You’re probably right. Ford doesn’t know what wire gauge to use with what amperage rating. Best to go to an overly complicated relay box.
Kidding aside, using a relay box is not wrong, but it does add a lot complexity that then results in more opportunity for failure.
I did not say Ford did not know what wire gauge to use, I said I was uncomfortable running the power I might need to use through the supplied wiring.
For 2018 (and I assume 2017 to 2020) Aux 1 and 2 are 17 awg multistrand , 4 and 5 are 20 awg multistrand, 5 and 6 are 22 awg multistrand. Aux 1 and 2 are rated at 15 Amps, 3 and 4 are 10 Amps, and 5 and 6 are 5 Amps.
That means that the potentially 15 Amps of Aux 1 and 2 pass through several feet of 17 gauge wire, even if you go to larger wire from the firewall out, the earliest you can connect to the factory wire. Call it about 10 feet total from the control relays to the firewall. I do not know if the entire wire run is 17, 20, and 22 awg, however all of it I can see is.
At 12 VDC and 15 Amps the potential voltage drop in 10 feet of 17 awg copper multistrand wire is around 12%, or up to 1.5 VDC. If the system is at 12 VDC and you need the full 15 Amps then there might be only 10.5 VDC at the end of the Aux 1 or 2 wires. Yeah, sure, I know that most of the time the supply voltage will be more than 12 VDC, up to about 13.8 (even higher is seen pretty regularly while the vehicle is running and charging) VDC. At 13.8 VDC and 15 Amps of draw the voltage drop in 17 awg multistrand copper wire will be around 11% or a bit more, or about the same 1.5 VDC, leaving maybe as little as 12.3 VDC at the firewall to be used. And then add the voltage drop of whatever wire you run from the firewall to the load. Some things will work fine on that, others not so much.
The wiring used by Ford is not unsafe, but it is far from optimal for some high current loads. For anything more than a few Amps I would not (and do not) use the Upfitter switches directly applying energy to the device I want to turn on or off. Sure, for my radar detector or a police scanner it is fine, a couple of amps max so no big deal. But when I need to draw 10+ Amps the Upfitters and associated wiring are just going to drop too much voltage in the line for some things to function properly. Even if the Ford factory relays and wires can safely handle it.
So for everything attached to Aux 1 to 4, all the heavier current draw items, I use the Aux output to control a relay that I install. And then properly sized wire, to match the required current and wire run length, is used from my relay box to the load. The light current loads I have on Aux 5 and 6 are perfectly fine on the factory wire and no relays. The measured voltage drop (from the Aux power source to the device under power) of my 1 Amp load (Uniden BCD536HP scanner) that is on Aux 5 is right at 1 Volt, or around a nominal 12 VDC to the device when it is turned on. This total run is the Ford wiring up to the the firewall and back inside, and my upsized 16 awg wire from the inside of the firewall to the console mounted radio. Remember that at 5 Amps it would have about 5 times that much drop and would be well under 10 VDC to the load.
Besides which, relays allow me to do things with the source voltage that the simple Aux switches do not, for example, selecting Aux 1 drives my light bar to the down position, while Aux 2 drives it to the up position (and I have fail safed it, accidentally selecting both will drive the light bar down, but not harm anything). The Aux switches alone could not do that.
T!
(edited after posting for spelling, apparently I can't)
(second edit, the original wire sizes I used were wrong, grabbed from a post on these forums, the correct wire sizes are documented in the manual and I corrected my calculations based on those sizes)
