Another electrical question: How many amps?, New headlight switch, don't want a fire or melt down Options

[Krieger]

As some of you know my car has different "headlights" I no longer have the pop up style. The lights are in the turn buckets. I bought a 15 amp toggle switch instead of the stock unit. This is what I would like to run on my new switch, but I do not know how many amps these items will take: front and rear running lights, license plate lights, dash lights and the stock relay to trigger the fog lights (this is how I chose to power my new "headlights". I was using the original hot wire that was on the oem headlight switch. It comes directly from the ignition switch to the headlight switch. Maybe I should put a 15 amp fuse in line on that hot wire? Other minor issue is the toggle switch has two posts and I have 5 wires to put on one end...

[GeorgeRud]

Don't run the lights through the toggle switch alone. Use the switch to energize a simple relay and you'll be in good shape.

[Krieger]

Why not use a toggle switch?

[Mike Bellis]

Why not use a toggle switch?

Toggle switch will arc internally, just like a set of points and eventually stop working. Use a relay with the toggle switch. You should never exceed 85% of the switch rating.

Additionally, most switches are made in China now and not really rated for the stated amount.

Ohms Law:
E=IxR
P=IxE

E=Voltage
I=Amperage
R=Resistance
P=Wattage

[Krieger]

Thanks for the added nudge Mike. I think I will use a relay. So do I add up the wattage of all the goodies and divide by 12 and that gives the Amps? Still not sure how to get some of them watt values. Oh hell, I have a Bosch relay that says 20/30 Amps. Think thats good enough? I'll even put a 20 amp fuse before it.

[GeorgeRud]

The relay should work fine.

FYI, these square relays and associated wiring are available at Parts Express at very reasonable prices if anyone is adding circuits to their car.

[Krieger]

Thanks George.

[Dave_Darling]

Still not sure how to get some of them watt values.

All of your bulbs should have a rating in watts. Add all of those together.

--DD

[Krieger]

What value do I assign to the lead that goes to the fog light relay?

[Tom]

Use an ohmmeter and measure the coil resistance of the relay. Divide that number into your running voltage. I=V/R That will give you the rated ( approximate) current in amps. Probably something like 150-250 mili-amps.
Tom
Edit: a good safety tip when working on old car electrical systems, when you hook the battery back up, leave the bolt loose in case something is shorted. A better way is take a reading of the resistance of the positive batt cable to ground before doing the work, then again after your mods are complete. If there has been a significant change, especially lower resistance to ground,recheck your work.

[Krieger]

Well, there might be an easier way. I wired it up with just an inline fuse. I guessed at 15 amps. I turned it all on and.....nothing but light. I left it on for maybe 5 minutes. No blown fuse. So I think its less than 15 amps. Should I try a 12 or a 10?

[Mike Bellis]

The size of your fuse is determined by the wire size. Not the load. If you have too much load, you shed some to another circuit. Too large a fuse on too small a wire = the wire becoming the fuse.

Here is some good reading for you. (IMG:style_emoticons/default/smile.gif)

http://www.rowand.net/Shop/Tech/WireCapacityChart.htm

[Krieger]

Thanks Mike. I did keep feeling the wires every couple of minutes to see if they were getting hot and they never did. I should have said that before. The wire for the fuse is 12 gauge and the assembly is rated for 30 amp. I will check out the reading later.