wire size variations at different points in a circuit

Submitted: Friday, Apr 16, 2010 at 01:30
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Any one have a view or better still, specific knowledge.

I am getting a hella plug in the wagon connected to the auxilliary, mostly for Waeco 80l. The positive cable, around 3 metres, will be doubled up 6mm cable (red and black from twin sheath joined together - possibly more than needed). The negative will earth to the car chassis at a factory earthing point. The auto leccy reckons the negative cable to earth (about 300mm long) need only be half the size of the positive cable that we are using.

The negative from the engine to the battery is decent size so why would it be OK for there to be 300mm of something smaller anywhere in the system.

Is the chain only as good as its weakest link? I always thought it was.
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Reply By: fisho64 - Friday, Apr 16, 2010 at 02:23

Friday, Apr 16, 2010 at 02:23
The negative from battery to engine handles all current so must be big enough to cope with the total current draw ie starter motor plus anything else that happens to be on. It is common to any circuit that is earthed thru the chassis.

The Waeco power is a separate circuit that doesnt need to carry starting amp draw. Resistance increases over the cable length so he may be right in what he is saying HOWEVER

doesnt he have any suitable cable to avoid having to double up the red/black (and confuse future fault-finding), and why does he want to save a few cents on a piece of cable 300mm long? Is he going to split the red/black and throw away the other 300mm offcut from the negative??

Seems a bit odd that he is quibbling over something so insignificant? Id have thought that the time it would take him @ $60+ an hour to get another roll of wire out and cut 300mm off would negate the saving?
AnswerID: 413093

Follow Up By: Fatso - Friday, Apr 16, 2010 at 10:25

Friday, Apr 16, 2010 at 10:25
You have to wonder eh Fisho, how these blokes put things into perspective.
$60/hour is what they would charge for the first year apprentice. Oh, plus GST.
The wire is probably 60 cents/metre difference in price.
Labour to do the job $180.
Earth wire $2.20
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Follow Up By: Spade Newsom - Friday, Apr 16, 2010 at 17:47

Friday, Apr 16, 2010 at 17:47
fisho and fatso, I already have the wire and it was free so this is not an issue, more of me trying to understand a bit more in depth how voltage drop works using different guage wire across a circuit. I found it difficult to accept his advice that using 300mm of smaller guage wire would make negligible difference.

It does make the earthing point on the chassis look tidier and a little easier to work on with a single wire rather than twin sheath.

I guess I will run with it.

Thanks for the comments.
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Follow Up By: fisho64 - Friday, Apr 16, 2010 at 21:02

Friday, Apr 16, 2010 at 21:02
On closer examination-the earth and the positive wire are part of the same circuit. Using 300mm of smaller wire for the earth would have exactly the same effect as putting 300mm of smaller wire ANYWHERE in the circuit. All the electrons must pass along the wire and thru the appliance then complete the circuit.
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Reply By: Member - Toyocrusa (NSW) - Friday, Apr 16, 2010 at 06:18

Friday, Apr 16, 2010 at 06:18
Hi. All replies from the gurus on this forum say you should have 6mm squared wire size both pos. and earth. The earth should always run back to the Battery as well. An 80l Waeco draws quite a bit of current so I have followed their advice with my wiring and no problems in 2 years so far. Bob.

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Follow Up By: Maîneÿ . . .- Friday, Apr 16, 2010 at 07:42

Friday, Apr 16, 2010 at 07:42
Bob,
I'm on record as recommending 10 mm sq cable
maybe using smaller 6 mm sq as a 'minimum' in very short runs only, and both pos & neg should be identical cable.

Yes, as you say the "earth should always run back to the Battery"

Maîneÿ . . .
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Reply By: Olsen's 4WD Tours and Training - Friday, Apr 16, 2010 at 06:20

Friday, Apr 16, 2010 at 06:20
DC resistance
The resistance R of a conductor of uniform cross section is a function of the conductor length and cross sectional area. So if the run is much shorter, the conductor can be thinner for the same resistance. Normally one would not bother because having lower resistance can only be helpful, however in your case, since you are doubling up one wire, not doubling up the other represents a small saving of time and money, so using a thinner (not doubled) wire for the negative makes sense.
AnswerID: 413100

Follow Up By: Maîneÿ . . .- Friday, Apr 16, 2010 at 07:46

Friday, Apr 16, 2010 at 07:46
"however in your case, since you are doubling up one wire, not doubling up the other represents a small saving of time and money
so using a thinner (not doubled) wire for the negative makes sense"

Bet you were looking for the 'edit' button when you read this ha ha
did you mean 'saves cents' ?

Maîneÿ . . .
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Follow Up By: Member - Stuart P (WA) - Friday, Apr 16, 2010 at 08:24

Friday, Apr 16, 2010 at 08:24
it depends on dual battery set up it is qiute acceptable and is in fact neater in appearnce to have all earths going to a body earth and not directly to the battery. imo as an autosparky there is no uglier sight than the pos and neg terminals of a battery looking like a xmas tree, with wires going evey where. as regards using a smaller wire diameter for an earth , volt drop over 300mm is negligable.
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Follow Up By: Olsen's 4WD Tours and Training - Friday, Apr 16, 2010 at 19:05

Friday, Apr 16, 2010 at 19:05
quote "Bet you were looking for the 'edit' button when you read this ha ha
did you mean 'saves cents' ?" unquote

You must have a different auto electrician to the rest of us. None I know work for cents :-)
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Reply By: Ozhumvee - Friday, Apr 16, 2010 at 07:23

Friday, Apr 16, 2010 at 07:23
Never ever connect the negative to the negative battery terminal, connect it to where the negative lead connects to either chassis or body.
The reason for this is that in the event that the main vehicle negative connection to the chassis/body is broken or poor then the starting current will take the path of least resistance through the next biggest negative connection whether it be your frig, HF or compressor.
Believe me it does happen, the accessory involved never works again.
If you must connect the negative lead direct to the battery terminal then fuse it as you would the positive.
Peter
1996 Oka Motorhome

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Follow Up By: Member - Allan B (QLD) - Friday, Apr 16, 2010 at 07:56

Friday, Apr 16, 2010 at 07:56
Peter, although it is good practice to not have a whole bunch of wires (either pos or neg) connected directly to a battery post it makes no difference from the electrical point of view. There is absolutely no way that the starter current of hundred of amps can flow through your fridge or anything else. The current flow through these circuits is determined by the applied voltage only.

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Allan

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Follow Up By: Maîneÿ . . .- Friday, Apr 16, 2010 at 08:00

Friday, Apr 16, 2010 at 08:00
Peter,
You say; "The reason for this is that in the event that the main vehicle negative connection to the chassis/body is broken or poor then the starting current will take the path of least resistance through the next biggest negative connection whether it be your frig, HF or compressor.........
If you must connect the negative lead direct to the battery terminal then fuse it as you would the positive"

Lets agree to disagree :-)

Maîneÿ . . .
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Follow Up By: Member - Captain (WA) - Friday, Apr 16, 2010 at 08:59

Friday, Apr 16, 2010 at 08:59
Guys, I think you need to listen to Peter, he is spot on. Fail to listen at your own peril....

Cheers

Captain
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Follow Up By: oldtrack123 - Friday, Apr 16, 2010 at 09:53

Friday, Apr 16, 2010 at 09:53
Hi
Mainey & I rarely agree, But on this both he & Alan are right.


You may damage the wiring but there is no way you can damage the equipment
The equipment can only pass the current it is designed for

However for the small extra cost involved I would strongly advise a cable of similar rating be run the full length & yes preferably earthed @ the main battery to chassis earth point .but no need for a fuse in earth lead,
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Follow Up By: Member - Captain (WA) - Friday, Apr 16, 2010 at 10:44

Friday, Apr 16, 2010 at 10:44
So, you agree you can burn out the wiring, but its not going to damage the equipment? .... whatever... you have been warned :(

Cheers

Captain
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Follow Up By: Maîneÿ . . .- Friday, Apr 16, 2010 at 16:28

Friday, Apr 16, 2010 at 16:28
Remember we are talking about the neg (-) cable, which has to flow to the earth of the battery, to form an electrical circuit.

Maîneÿ . . .
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Follow Up By: Ozhumvee - Friday, Apr 16, 2010 at 17:56

Friday, Apr 16, 2010 at 17:56
Having installed HF's for many years in all sorts of vehicles from sedans through to prime movers believe me the equipment WILL TRY to carry the starting current with disastrous results.
Pulling the case off a Codan transceiver to show the owner the TOTALLY cremated internals is heartbreaking for them. Unfortunately despite coming across at least half a dozen over the years I never bothered to take any pics.
An ARB compressor won't allow the vehicle to start either ;-)))) with total meltdown of the motor windings as the result. It was in a 60 series with the compressor mounted under the drivers seat, the driver complained that it was hard to start, when glowing the glowplugs he got a strong electrical smell which stopped as soon as the engine started. Eventually on one memorable morning as we used the vehicle to repeatedly winch another the carpets caught fire and it wouldn't start. The main earth from battery to body/chassis had broken and the compressor was the path of least resistance.
I've also seen the result when the actual wiring to the accessory (in this case a fridge in the rear) couldn't handle the current flow, melted the insulation on it and the main wiring harness on a Patrol.
When the accessory is the ONLY path for ground return to the battery you WILL have problems.
Peter
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Follow Up By: Member - Allan B (QLD) - Friday, Apr 16, 2010 at 18:31

Friday, Apr 16, 2010 at 18:31
Peter, I can see where you are coming from........err, to some degree.

As I have already said, it is not good practice to have all your negative return conductors perched on you battery negative post.

But the only way trouble can happen with that plus a broken connection between battery negative post and the body/chassis is when some device such as the air compressor has both a negative cable to the battery AND the same cable is connected to earth either at or within the air compressor. That cable now substitutes for the broken connection between the battery and body and will try to carry the current. However it will be the negative wire between the battery and compressor which will have the problem, not the compressor windings.

In the case of a HF radio, it may be that the negative supply terminal is connected to the radio chassis within the case and it is usual to have the case independently grounded to the vehicle so in a similar situation to above the current will flow through conductors within the radio case and cause destruction.

So Peter, you are right. Damage can occur in this situation but only where the device negative is also joined to the earthed body. There could however be plenty of those!

Cheers
Allan

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Follow Up By: Geoff (Newcastle, NSW) - Friday, Apr 16, 2010 at 18:47

Friday, Apr 16, 2010 at 18:47
How about we do the old "A picture tells a thousand words" thing?

I knocked these up quite some time back to illustrate Peter's exact point to a mate of mine!





Geoff,
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Follow Up By: Member - Allan B (QLD) - Friday, Apr 16, 2010 at 19:03

Friday, Apr 16, 2010 at 19:03
Yes Geoff, quite so, however.............

The thin cable from the battery negative post is shown connected to the positive cable going into the UHF radio! Also the coax screen is shown connected to the coax inner conductor. Unless you make it clear that the neg supply cable is connected to the UHF case AND the coax screen is also connected to the UHF case then the point of illustration maybe lost to someone trying to follow the true current path.

Cheers
Allan

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Follow Up By: Geoff (Newcastle, NSW) - Friday, Apr 16, 2010 at 19:10

Friday, Apr 16, 2010 at 19:10
Hello Allan,
Quite so!

PowerPoint just doesn't make that good a drawing package!

Did you sort out the Iridium battery problem successfully?

Geoff

Geoff,
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Follow Up By: Member - Allan B (QLD) - Friday, Apr 16, 2010 at 22:09

Friday, Apr 16, 2010 at 22:09
Sort of Geoff, it's a long story and I don't want to be had for chit-chat!!!!

Cheers
Allan

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Reply By: Member - Allan B (QLD) - Friday, Apr 16, 2010 at 08:05

Friday, Apr 16, 2010 at 08:05
Hi Spade, what your auto leccy has done is OK from a technical point of view but it is rather sloppy from a practical viewpoint. (It's the sort of thing that I may do myself sometimes when no-one is watching. LOL)

As for the chain weakest link?............well yes, but in this case the "weakest link" is adequate for your fridge current load.

Cheers
Allan

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Follow Up By: Spade Newsom - Friday, Apr 16, 2010 at 09:04

Friday, Apr 16, 2010 at 09:04
So is it reasonable to say that if the short negative to the chassis is adequate then positive may as well be the same cable size because both are adequate.

But is it defeating the purpose of doubling the cable size on the negative (even if someone might argue the purpose is purposeless) and then not doing it on the negative even for a short distance.
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Follow Up By: Member - Allan B (QLD) - Friday, Apr 16, 2010 at 10:51

Friday, Apr 16, 2010 at 10:51
Spade, There are two considerations about cable size.

The first is that the cable has to be large enough to carry the current without melting. Just look at the size of the cable to the starter motor. So quite a small cable can carry your fridge current without melting.

The second consideration is about power-loss along the cable, also referred to as "voltage-drop". This is a function of both the cable copper diameter and the cable length from the source (battery) to the load (fridge).

So in your case, whilst the 300mm long cable from the fridge to the chassis is thinner, it is quite capable of carrying the fridge current without damage. Also as it is very short there will be little power loss in this section of cable. The cable back to the battery however is several metres long and will have power-loss if it is too small so your auto leccy has made it larger by doubling-up two conductors. My guess is that he did this because he had no stock of larger cable.

Incidentally, using the chassis as a return path for the negative conductor is acceptable (and economic) for small currents such as fuel gauges etc. it can create problems for devices of larger current draw. The steel of the body and chassis does not conduct electricity as well as copper and there are a number of joints along the way which introduce more resistance to the current path. It is better, even if more expensive, to run negative return conductors of the same size as the positive conductor back to the earth point near the battery.

Cheers
Allan

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Follow Up By: Spade Newsom - Friday, Apr 16, 2010 at 11:47

Friday, Apr 16, 2010 at 11:47
Allan,

I understand that that cable size will not burn and it has a fuse rated lower than the cable anyway. I also understand what voltage drop does and generally how it occurs but I have never understaood how it might work on say a length of cable that has varying widths along its length as will be the case here.

If I have, say, two metres of cable and one metre is 4mm and one metre is 6mm, is the voltage drop maths calculated separately and then added together or am I now getting the 4mm equivalent voltage drop across the whole two metre length.

This is really the guts of my initial question.

You could extrapolate this and say have 10 metres of positive 6mm cable and 10 metres of 4mm negative cable. Same question just different dimensions.

I suppose my lack of understanding comes from how voltage drop was originally explained to me many years ago being like water pipes, ie pressure versus pipe diameter. (Some fridges need more "water" and "water pressure" to run them than others)
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Follow Up By: Spade Newsom - Friday, Apr 16, 2010 at 11:52

Friday, Apr 16, 2010 at 11:52
Also Allan I have melted cable before so I have first hand experience at that although never worked out what actually happened.

The fuse was still in the packet (this was the first and obvious error but still doesn't explain how the short occured), the cable that was pretty decent size caught fire, all the connectors were fine, the battery died and the fridge carried on as though nothing happened.

I am not sure I should press the send button on this one.
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Follow Up By: ob - Friday, Apr 16, 2010 at 16:54

Friday, Apr 16, 2010 at 16:54
A lot of car makers use negative switching for lights, wipers etc. Can anyone enlighten me as to the advantages over positive switching please? If wire sizes are the same for both parts of the circuit there would me no gain from that.
So why do it as now a positive feed is always live, and yes I realise fuses would protect in the event of a short.

Thanks
ob
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Follow Up By: Member - Allan B (QLD) - Friday, Apr 16, 2010 at 17:09

Friday, Apr 16, 2010 at 17:09
Hi Spade, Yes, you got it right. The total voltage-drop is the sum of the voltage-drops of each section of cable. You need to consider the volt-drop of say the 4mm section and add it to the drop across the 6mm section. You are not getting the 4mm equivalent drop across the whole 2 metre length. Same for the 10 metre example or any lengths or combinations.

Your water pipe analogy was correct. It works the same way. There will be some pressure lost in the large pipe and some more lost in the smaller pipe. just add them up.

Be aware though that the current limit before cable overheating is related only to the smaller cable. If the rating is exceeded it will overheat without the larger cable being affected.

Don't have enough info to comment on your cable fire event. Must have been a short to set the cable on fire. That is the purpose of fuses but you know that now don't you? LOL

Cheers
Allan

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Follow Up By: Member - Allan B (QLD) - Friday, Apr 16, 2010 at 17:29

Friday, Apr 16, 2010 at 17:29
Hi ob, The advantages of negative switching is cost-saving for the manufacturer. In most cases it is things such as door switches for courtesy lights etc. where it is only necessary to run a single wire to each door switch and through the switch to the body to complete the circuit. It also simplifies the construction of the switch itself where only one switch contact need be insulated, the other being the earthed switch body.

It is also the norm for instrumentation contacts such as oil pressure and thermostats where the device is in good contact with the engine (earth or negative) and the single wire is then connected via the device contact to earth to complete the circuit.

So wire sizes being "the same for both parts of the circuit" does not apply as the positive wire is probably just a short connection, possibly even a copper track on a dashboard circuit board.

The reverse applies to such things as tail lights where it is more economical to run a single switched positive from the dash switch to the lights at the rear of the vehicle and return the current via the body/chassis. But in the case of say headlights where there is a relay at the headlight it may be easier to switch the relay negative.

It all gets down to vehicle manufacturing design and based on economics. It has nothing to do with electrical engineering.

Cheers
Allan

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Follow Up By: ob - Friday, Apr 16, 2010 at 18:05

Friday, Apr 16, 2010 at 18:05
Thanks Allan, I had a feeling economics may have played a part in this.I couldn't see what electrical advantage there might be.

Cheers
ob
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Reply By: Member - Jeremy W (SA) - Friday, Apr 16, 2010 at 08:06

Friday, Apr 16, 2010 at 08:06
Re:

... negative cable to earth (about 300mm long) need only be half the size of the positive cable that we are using ...

Now this is quite wrong! The fridge and battery is one circuit (series) so the current in the negative and positive leads is the same so the wire sizes should be the same.

As said before, because of the unreliable chassis earth (return path) it is best to run a separate wire for the return path (earth).

If the positive wire is 3M long then the negative wire is 3M long and the total resistance in circuit is due to 2 wires and not one ie. a 6mm wire has a resistance of 0.0028 ohms/Metre so 6M of wire has a resistance of 0.017 ohms and at a current of 10A 0.17V is "lost" in the wiring. For 4mm wires and with 10A current the voltage drop is 0.25V, So it looks as if 4mm wires for your fridge is more than adequate and slightly cheaper!!


You need only 1 fuse in circuit - place this fuse in the positive lead as close to the battery as possible
AnswerID: 413115

Reply By: Alloy c/t - Friday, Apr 16, 2010 at 10:44

Friday, Apr 16, 2010 at 10:44
Twas me I'd be looking for a different Auto lekie , or at the very least get him to read the Waeco wireing recomendations concerning cable size - voltage drop over distance ect ,ect. You will see what I mean not if but WHEN your 80lt error light starts to flash,,
AnswerID: 413134

Reply By: Maîneÿ . . .- Friday, Apr 16, 2010 at 16:42

Friday, Apr 16, 2010 at 16:42
As mentioned above get a second opinion from another Auto electrician, think about it for yourself and make the decision based on logic and not on costs or something you admit you don't understand (said nicely - insert huge smiley face here)

Ask other Waeco 80lt owners how they have attached their fridge to the 12v power source :-)

As you say, "the chain only as good as its weakest link" this includes cable and connections as well.

Maîneÿ . . .
AnswerID: 413163

Reply By: Member - Stuart P (WA) - Sunday, Apr 18, 2010 at 17:30

Sunday, Apr 18, 2010 at 17:30
which is why there are one or more earths on vehicles , crimping these days is better than it was done years ago because computor systems are a little sensitive,if one earth breaks you will find the chassis earth will compensate
AnswerID: 413448

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