John Frum
Tell me your problems
- Joined
- Nov 30, 2019
- Messages
- 15,234
Well explained.You've made an interesting question. Here's my answer, involving some relevant details - but I agree with you, I would rather have used AWG-8 on the Tow Vehicle "12V" side leading into the Boost Converter. The input lead to that Converter Box may be even smaller than 10-AWG, so I cut it fairly short before plugging into my relay output lead. My "automotive Relay" socket base couldn't handle anything larger than 10-AWG (and even that was a bit of struggle, I ordered custom bases to make them take #10 AWG on the 3 "power legs" in question, and I blew up one of the two I bases which I ordered while trying to squeeze in the wire).
But you're assuming that I attempt to pull the full RATED watts of the Boost Converter. I'm only pulling about 480 watts maximum for wire losses to reach the MPPT, MMTP inefficiency, and the MMPT's rated maximum output power 28A at about 14.4 Volts. 480W /13.4V = 35.82 Amps. 35.82 Amps / 0.90 efficiency yields 39.8 Amps total draw at maximum output, a figure slightly lower than your 43 amp calculation.
These wire distances are also short (heat can dissipate at the copper ends, not just through the insulation) and most of what goes on in here can arguably be called "chassis wiring". This is also THWN-2, for which I typically see ampacity ratings of 40A @ 90 degrees C. That's with conduit allowed, while the hood underside at this location (with wires in free air) has substantial airflow when driving at speed. But temperature is already a bit high in that location, somewhat reducing ampacity back down a bit.
The bottom line for wiring size in my scheme is whether it has performed OK in the real world, leaving its insulation undamaged. And for that question, I'm happy to report that I have seen no heat-related issues on the connectors, exposed insulation, or even the electric tape, over multiple years. Neither have the friends who use the same scheme.
Thanks for reminding me about chassis wiring.