64a cable for fridge?

[Gavin Brunton]

Hi I am installing a 12v fridge on my girlfriends boat and wanted to know if 10mm 64a twin and earth from b&q would be suitable . We need about 10m of cable and the fridge is 70w so 5.83amps at startup. I have read a few posts with complicated formulas but surely if the cable is rated at 10.97 times the rated amperage than this would be suitable

[Chalky]

The current isn't the problem, it's the volt drop. This is why you need the big cable. If the volt drop is too low then the fridge won't run or will cut out unexpectedly. What you need to do is work out what volt drop is acceptable then work out the cable size required for the cable length you need. Remember to double the distance since the cable goes between the batteries and the fridge and back to the batteries. The cable size will easily cope with the current. Also put a fuse in the circuit. This should be rated to protect the cable not the fridge. I can't give you approx cable sizes since I use a mains fridge and inverter.

 

I assume that it's a compressor based fridge. If it's an adsorption fridge (type used in a caravan with gas/electrical) then you're going to have problems with it flattening the battieris very quickly.

[nicknorman]

Couple of points, firstly using solid core twin and earth on a boat is frowned upon. I'm pretty sure it is disallowed under the RCD rules, although if it is an older boat you don't HAVE to comply with these rules. Multistrand cable is preferred because it is much less pone to vibration-induced fracturing.

 

Secondly, cable selection is made on two criteria 1/ the capability of the cable to pass the current without overheating and 2/ the voltage drop in the cable. For 12v systems in cables of fairly long length, it is always 2/ that is the limiting factor. If you drop a volt or two in the wires at 230v, in percentage terms it is irrelevant. But dropping a volt or two at 12v represents a high percentage and is probably unacceptable.

 

To work out whether you 10sq mm is adequate you need to consider the total length of each run (ie in your case it is 20m total), the current, and the acceptable voltage drop. Clearly there is no "absolute right answer" for this last one. If you accept a higher voltage drop, the fridge will start to play up earlier in the batteries discharge profile.

 

As to whether 10sqmm is reasonable, I'll go away and think about it (formulas etc!). If you are determined to use the twin and earth (I don't recommend it) you could put the earth in parallel with one of the other cores to increase the effective cable size.

 

Edit: ok so 20m at 10sq mm at 6A it will drop 0.4v. A battery getting down to 12v (50% SoC under load) would result in 11.6v at the fridge. To know whether this is acceptable you need to know the minimum operating voltage of the fridge. It could also be that the startup current from the fridge is momentarily more than the figure you quoted.

 

My gut feeling is that this is probably OK.

 

Further edit: this graph http://www.aquafax.co.uk/images/aceimages/Product%20PDF/Shoreline_Product_Update_22-02-13.pdf

Suggests that 10sq mm is just about right for their fridges.

Edited September 7, 2014 by nicknorman

[John V]

Nicknorman is obviously quicker at the maths than me.....yep 10mm is fine (6mm just a bit too small)....10mm T/E will be stranded (but thick strands) but its B awful cable to run and terminate, you would be better off with more flexible stuff, either pukka marine, automotive or tri-rate cables.

 

eta......10mm welding cable in black is available (singles) and nice cable to work with

Edited September 7, 2014 by John V

[AndrewIC]

The startup current for a compressor fridge is, momentarily, very high, much higher than the nominal rating of the fridge. It's reducing the volt drop along the cable during startup that needs the very thick cable.

[Gavin Brunton]

Each core 64a twin and earth has 7 strands is this ok?

[Gavin Brunton]

I'm trying to keep costs down so it seems 64a twin and earth is this cheapest option. Just out of interest has anyone ever considered a dc-dc voltage booster. They are pretty cheap on ebay

[nicknorman]

Each core 64a twin and earth has 7 strands is this ok?

 

I'm trying to keep costs down so it seems 64a twin and earth is this cheapest option. Just out of interest has anyone ever considered a dc-dc voltage booster. They are pretty cheap on ebay

As said previously, it is 7 strands of nasty heavy cable. This has issues with rigidity so it is easy for the cable to put a big strain on the connection points, resulting in them failing. Will it work? Yes, in the short to medium term. Is it the right way to do it? No.

 

As to the cost this:

 

http://www.barden-ukshop.com/black-flexible-10mm-battery-cable---cab003-1224-p.asp

 

is just over £3 per metre for the correct stuff, is your B&Q stuff so much cheaper that it warrants using the wrong stuff? 10 metres of red and 10 metres of black will be just over £60, free delivery.

 

Edit: Looks to me like the B&Q cable is more expensive = no contest!

Edited September 7, 2014 by nicknorman

[John V]

I have just been trying to look this up for you.....my boat safety scheme books are October 1999 and don't state actual spec for cable only that it should have multi stranded conductors........IMO you still would be better off doing it properly

 

 

eta ....Lloyds would require 80/.40 which is very flexible (80 strands) but I still can't find the recommended for BSS

Edited September 7, 2014 by John V

[nicknorman]

I have just been trying to look this up for you.....my boat safety scheme books are October 1999 and don't state actual spec for cable only that it should have multi stranded conductors........IMO you still would be better off doing it properly

I agree with the principle (by the way it is not required in the BSS now, but is in the RCD) but of course the cable he is proposing is multistrand. Just horribly thick strands! Probably within the letter of the RCD but not the spirit!

Just out of interest has anyone ever considered a dc-dc voltage booster. They are pretty cheap on ebay

A dc-dc booster might be an option but as previously said, a fridge can have a very high startup transient current. The converter would need to be able to cater for that. Additionally, you are introducing another thing to go wrong and ruin the contents of the fridge. Electronics on a boat have to be protected against the significant transients that occur when inductive stuff such as pumps, fridges and starters switch off so a cheapo converter might not survive the electrical environment. Also bear in mind that power is a precious resource on a boat and the fridge can be one of the bigger consumers of it due to its 24/7 nature. Having thinner cable and a voltage booster will waste more power than having the correct sized cable in the first place.

[John V]

Most of the jobs I do are to Lloyds but I have to be careful with private as sometimes Lloyds are sometimes easier than private boat recommendations,

the normal max volt drop on 12V is 0.5v or 0.6v depending on which rules you read but Lloyds allow 0.72v!

[Chalky]

I'm trying to keep costs down so it seems 64a twin and earth is this cheapest option. Just out of interest has anyone ever considered a dc-dc voltage booster. They are pretty cheap on ebay

 

A typical ebay DC-DC converter is about 90% efficient, so straight away you're throwing away about 1/5 of your battery supply in power conversion losses. Also unless you get a good one they won't be able to cope with the start up surge of the fridge. You could get round this with some big capacitors but then you need some form of pre-charge circuit to charge them up slowly. Also the chances are they'll radiate a lot of RF noise so you'll need to screen them carefully unless you want to block out your radio. They'll also need cooling - power electronics don't take kindly to running hot...

 

I've used a number of small (and large) DC-DC converters. The small ones (5A nom) from ebay work really well with USB batteries to generate 12-14v ish to run stepper motors on small robots. The big ones I've used convert +300 V DC down to 12V at about 200A. These are water or oil cooled about 98% efficient and cost ££££. Use the correct cable - it's cheaper and more reliable.

Edited September 7, 2014 by Chalky

[Alan de Enfield]

.......the fridge is 70w so 5.83amps at startup.....

 

Dont you believe that.

 

Some time ago (as an experiment, and I should have know better) I connected a 75 watt (6.25 amps) fridge via a 10 amp cigarette lighter socket.

 

The fridge started up - no problem - but the entire lighter socket, lighter plug, and cables were a "molten mass"

 

It drew much. much more than 6.25 amps (75 watts) on start up.

Edited September 7, 2014 by Alan de Enfield

[smileypete]

This'll do, £35ish delivered for both 10m of 10mm² red, and 10m of 10mm² black:

 

http://www.ebay.co.uk/itm/2-5-4-0-6-0-10MM-SQ-TRI-RATED-POWER-CABLE-WIRE-RED-OR-BLACK-10M-LENGTH-/141006420570

 

To use smaller cables or converters you have to really know what you're doing, not really worth any potential saving.

 

 

A typical ebay DC-DC converter is about 90% efficient, so straight away you're throwing away about 1/5 of your battery supply in power conversion losses. Also unless you get a good one they won't be able to cope with the start up surge of the fridge. You could get round this with some big capacitors but then you need some form of pre-charge circuit to charge them up slowly. Also the chances are they'll radiate a lot of RF noise so you'll need to screen them carefully unless you want to block out your radio. They'll also need cooling - power electronics don't take kindly to running hot...

 

I've used a few of these myself, pretty good but almost doubled in price recently:

 

http://www.ebay.com/itm/DC-DC-Adjustable-Step-down-Power-Module-LM2596-4-75-24V-To-0-93-18V-04271-/200768739773

 

The more efficient converters usually have much thicker wire on the inductor than normal.

 

cheers, Pete.

~smpt~

Edited September 7, 2014 by smileypete

[nicknorman]

 

This'll do, £35ish delivered for both 10m of 10mm² red, and 10m of 10mm² black:

 

http://www.ebay.co.uk/itm/2-5-4-0-6-0-10MM-SQ-TRI-RATED-POWER-CABLE-WIRE-RED-OR-BLACK-10M-LENGTH-/141006420570

Good price!

 

I've used a few of these myself, pretty good:

 

http://www.ebay.com/itm/DC-DC-Converter-Buck-Step-Down-Voltage-Module-4-75-24V-To-0-93-18V-2-5A-SS-/170848141910

 

The more efficient converters usually have much thicker wire on the inductor than normal.

 

cheers, Pete.

~smpt~

 

Yeabut - that is step down only.

[smileypete]

Yeabut - that is step down only.

 

I originally mentioned another one that was more powerful and step up/down but there was an editing screwup somewhere.

 

ETA: This is the more powerful and step up/down one. Would consider it myself for a lappie but not powerful enough for a fridge and where something off the shelf is needed:

 

http://www.ebay.co.uk/itm/High-Power-Automatic-lifting-pressure-module-LTC3780-Car-PC-Power-Supply-/151261763588

 

 

Used a few of the step down ones for getting 12V/9V/5V from a 15V supply:

 

 

Also used it to charge a tablet at around 2 amps, despite that those little chips don't get too hot, as they have built in mosfets.

 

cheers, Pete.

~smpt~

Edited September 7, 2014 by smileypete

[Phil Ambrose]

When I fitted my 12v fridge having read all the warnings about voltage drop I decided not to sail close to the wind and used 25mm sq, never had any problems, yes cost more but you onlt have to do it once.

Phil

[Chalky]

 

I've used a few of these myself, pretty good but almost doubled in price recently:

 

http://www.ebay.com/itm/DC-DC-Adjustable-Step-down-Power-Module-LM2596-4-75-24V-To-0-93-18V-04271-/200768739773

 

The more efficient converters usually have much thicker wire on the inductor than normal.

 

cheers, Pete.

~smpt~

 

That's the sort of thing. I've used single chip Boost converters based on the LM2587 - about £1.50 each.

 

I'm looking for some cheap buck converters that are current limited at 700ma to drive some power LEDS. Since I last bought some the price has rocketed and the range dropped. Got a couple of cabin light to convert...

[smileypete]

 

That's the sort of thing. I've used single chip Boost converters based on the LM2587 - about £1.50 each.

 

I'm looking for some cheap buck converters that are current limited at 700ma to drive some power LEDS. Since I last bought some the price has rocketed and the range dropped. Got a couple of cabin light to convert...

 

I'd use something like this if space allows:

 

http://www.ebay.co.uk/itm/5A-Lithium-Charger-CV-CC-buck-Step-down-Power-Supply-Module-LED-Driver-New-/201114996886

 

Not the most efficient, but... life is too short. There are tiny LED drivers, though apparently can affect radio reception.

 

That said I've found if heatsinking is good and LED run at lower power, a constant voltage will do fine. Got 4 3W LEDs in series running at 12V on one of the above converters, running about 1.5W each.

 

cheers, Pete.

~smpt~

[tommytelford]

before I had a proper inverter fitted I I bought a 240 volt fridge and a 100watt inverter. Fridge cost less than half what a 12 volt fridge, the inverter was about £30. Short length of 10mm cable to the inverter and a 10 metre length of 1.5mm connected to the fridge, job done total cost about £160

[Tom Richmond]

before I had a proper inverter fitted I I bought a 240 volt fridge and a 100watt inverter. Fridge cost less than half what a 12 volt fridge, the inverter was about £30. Short length of 10mm cable to the inverter and a 10 metre length of 1.5mm connected to the fridge, job done total cost about £160

 

But how efficient is it?

[Phil Ambrose]

 

But how efficient is it?

Very true, often people just go for the cheap option ignoring the best solution.

Phil

[ditchcrawler]

 

But how efficient is it?

Hard to say because you don't seem to be able to get official consumption figures for low voltage fridges.

[smileypete]

Hard to say because you don't seem to be able to get official consumption figures for low voltage fridges.

 

Manufacturer should be able to advise, sometimes they give Ah @ 12v per hr or day, which can be turned into kWh/yr.

 

cheers, Pete.

~smpt~

[tommytelford]

A whole lot more efficient than the gas one I replaced