12v wiring for new fridge from old

[KevinW]

Hi
I have taken out an old 3 way electrical fridge from my 1991 Talbot took photos as I went along as it was part of a bigger refurb but come to put new fridge in where the new fridge has a black and a red wire to the 12v side, my old fridge had a blue/white stripe cable and a black cable any help would be appreciated as I need to finish the refurbished inn2 weeks for my holiday YES in the van .
Many thanks
Kevin

 

[wildebus]

Well, the new fridge wiring will be RED for +ve and BLACK for -ve, so you need to go on that basis.
You haven't said what kind of fridge the new one is. If it is a 3-way again, then yu can use the old wiring (assuming it was still working correctly, of course) but will need to double-check which wire is +ve and which is -ve (can't assume the black on the old one was -ve).
If it is a Compressor Fridge you are fitting, then you need to forget the old wiring (make sure you don't leave any bare ends though) and run new cable from the Leisure Battery to the Fridge.
For a compressor fridge, you should go for 4mm2 absolute minimum, 6mm2 would be better and if a fair length of cable, consider using 10mm2 cable. And again, if a compressor fridge, you need to consider if your battery storage is sufficient for your style of camping with that type of fridge - reckon on it using 40Ah a day if you are lucky, and more likely 50Ah per day.

 

[KevinW]

Hi
Yes correct for the new fridge which is a Smad 3 way, someone had added a red and a link from the black that was with the blue/white, which went to the igniter on the old 3way fridge

 

[hayhiker]

I replaced my 3 way in November blanked off the old wiring and ran new as Wildebus has said and I agree about the power usage

 

[KevinW]

Thanks for your input, I have blanked the old wiring off and rewiring, as it’s a 3 way power usage isn’t really a problem as it will be mainly on 240/gas and 12v while driving.

 

[Chasn]

Well, the new fridge wiring will be RED for +ve and BLACK for -ve, so you need to go on that basis.
You haven't said what kind of fridge the new one is. If it is a 3-way again, then yu can use the old wiring (assuming it was still working correctly, of course) but will need to double-check which wire is +ve and which is -ve (can't assume the black on the old one was -ve).
If it is a Compressor Fridge you are fitting, then you need to forget the old wiring (make sure you don't leave any bare ends though) and run new cable from the Leisure Battery to the Fridge.
For a compressor fridge, you should go for 4mm2 absolute minimum, 6mm2 would be better and if a fair length of cable, consider using 10mm2 cable. And again, if a compressor fridge, you need to consider if your battery storage is sufficient for your style of camping with that type of fridge - reckon on it using 40Ah a day if you are lucky, and more likely 50Ah per day.

I am puzzled why you might need a thicker cable for a replacement compressor fridge. One of the main reasons to change to a compressor fridge is the lower power consumption. I am about to make this change. My existing absorption fridge is a 170W model whereas its replace to compressor fridge is 80W. Perhaps it is to cope with initial start up surge currents?

 

[wildebus]

I am puzzled why you might need a thicker cable for a replacement compressor fridge. One of the main reasons to change to a compressor fridge is the lower power consumption. I am about to make this change. My existing absorption fridge is a 170W model whereas its replace to compressor fridge is 80W. Perhaps it is to cope with initial start up surge currents?

This "puzzlelation" is precisely why so many compressor fridges have been installed with inadequate cabling which stops them working with anything but a fully charged battery and why so many owners end up starting their engines to start their fridges . I lost count how many folk I spoke to at VW Festivals when I had a Trade Stand asking me why their fridge wasn't working properly in the van either they converted or had converted for them.

The issue is nothing to do with the running power of the compressor fridge (which is not 80W but more like 50W typically in fact) but as you have considered, the surge power/current is the issue for undersized cables, where the voltage drop on surge on thin cable can be dramatic and so much that the fridge detects a low voltage situation and refuses to start.

A couple of screen captures showing the difference between the surge current and the running current once compressor has started
IN-RUSH

And this doesn't actually capture the full power spike, which can be upto 20x the running power and is why if you are fitting a 240V Compressor fridge into a camper for off-grid use, you don't get a tiny inverter delivering 100W max, but need one that can provide 800W or more momentarily.


Post IN-RUSH

This was for a 240V Fridge powered by a Victron Inverter and even with good sized cables and a short run, there was still a voltage drop of >0.5V. Check on a on-line calculator such as here what the voltage drop would be on say a 2.5mm2 cable on a run of say 3 metres and a current of 40A


This video shows the full power-on sequence for the Compressor Fridge. And note this is a sequence that is repeated many times a day.

0:05 - Inverter Switched On
0:20 - Fridge Plugged in
0:30 - Fridge Door Opened (so Light comes on)
0:41 - Fridge Temp Dial turned to Max - Compressor In-rush Current comes on
0:45 - In-Rush Current ends

Note that the power use you are seeing from 0:45 is the maximum power draw all the time the fridge is actively cooling, which is a small percentage of the time on a Compressor Fridge (this is what makes them so efficient in use).