I want to add a cutoff switch into the primary lead that feeds the HEI. I know I can't put in any old switch 'cause it might melt under the load. What is the minimum rating I should use for this application?

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Tom H.
Member #259
Indianapolis,IN www.iquest.net/~bharold/chevelle/ (http://www.iquest.net/~bharold/chevelle/)

20 amperes

Thanks, Ken. Now, how would I apply that to different voltage when I decide to set this up for some other application? Put simply, what's the formula for ampere rating vs. voltage?

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Tom H.
Member #259
Indianapolis,IN www.iquest.net/~bharold/chevelle/ (http://www.iquest.net/~bharold/chevelle/)

Should be IxR=E E/I=R E/R=I E= volts I= amps R= resistance

I hope this will not be a confusing answer.

The above formulas, while absolutely correct, aren't much help. You would have to know what the resistance per foot is for each wire type, the tollerable voltage drop, the power dissipation capability for the wire, etc.

For your house, unlike your car, there are codes that must be followed by law. A 20 amp circuit uses 12ga wire and a 15 amp can use 14ga. That's solid core copper wire, for runs of a hundred feet or more and under high constant load. If your car were wired like that, it would weigh a lot more than it does now!

With the exception of your spark plug and coil ignition wires, voltage is not an issue. Everything else in your car is nominally 12 volts. It is the CURRENT that will determine the thickness of the wire. (The voltage would determine the thickness of the insulation.)

The things that use the most current in your car are your lights, horn, power windows, heater fan, AC, etc. If you look at those wires, you should find that they are 16ga to 18ga stranded wire. Stick with those and you should be fine.

Hope this helps.

Bob

The HEI primary wire is supposed to be 14 gauge to handle the high current demand. If you're going to add a switch in series your wire would have to be at least this thick. I would isolate this circuit with a relay and wire the hidden switch for low current relay control.

Kevin: Huh?

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Tom H.
Member #259
Indianapolis,IN www.iquest.net/~bharold/chevelle/ (http://www.iquest.net/~bharold/chevelle/)

A relay is basically a switch that handles high current switching using separate (low cuurent) control lines. Think of it as your starter solenoid; two little wires control a large plunger that switches direct battery power to the starter motor. For this application one would connect the HEI primary wire between the high current terminals of the relay and then the relay's control terminals would be connected to smaller 18 guage wires that would switch in 12 volts.

Hmmm...do I need this, or is it okay to use a standard switch? It sounds okay, but I think I've already got a 20 or 25A switch sitting around in the garage.

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Tom H.
Member #259
Indianapolis,IN www.iquest.net/~bharold/chevelle/ (http://www.iquest.net/~bharold/chevelle/)

The switch only will be fine. Avoiding extra components will reduce the chance of something extra that can fail. Solder all connections for long lasting work and insulate. Make sure you run through grommet in firewall.

Tom,

If you want to "Mickey Mouse it" then do it Greg's way (you'll appreciate the new noise in your radio as well). Just remember that auto manufacturer's have reliability concerns too and they elect to use relays for high current switching.

So where do I get this nifty piece of hardware? (BTW, there's no radio in the car right now, but I'm not discounting anyone's advice on either side of the fence.)