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Wiring for 12v fridge

I am going to buy a shoreline fridge and have a spare connection on my control panel. I have been told that standard 12v cable would not be sufficient for this and would need something thicker? Fridge will be approx. 25feet from the control panel. Any advice appreciated.

Asked by: Paul Jones  | 7.27pm, Sunday 24 February


WW says:

To be fair, there is no "standard" 12V cable- different loads require different thicknesses of conductor, to avoid voltage drop and potential overheating of the cable.
All marine wiring needs to be multistrand cables, rather than the solid cables often used in domestic "twin flat and earth" types cables.
Fridges running on 12V are particularly at risk of voltage drop as often wiring that is too thin is used, which lowers the voltage by the time it gets to the fridge, which then cuts-out, sensing an artificial low voltage.
If your fridge is 25ft from the fridge, then shoreline themselves in their technical notes, suggest that you need to use 10mm2 cross sectional cable. This might seem excessive, but it will guarantee that voltage drop won't cause problems.
You will need a 15amp fuse on the supply board- and if you use in inline isolation switch (recommended) then that should be rated at 20A current.
Also, if you ensure that there is ample ventilation all around the fridge, which will help reduce the load on the compressor and reduce running times. The installation advice for the fridge should be very carefully followed.
Hope that helps, if you need any further advice, please do ask.

Mark Langley  | 8.08PM, Sunday 24 February

It is unlikely that your spare fused circuit will be suitable for the reasons that Mark has outlined. To get the best trouble free service from a 12V fridge it needs a good low resistance supply and a lot of boaters put in a dedicated supply straight from the battery, thus bypassing any superfluous connections. If you do go down this line do ensure that a fuse is fitted as close to the battery as possible.

Rupert Smedley  | 11.06PM, Sunday 24 February


Readers say:

I rewired a lot of the 12 volt system on our boat last year and found two resources really useful to try to get my head around voltage drop and cable sizing. They were Nigel Calder's "Boatowner's Mechanical and Electrical Manual" and the Smartgauge website (http://www.smartgauge.co.uk/cable_type.html). The latter gives a useful description of how to specify the right cable for the job, and can be summarised by this formula: cable size[mm2] = 18/((Volt Drop*1000/Amps)/Metres), where Volt Drop is the difference between the typical voltage at the battery (e.g. 12 volts) and the acceptable voltage at the device (e.g 11.75 volts), Amps is the power consumption of the device and metres is the cable run from the battery to the device and back again (i.e. twice the distance between the two plus a bit for going round the corners).
So for Paul's installation you could calculate:
Volt drop = 0.25 volts (giving 11.75 volts at the fridge, or a 2% drop)
Amps = 5 (This is my guess, but it will be in the specification)
Metres = 17 (25ft x .305 x 2, plus 10% for luck)
Taking these variables, cable size[mm2] = 18/((1*1000/5)/17) = 6 mm2, and following the Smartgauge advice to build in an extra 30% capacity you get 8mm sq. Going for a bigger than absolutely necessary cable size for the fridge (such as 10 mm2) will help avoid the fridge's compressor cutting out due to the battery voltage falling temporarily when other power drains are applied, such as flushing the loo.
N.B. I found it was easy to get confused by the units in which cable is sized and sold, "mm2" refers to the area of the cable and not to its diameter. Pi x R squared dictates that 10 mm2 cable has a diameter of 3.6mm and 6 mm2 cable has a diameter of 2.8mm, so if you can measure the diameter you can estimate the area (mm2) of the cable.

Colin Wilks  | 10.58AM, Tuesday 5 March

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