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Wire sizes for start system

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Hi Guys

I am just gathering materials to rewire a 12HT swapped Landcruiser. it is a mechanical engine

It is a 12V truck and really basic but i need clarification on what gauge wire to use.

the components in the engine bay are as follows including power rating information I have.

Starter motor. rated 2.5KW so the power equation gives me 166A at 15V

Alternator with built in voltage regulator. Label says 12v 80A so at 15V 100A

glow screen. cannot find information on current draw.

I just need to know what size wire to run on the Hot side of the relays. can anyone shed some light?

You size the wire for the relays based upon the relay capacity. Usually 12-14 ga is sufficient.

BTW, the current goes down as the voltage goes up if the same power (measured in watts) is being supplied. So

Watts = Volts x Amps = 12v x 80A = 960W

Amps = Watts / Volts = 960W / 15V = 64A

Your 2500W starter will likely turn at 10V when it's cold, so you can expect it will draw 250A then. Wires between a starter and the battery (and ground) will typically be 2-6 ga (although I have seen many British formula cars that try to use smaller than 6ga)

[edit] IMPORTANT, seems some variants of the 12HT used a 24V system - if so, you'll need to swap out the starter for a 12V version, same with the glow screen, alternator if it is 24V, etc.

You're forgetting something quite important - the starter isn't operating at 15V but cranking voltage will be around 9-10V, as David said.

Since you have a nominal power for the starter motor, that suggests it's an aftermarket unit, if so you should be able to find some information about stall current Vs voltage on the manufactures web site.

If you don't, I'd use closer to 8V for the stall and figure on ~300A for the peak current. The next thing to consider is the battery location, as that will govern the length of cable - not forgetting the return path for the circuit - and the general rull of thumb is a maximum of 0.1V for each side - less is better.

If in doubt, round up - there will be a very small weight gain but the extra cranking voltage will give more current and hence more power to turn over the engine. That can make a crucial difference if it's really cold and/or the battery is almost discharged.

Similarly, the alternator output, 100A, will normally be rated at 13.2 to 14.4V. It will also be susceptible to the voltage drop through the wire - this may not be critical, though, as some use a battery reference, rather than their output terminal, for the voltage. Don't forget the alternatoor earth path too - one of my cars has a 100A alt' that uses vibration dampening mountings and a separate earth to the engine - I doubled it up because the OEM single earth wire actually melted the insulation.

That said, for the most part, the alternator will only output what is required to balance the electrical load and/or charging any discharge of the battery. Even then, if you look at any alternator output chart, you will see that at lower rpm it can be a lot less than the rated output current - this is the primary reason many modern vehicled are fitted with such high outpur alternators - 200A isn't uncommon - it's to ensure the alternator can supply enough current at idle/low rpm to power the greater draw from the electical and electronic systems of modern vehicles without discharging the battery.

As you mention, these engines use a pre-heat screen, rather than glow plugs, but I can't find out much about them. With any swap, I would advise getting hold of the workshop manuals for both vehicles - it 'should' be found there what the current is, if it isn't, I would use 40A as a working value, but perhaps an auto electrician in your area can better advise you?

As a general rule, with relays I try to over spec'them by at least 50% - seen them stick, burn contacts and even melt when run close to their nominal ratings - and those were some 'good' name brand ones!