wire / cable size calculations

Submitted: Saturday, Feb 23, 2008 at 20:38
ThreadID: 54859 Views:21406 Replies:6 FollowUps:17
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To those sparkies out there ...I understand that when calculating the correct size to use it is the entire circuit to use for length calculations that is.... positive feed and the earth as well....and I have alwyas done so with good results on past systems using available tables for calculations

as stated, in the past I have always run a seperate earth for all accessories however I am about to run the new circuits on new LC purchase and I will be installing A PL20 with a shunt in the negative line in a rear canopy of the LC.........the batteries to be charged by the alternator and the solar ....the PL20 will control a low voltage disconnect (via relay switching the positive) and it is being fed info of amps in / out via the shunt etc so it knows SOC as well

I have been encouraged by Plasmatronics (PL 20 manufacture) to use the chassis as the earth in order to pick up all potential sources of use so.....I have a good handle on all of that from past experience.....

this leads to my question .

IF i am use to use the chassis as the earth for all accessories as against a separate earth feed.......is the cross sectional area of the chassis sufficient in size to negate the need for inclusion in "wire length"...

that is, do I simply calculate cable size using the length of the positive feed or do I add some factor in for a "nominal length"

I hope that is clearer than mud......thanks
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Reply By: Member - Graham H (QLD) - Saturday, Feb 23, 2008 at 20:58

Saturday, Feb 23, 2008 at 20:58
Have no idea but these people will


http://www.springers.com.au/default.aspx
AnswerID: 289029

Follow Up By: jskogsta - Saturday, Feb 23, 2008 at 21:20

Saturday, Feb 23, 2008 at 21:20
Thanks for that.. the Patrol GU panel and the general one is actually 'cheap' compared to other places I've seen them. ;-) Might be looking to buy one of them... talking about this one: http://www.springers.com.au/Products.aspx?ProductID=587.
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Reply By: Dunaruna - Saturday, Feb 23, 2008 at 23:53

Saturday, Feb 23, 2008 at 23:53
The answer to your question is yes.

A bolted (not self tapper) chassis earth offers far less resistance than a wire earth so your return path calculations need to change. Calculate for a single wire resistance and add a nominal amount (as you already suspected).

What I like to do is connect a chassis earth AND still run a return earth back to the battery, that way you have created an almost failsafe circuit.

BTW, the PL20 is an excellent unit.
AnswerID: 289043

Reply By: Bonz (Vic) - Sunday, Feb 24, 2008 at 08:16

Sunday, Feb 24, 2008 at 08:16
Out of interest, what size do your calculations tell you to run? IMHO for a cab-chassis or normal wagon the lengths of the run are not long hence the length really isn't a worry, its more the current carrying capacity of the cables. I ran 6mm to the back of the Patrol for accessories (active and neutral) and everything runs well but I wouldnet be taking more than 20 amps at any one time.

In my old memory 2.5mm is good for 28amps so 6 mm is easily big enough. If I was charging batteries back there I would run 16mm or 25mm (16mm is around 70amps by memory). Battery to battery under the bonnet I ran 35mm so I could start from the aux battery.

Anyways what'd you end up running?
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Follow Up By: Mike Harding - Sunday, Feb 24, 2008 at 08:31

Sunday, Feb 24, 2008 at 08:31
>(active and neutral)

Got a 240V alternator and of them new fangled AC batteries eh? :)

>2.5mm is good for 28amps

Are we talking about 2.5mm CSA and not this stupid bloody "automotive" wire sizing? The Farnell book informs me that 2.5mm switchgear wire to BS6231 is good for 29A and 6mm CSA is good for 50A but I doubt the public can buy it by the metre? See if you can find a friendly electrician to donate a few metres. I'd go with 6mm too Bonz.

Mike Harding
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Follow Up By: Bonz (Vic) - Sunday, Feb 24, 2008 at 08:34

Sunday, Feb 24, 2008 at 08:34
my background in the electricity industry belies my knowledge

Of course I mean positive and negative hahahah

I mean from AS3000 and 30 yrs ago when I was studying it
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Follow Up By: Dunaruna - Sunday, Feb 24, 2008 at 10:24

Sunday, Feb 24, 2008 at 10:24
Current flow (amps) is directly related to voltage, the more voltage you can produce at the end of the circuit will determine max amps. Of course this comes back to wire size/construction & quality connections.

Auto 6mm (which is 4.5mm sq) is usually more than enough although if you want to future proof the circuit use 6mm B&S (true 6mm sq).
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Follow Up By: Mike Harding - Sunday, Feb 24, 2008 at 14:27

Sunday, Feb 24, 2008 at 14:27
>Auto 6mm (which is 4.5mm sq)

I was under the impression that automotive wire was sized by its outside diameter only ie. 10mm wire requires a 10.1mm hole to pass through and that's the only information one can derive from the given size. It's conductor size could be anything and has no relevance to the given size - is that not correct?

Mike Harding
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Follow Up By: Dunaruna - Sunday, Feb 24, 2008 at 18:36

Sunday, Feb 24, 2008 at 18:36
Your impression is correct (in Australia only). The 4.5mm sq that I quoted was the actual wire size not including insulation. Automotive wire sizing in Australia is decieving, always look for actual wire size (it's usually on the packaging). To make it even more confusing, battery and starter cable does not include the insulation in the sizing. A 6mm auto wire and a 6mm battery wire are different sizes!
Alternatively, use marine wire, it's rated correctly and being tinned it handles more current.

Mike, are you related to Greg or Tom?

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Follow Up By: Mike Harding - Sunday, Feb 24, 2008 at 19:14

Sunday, Feb 24, 2008 at 19:14
>The 4.5mm sq that I quoted was the actual wire size not including insulation.

But there is no consistency or standard for that in Oz (or any other country - unless you know one?) so your 6mm auto wire with a 4.5mm CSA conductor may not exist anywhere outside your particular piece of cable? ie. My mate Fred with a roll of 6mm automotive wire may only have a conductor size of 1.5mm CSA or, indeed, 0.5mm, CSA - this silly "automotive" measurement has no standard basis.

My understanding is that automotive wire sizes are a heap of dingos kidneys and not, in any sense, related to proper engineering cable descriptions - and we won't even mention temperature, cable bunching or the conduit multiple carriers are run in.

Mike Harding
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Follow Up By: Member - Mike DID - Sunday, Feb 24, 2008 at 19:30

Sunday, Feb 24, 2008 at 19:30
Australia seems to be the ONLY country that accepts this deceptive auto wire sizing - at least up to 6mm Outer Diameter over the insulation.

Above that size the AWG (American Wire Gauge - same B&S=Browne & Sharp ) is used, that specifies cross-sectional area !
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Follow Up By: Dunaruna - Sunday, Feb 24, 2008 at 19:48

Sunday, Feb 24, 2008 at 19:48
The yanks most definetly have a standard (AWG - American Wire Gauge) and the Europeans have recently decided on the sq mm system for automotive and marine.

It's only downunder that has this stupid system of measuring the total diameter only (not sq) including insulation.

All the wires I have been using have the actual core diameter squared listed on the roll, most 6mm is somewhere between 4.5 & 4.8mm sq but I take your point about no standard. It pays to check.

So what about Tom or Greg?
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Follow Up By: Mike Harding - Sunday, Feb 24, 2008 at 20:24

Sunday, Feb 24, 2008 at 20:24
>So what about Tom or Greg?

Qui?

Mike Harding
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Follow Up By: Dunaruna - Sunday, Feb 24, 2008 at 20:55

Sunday, Feb 24, 2008 at 20:55
I know a Greg Harding who was (until recently) into caravan repairs. Wondering if you were related?
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Follow Up By: disco1942 - Sunday, Feb 24, 2008 at 23:26

Sunday, Feb 24, 2008 at 23:26
For more on this subject, go to the link I posted in FollowUp 1 in Message 5

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Follow Up By: Mike Harding - Monday, Feb 25, 2008 at 05:56

Monday, Feb 25, 2008 at 05:56
>So what about Tom or Greg?

Nope, don't know them.
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Reply By: Member - Mike DID - Sunday, Feb 24, 2008 at 08:31

Sunday, Feb 24, 2008 at 08:31
On a Pajero with monocoque (integrated chassis) construction, I measured the resistance from battery (front left) to the rear right to be the same as 4 Gauge B&S wire run over that distance.

If you have a separate chassis, you would need to bond body and chassis at front and rear, to get the same sort resistance.
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Follow Up By: Dunaruna - Sunday, Feb 24, 2008 at 10:18

Sunday, Feb 24, 2008 at 10:18
How did you measure the resistance over such a distance?
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Follow Up By: Member - Mike DID - Sunday, Feb 24, 2008 at 15:12

Sunday, Feb 24, 2008 at 15:12
I connected a 100 watt lamp at the back and measured the voltage drop.
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Reply By: Keith_A (Qld) - Sunday, Feb 24, 2008 at 10:34

Sunday, Feb 24, 2008 at 10:34
Question well asked - and answered.
You probably already know of this site - but if not - worth a look ;
Site Link

regards........................Keith
AnswerID: 289083

Follow Up By: disco1942 - Sunday, Feb 24, 2008 at 23:23

Sunday, Feb 24, 2008 at 23:23
A better link that will take you directly to the article on Wiring Woes and Auto cable

There was discussions on the amount of copper in a 6mm cable in a long series of FollowUps above. In this article Collyn quotes 5 different amounts of copper found in typical 4mm cables - the lowest was 1.25squmm. Also in the article is the formula for working out the voltage drop if you know the length of cable you are going to use.

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Reply By: disco1942 - Monday, Feb 25, 2008 at 00:38

Monday, Feb 25, 2008 at 00:38
bungarra

Have you received full instructions from Plasmatronics regarding the connection of additional charging sources to (vehicle alternator) to your solar system? If Plasmatronic (and Steca as well) regulators are not wired correctly then the State Of Charge (SOC) function of the regulator will not function properly (be totally useless.)

There is a good article on how so called experts have wrongly wired up motorhomes - Plasmatronics Regulators

You are talking about using a chassis earth for all the accessories that are forming the loading on the battery. You are also about using the alternator also for charging your batteries. (As you are talking a shunt you may be aware of the problems posed here, but I will continue for the benefit of others reading this.)

The regulator has 3 negative terminals. One for the solar panel, one for the battery and one for the load. If you connect more than one of those to earth you upset the operation of the regulator. This applies under normal simple operation. However if you wish to use an inverter or other load that is bigger than the rated load current for your regulator you must use an external current shunt to increase the load current rating of the regulator.

There is a good article on how so called experts have wrongly wired up motorhomes -

I have seen additional external circuitry to allow an external battery charger used to charge the batteries in addition to the solar cells. Is this what you are intending to use for your generator? May bee you can tell us what Plasmatronics are proposing that you do to allow your alternator to be used for supplementary charging without upsetting the SOC monitoring.

I would advise against using your alternator to assist in charging your battery when there is good solar charging taking place. Your regulator will charge your batteries at over 14V. When your vehicles motor is up to working temperature, The alternators output voltage will be less than 14V. Your battery will charge faster with an applied voltage over 14V than it will with less than 14V.

Consider the case where your vehicles electrical system is connected to the auxiliary battery system and the motor has run for half an hour. If some external power source that is greater than the alternators regulator thinks the system should be running at, the regulator will reduce the output current to reduce the system voltage to its normal level. This means that your solar system will have to make up the shortfall in current to the car that the alternator is now not giving out. This will hold the solar system voltage down and the battery will not receive as much charge as it would if the vehicle system is disconnected.

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Follow Up By: Member - bungarra (WA) - Monday, Feb 25, 2008 at 17:56

Monday, Feb 25, 2008 at 17:56
Hi disco 1942

Thanks to your and also to all the others that took the time to write a reply

I believe I understand and are conversant in wiring and using a PL20 togther with solar and alternator charging as I have the identical set up in my off road camper trailer. I wired that up a few years ago when I "inherited" a poorly designed system at purchase..works perfectly

I guess I did not explain more clearly that the alternator and solar will never charge simultaneously. Either one doing so or the other. When camped for extended periods then obviously solar is the sole charging source and is in balance with usage (actually insurplus).

The alternator system is used because when driving most of the day, camping overnight (2 fridge, 1 freezer) then driving all day then camping etc the battery bank will run down before we get to our ultimate destination when we ultimately park up and camp.

On the camper setup I ran a separate earth wire from the alternator to the back of the vehicle etc via an anderson plug.

It was a comment from Plasmatronics that suggested I use the vehicles chassis as the earth return that led to my question of wire size calculations because of the cross sectional area of the chassis

For the record the PL20 and the bi directional shunt togther with the shunt adaptor has enabled the existing system to work a treat and I am simply repeating the sytem again and maybe saving a run or runs of wire for the earth

I use the negative output from the PL20 to switch a relay on the positive side of the accessories supply

Thanks for your interest and input





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