Have you ever wondered what size wire to use for DC?

Mike6158

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I was going to run about 20' ish of a #8 pair of wires back to the cargo area to feed a 100A Blue Sea distribution box. Was... Since it's a distribution box rated for 100A I should feed it with a fuse at the battery and a #2 pair of wires according to the table (3% voltage drop since its going to a distribution box). I'm trying to imagine what that even looks like. A pair of #8's was bad enough.

I can't see ever needing 100A back there. I plan to feed (1) HF Ham rig, (1) VHF/UHF Ham rig with APRS, an ARB fridge/freezer, and maybe some lights or something. I'll also have a solar panel input so I can charge the aux battery (that I don't have yet) when stopped during the day.

I went backwards to check how many amps 20' of #8 would allow and it's a dismal 20A at 3% voltage drop. Twice that (40A) at 10% voltage drop.

It's easy enough for me to mentally derate the panel and not connect anymore than I have wire for but what if I sell it? Or what if I decide to put an AC inverter and run a drill, small machine shop, you know... common every day things like that :D

What kind of amperage will the ARB fridge need (it's the big one). The Ham rigs have very low idle current and shouldn't transmit at the same time. Well? Actually, if I was out with another Ham operator one could be on HF and the other on VHF. Either way, it's 100W on the HF rig and less on the VHF/UHF/APRS rig.

Part 1: Choosing the Correct Wire Size for a DC Circuit - Blue Sea Systems

Here's the chart in the link if you don't feel like clicking the link:

 

C6H12O6

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You could run 100A at the same time through that panel, but you never will with your set-up, even if both radios are transmitting on high power, the fridge kicks on, and big set of LED lights are on. The voltage drop is a real issue, but make the run with rope first and measure it out. I ballparked it at 20' when I was planning the same thing out for my ARB 50qt. fridge and sub amp, but in reality, it was quite a bit shorter. I don't remember exactly how long, but short enough that I was in the zone of acceptability for 8ga. wire. I already had 8ga. run for the sub amp and a dedicated 12ga. fused wire right to the battery for the HAM radio, so no issues there for me. I bet you'll be fine.

All that said, run the biggest wire you think you'll ever need. Pulling all the trim off and running the wire isn't hard, but it's enough of a pain in the butt that you'll not want to do it again anytime soon.
 

Mike6158

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I'm not worried about how much current I have now, it's what happens in the future. 20' came from me stepping off about 15' and then upping it some just to cover whatever I "might" need so yeah, 15' is probably longer than I'll need if I come off of the aux battery that I don't have yet. I was going to run #8 because I had it. I can go with larger wire. I was surprised at how little #8 can handle at the lengths I was looking at (good old voltage drop strikes again).

My little Kenwood VHF/UHF/APRS rig "kind of" reboots when I try to transmit. I just have it plugged into cigarette jack with a powerpole adapter. As I recall the factory wire feeding the jack is tiny. So my solution was to run a big enough circuit to power everything that I need, might need, forgot that I needed, and probably won't ever need :)
 

Mike6158

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By the way, does anyone know of a way to get wire through the firewall? I'd rather not drill (wouldn't). Is there a place to pass through an existing grommet?
 

C6H12O6

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My PO had already gouged a hole through the firewall for something else, so I just drilled that out and snapped a plastic grommet in there that was just about the same ID as the OD of the 8ga. wire.
 

spressomon

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By the way, does anyone know of a way to get wire through the firewall? I'd rather not drill (wouldn't). Is there a place to pass through an existing grommet?
For me, unless its smaller AWG wire that can be easily 'poked' through one of the existing rubber grommets and/or needed in the first or second row, I run larger AWG wire from the engine bay/battery, down along the top of the frame and then into the rear 3rd seat area using waterproof Ancor Wire Seals.

https://www.amazon.com/dp/B000NI019...FCZJX0N4A3WWJP921&ie=UTF8&qid=1504636491&sr=1
 

Mike6158

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Spressomon- Thank you! Good stuff

george_TLC- Much appreciated

Boltripper- Very nice work. Very clean! I'll check my LC Friday or this weekend. Since I'm going to install a second battery it makes sense to route the wire on the passenger side.
 
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Actually the ground wire is very important and you should choose the largest one you can get (within reason here, ok).
At least the same gauge as the hot wire if not slightly larger in diameter. I would not cheat here.
 

Rob Faucett

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What I should have asked was is it OK to split the ground wire "responsibilities" between the chasis and a wire so as to enable
use of a smaller wire gauge.
 
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