Back of cab power: questions (overthinking, etc.) (1 Viewer)

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So I have read through a lot of the threads on here about running power to the back of the cab, specifically the passenger side. I have ordered the rear panel mount from Wits and I am waiting for it to arrive. In the mean time I have figure out the following.

I have a single battery for the time being so Ill be running power from that. From the battery ill go to a 60 amp bus breaker, then ill go through the firewall driver side, over the trans bay and run along the floor boards to the rear pass quarter panel to fuse box.

Question 1: Do I run BOTH a pos and neg or JUST pos wire and ground the neg side..? Is there upsides to wiring both? If so what?
Question 2: What gauge do I run? 4, 8, 10? I assume run big wiring from the battery to the fuse then from there run smaller based on the application, ie 12 or 14 for USB, running the fridge, dual compressor etc.?
Question 3: Do I have another breaker in the back of the truck too? ie before the fuse box? Or is the one in the front good enough?

I am a trained engineer and mathematician.. but ee was never my strong point :).

Many thanks y'all!
 
Joined
Nov 9, 2012
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Location
Olathe, KS, USA
So I have read through a lot of the threads on here about running power to the back of the cab, specifically the passenger side. I have ordered the rear panel mount from Wits and I am waiting for it to arrive. In the mean time I have figure out the following.

I have a single battery for the time being so Ill be running power from that. From the battery ill go to a 60 amp bus breaker, then ill go through the firewall driver side, over the trans bay and run along the floor boards to the rear pass quarter panel to fuse box.

Question 1: Do I run BOTH a pos and neg or JUST pos wire and ground the neg side..? Is there upsides to wiring both? If so what?
Question 2: What gauge do I run? 4, 8, 10? I assume run big wiring from the battery to the fuse then from there run smaller based on the application, ie 12 or 14 for USB, running the fridge, dual compressor etc.?
Question 3: Do I have another breaker in the back of the truck too? ie before the fuse box? Or is the one in the front good enough?

I am a trained engineer and mathematician.. but ee was never my strong point :).

Many thanks y'all!
First, you must calculate your anticipated electrical load that you will be feeding from the RR of the truck.

In my case, I need a 3000 Watt pure sine wave inverter. Yes, I can actually run a 120 VAC circular saw from the back of my truck. I have it because I need to charge a power wheelchair from my truck.

3000W / 12VDC = 250 Amps

I need a 250 Amp breaker by my battery and I need cables that can handle 250 A to the rear of the truck.

The larger the cable, the more Amps it can handle without overheating and burning down the truck.

I ended up using 00 (2/0) gauge welding cable to run fro the battery through the firewall, down the left side to an Anderson connector in the rear. I can then plug in or unplug the Inverter or other necessary equipment as I need it.

I intentionally installed a 200A circuit breaker at the battery to have it trip before I reach maximum load.

Use the tools below to help figure the correct size wiring to get you there.

I ran a red power wire and a black ground wire from the battery to the point so I get less "transient" voltages to end up screwing with lights, computers, radios, antennas, and cell phones.

Sizing Automotive Circuits
 
Joined
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Messages
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Location
Bay Area Ca
First, you must calculate your anticipated electrical load that you will be feeding from the RR of the truck.

In my case, I need a 3000 Watt pure sine wave inverter. Yes, I can actually run a 120 VAC circular saw from the back of my truck. I have it because I need to charge a power wheelchair from my truck.

3000W / 12VDC = 250 Amps

I need a 250 Amp breaker by my battery and I need cables that can handle 250 A to the rear of the truck.

The larger the cable, the more Amps it can handle without overheating and burning down the truck.

I ended up using 00 (2/0) gauge welding cable to run fro the battery through the firewall, down the left side to an Anderson connector in the rear. I can then plug in or unplug the Inverter or other necessary equipment as I need it.

I intentionally installed a 200A circuit breaker at the battery to have it trip before I reach maximum load.

Use the tools below to help figure the correct size wiring to get you there.

I ran a red power wire and a black ground wire from the battery to the point so I get less "transient" voltages to end up screwing with lights, computers, radios, antennas, and cell phones.

Sizing Automotive Circuits


This is awesome! And yeah I have a general notion of what Ill be running from the rear of the truck, tool charing station, fridge, USB, dual compressor, lights.... So I am thinking 4 awg should be plenty. Thanks for the input!
 
Joined
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Messages
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Location
Olathe, KS, USA
200A Circuit Breaker

Anderson SB350 Connector

Power Welding Cable 00

Cable Lugs except I ordered 2/0

3000 Watt Pure Sine Wave Inverter

Typically you should oversize to plan for future upgrades so you only have to do all of it once.

Add up your amperages from all that stuff AT START UP (not just operating amperages) then design your system from there.

You can set smaller circuit breakers near the rear of the truck to be "convenience" breakers near the operating source, then have the main feed one at the battery so it protects the whole circuit from burning down the truck.
 

Road Apple

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Santa Cruz, CA
Getting ready to do the same thing. Currently have 1 battery, but want a second battery for a switch pro and accessories to be run off the QPM (quarter panel mount). I’m going to install the second battery now and run all accessories off that “house battery”.

Read up on the electrical forum. Everything I read says wire it right, wire it once. Re-wiring later with the second battery in a pain in the a**.

Even though I don’t have the frig, compressor, solar, and inverter yet, I’m going to wire the truck in preparation to install these in the future. I’ll run 4 gauge all the way back to the QPM. You’ll need to figure out your expected load in amps and how far the wire run is. That will determine the gauge wire you need. See chart below. Over building the system will be safer and allow for additional capacity in the future.

I’ve seen most installs with both pos and neg wires run from the battery back to the QPM with the fuse as close to the battery as possible. Also, they wire a fuse block on the QPM with smaller fuses for specific devices.

00D16C8E-8696-4BD7-88C7-5E0DE15F1EF4.png
 
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Joined
Aug 2, 2020
Messages
189
Location
Bay Area Ca
Getting ready to do the same thing. Currently have 1 battery, but want a second battery for a switch pro and accessories to be run off the QPM (quarter panel mount). I’m going to install the second battery now and run all accessories off that “house battery”.

Read up on the electrical forum. Everything I read says wire it right, wire it once. Re-wiring later with the second battery in a pain in the a**.

Even though I don’t have the frig, compressor, solar, and inverter yet, I’m going to wire the truck in preparation to install these in the future. I’ll 4 gauge all the way back to the QPM. You’ll need to figure out your expected load in amp and how far the wire run is. That will determine the gauge wire you need. See chart below. Over building the system will be safer and allow for additional capacity in the future.

I’ve seen most installs with both pos and neg wires run from the battery back to the QPM with the fuse as close to the battery as possible. Also, they wire a fuse block on the QPM with smaller fuses for specific devices.

View attachment 2587365
Are you going to run a single pos wire back or both pos and neg?
 

cme4lyt

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Walnut Creek, CA Bay Area
I’m planning to run both pos and neg wires back to the QPM.

@Shoredreamer is very knowledgeable, especially when it comes to wiring. He might chime in with suggestions.

I’d run a very robust ground from your battery to the frame in the front, many points to attach to, and then you run a ground from the QPM to a rear frame bolt, again many to choose from. This way you don’t need to drag a neg wire from the front to the rear and you also make sure you are absolutely grounded.

Just my 2 cents....
 
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You don't technically need to run a ground wire back to the battery, but make sure you have a really solid earth strap from the battery to the chassis/body/engine block. (The ground wire should be equal to or larger than the 12v feed).

A fuse should be used as close to the battery terminal as possible and should be rated at less than the rating of the wire (EG if the wire can handle 100amps, you should run a 80amp fuse).

Whenever running wire in an engine bay/along the chassis, have it wrapped in flexible conduit. When going through a panel (eg into the cab) use a rubber grommet.

Buy yourself a hydraulic crimping tool (8t or 12t ideally). Buy yourself some stainless steel crimps to terminate the wire.

I personally prefer to use an OEM fuse/relay panel from an expensive donor vehicle (Mercedes, Audi etc) rather than an off the shelf product. There are some great fuse panels from early 2000s cars (before canbus) which will have 15+ fuses and 10+ relays. One of my personal favourites is a fuse panel from a Saab 9000.

headlamp_relay01.jpg


It has 11 normal fuses (5-40amp), 2 large fuses (40-100amp), 4 small relays (20amp), 4 large relays (up to 40amp) and 2 high powered relays (up to 80amp IIRC). Its also comes in its own standalone mounting box and has a nice cover/lid.

Ideally try and run heavy gauge wire (welding wire is a good/cheap option) to give yourself flexibility in the future. If you plan on running a LOT of stuff i recommend getting an entire wiring loom from a high spec car from the early 2000s as it'll have a LOT of uniquely coloured wires you can use. I also personally draw up a full wiring diagram (in software) and keep a printed/laminated copy in the car. Makes troubleshooting or adding functionality super easy in the future.

Finally, I also install a bluesea 9002e battery switch in every landcruiser i own. It allows for a secondary battery to be installed (and using a VSR automatic charging) which allows you to quickly isolate the battery (and stop the engine using the Alternator Field Disconnect) in the event of an electrical fire.
 

LandLocked93

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Jun 11, 2018
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Colorado
When I went thru this my biggest concern was the amount of current passing from front to rear of the vehicle at any given time.
There are many ways to skin that cat, but I ended up with a maximum of 20a (12v) passing from front to rear (controlled by a BCDC) feeding 200aH capacity batt bank (24v) to run twin compressors, 2000w Lo-Freq pure sine, water pump, lights, etc.
And because everything in the rear is 24v, the current requirements for the cabling/fusing/relay is half that of 12v. And run time is about 1/3 longer. 2/0 cabling costs. The thought of up to 200amps or more running under the carpet was a concern for me.
My situation used a single 2AWG wire, but I could have gone with 6 or 8AWG having only 20amps moving thru 25ft of cable. And FWIW, those 20 amps are coming from an auxillary battery up front, so no direct load to the crank batt unless I combine them and even then the vintage '93 alternator can more than keep up.
The grounds are local to body and frame.
 
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I ended up using 00 (2/0) gauge welding cable
Holy crap thats some big wire, were you planning on running a washing machine or a household HVAC haha


I've ran 4gauge red from the aux battery, thru the firewall (cord grip), to the back of the panel. The wire was covered in anti-abrasion sleeving off amazon. The positive is fed off a 150A breaker For negative Ive ran 4gauge wire to the frame thru a cord grip above the exhaust muffler and 4gauge to a random body bolt. The aux battery is tied to frame via 1/0.

I dont have the wits end panel but i did post some electrical wiring stuff in my build if you are looking for ideas.
 

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