wiring for fridge

16mm² if completely made up of copper would be gross overkill.

I was going to chastise you for being so wasteful but I did some maths.

The narva cable I specified is made up of 57 strands of 0.32mm diameter copper.

0.32 mm diameter copper has a cross sectional area of 0.0804mm².

57 x 0.0804mm² equals 4.58mm².

This has a diameter of about 4.8mm and some of the middle bit is obviously air.

But a 4.8mm diameter solid conductor has a cross sectional area of just a little more than the 16mm² you were suggesting, at about 18mm².

You couldn't use solid copper (like house wire in an automotive application) you have to use a stranded cable for flexibilty and vibration resistance.

A 16mm² cross sectional area STRANDED cable made of 0.32mm dia strands would have a strand count of about 200 strands and a need a cross sectional area of about 63mm² allowing for the same air wastage between conductors (if my fuzzy night shift brain is working) and therefore a diameter of about 9mm plus insulation - about 11mm. This is getting up into starter cable territory capable of carrying hundreds of amps over reasonable distances.

My conclusion is that we were probably recommending one and the same thing in a different fashion. Correct me if I am wrong.

As someone once said on TV (so it must be true) Oils ain't oils. And cables definately aint just cables.

The moral of the story is that the only way to refer to cable is total cross sectional area of conductor. Not diameter and not overall area.

Dave

Hi Dave

You totally know more about it than I do, I simply read the printing on the out side of the roll of wire i have. hahaha But whta you are saying makes sense.
Total cross section is about 10mm, wire is about 8mm

cheers mate

Adam

Deano

Run your earth wire from the battery as well.

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geocatcher,
My Waeco hand book told me the 6mm twin sheathed auto cable was not large enough for a five metre round trip from battery to fridge. I was intending to get the 4mm twin sheathed auto cable and double it up. It's cheaper than buying 8mm cable.

Prado 4 x 4,
Shouldn't the Waeco and Engel plug be able to fit straight into the hella/narva socket without an adapter.

2 x 4mm doesn't come close to 8mm for conductor cross sectional area. It's not a linear relationship.

6mm narva cable is 4.58mm² and is plenty for your application.

At 2.5 amps the voltage drop allowing for supply and return cable run and factoring in crimp terminals is well less than 3% of supply voltage which is the acceptable loss in such a system.

There's a lot of confusion in discussion around automotive cable applications as the diameter referred to in automotive cable refers to the outside diameter including insulation. It bears no reference what so ever to the quantity of conducting material available in that cable.

To illustrate this point compare a set of $10 jumper leads from super cheap or kmart or the reject shop with a set of Matson ones. You can tell just by a weight comparison but if you cut both cables the crappy chinese ones will be 95% insulation 5% copper and the matson ones will be 95% copper and 5% insulation. They'll probably be thicker too.

If you do the test - jump start a flat V8 with both o - one will work the other will not.

That is why I specified cable by part number and not just diameter. I know the specs on the Narva cable as I use it myself when setting up other peoples fourbies and I use it in my own running my Codan 9323 - pulling 25 amps and sensitive to low voltages. It runs the Codan just fine. It'll definately run a fridge pulling 10% of the current.

Practical application and theory back my recommendation.

I didn't spend nearly 10 years in GM's Engineering Dept as an Electronic Tech without learning a few things.

I wasn't always an Ambo.

Dave

Dave,

I don't disagree with your numbers except my Waeco hand book says I need 8mm2 cross section to run a 5m cable. ie the 4.58mm2 (your calculation) that the 6mm twin sheathed cable will give you would not be sufficient. Appologies my original response was not quite specific.

The doubled up twin sheathed 4mm cable gave a total of about 7mm2 cross section when I did the radius squared x pi (sorry don't know how to do fancy characters) to work out the area of the circle. I figured this was close enough and much cheaper than buying equivalent cable with an 8mm2 cross section.

Any idea where you think the discrepancy could be between your understanding that 4.58mm2 should be sufficient when the waeco book says otherwise? Are my figures flawed?

I have tried the jumper lead experiment in the past and have a half melted set as a momento. The flat did eventually start. Now have a good set of jumpers.

Spade

You can't use the diameter though. In automotive cable it includes the insulation.

Narva 4mm cable has an actual copper cross sectional area of 1.85mm². Double it and you have 3.7mm². The cable I recommended has a cross sectional area of conductor of 4.58mm².

Cable cost is basically proportional to weight of copper. If it is cheaper to double up the 4mm that would nearly always mean there's less copper in it. The copper is the expensive bit that you are buying after all. The plastic is just there to stop it touching anything else.

Even recalculated based on the current draw of 3.7Amps for a Waeco which seem high but given that Waeco don't quote a current draw on their fridge specs on their website (fairly useless technical specs given) I'll go with it. The loss is about 1.5% of supply.

Go for bigger cable if you feel you have to but I hope you're sitting down when they tell you the price of 8mm cable. Then check the actual copper area and make sure you aren't just buying more insulation.

If it were mine I'd set it up the way I've recommended. If you were paying me to do it I'd do the same and guarantee it.

Dave

If you could humour me for a bit longer would be appreciated.

The 4mm cable I have is a brand called Ziel. Could not find a quoted cross section measurement. The diameter of the copper is 2.1mm as best as I could measure it. I worked out the cross section as 1.05mm x 1.05mm x 3.1415 = 3.45mm2. Doubled = 6.90mm2. Many years since doing maths at school, but I thought I was going pretty well.

By my calculations (assuming my sums aren't flawed) a 1.85mm2 cross section would have a diameter of 1.5mm.

Maths check here would be appreciated.

My multimeter shows a draw of 3.2.

I already had the 4mm cable hence my plan to use it. Will buy the 6mm on your recommendation. A lot easier than doubling up.

Spade

All true spade but it's not solid copper (I hope - if it is it's not good for automotive use) it's made up of a twisted bunch of smaller strands.

The strands have to have air filling the gaps and air doesn't conduct electricity (lets not mention lightning & stuff.)

To work out the true cross sectional area you need a good vernier or micrometer. Find the thickness (diameter) of an individual strand and do an area calculation on a circle of that diameter.

Then count how many strands.

(No. of strands) multiplied by (Area of one strand) = total.

It will be much less than for a solid copper conductor of the same overall diameter as the bunch of strands.

Dave

Dave,

Now we have reached that stage in the discussion where I have to think up something real clever because I now have no further facts or figues to back up my arguement.

On second thoughts I think I might just concede.

Thank you for the information here and helping me understand. Your position (whether true or not LOLOL) sounds convincing enough for me.

Spade

Won't type much - I'm on the pda.

There sre many good reasons to run the earth as well.

Not running it can set up ground loops, induce electrical interference - particularly on HF, and cause other headaches.

Body movement, dust & wear & tear all have an effect on that earth over time and it won't remain as good necessarily in the future.

You'd need some pretty sensitive gear to measure resistance that low - how did you do it?

Deano, run the neg supply from the battery.

It is extremely poor electrical practice to do otherwise.

Dave

Mike,

Lovely to hear from you. Such attitude is rare and hard to come by - in person where as it's rife in these forums unfortunately. Your arrogant rude condescending tone, criticising someone qualified to make the comments they did is becoming far to common on this site. The fact that you were so ready and willing to flame away at me without stating your own expertise in the area is noted.

Running HF gear it is amazing to hear how much faith you have in manufacturers earthing. All the guys on the codan forums, and the amateur radio buffs must have rocks in their head chasing sources of interference and bonding bonnets to bodies, chassis to body, rear door to bodies, motor to body & chassis etc. I will pass on to them that you assure us that the interference they hear is a figment of their imagination and they need not spend any more hours or days solving it. They will be thrilled.

Find me a manual from Engel or Waeco that assures us that running a single feed is enough. For that matter find one from a reputable radio manufacturer like GME or Uniden that doesn't suggest doing it my way for optimum performance. Even Nokia suggest it with their car kits. But hangon a minute, hold the phone, Mike DiD said that all of their millions of dollars of engineers and designers are wrong.

You must stay on the bitumen because I am already aware of GQ's, GU's and 100 series that have been on trips this year and have required spot weld repairs, and many of our modern vehicles either in 4wd or 2wd have bits like firewalls glued in with minimal if any spot welding on some parts - particularly in 2wd and softroader vehicles. The use of adhesives which have been found to be stronger than welding and more resistant to fatigue due to twist and deformation must do wonders for earthing.

Particularly in a set up like mine where the 12v accessories are all running in the load area of the ute it becomes even more important not to earth to body work cos guess what - shock horror - the tray is bolted on to the chassis. Deano did not specify the sort of vehicle and my answer is technically correct for all vehicles. Your answer is fairly specific to your vehicle. I beleive I would be correct in assuming that your extensive "actual" testing involved exactly one vehicle - that is what your post suggests.

Your pajero is a different case to many, particularly due to the fact that it doesn't run a chassis and body as do many other vehicles in the large 4wd market and you probably have a better body return earth than most fourbies. Doesn't make it correct for everyone else though.

The OEM devices that need to be earthed to a point at or extremely close to the battery already are earthed that way in modern vehicles. Manufacturers spend millions of dollars engineering wiring harnesses to meet stringent specifications for susceptability and generation of RF interference and to try to save money by eliminating needless length. You have no way of knowing where the wires end up once they disappear into the harness.

Every other electrical device I have added to my vehicle is wired in a fashion that is technically correct regardless of whether it has been run off the aux or main battery. So no I won't be rewiring it - I did it right the first time.

Mike, what electrical qualifications if any do you hold?

A little bit of knowledge is a dangerous thing. Until proven otherwise I figure you fit the "little bit" category.

I rarely fire up on forums like this but the attitude and accusations you implied really pi$$ed me off.

Dave

"Mike, what electrical qualifications if any do you hold? "

- 35 years of designing, building, testing and using a large variety of electronic and computer equipment. Honours Degree in Electrical Engineering from Sydney University.

I don't need any qualifications to see how all electrical equipment in modern vehicles is earthed - to the body. Even when Pajeros had a chassis, it was earthed the same way.

"Your arrogant rude condescending tone, criticising someone qualified to make the comments they did is becoming far to common on this site. The fact that you were so ready and willing to flame away at me without stating your own expertise in the area is noted. "

I least I have learnt to accept people criticising my ideas or saying they are wrong without resorting to calling them rude, arrogant or flaming.

That's the way I read it.

I stand corrected on your qualification.

Running a positive feed and negative return is still the best way of ensuring minimal loss due to factors beyond control.

If you reckon that your response wasn't rude, arrogant or flaming maybe I misinterpreted it. If you had said the same thing over a beer with a smile on your face and a relaxed body language it may have come across differently.

The way you typed it left very little room to move on interpretation.

I can agree to disagree with you on this one.

I have spent too many years in automotive engineering chasing electrical problems to agree with you.

Dave

From another forum:
It might just be a crook poem
But the messages are clear.
Lets take our arguements elsewhere
And not have them in here.

"The trouble with this email crap
We cannot see a face
You might just be an alien
Come in from outer apace

Did you just insult me
Oh yeh I rekkon so
Do you think I'm just a dill
Oh maybe rekkon slow ?

I noticed hesitation
Your messages are cold
Maybe you meant the bit about
My memory gettin old

How dare you tell me something
I've known for years and years
I cannot hide frustration
I'll burn ya blasted ears

Ya mongel dog you're whingin
I never asked for this
I rekkon you're one dillbrain
I'm never gonna miss

But wait; I see the smilie
Maybe I spoke too soon
I wish that I were somewhere else
Maybe upon the moon

I'm sorry mate
I couldnt see
the smile upon your face
I need a life
To go get out
And leave this blasted place

The trouble with this email crap
We cannot see a face
You might just be an alien
Come in from outer space"

Nah, mainey yer a deakhid... :-((
-> deano8it posted about wiring for a FRIDGE !

What does the fridge manufacturer recomend?
I would still go a bit large, min maybe about 16mm sq for the fridge.

hope I have not offended no 1 :-)