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Cable size for 12 volt fridge.


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I am getting a 12 volt freezer and moving my existing inlander 12 volt fridge with icebox to the other side of the galley as not as deep as the freezer.

What size cable should I buy and what fuse rating? It's an 8 metre length of cable I need and needs to be run in conduit. Would connecting it direct to batteries with fuse be ok as 12 volt distribution is full?

Many thanks. James.

Edited by canals are us?
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Just now, WotEver said:

Presumably if the fridge was happy with the existing wiring the freezer will be too :)

Yes i agree, but folk have given a size of cable without knowing if the freezer has the same consumption.  Just though i should ask as its what i would always do being a newbie.

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21 minutes ago, Alan de Enfield said:

Pretty much - NO.

Its volt drop that is the deciding feature.

 

How do you work out volt drop with not knowing the Amps of the freezer then ?

 

The OP did not specify any Amps or same appliance etc.

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8 minutes ago, W+T said:

How do you work out volt drop with not knowing the Amps of the freezer then ?

 

The OP did not specify any Amps or same appliance etc.

To work out the accurate volt-drop, you do need to know the current.

However thinking that "the fridge is 5 amps - I need 5amp cable" would result in disappointment.

It may be a generality but if someone asks what size cable do I need they are probably not aware of the importance of volt drop.

There are a huge number of 'guidelines' available which give a 'quick & dirty' guide of 1mm2 per metre run, and there is such a big difference between what cable 'needs' (say) 5amps rating and what is actually the 'size needed'.

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1 hour ago, canals are us? said:

I am getting a 12 volt freezer and moving my existing inlander 12 volt fridge with icebox to the other side of the galley as not as deep as the freezer.

What size cable should I buy and what fuse rating? It's an 8 metre length of cable I need and needs to be run in conduit. Would connecting it direct to batteries with fuse be ok as 12 volt distribution is full?

Many thanks. James.

As others have said 10mm cable. You should be wiring it via the master switch and not directly onto the battery.

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1 hour ago, W+T said:

How do you work out volt drop with not knowing the Amps of the freezer then ?

 

The OP did not specify any Amps or same appliance etc.

You can't without knowing the current draw as Alan has stated. Also the fridge compressor starting current will probably be twice the running current albeit for a very short time.

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1 hour ago, W+T said:

How do you work out volt drop with not knowing the Amps of the freezer then ?

 

The OP did not specify any Amps or same appliance etc.

These are well know items as is the problem with volt drop that is why the manufacturers recommend the cable size to be used. I understand there is at least one professional boat wiring company that doesn't agree with them.  

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ISTR the DC compressor fridges have a 15A surge on 12V (according to the Danfoss data sheet) and can be very touchy about volt drop, so the cable size is ~5x what the running current will suggest.

Seems the rule of thumb for 12V fridges is 1mm2 per metre distance between fridge and batts, and the info Brian posted appears to confirm that. Gotta remember that the fridge needs to start OK when there's another heavy 12V load pulling the terminal voltage down, eg big inverter.

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3 hours ago, Flyboy said:

As others have said 10mm cable. You should be wiring it via the master switch and not directly onto the battery.

I have just had a look at my 12 volt fuse board and there is a spare fuse holder except switch is broken/ missing. Can just the switch be changed?

Does the main feed wire look beefy enough? Boat built between 1998 and 2002. Usual 12 volt boat items.  Webasto wired direct to batteries and fused. Any other main switch that's also fused that I could surface mount?

Thanks. James:cheers:

 

12 volt 1.jpg

12 volt 2.jpg

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10 hours ago, canals are us? said:

I have just had a look at my 12 volt fuse board and there is a spare fuse holder except switch is broken/ missing. Can just the switch be changed?

Does this make the run even longer? General advice is to run a dedicated cable, as short as possible, fused at the battery end. Finding a beefy enough switch (20A DC) to fit the fuseboard might also prove difficult. 

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Yes you could connect the 10mm square cable at this point, but not to the type of fuse holder or switch that appear to be fitted - they are probably on the limit at 10amps . You would need to fit the industrial 20 or 30 amp bottle fuse type which are much larger, as well as find a DC switch that is good for 30 amps, to ensure you do not get a significant voltage drop as the compressor tries to start. The existing supply cable also looks like 10mm but assembled with some very light duty crimp terminals, so likely to give significant voltage drop under load .

As others have said, you would have a much higher chance of every thing working correctly if you pick up a new supply from the battery isolator through an 20 amp inline fuse fitted with a cover, also you need to run a return cable of 10mm back to the neutral anchor point adjacent to your batteries.

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