Wire size

[akd]

Hello All,

Would like to hard wire my fridge on the back of the ute,I have been given some
15amp auto wire,will this be heavy enough,and which type of plug would you recommend.?T plug or Anderson plug.

Thanks
AKD

[Mick_Marsh]

Hello All,

Would like to hard wire my fridge on the back of the ute,I have been given some
15amp auto wire,will this be heavy enough,and which type of plug would you recommend.?T plug or Anderson plug.

Thanks
AKD
Fridges usually use about 2.5A and you need to take into account volt drop.
15A cable will be plenty.

[lebanon]

Hello,

to determine the correct thickness of electric wires (conductor) you can use the following nonogram.

Example: for a cable length of 12.50 meters and a current of 18 A,
a wire of 6 mm ² is to be used.

For a current of 15 A and for the same length, the nomogram indicates a value between 4 mm ² and 6 mm ².
The choice will be on the upper section that is to say 6 mm ².

Regards

[drivesafe]

Hi akd, need a bit more info, like total cable length and amp draw of your fridge.

I would be surprised if your 15 amp cable is anywhere near thick enough.

The current rating of automotive cable is very misleading and you will most likely need to use much thicker cable to reduce voltage drop.

Voltage drop, particularly with some brands of fridges, will cause loads of problems and the only way to reduce voltage drop is to increase the thickness of the cable.

[p38arover]

Hello,

to determine the correct thickness of electric wires (conductor) you can use the following nonogram.

Example: for a cable length of 12.50 meters and a current of 18 A,
a wire of 6 mm ² is to be used.

For a current of 15 A and for the same length, the nomogram indicates a value between 4 mm ² and 6 mm ².
The choice will be on the upper section that is to say 6 mm ².

Regards

The thing I don't like about that diagram is that it doesn't give voltage drop.

A few volts with a 240 volts supply is less significant than the same drop with a nominal 12 volt supply.

[one_iota]

Attached is a graph from Waeco that shows cable size vs length required for both 12 and 24 Volts.

[p38arover]

Note how the Waeco graph shows a much bigger wire size for the same 12 metre run for a 12 volt fridge than does the first graph.

[pawl]

I use a formula to calc voltage drop. This only applies to copper and at 25C.
cable length(metres) x current(Amps) x 0.017 and divide that result by the cross sectional area of the conductor in mm2.
So for the first graph above, 12.5m x 18A x 0.017=3.825 /6mm2 then gives a voltage drop of 0.64 V which is too much. 3% or 0.36V or less is regarded as permissable, from what I've read. 6 B&S approx 13mm2 would be a better choice in the above example.
I ran 6 B&S all the way down the back for the fridge as well as for the 3 way in the c'van, although I did skimp and use the chassis as the earth return. The 15A auto wire is probably a little on the light side, as drivesafe said, auto cable can be VERY misleading. Does it give the area of the wire(not including insulation)?

[akd]

Thanks to you all for your imput.

At present I have not measured the length,needed for the cable,BUT it looks like
the cable I have is too light.

Will be going to the 12v shop in Kewdale on thursday and see what cable they recommend.

The fridge is a 40l engle.

Cheers
Ann
akd

[Mick_Marsh]

I have an Engel 40l fridge. When it does draw current, it only draws 2.5 Amperes.
It seems most seem to be assuming the fridge draws 15 to 18 Amperes.
The waeco cable size chart recommends 16 sq mm. seems to be overkill to me. My jumper leads are less than that.
I'd be interested in what the shop recommends Ann.
regards
Mick

[Homestar]

Theoretical voltage drop is very different to real voltage drop. As an Electrician, we need to calculate this almsot every day based on AS3008 - to keep the inspectors happy. I have done some experiments with both 240 volt and 12 volt on this. In both cases the calculated voltage drop was well over twice as much as the actual I was getting under a given load. These calculations for voltage drop and current ratings are VERY conservative, so if you needed anything more than 4mm2 I would be supprised - you can run a large AC unit in a house on this over 20 metres from the switchboard, but with 12 volts, as mentioned, voltage drop becomes a much bigger factor. Also worth noting is that there are different terminologies for cable size. The term mm2 is actually very misleading, it stands for square millimetres, not millimetres squared - a big difference. I have a piece of 2.5mm2 building wire in the old commodore running 2 sets of driving lights - been there for years with no problems, and if you work it out, with 4 x 100 watt lamps, it is supplying nearly 30 amps (at 13.5 volts - engine running). If you are in doubt, run the cable out you have and connect the fridge to it. Once the fridge is running, and with the engine off, measure the voltage at the battery, then at the fridge. If there is a difference of more than about 0.6 volts (about 5%), then the cable may be too small. I doubt you would see this much on a bit of 4mm2 cable. As an aside, the electric brakes on my RRC are wired up with 1.5mm2 cable from the controller - done by an autolec before I got it. This runs the brakes on the trailer fine, and can draw over 10 amps easily depending on where the controller is set - all with no problems.

Cheers - Gav