Fridge wiring size

I paid an auto electrician big money earlier this year to supply fit a tray/dual battery under the bonnet (55amp), redarc isolator and run cable to the tray for a fridge and a light. For my $1300 he ran a 26 strand (1.84mm2) twin core wire for the fridge and a 14 strand (1mm2?) for the 14 watt fluro light. Neither of the wires had conduit protection whatsoever and the black wires were just self tapped under the bonnet without scraping off the paint. From what I've read on this forum, it seems like a pretty shoddy job to me in regards to the cable size for the fridge. Just wondering if anyone else agrees before i querie his work with him. Any advice apreciated. 2007 Navara dual cab st-r. he also said that the biggest and best battery he could fit was an R&J 55amp but I can't find any info on it as to wether it is a deep cycle or cranker

You have every right to complain.

While the gauge of wire chosen might be suitable for a short distance, the voltage is reduced over distance.

It's a simple process really - every wire represents a resistance, which causes the voltage to drop simply by the wire's existence.

The problem in the automotive arena is that with something drawing power (watts) on the other end of the wire, if it can't get (say) 12V times 20A = 240watts, if it needs 240watts and the voltage has dropped to 10V, it will try to draw 24A. The increase in current actually (at an atomic level) represents an increase in the number of electrons trying to shuffle along the cable - because electricity is really the passing of an electron from one metal atom to another, and the voltage is proportional to the energy that each electron has. This increase in the number of electrons creates an increase in heat - basically, the more current flowing through the wire, the hotter it gets.

This problem really becomes evident with large trailers and caravans. Each wheel will draw about 6.5A on its electric brake, so 4 wheels draw 26A. I was reading somewhere where an auto electrician had used 40A cable considering it to be large enough - but because of the tyranny of distance, the cable ended up melting through its conductor and catching alight.

For distances over about a metre I'd be going up to the heavier gauge wire. For 20A installations - and really, that's what any power to the tub should be rated at - use 60A cable (6 sq mm) at an absolute minimum. There's a guy that writes a lot of stuff about electrical wiring and I've learnt a lot from him - Collyn Rivers - he actually says to use a much heavier gauge for that distance - read the section on "12V Fridges" on this page.

The cable that I use is 6 sqmm and it comes as a doube-insulated twin core (so positive and negative surrounded by a sheath). It's adequate, not perfect. But it's a damn sight better than <2sqmm cable, which is not only ridiculous it's downright dangerous.

Thanks for the reply Tony. Would I be right in assuming that 4mm (1.84mm2) cable is rated at 15amp? For both the 1.84mm wire and the 1mm wire to the fluro light, he fitted a 20amp blade fuse. To me that's pointless. Not sure if I should have a fuse on the wire from the Aux battery to the Redarc isolater. There's none at present

You're absolutely right about it being pointless - the wire will overheat and burn something before the fuse blows. At best I'd use a 5A fuse on that size wire - I want the fuse to do the job, I don't want the insulation to melt!

The RedArc isolator will need a hefty fuse on it if it doesn't already have one. You can get fusible links of 100, 120, 140Amp etc. Might be worth checking the RedArc unit - might have one built in, and should be mounted close enough to the battery to be meaningful.

After much reading at the weekend including Collyn Rivers I decided today to run 2 X 6mm2 cables in 10mm conduits tucked up securely and pull out the auto sparkys crap. One cable for the fridge, the other cable for a big invertor and lighting (6 x led lights/floodlamps from a 6 way fuse block) my last 2 questions are:

If I run positive and negative directly from the aux battery, can I/ should I disconnect the big fat negative cable that the sparky attached to the engine block (looks terrible)

Is there a formula that can be applied for fuse size? Even tho I have used 50amp cable, if the max draw from the fridge is say 7amps, do I go 10amp fuse? or more?

Next project will be a solar setup and I've already read many of your posts on the subject. Thanks Tony

When I did it I went proper overboard, ran 100amp circuit breakers on both ends, 50mm twin core cable and a blue sea dual battery system..

I ran 9mm cable to the fridge and battery (seperate cables)
and a 6mm cable for the lights

With circuit breakers.

I really know nothing about all those things..

Peeeeda said:

After much reading at the weekend including Collyn Rivers I decided today to run 2 X 6mm2 cables in 10mm conduits tucked up securely and pull out the auto sparkys crap. One cable for the fridge, the other cable for a big invertor and lighting (6 x led lights/floodlamps from a 6 way fuse block) my last 2 questions are:

If I run positive and negative directly from the aux battery, can I/ should I disconnect the big fat negative cable that the sparky attached to the engine block (looks terrible)

Click to expand...

Ok, sould like the sparky hasn't done many dual batt systems. $1300 is outrageous! (did you get any other quotes)

Using a redarc, You should have 3 wires +ve main battery to redarc. Redarc to +ve aux battery. And earth main to earth aux. Thats it, and all should be 8 gauge or better.

All you accessories then should be hooked up to the +ve aux, with the appropriate fuse. (see below)

Peeeeda said:

Is there a formula that can be applied for fuse size? Even tho I have used 50amp cable, if the max draw from the fridge is say 7amps, do I go 10amp fuse? or more?

Click to expand...

It's just a best fit senario. I would go the next size fuse available ( the example you gave is perfect)

Plenty of good info here as well Technical Information

If you are running the fridge in the tub (where else does it fit) You will need to run some medium duty cable down from the aux battery and either earth it to the chassis or back to the battery. A fuse box or bus bar might be of some use if running a few accessories (sounds like you are)

Peeeeda said:

the other cable for a big invertor

Click to expand...

How big is the big inverter?

Hey! I've just been through the same process myself, but being a Sparky by trade, i decided to do the work myself, after much deliberation and consulting of auto electician mates. i ran an 8AWG cable from my cranking battery under the hood, through a 30A self resetting circuit breaker, and all the way to the tray (about 5-6m of cable (1-way)) and picked up an earth half way along the chassis. Once inside the tray i fit an anderson plug, and my no load voltage reading at the plug was 12.8VDC.

Incidentally, i also ran an Anderson Plug out the front and onto the top of the bullbar, (about 2m return) and also read 12.8VDC at that plug.

I then use a REDARC BCDC1220 as my isolator/battery charging system in the tray. look it up and you'll see why i chose it, especially for deep-cycle applications.

The good part of the whole thing is...materials really only cost me about $900 or so, including a Century 100Ah Gel Battery, and the REDARC BCDC1220 (found it on eBay for $350), cable, battery box, etc.

outside of that investment was the time to wire it all up. having done it once and learnt a few things here and there about the efficient way of doing it, and what materials are needed, i reckon i could knock the whole thing over in 2-3 hrs.

For doing what I did, I'd charge $1200-$1300, and you'd get a hell of a better system than the standard dual battery system. I also plan on putting a second cranker under the hood.

Peeeeda said:

If I run positive and negative directly from the aux battery, can I/ should I disconnect the big fat negative cable that the sparky attached to the engine block (looks terrible)

Is there a formula that can be applied for fuse size? Even tho I have used 50amp cable, if the max draw from the fridge is say 7amps, do I go 10amp fuse? or more?

Click to expand...

I'd leave the big fat cable attached to the engine block. It won't hurt anything.

Fuse sizes are quite important (and watch my next reply in this thread for a perfect example). The fuse is protecting two things: the cable that carries the power, yes, but more importantly the device on the other end of it.

We have to protect the cable, because the cable can be damaged. A stray stick that the wheel runs onto that pokes up, stabs the cable and tears through the conductors making them join onto the chassis. Suffice to say that in a long cable run, the fuse should be near the main power source - so if you have a battery on both ends of it, BOTH ends should be fused.

Protection should be "just enough". A 5amp fuse will allow 5A to run through it constantly but it will get a little warm, which will make it less efficient but won't compromise anything. In practice, "the next size up" is a good, reasonable safe general guide, so if you're using 7.5A on the end of the cable, a 10A fuse is more than enough.

scarecrow said:

When I did it I went proper overboard, ran 100amp circuit breakers on both ends, 50mm twin core cable and a blue sea dual battery system..

I ran 9mm cable to the fridge and battery (seperate cables)
and a 6mm cable for the lights

With circuit breakers.

I really know nothing about all those things..

Click to expand...

Can I suggest that those circuit breakers are likely to cause problems if something goes wrong?

100A is a LOT of power in a 12V system. D22 alternators can't put out that much power, and a D40 has to be revving fairly well to pump that much out. It's enough power to light 12 driving lights.

The circuit breaker should be enough to carry the maximum intended current and not much more. In mine, I have 26A heading for the brakes (30A thermal breaker) and 20A-> caravan power plus about 9A into the tub running through a second 30A breaker.

I could use a larger breaker, of course, but I need the thing to trip when there's a problem before it can cause damage to any other part of the system, and 100A is just too much problem.

It's a suggestion only, please don't think I'm telling you that you must do it! I do recommend going and asking an auto electrician and get their opinion, but I'm fairly sure they'll say that a 100A breaker on a line that really only needs to carry 20A - 30A at best - is too high.

Krankin said:

How big is the big inverter?

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The missus won't go camping without her hairdryer It takes her 8 minutes to dry her mop. I need to work out what size of inverter will cope with 1500+ watts for 8 minutes and how much amps it will suck in 8 mins. Anybody know?

Thats an easy one mate.

Power = volts x current.

Therefore, current = power / volts

Doing a rough calc (in my head), a 1500w hairdryer would draw roughly 120A from a 12v system while running.
Thats not counting the extra power draw required because of inefficiencies in the inverter.

However working on the 120A current draw, using the dryer for 8 minutes will suck 16Ah out of your battery. Considering the current draw, you're going to need some damn hefty cable from your battery to inverter. And your battery will most likely need to be a cranker.

Id be tellin her to get a haircut and go without washing her hair for a few days.

+1 on that and the math is confirmed. 1500/12=125A. 125*8/60 = 16.6Ah.

You'd need 0 gauge cable to carry that load. Anything less and you risk a fire.

I second the move to a starter (cranker) battery as well. I would NOT use a deep cycle to drive it, because hauling 125A out of any single deep cycle battery will kill it fast (in terms of warping the plates, not its capacity).

You could use a bank of deep cycles in parallel to spread the load - say, using 10 of them to reduce the current draw per battery to 12.5A - but that's an outlay of $3,000+ and then you have to haul the weight around too.

The best solution for a hair dryer - seriously - is a portable generator, like the Honda Eu20si. It won't have a problem delivering 1500W.

Generator seems the way to go then. Just wondering what people would do with a high power inverter? Ebay has loads of big ones even as high as 5000 watt inverters. No battery system could supply that then?

From the 55amp aux batt under the bonnet I've run not one but two 6mm2 twin core cables to the canopy. Is that the equivalent of a single 12mm2 cable or less? Just wondering if I'm able to throw in a third battery in the canopy to let my fridge run longer?

Large motor homes use inverters that size but they'll have a bank of 10 or even up to 20 deep cycle batteries. They may even have sophisticated charging systems - some buses run two alternators, with multiple solar panels ... very expensive. It's so that they can run normal household appliances - fridges, microwaves, washing machines and TVs usually. 12V stuff just doesn't cut the mustard for size, usually - the compressors aren't effective enough (they are efficient, just not effective in large volume spaces).

I just saw a 12 volt hairdryer on eBay, but still said 16 amp fuse minimum.
Would be rubbish anyway you would think.
What about getting her to put her hair in front of the exhaust while you rev the engine?
My missus hasn't had this concern yet but our next bush camping trip will be at least 6 days so could be interesting for her, I give her 4 days!

If only a hair dryer was all that was stopping my missus from going camping. She barely even comes out on the farm, I think she actually thinks if she shows any interest I'll have her mowing the grass.... and she's probably right too.

Think I'll set up a mirror array that focuses the suns energy to one spot. Might even light her fag while she's getting her hair cooked

Old.Tony said:

Can I suggest that those circuit breakers are likely to cause problems if something goes wrong?

100A is a LOT of power in a 12V system. D22 alternators can't put out that much power, and a D40 has to be revving fairly well to pump that much out. It's enough power to light 12 driving lights.

The circuit breaker should be enough to carry the maximum intended current and not much more. In mine, I have 26A heading for the brakes (30A thermal breaker) and 20A-> caravan power plus about 9A into the tub running through a second 30A breaker.

I could use a larger breaker, of course, but I need the thing to trip when there's a problem before it can cause damage to any other part of the system, and 100A is just too much problem.

It's a suggestion only, please don't think I'm telling you that you must do it! I do recommend going and asking an auto electrician and get their opinion, but I'm fairly sure they'll say that a 100A breaker on a line that really only needs to carry 20A - 30A at best - is too high.

Click to expand...

Told you I knew nothing I called a buddy and he explained how it works..
Starter battery -> Dual Batt Controller -> Circuit breaker -> Cable to Dual Battery (in tray) -> 2nd Circuit Breaker -> Cables running to fridge / cigarette points both have 30amp breakers or maybe 25... can't remember!

That sound a little more logical?