I am not very familiar with the Redarc products but I will give this a go. Hopefully the real experts will weigh in.
My response is somewhat generic to apply to either DC-DC charger or regular battery to battery charging connections vis say a Blue Sea ACR.
The breakers on the truck battery and camper battery sides are to protect the wiring in the event there is a short. The Redarc device may have a breaker or fuse to protect the electronics but that may or may not protect your wiring (you need to read the installation manual - hopefully it will tell you).
The three questions that are important are:
1) What is the largest load you will draw in the camper at any one time from a combination of all devices being used (is say charger, lights, heater, refrigerator, water pump were all on) at the same time)? Maybe the battery is low and there is a big initial current draw. The DC-DC charger will probably liniit that but an ACR may not.
2) What is round trip distance of the wire between the truck and the camper?
3) How much current at say 13.5v can that length of wire between the truck and the camper handle before being damaged?
A short would cause far more current than the 6 Gauge wire will handle so we want to know the max steady state current the wire can survive (knowing it may get pretty hot near max current even it t does not fail)
Most wire size charts calculate voltage drop (typically 3%) over distance for a given voltage not joule heating from the resistance in the wire.
The heating effect produced by an electric current (I) through a conductor of resistance ® for a time (t) is given by H = I2Rt
We need to find the resistance of the wire by looking up the wire size and getting the resistance value along with the cross sectional area for that wire size. I'll assume 30 feet round trip for the length (might be closer to 20 feet in your truck/camper).
You indicate that you plan to use 6 ga welding wire. Please note that different wire construction techniques for the same wire size will produce different resistance values (although they will be close). 6 gauge wire has a resistance of about 0.3951 ohms per 1000 ft and a cross sectional area of 13.3mm^2.
https://www.powerstr...m/Wire_Size.htm
However, the table also says the maximum current is 101 Amps. So we don't have to do the calculation. But I kept the details here in case you want to nerd out. :-)
Edit: The chert at this link rates 6 ga for only up to 75 amps. Maybe more conservative to use 75 amps as the max.
https://www.cerrowir...mpacity-charts/
Pragmatically, the breaker on each end of the wire should be large enough to allow the maximum current needed for the devices in the camper (probably not more than 40 amps) and less than 100 amps to protect the wires. So a good guess not knowing your setup is probably to use 40 amp to a max of around 80 amp breakers. There is no downside to using 80 Amp breakers because the wire can handle the current and you have 20% current margin vs the max current the wire can handle. On the flip side you want to make sure you don't go too small and then have the breaker pop because you exceeded what the breaker can handle even though the wire is not at risk.
So the best answer will be the max current your camper needs to run all devices plus a bit of margin say 10% - 20% to avoid annoying breaker trips. I going to guess that 40 amps to 50 amps turns out to be adequate .
https://www.omnicalc...wire-resistance
If desired and knowing the distance, resistance, and cross sectional area you can use the wire heating calculator here
http://hyperphysics....les/wirega.html
Here is another calculator that might be useful to some folks
https://www.rapidtab...Calculator.html
I hope this is helpful and I hope that if I made any errors the experts will comment with corrections.
Craig
Edited by ckent323, 09 September 2023 - 08:59 AM.
