Installing a second fuse box

Submitted: Thursday, Mar 16, 2006 at 13:45
ThreadID: 31799 Views:4693 Replies:3 FollowUps:5
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Coming to grips with 12 volt technology (?) is an absolute nightmare when you’re a complete novice like me, seems there’s definitely more than one way to skin a cat and a thick hide is needed if you’re going to offer/accept advice, so I hope this question doesn’t turn into a bun fight that only adds further to my confusion.

I want to get some power to the rear of my vehicle so I can run all the usual stuff (fridge, light, inverter etc) and it seems that an extra fuse box is generally regarded as a worthwhile investment by some (but not others), especially if you want to expand later.

What’s involved in installing one? I understand that the closer to the battery it is the better, but I assume that along with power, it would also need to be earthed?

Once installed is it just a case of running a suitable cable to the power outlet and then earthing the outlet? If so, what’s the best place to earth if you have installed the extra fuse box?

While on the subject of cable, how does one determine what is a suitable size cable for each outlet (not to mention extra fuse box), again this area seems to be harder than walking through a mine field with your eyes shut?

I already have a dual battery system installed, but it seems such a shame not to be using it.

Those who feel inclined to answer should apply the KIS principle (keep it simple) so I can understand. And searching the archives has only added to my confusion.
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Reply By: Member - Mike DID - Thursday, Mar 16, 2006 at 14:48

Thursday, Mar 16, 2006 at 14:48
The wire to the back needs to be thick enough so that voltage drop is acceptable under maximum load - for typical uses - fridg, light etc an 8Gauge cable is adequate - 6mm diameter of insulation.

A fuse has to be placed near the battery in case the wire is shorted anywhere along its length. the fuse must less than the cables current capability (50 amps) but more than the maximum load drawn - say 20 amps.

It's a good idea to have fuses at the back as well for each socket (say 10 amps) so that the thinner wiring to each appliance is protected and so that if you short out one socket the others will still work.

If you have a modern vehicle, you can earth the socket at the back the way every other device (excluding starter motor) is earthed - to the body. My tests show that the voltage drop at the baclk will be the same as using 4 Gauge cable. You can run a wire from battery negative and connect it to socket earth as well.

AnswerID: 160939

Follow Up By: V8troopie - Thursday, Mar 16, 2006 at 15:27

Thursday, Mar 16, 2006 at 15:27
Mike, in your last paragraph you mention earthing to the car body. While your tests confirm an acceptable voltage drop there is a compelling reason why manufacturers do not earth devices willy nilly to the body. They use certain, carefully chosen, earth points to minimise electrolytic corrosion problems.
In the long run it would be wiser to run a separate earth wire all the way to the battery. Using twin figure 8 cable of a suitable big gauge makes that task easy.

FollowupID: 415672

Follow Up By: Member - Mike DID - Thursday, Mar 16, 2006 at 16:58

Thursday, Mar 16, 2006 at 16:58
The current model NM/NP pajero already has equipment earthed in the engine compartment, dash centre left and right, transmission tunnel front and centre, roof, rear left and rear right. The existing rear power socket rated at 10 amps ??? is already earthed to the body at the rear.

If you are going to add the weight of more copper, you will get less voltage drop if you put the extra copper in the positive lead rather than in parallel with the huge sheet of steel running between the front and rear of the vehicle.

I don't see how electrolytic corrosion has any impact ??

FollowupID: 415684

Reply By: Member - Omaroo (NSW) - Thursday, Mar 16, 2006 at 15:15

Thursday, Mar 16, 2006 at 15:15

Here's a pic of the one I installed in the Jeep.

You'll notice that it is right next to the battery. The top row of contacts I've joined together as a common "bus rail" by splitting the wire filaments into two bunches which then criss-cross interleave the contacts. I then soldered it all up and covered with split tubing. That's the "supply side".

I then run individual wiring where I need it to go from the contacts on the opposite side - and install an aptly-chosen blade fuse when I need to connect a new accessory.

I typically run a separate earth wire to the desired location as well rather than use chassis ground. Some prefer not to do this, but I do because it's less problematic in the long run.


AnswerID: 160946

Follow Up By: Member - ROTORD - Thursday, Mar 16, 2006 at 16:32

Thursday, Mar 16, 2006 at 16:32
Hello Surf

Have a browse through a good boating shop . They have better quality gear , cheaper , and better suited for amateur installation . you may consider a fused switch board feeding outlet plugs . These come with a fuse on each switch .
FollowupID: 415679

Follow Up By: Member - Mike DID - Thursday, Mar 16, 2006 at 17:03

Thursday, Mar 16, 2006 at 17:03
Here's an example

FollowupID: 415685

Follow Up By: Member - Glenn D (NSW) - Friday, Mar 17, 2006 at 00:35

Friday, Mar 17, 2006 at 00:35
Have almost the exact same set up , but have a terminal strip with 10 fuses .

The feeder wire I used was the same as battery feeder cable with wires soldered onto it at 90 degrees in line with the fuses and a blade connectors to the fusebox/terminal strip.

Even after liberal sealant application water and dust gets in. After I did this have seen marine hardware and it looks to be heaps better quality.

Circuit breakers in a properly sealed box would be a better option.

Havent used local grounds but run twin core to where it is needed and use one earth point close to the battery.

hope this helps .

FollowupID: 415783

Reply By: VK3CAT - Thursday, Mar 16, 2006 at 19:46

Thursday, Mar 16, 2006 at 19:46
Surf. Try using Clipsal DC circuit breakers instead of fuses. cat # 4cb120/6DC corresponds to a 20 amp single pole circuit breaker.
A suitable enclosure would be the Clipsal #4CC4 or the weatherproof #56SB4
A suitable location for a 4CC4 in an 80 series is inside the cabin on the fire wall above the pedals.
In a pinch, DC cabling can be threaded through the chassis rails. This provides protection for the cables & can actually make the installation easier.
Note that there is a lot of confusion with cable sizes. Automotive cable sizes are based upon the hole that the cable will fit through (ie including the insulation) whilst mains voltage cables are calculated by the cross sectional area of the conductors in millimetres squared.
Both positive & negative lines should be run back to the switchbox with as large a cable as possible that suits your electrical loading. Voltage drop on 12 volts is critical. The amount of voltage drop will increase with Amongst other things, running the negative back to the battery / switchbox will reduce interference from appliances to radio equipment etc due to earth loops.
Cheers, Tony.
AnswerID: 160997

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