What amp cable best for fridge?

Submitted: Sunday, Feb 15, 2004 at 00:05
ThreadID: 10534 Views:20752 Replies:10 FollowUps:13
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Hi All,

i have been seraching the forum, with no luck, in regards to the best size wiring to run from the front of the vehicle to the rear of a SWB Patrol for power to an engel fridge. I was going to use 20 amp cable but have read about voltage drop over distance. it would appear from other threads that fridges draw up to 7amps, so is 20 amp enough or is the whole volt versus amp thing different.

thanx all
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Reply By: Wayne (NSW) - Sunday, Feb 15, 2004 at 00:18

Sunday, Feb 15, 2004 at 00:18
I tend to go on size of cable. 6mm twin core. Very important if you run a wire +pos from the battery, then a wire- neg should also go to the battery. Grab a multi meter check voltage at the battery and then check at the end of the cable, if they are the same or very close, right size cable.

AnswerID: 46740

Reply By: Bilbo - Sunday, Feb 15, 2004 at 00:19

Sunday, Feb 15, 2004 at 00:19
I used to a run a compressor fridge in the back of GQ trayback ute. My son's an electrical engineer and advised me to use 20 amp cable - minimum. There was no noticeable effect from voltage drop. In fact, though voltage drop is an effect of wiring length, if you use anything less than 20 amp cable, you will suffer voltage drop because smaller cable can't carry the current without overheating. I'd use 20 amp for any compressor fridge like an Engel. A compressor fridge will draw 20 amps when it's compressor starts up fro rest, after which the current flow will drop to about 40% of it's starting current. Use 20 amp and avoid overheated wiring.
AnswerID: 46741

Reply By: Member Eric - Sunday, Feb 15, 2004 at 00:21

Sunday, Feb 15, 2004 at 00:21
20 amp is not a problem , in the extention lead thats provided , its only 6 ml cable , I ran 400 amp cable to the rear of the car with anderson plugs , I use this to run inverters and do jump starts from the rear of the vehivle , I ran my 20 amp wire off that for the fridge . The only problem you will have with my set up , is somewere to run the cable , I had to do many mods for it to work , I am sure some one here would know the VD over length if you have major concern.
I personaly think that you will have no problems at all .Venus Bay
AnswerID: 46742

Follow Up By: Brett - Sunday, Feb 15, 2004 at 10:38

Sunday, Feb 15, 2004 at 10:38
What cross sectional diameter was this cable?

If you have rubber double insulated flexible cable you will be able to carry 400amp at a 10% duty cycle or 135 amps at 100% duty cycle with 16sq mm cable.These ratings are for the purpose of low voltage AC-DC situations welding, automotive and battery charging systems.

Be aware that the benefits of greater current carrying capacity are not always good. Enginners design elecrtical systems with AC impedance or DC resistance in mind to assist in controlling a severe fault. In short the cable or wire should be matched to the load and the maximum number of amp able to be provided by the supply under short circuit fault conditions.

Say you have a battery rated at 600 cranking amps and you were running your fridge directly from the battery with 6mm sq TWO CORE FLAT, PVC INSULATED AND SHEATHED cable and the frigde or fridge connector developed a fault in the form of a direct short. You would be correct in assuming that the battery will try to dump it's guts into the fault because the unintelligent battery thinks that there is just more load being applied. This can cause a dangerous situation. Fire smoke and equipment damage is possible, but the length of the cable, the conductor resistance and the short curcuit carrying capacity of the cable can provide a degree of protection if matched correctly to the nominal load of the installation. The above scenario would probably see a melted plug and a bit of smaoke. Imagine the same fault carried by 95 sq mm cable which can carry 430 amps 100% of the time and short curcuit capacity will be more than 10 times that, 4300amps +. The battery will continue to feed the fault untill there is nothing left creating a very dangerous situation. A fire will almost certainly occur. A rubbed through cable of this size contacting the inner guard of a car will melt the metal.

If using heavy cables make sure you protect the cable and appliance with some sort of current limiter (fuse/ circuit breaker etc) as close as possible to the supply to avoid this type of problem.

In addition, when purchasing cable, be aware that cable is rated at carrying a certain current at a certain maximum conductor temperature (V75 and V90 are common and refer to the temp in degrees Celcius this relates only to the cable and not the method of termination or connection..terminals melt too).
The rating of cable is also determined by the installation method. Spaced, spaced and touching, exposed to sun, open air, installed in conduit are all factors which do change the rating of the cable and it's ability to shed heat. Consider the engine bay temperatures of the average motor vehicle. It could be possible that a cable could be 40 or 50 degrees or even more before it begins to carry current.......... food for thought.


FollowupID: 308725

Follow Up By: Member Eric - Sunday, Feb 15, 2004 at 12:49

Sunday, Feb 15, 2004 at 12:49
Thanks for your detailed responce , i have insulated moth cable individualy and bent any metal withing its visinity and insulated that aslo . I however didn't think of fusing this cable , thanks for that, I will organise one MondayVenus Bay
FollowupID: 308741

Reply By: shortgq - Sunday, Feb 15, 2004 at 00:38

Sunday, Feb 15, 2004 at 00:38
Thanks for all your replys, it has given me the answer I require, much appreciated.

AnswerID: 46745

Reply By: Member - StevenL - Sunday, Feb 15, 2004 at 00:44

Sunday, Feb 15, 2004 at 00:44
I had TJM wire me in a Hella point in the boot to run a 40l Weaco fridge via a Power Pack. They used 6mm cable and wired it into the ignition so that it is only active when ignition is on. $125 including parts. Works a treat. The dealer that sold me the fridge pointed me to TJM and organised the price for me. He then put a multimeter on the rear point when it was done and said that there was minimal voltage drop from the battery. Fridge has been running great since then. Beer is cold and that's all that matters!

When I get the new Prado I will be have my fridge dealer checking the performance of the factory rear outlet. (He is actually a Marshalls Battery outlet that sells fridges as a sideline but offered the best price I could find).

StevenL Playdoe GXL TD Manual
It's on order, Delivery in April '04.
This pic will have to do till then. Can't wait!!!
AnswerID: 46746

Follow Up By: Phil G - Sunday, Feb 15, 2004 at 08:09

Sunday, Feb 15, 2004 at 08:09
Gday Steven,

Just interested in the method the dealer used to measure voltage drop. Putting a multimeter in the socket is useless unless its a second socket and your fridge is running from the first socket. You need maximum current flow through the cable to measure a voltage drop.
FollowupID: 308701

Follow Up By: Brett - Sunday, Feb 15, 2004 at 10:41

Sunday, Feb 15, 2004 at 10:41
You need not use the fridge. A carbon pile rheostat used to test batteries or a poewr resister can provide the appropriate load to make an accurate voltage measurement. Much easier than pulling the fridge apart.
FollowupID: 308727

Follow Up By: Phil G - Sunday, Feb 15, 2004 at 11:00

Sunday, Feb 15, 2004 at 11:00

Yeah, can't say I've ever pulled the fridge apart to measure voltage drop!!! If you install more than one socket, simply measure the other socket while the fridge is running, alternatively, unscrew the fridge plug and measure while running.

I think its best to measure it in action. That way you detect any resistance problem with the fuses, connectors etc as well.

FollowupID: 308729

Follow Up By: Phil G - Sunday, Feb 15, 2004 at 11:03

Sunday, Feb 15, 2004 at 11:03
Whoops, I meant unsrew the socket not the plug :-)))
FollowupID: 308730

Follow Up By: Brett - Sunday, Feb 15, 2004 at 11:27

Sunday, Feb 15, 2004 at 11:27
Good Idea Phil.
FollowupID: 308731

Follow Up By: Member - StevenL - Sunday, Feb 15, 2004 at 12:41

Sunday, Feb 15, 2004 at 12:41

One thing I did mention is that I use a Weaco power pack that is plugged into the point. The fridge then runs all the time off the power pack.

Whether the power pack draws differently than if the fridge was plugged in direct I am not sure.

When the Marshalls bloke tested it I remember he did not use his regular little multimeter but another larger one which then displayed 12+ volts available at the rear point. The fridge was in the lounge room at home at the time! He just said that TJMs had done a good install job and that I would have no troubles. Maybe this thing was a resistor or rheostat as suggested by Brett? As a battery dealer I would hope he would have the right tools for the job!

So far in many thousands of Kms touring it has performed flawlessly so he must know what he is talking about. I am not an auto electrician so that's why I took his advice and is why I will be seeking his opinion on the rear point in the new Prado when it is delivered. I've already spoken to him and he is only to happy to help out.

StevenLPlaydoe GXL TD Manual
It's on order, Delivery in April '04.
This pic will have to do till then. Can't wait!!!
FollowupID: 308740

Reply By: Phil G - Sunday, Feb 15, 2004 at 08:24

Sunday, Feb 15, 2004 at 08:24
Why are we worried about voltage drop? So what if it measures 12.0 volts at the fridge instead of 12.8?

Well, to answer my own question, Danfoss fridges such as Waeco incorporate a low voltage cutout and Engel owners may fit them after. If you're losing a volt while the fridge is cycling, then your low voltage cutout will cutout much earlier than it should, making you think the battery is down. Good for the battery, frustrating for the owner. Also, there is a little lost efficiency of the fridge being supplied a lower voltage. Can also lose voltage at the cig socket, hence the benefit of better socket or connectors.

Also, out of interest, my latest 40 litre engel draws 5 amps when cycling. 52 litre Waecos draw 5 amps as well, unless in "turbo" mode which increases it to 7 amps.

Like the others said, 20 amp cable is fine over a 4 metre length for a 5 amp draw.
AnswerID: 46751

Reply By: ThePublican - Sunday, Feb 15, 2004 at 10:40

Sunday, Feb 15, 2004 at 10:40
Shortgq, the waeco fridge handbook has a graph of recommended wire xsection in mm over cable length in mtr,,,, to minimize voltage drop over a cable length of 4 to 6 mtre a minimum xsection of 6mm is recommended ,increasing xsection by 2mm per 2mtr,,upto 10mm xsection at 10mtr and longer. Hope this clears some cofusion.
AnswerID: 46761

Follow Up By: shortgq - Monday, Feb 16, 2004 at 19:16

Monday, Feb 16, 2004 at 19:16
Plain english! THANKYOU..
FollowupID: 308918

Reply By: CHRIS - Sunday, Feb 15, 2004 at 16:15

Sunday, Feb 15, 2004 at 16:15
I ran a 240 Volt 3 core standard power cable from my battery to a socket in the back of the Cruiser to run my WAECO 45 ( with 20 amp inline fuse). Plenty of insulation ---nice and flat, and no voltage drop. Not many people realize that the WAECO also has a 8 amp fuse in the plug. Also the cable is cheap to buy and if it runs household fridges and other appliances off the one circuit with no trouble Im sure it will run a lonely little Engel easily.
AnswerID: 46787

Follow Up By: Phil G - Sunday, Feb 15, 2004 at 17:40

Sunday, Feb 15, 2004 at 17:40
Sorry Chris,

Doesn't work like that. 240volts AC is a different animal. The reason why your house is wired with single strand wires carrying alternating current is that voltage drop is minimised with AC.

You're using AC wire for a circuit needing 7 amps DC. Yes you will have voltage drop. Every wire has voltage drop, especially house wiring used for DC.
FollowupID: 308776

Reply By: Rob - Monday, Feb 16, 2004 at 12:01

Monday, Feb 16, 2004 at 12:01
Using house wire in a mobile situation is DANGEROUS !!! This type of wire is not flexible enough to take the constant vibrations. Especially the single stranded stuff. It will eventually break and/or create a high resitance joint and cause a FIRE!!
IMHO as an electrician.

AnswerID: 46872

Follow Up By: Brett - Monday, Feb 16, 2004 at 12:46

Monday, Feb 16, 2004 at 12:46
TPS should stay in houses.
FollowupID: 308878

Reply By: shortgq - Monday, Feb 16, 2004 at 19:34

Monday, Feb 16, 2004 at 19:34
Thanks all for the contribution. I believe I have made myself a wiring loom that will be effective and most of all safe, thanks to all info supplied.

From my aux battery, I am using 14mm 125amp Narva cable for about 800mm were the wire then splits into 6 individual 25 amp (6mm) cables which then plug into a Narva 6 way blade fuse box. From there I will be picking up my power for different accesories inc fridge, running different size cable and fuses to each accessorie as required by it's power output. I have run 25 amp cable to the rear for the fridge and wiring in a flush mount marine socket, which is 16amp (the best i could find). HEAPS of insulation used everywhere and then all wrapped in convuluted sheath.

If there are any flaws in my design, please comment.

AnswerID: 46940

Follow Up By: Hoffy - Tuesday, Feb 17, 2004 at 14:15

Tuesday, Feb 17, 2004 at 14:15
This is a site that gives a good insight into the Auto Electrical cables. Collyn Rivers has a very good understanding of the electrics and cable sizes and explains how to find the correct size cable for the purpose. It makes good reading also. The URL is


FollowupID: 309016

Follow Up By: shortgq - Tuesday, Feb 17, 2004 at 19:59

Tuesday, Feb 17, 2004 at 19:59
Thanks Keith,

That is a very good website, i am happy i am not going to have a meltdown. Cheers..
FollowupID: 309063

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