Wiring a 'relay based" starter switch circuit

[WagonCrazy]

Hang with me here for a minute folks. Listen to the essence of what I'm after. Tell me if you have experience doing this, or see major issues...

I'm wiring up my 57 Nomad restomod project. All fuse panels, relays, ECM (LS1), and most electronics/wiring is going into a custom center console that I'm building. Packed with wiring and components.

I want to keep the original "keyed ignition" on the dash. But one of the things I want to accomplish is to wire the starter circuit "outside" of the key ignition switch. In other words, don't want that heavy "12v red battery wire-to-purple starter wire" voltage/amp draw traveling thru the ignition switch. Want to use a heavy duty "relay" to energize (from the start position via the ignition switch) the purple wire to the starter. Via a relay I suppose? What is the appropriate relay to use for this. Is a 40amp relay "heavy" enough?

Another way of asking is how many amps pass thru the ignition switch when you turn the key to "start" and the starter cranks?

The starter itself is getting its voltage from a 1 ott stranded cable coming from the battery, so it seems to me the purple wire just "energizes" the solenoid on the starter that allows it to "pass" that 12 volts from the battery cable to the starter windings...

So how much draw does this (approx. 12 gauge) purple wire get? And what amperage relay is "too small" for that?

Stuck in automotive wiring theory hell right now. Can't seem to find the answers with google searches...

[chevynut]

A 40A relay for the solenoid wire is overkill, but if you have one use it. Nothing wrong with doing it the way you want to, but the ignition switch will handle the current just fine...it has for decades. I'd say it draws 10-20 amps or so. Measure the resistance of the solenoid and use Ohm's Law. ;). The advantage of a relay is if you want to interlock the starter in some way.

[55 Rescue Dog]

A 40 amp relay should be fine. The short duty cycle operating current on the solenoid (which is a relay too) is probably under 10 amps, but the "pull-in" current is as much 5 times higher for a fraction of a second, and the heavier relay contacts will last much longer. Originally most of the cars electrical power other than lights had to go through the ignition switch too, which was not much compared to modern/modified cars. The more relays the better.

[WagonCrazy]

Thanks guys. I'm drawing up a wiring diagram (in my head) and need to get it on paper soon. To keep me on track with wiring in general...
It's so easy to look at the mess of wires and get overwhelmed with all the circuits at once. That's the challenge as you build a custom hot rod...plan ahead to keep on track, avoid getting too confused, avoid running out of money, avoid running out of time! Did I miss any? :)

[55 Rescue Dog]

Thanks guys. I'm drawing up a wiring diagram (in my head) and need to get it on paper soon. To keep me on track with wiring in general...
It's so easy to look at the mess of wires and get overwhelmed with all the circuits at once. That's the challenge as you build a custom hot rod...plan ahead to keep on track, avoid getting too confused, avoid running out of money, avoid running out of time! Did I miss any? :)
Although there are a few wire harness set-ups made for tri5's, if you are building custom the Ron Francis fuse/relay panels make it much easier. The panel is basically the terminating point for all the color coded, labeled wires, so you can run from the device back to the panel, or from the panel. A lot of options for routing the wiring. I used the drop down bracket on mine, which allows me to reach up under the dash and drop the whole fuse panel down if needed.7359

[55 Rescue Dog]

Plus with something like one of the Ron Francis kits, the wiring diagram is already done. You just connect all the dots! They have a lot of cool stuff for wiring.
http://www.ronfrancis.com/
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