.

For the "total" for the box, figure your entire load, add some extra overhead for future expansion, then buy the closest master switch listed above.

Dave, if you need anything else, lemme know, either AIM me or email.

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