What cable for running accessories?

[Animal Mother]

Ploughing on with my overland setup, I'm at the stage now where I can start to prep for electrical accessories.

I plan to run a twin core cable from the deep cycle/aux battery in the front of the car (120) through the car into the rear door. I'll be running a fridge (up to 5A 12v (66W motor)), a couple of LED lights, tyre compressor now and again, USB charger, and maybe a radio.

I'll put a large fuse battery side, and be running a fuse panel at the accessory end of the circuit. What I can't figure out it what cable spec to use.

Here's a selection: https://www.autoelectricsupplies.co.uk/product/1359/category/281
Also there's 2x2.5mm², I'm thinking this might be best.

Any ideas or advice would be very helpful. Thank you in advance.

 

[Animal Mother]

Doing some digging and I've found the info below.

On the basis of the above guidance, Im at 10ga, which is 2x5mm which, to me, feels like overkill.

 

[StarCruiser]

Do a little drawing with lengths of run, full load currents and load names.
If you can avoid putting your fuse panel in the door that would be preferable and save access and cable damage issues. You don’t need to use twin cable as you can use the body as earth which is far better.

 

[StarCruiser]

Doing some digging and I've found the info below.
View attachment 144933
View attachment 144934

On the basis of the above guidance, Im at 10ga, which is 2x5mm which, to me, feels like overkill.

My guess without all the loads was going to be no less than 6mm. Positive AND negative is overkill and an extra problem.

 

[Animal Mother]

The rationale behind twin core is the due to potential bad earths when trying to earth on the door, as there is likely to be poor earth connectivity to the rest of the body/chassis (Krown oil will have penetrated into all hinges, etc by now).

The rationale behind fuse panel in the rear door is that it's a combined switch/accessory/fuse panel which would be mounted on the rear door for practicality purposes.

6mm? Wow. That's significant.

 

[moggy1968]

The rationale behind twin core is the due to potential bad earths when trying to earth on the door, as there is likely to be poor earth connectivity to the rest of the body/chassis (Krown oil will have penetrated into all hinges, etc by now).

The rationale behind fuse panel in the rear door is that it's a combined switch/accessory/fuse panel which would be mounted on the rear door for practicality purposes.

6mm? Wow. That's significant.

I followed starcruiser and others advice and moved away from the idea of using flex type cable and used instead proper dc rated multi strand singles. I was going to run 4mm but found that caused problems getting a good grip on crimps rated high enough for the 32 amp expected load so I ended up using 6mm and it was much better. I protected the cable in some sleeving stuff and then flexi conduit. The key is to ensure everything is well protected and properly secured because it is a harsh environment.

 

[moggy1968]

 

[StarCruiser]

The rationale behind twin core is the due to potential bad earths when trying to earth on the door, as there is likely to be poor earth connectivity to the rest of the body/chassis (Krown oil will have penetrated into all hinges, etc by now).

The rationale behind fuse panel in the rear door is that it's a combined switch/accessory/fuse panel which would be mounted on the rear door for practicality purposes.

6mm? Wow. That's significant.

Like I say, steer away from mounting on the door itself. The constant bending back and forth caused a fire in Clive’s original wiring loom so it’s best avoided unless you have the right space and method and highly flexible wire. You are correct not to try and use the hinges as earth, but to avoid the door altogether and earth to a bodywork point behind the side panels or wherever is fairly close to your accessory. There was a spate of dishwashers that caught fire due to folding wires in the door connection hence they do a Z shaped loom with a twist in it which makes it far better but if you need to add to it later it will be a faff.

As Moggy has said, you need to protect the wiring against chafing and vibration induced wear. 6mm is the barest minimum I would run front to rear fused accordingly. It MUST be fine multistrand flexible cable with well crimped ends. Vibration causes loose connections which need to be avoided on higher current stuff.
There are some decent common bus bar type fuse boxes out there. The better ones are those for marine use, Blue Sea being some of the best but of course more money.
When sizing cable you can never go too big and the cost of going one size bigger is minimal. Insulation rating needs to be high temp and at least equal to twice the voltage. I’ve used mains wiring just because I had easy access to it and could get a reel of 10mm of a colour that’s going out of fashion for a good price. ‘Tri rated’ cable is ideal. It’s 105c rated with a flexible tough insulation. Don’t go smaller than 0.75mm for the small stuff, ideally 1mm just from a strength point of view.

 

[nick_the_fish]

Also consider voltage drop in your calculations which can be significant when running cable for a few meters. I have put a few Blue Sea items in the recent rewire and they are top kit. The battery terminal fuse is excellent and avoids any un-fused wire

https://www.mudstuff.co.uk/blue-sea-fuse-block.html

 

[moggy1968]

You shouldn’t have any issues with voltage drop on normal vehicle wiring, it’s like your running a 40 foot trailer! Consider the distance to a caravans wiring and that manages fine.

 

[StarCruiser]

You shouldn’t have any issues with voltage drop on normal vehicle wiring, it’s like your running a 40 foot trailer! Consider the distance to a caravans wiring and that manages fine.

But you do need to size the cable correctly considering voltage drop and grouping as well as ambient temperature and where it will be running, under carpets, through insulation etc. Nick calculated his out very well with a good margin. It’s easy to exceed the volt drop limit at 12v.

 

[GeekOKent]

AM, that compressor is going to draw a bit. Really, dont skimp it.. 6mm it i found a good -ve in the bolts that hold down the back seat/bench and pull a black cable from all over the back wiring to there.

 

[moggy1968]

But you do need to size the cable correctly considering voltage drop and grouping as well as ambient temperature and where it will be running, under carpets, through insulation etc. Nick calculated his out very well with a good margin. It’s easy to exceed the volt drop limit at 12v.

At 5 amps with 4mm cable over 35 feet the voltage drop is under 5% (5% being the generally accepted threshold) so it shouldn't be an issue on this type of install. You'd have to really go some on packing in the cables and surrounding the cable with insulation as well for 5a to be an issue on 4mm cable, I couldn't even make a scenario that would do that!

 

[StarCruiser]

The thing is Andy, most folks would use 5A rated wire for a 5A load and think that’s ok without considering volt drop, or other de-rating factors. Your example uses 41A max cable so, as you rightly say, would take some doing to de rate it to be no good for 5A.

As an example, 0.5mm Tri rated is rated at 11A max so could be chosen for a 5A load. Let’s say it’s bunched with 2 other circuits in a loom. Derate by 10% for each that’s 20% so it’s down to 8.8A max to start with before even considering ambient temperature or thermal insulation.

More info on this thread. https://www.landcruiserclub.net/community/posts/1483048/
The main thing is that under short circuit conditions, the fuse blows before the cable does any damage. This didn’t happen in Clive’s case on original wiring. If the rules are followed then you don’t need to worry about overload causing damage. If the cable is too small you risk the fuse just sitting there while the cable gets hot and causes damage or a fire.

 

[Animal Mother]

As per the sage advice, I won't be going with a door mounted witch panel, although it would have been more aesthetically pleasing.

Here is a rough diagram.

With my current plan, if I'm running everything together, I'll be drawing 25A. If I add an inverter and water pump, for example (but unlikely), I've still got 15A to play with.

This will all be fused at the load end and with a midi fuse of 40A at the battery end.

 

[StarCruiser]

Ok, good drawing.

I think the 40A fuse will probably be fine for what you want now. Any later additions should be considered at this stage, no point doing the job twice. There is a very good chance you won’t be running everything at once but perfectly possible that most could be when you start the air compressor and that will take a heavy current to start it.

Where do you plan to run your front/back cable?
How many other circuits are there likely to be with it if any?
What size inverter do you think you might consider in the future?
What do you want the voltmeter to read, battery voltage? If so you need to take a dedicated cable of probably only 1.5mm direct to the battery fused as close to the battery terminal as practical.
What would be the current draw of the pump you could add?

 

[moggy1968]

Need to watch your total load there, depending on what your going to run from the accessory socket, your could be above 40a total load, mine are rated at 20a each + a few other accessories, but as they generally only run things like satnav and phone chargers it's not been a problem

 

[Animal Mother]

Ok, good drawing. Thank you.

I think the 40A fuse will probably be fine for what you want now. Any later additions should be considered at this stage, no point doing the job twice. There is a very good chance you won’t be running everything at once but perfectly possible that most could be when you start the air compressor and that will take a heavy current to start it.

Where do you plan to run your front/back cable? O/S battery positive terminal straight up inner wing through firewall and along existing clipped cable run under protective covers along the sills, then behind the cargo area side covers.
How many other circuits are there likely to be with it if any? None. Dedicated accessory circuit with dedicated battery.
What size inverter do you think you might consider in the future? It's unlikely I'll use one TBH. If I do, it'll be something like 100W max as most of my requirements are 5v or 12v and are serviced by other charging points.
What do you want the voltmeter to read, battery voltage? If so you need to take a dedicated cable of probably only 1.5mm direct to the battery fused as close to the battery terminal as practical. Yes I do. Ok, thank you.
What would be the current draw of the pump you could add? Up to 3A.

 

[StarCruiser]

You say you will run along existing clipped cable run along the sills, then go on to say you aren’t running with any other circuits. I’m not familiar with the 120 wiring but do you mean the existing clipped cable run contains no existing vehicle cables? If so then 6mm will possibly be fine, if other circuits are present then we need to consider them and the heat that they produce in conjunction with the added cable and the enclosed nature of the run.

 

[Animal Mother]

You say you will run along existing clipped cable run along the sills, then go on to say you aren’t running with any other circuits. I’m not familiar with the 120 wiring but do you mean the existing clipped cable run contains no existing vehicle cables? If so then 6mm will possibly be fine, if other circuits are present then we need to consider them and the heat that they produce in conjunction with the added cable and the enclosed nature of the run.

Ah, ok. I misunderstood. There are existing cables in that run. As well as the rear screen wash line, but the number of those cables is low (from what I can see). I believe the existing circuits to be rear lighting, rear wiper motor, rear washer pump, rear demister, reversing camera power and signal cable, boot space power socket (this will be replaced by my power supply so no feed/draw through those cables), central locking power (for rear door).

There's loads of space in the run. In fact, it appears to be two cable runs, of which one is only partially used (by cables, the rest is water pipe). The second cable run (separated by plastic clips) is completely free.