Starters are an inductive/resistive load, and inductive loads react very differently than purely resistive loads, which is why I've been careful to use lighting as an example since it is resistive. And, since the headlight circuit/switch was the topic that started this discussion.

 

So, what about the blower (which is a motor -speed regulated by a resistor-?

 

That would be both an inductive and resistive load. And a lot more resistance than the starter. I'll have to research that combo. But, we already know that the current goes down as the resistance goes up. Flip the switch from high to low and more resistance comes into the circuit, which causes the motor to run more slowly and the ammeter to swing less and the volt meter to go up.

 

Ok, I spent the day testing, and put some of the results on a page on my web site: Electricity 099 - Gary's Garagemahal. There was waaaaay too much to put in a post here, so that approach seemed best.

Please take a look and let me know what you think.

 

Nice Gary, well done! FWIW, the pull-in and breakaway current on the Ford movable pole piece starter is somewhere well over 250 amps. Couple that with super soft copper alloy the Chinese flog to us and you have a welded solenoid. That is why the PMGR starter is the best solution.

 

Thanks, Bill. I think it needs some cleanup as well as the add-on navigation, but I don't know what else to do to it.

Yes, I plan to go with a 460's PMGR when the engine goes back in.

 

And, in case any of y'all don't go look at the page I guess I should add that it says: Conclusion: If the voltage doesn't change but the resistance goes up then the current goes down.

But, if you want mind-numbing detail, empirical evidence that supports that position, go read the page.

 

More importantly, when it comes to maintenance/troubleshooting, especially as vehicles age, is that generally when resistance goes up the voltage goes down.

 

Amen. That is a corollary. And probably a better way to present the results since it is the reduced voltage that begats reduced current in the item in question.

Having said that, do you have an exception in mind that prompted the "generally" caveat?

 

I phrased it wrong. Installing different wattage bulbs would be a case where the same would happen, but it wouldn't be a wiring defect.

 

In that case the resistance of the circuit wouldn't change - at least not initially. But the voltage would go down and the current would go up. However, the extra current draw may well be the death knell for the wiring. If the majority of the voltage drop is at one point it'll cause localized heating, which may cause connections to fail, switches to burn up, and/or insulation to melt.

 

Gary, I can't find it to quote but you linked to a wiring chart at engineer's toolbox. I'm pretty sure you linked to the one for AC premise wiring. They have a different one for 12v systems.

In any case, the Ancor marine wire folks have a lot of good information on their site. Keep in mind though that it's very high quality so the stuff one would by at the local auto parts store would likely need to be sized larger.

Here's one chart from Ancor.

Conductors Sized (AWG) for 3% Voltage Drop | Ancor

For a stock location of battery and alternator in a bullnose #4 should be fine for a 130A alternator. The only caveat IMO would be if you're running an electric winch and want maximum voltage from the alternator to the battery.
Probably obvious, but the larger you go the more important strain relief is. For #2, even #4 probably, I'd anchor it well on the alternator bracket to prevent stress on the main feed stud.

Blue Sea systems has a circuit wizard, available here: Circuit Wizard - Blue Sea Systems and also as an app for IOS and Android.
Unfortunately, using the online wizard says #2 vs. the app, using the same parameters, which says #4.
When they open I'm going to call Blue Sea and ask them what's up with that.

 

Actually it would, that's how you get different brightness bulbs. Don't forget that the bulb is part of the circuit.

 

Yes, that is the more accurate way to say it. I should have said the resistance of the truck's "wiring" won't change.

 

Tom - I missed your correction on the amp/distance/gauge chart. Thanks for that.

And, I agree that #4 is adequate for the charge cable, especially as it is quite short in our trucks. But, when sizing the ground we need to think about everything that will be running through it - at the same time.