Brace yourselves... It's ANOTHER 12V/230V fridge question

[Up-Side-Down]

On 01/11/2023 at 18:19, Alan de Enfield said:

 

They are well undersize.

 

A cable is measured by the CSA (Cross sectional area) of the copper and NOT the overall diameter of the wires. a 3mm diameter wire will be less than 3mm2 (CSA)

 

The MINIMUM CSA should be 1mm2 (1mm square) per metre of length of run.

 

EACH cable should be 6mm2 (bigger would be better - if it is a 7mt run then it will be undersized.

If it's any help, Shoreline specify a 10mm2 cable for a 10m run (that would be for each cable – pos and neg!).

[Alan de Enfield]

16 minutes ago, Up-Side-Down said:

If it's any help, Shoreline specify a 10mm2 cable for a 10m run (that would be for each cable – pos and neg!).

 

Yup - the oft quoted 1mm2 per metre run, but other manufacturers (eg Dometic) quote much larger pe mt run. (as per the example I quoted earlier).

Edited November 3, 2023 by Alan de Enfield

[Tony Brooks]

Just now, Alan de Enfield said:

 

Yup - the oft quoted 1mm2 per metre run.

One way run, use the same for the return (Danfoss recommendation).

[Alan de Enfield]

35 minutes ago, Tony Brooks said:

One way run, use the same for the return (Danfoss recommendation).

 

Maybe each manufacturer has differing specifications. or maybe their fridge is more / less volt drop sensitive ?

 

Dometic quote 10mm2 for a 6 metre run and 14mm 2 for a 10 metre run (total length) based on 12v DC

 

 

 

 

[captain flint]

I know there's no way of telling me for sure, but if cables disappear into the cabin linings on one side of the boat, and reappear on the other, am I right to assume that most fitter-outers (professional) would run them round the cabin sides, or might they be run the shorter route direct across the floor or through the top side of the cabin lining? 

 

I guess the answer might be to actually measure the voltage drop at the fridge end. I'll do some googling to find out how to do that, but maybe it's just a question of detaching the wire from the fridge and sticking a voltmeter on... Seems kind of obvious but with the massive caveat that I'm totally ignorant... 

[Iain_S]

3 minutes ago, captain flint said:

(snip)

 

I guess the answer might be to actually measure the voltage drop at the fridge end. I'll do some googling to find out how to do that, but maybe it's just a question of detaching the wire from the fridge and sticking a voltmeter on... Seems kind of obvious but with the massive caveat that I'm totally ignorant... 

If the wire's detached from the fridge, there won't be any voltage drop. Volt drop is the result of current flowing through the resistance of the cable. Even measuring with the fridge connected and running, the result will be on the low side; it's the volt drop when the compressor is starting that is the important one.

[David Mack]

18 minutes ago, captain flint said:

I know there's no way of telling me for sure, but if cables disappear into the cabin linings on one side of the boat, and reappear on the other, am I right to assume that most fitter-outers (professional) would run them round the cabin sides, or might they be run the shorter route direct across the floor or through the top side of the cabin lining? 

If you pull one end of the cable does the other end move? Are both ends the same size and colour?

Cables should not be run under the floor, but they could be behind hull side, cabin side or roof linings. But most likely to be run under the gunwale, in the angle between cabin side and roof, or occasionally down the middle of the roof (usually ceiling lighting only). With any of these there may be a vertical run behind the lining to get to the outlet point. On well fitted boats there will be access to this change of cable direction, but on others maybe not.

If a cable runs diagonally behind the lining that is indicative of a less than professional installation.

Edited November 4, 2023 by David Mack

[captain flint]

Thanks, David. It's a professional fit out, and not at all bad, but by no means perfect. I'm not aware of any way of accessing them, but I might well be missing something. I'll get someone more clued up than me to have a look. The question of size and colour had occurred to me and I can't see cables the same colour coming out of the fuse box, so I'm unclear what's going on there. 

 

I had assumed they ran round the cabin, beneath the gunnels, but there's an obvious place in that run where they'd be visible and I can't see them there. So it's hard to say, really... but maybe they're under the roof. 

 

I guess an option would be to get a new fridge having discussed the returns policy with the vendor, on the basis that I can return it if it doesn't work, though the shipping costs I'd incur would not be ideal. At 6mm2 I'm guessing there's a chance I'll run into voltage drop problems, but not much risk of it being dangerous. Since I've moved over to LiFePO4 batteries, which, I'm told, have a slightly higher voltage output, I may be OK...(?)

On 04/11/2023 at 11:50, Iain_S said:

Even measuring with the fridge connected and running, the result will be on the low side; it's the volt drop when the compressor is starting that is the important one.

Right. Of course. That makes sense. Thanks for setting me straight, Iain

Edited November 6, 2023 by captain flint

[Tracy D'arth]

On 04/11/2023 at 11:38, captain flint said:

I know there's no way of telling me for sure, but if cables disappear into the cabin linings on one side of the boat, and reappear on the other, am I right to assume that most fitter-outers (professional) would run them round the cabin sides, or might they be run the shorter route direct across the floor or through the top side of the cabin lining? 

 

I guess the answer might be to actually measure the voltage drop at the fridge end. I'll do some googling to find out how to do that, but maybe it's just a question of detaching the wire from the fridge and sticking a voltmeter on... Seems kind of obvious but with the massive caveat that I'm totally ignorant... 

You cannot measure the STARTING voltage drop which is what matters as the starting current is much higher than the running current and only for a very brief but vital period..

[IanD]

39 minutes ago, Tracy D'arth said:

You cannot measure the STARTING voltage drop which is what matters as the starting current is much higher than the running current and only for a very brief but vital period..

 

Yes you can, some more professional DVMs measure not just average voltage (over the sampling period) but also maximum and minimum, with sampling rates high enough to spot mains transients. Have done this to spot exactly this kind of problem, though on something rather bigger than a fridge (industrial compressor).

 

But most people are unlikely to have one of these, and the common cheapo 3.5 digit ones don't do this, so they can't do it. But that's an equipment problem, not because it's not possible... 😉

[cuthound]

One of these will make tracing hidden cables easier.

 

https://www.amazon.co.uk/Proster-Wire-Tracker-Multifunction-Continuity/dp/B0811P7SCT?source=ps-sl-shoppingads-lpcontext&ref_=fplfs&smid=A1UK0NFRCCP9A&nis=6&th=1

 

Note I have only ever used the professional versions which cost an order of magnitude more, but this ine seems to have generally good reviews.