wire size variations at different points in a circuit

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

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.

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.

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 . . .

"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 . . .

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.

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 :-)

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.

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 . . .

Guys, I think you need to listen to Peter, he is spot on. Fail to listen at your own peril....

Cheers

Captain

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,

So, you agree you can burn out the wiring, but its not going to damage the equipment? .... whatever... you have been warned :(

Cheers

Captain

Remember we are talking about the neg (-) cable, which has to flow to the earth of the battery, to form an electrical circuit.

Maîneÿ . . .

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, 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!

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!

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.

Hello Allan,
Quite so!

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

Did you sort out the Iridium battery problem successfully?

Geoff

Sort of Geoff, it's a long story and I don't want to be had for chit-chat!!!!

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.

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.

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)

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.

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

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

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.

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