Basic Switch and Wire Gauge Question

Thread Starter

soma56

Joined Nov 11, 2018
12
I'm converting a VW diesel to run on waste veg oil. Long story short: I need to rig up several switches to a fuse box. In the amateur, grade 3 drawing I made below I'd like to know what gauge wire would be appropriate for the main power on/off switch in addition to knowing what type of switch would be appropriate (how many amps should it be rated for?). I don't anticipate having more then 25 amps in total with all the various devices connected.

Secondly, would a relay be appropriate to implement in this setup? If so then where?

 

wayneh

Joined Sep 9, 2010
16,164
I'm converting a VW diesel to run on waste veg oil. Long story short: I need to rig up several switches to a fuse box. In the amateur, grade 3 drawing I made below I'd like to know what gauge wire would be appropriate for the main power on/off switch in addition to knowing what type of switch would be appropriate (how many amps should it be rated for?). I don't anticipate having more then 25 amps in total with all the various devices connected.

Secondly, would a relay be appropriate to implement in this setup? If so then where?
You could certainly accomplish the switching with a switch rated for the max current, times two (for a safety factor - you never want to take a component right to the edge of its rating). But in my opinion a relay, plus a small switch to control the relay, would be the better choice and might be less expensive. It's no problem to find an automotive relay rated to 30-40 amps, and just an inexpensive switch would be adequate to power the relay coil.
 

Reloadron

Joined Jan 15, 2015
4,918
Your main feed length from the battery is a concern. So the Max Current will not exceed 25 Amps I would go higher and run with AWG 8 or even AWG 6 assuming battery to fuse block of about 10' to 20'. Then rate your load lines accordingly. You are not going to hurt anything using a heavier gauge wire and will reduce any I*R losses. This chart should be of some help. Make sure as mentioned your switch can handle the load or just switch an automotive relay and let the relay handle the loads. That allows use of nice small switches. I would do as Wayneh suggested above.

Ron
 

Thread Starter

soma56

Joined Nov 11, 2018
12
Thanks gentlemen (much appreciated). I've updated the diagram with a relay. Can you someone let me know if it's accurate? Also, would a fuse be appropriate before the relay (after? switch wire and/or main wire?).

 

Reloadron

Joined Jan 15, 2015
4,918
You have 3 switched items on the fused DC bus. I guess I fail to see what you are switching the fuse block? Will it work? Yes, but why switch a fuse block?

Ron
 

Thread Starter

soma56

Joined Nov 11, 2018
12
You have 3 switched items on the fused DC bus. I guess I fail to see what you are switching the fuse block? Will it work? Yes, but why switch a fuse block?

Ron
There's more then 3. More like 7 with a total amperage of about 25 amps. The 10 channel fuse box made sense to me. The reason why I wish to turn off the fuse block is to have a one-switch default change to the original diesel settings with respect to switching back and forth for running on waste veg oil. Also, as a safety measure in case something goes wrong within the system - I can simply flick one switch to resort to VW factory default.
 

Reloadron

Joined Jan 15, 2015
4,918
Then it will work fine as drawn. Typical automotive relays are good for 30 Amps to 40 Amps, I would use a 40 amp version just to allow some overhead.

Ron
 
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