Issues with voltage drop and melting wires

G'day Phil

I would suggest the professional installation on your mates car wasn't up to scratch. If it had been fused close to the battery the dead short would not have occurred.

Another point, and possibly pedantic but ... the term voltage drop is used here regularly, however this is a bit of a misnomer as it's not voltage drop at all, rather it's current (amperage) drop due to resistance of the wire. If you were to check with a multi you'd find you still have 12.4V or similar at the end of the wire.

A good basis of comparison is a long garden hose. Connect it to a tap and you will find the pressure (read voltage or potential (to do work) at the end of the hose is the same as at the tap). However, due to resistance (friction) you will only get a small dribble of water out of the end when compared to the unrestricted flow at the tap.

Think of electricity along the same lines .. a thin wire is the same as a thin hose where resistance = friction and voltage = pressure.

So .. for wiring and Ohm's Law V=iR. V is constant so i=V/R ... the availabe current (i) = voltage divided by resistance. The greater the resistance (thin wire) the lesser the current (i). So your fridge won't get enough amps to work at the end of a long thin wire.

Get my drift ... ;o))

G'day PJR Phil
I have seen or been in situations where the word "professional" is used to demand respect of someone ability and that it should never be questioned.
It has caused me to never want to be "professional".

I also find from reading various forums where electricity is used that many people who went to school never seemed to learn anything about electricity.
The good old Tech school situation prepared people for, or alerted them to many situations. They then developed their understanding from a sound grounding in basics.
Things I learned then prepared me and began a developing life long learning process.

I find many people now are having problems with items they either do themselves or have fitted by "professionals" who really don't know either.
Experience might stop an Auto sparky from putting a grommet where it will fail. Basic periodic checking of wiring will pickup the developing failure if looked at.

Many can grasp the idea of connecting something to a battery with a piece of cable but few actually start thinking about the issue in any depth so as to avoid future problems with the design or modification or realize the consequences of what they are doing.

It all looks so simple but each field of endeavour has it's own intricacies which must be under stood to at least some degree.

Therefore many problems seem to be caused by the people themselves.

PS No one should tie 12v cables near brake lines.Easily a foreseeable safety issue.
But if you don't know about the danger or can't perceive it, then there isn't a problem with a "professional" installation is there?

Ross M

So after the preliminaries, has (a) the voltage drop team or (b) the current drop/resistance team taken the points? My science hat suggests the latter but my brain's in neutral and doesn't want to do the work required to confirm.

Solution:
Be careful selecting the correct cable and where you choose to run the cable. Go up a size to be safe. Allow for flexing and protection against sharp edges, shorting and rats/mice eating the insulation.


And put a fuse at both ends were there are two source's of power..

cheers phil.

Phil.
Have been reading this post with some interest, and will admit to not being very smart in electrics at all, but willing to listen, read and learn.
May I say to you and all other posters thank you for the information in this post.
It lets the average person like myself know that it's not all beer & skittles when it comes to this type of thing.
While admitting to having a basic understanding of what is required for any electrical works when I need it fitted to my vehicle, I do realise the need for an extended knowledge which is from training and experience in this field.
This I can see from this post !!!
I can only hope that it continues this way.
Am beginning to get a bit sad with the continual battering some people are getting from a simple question.
Once again thanks for your post on this subject, and thanks also due to all other contributors for your excellent input.
Cheers & Regards
Flighty

Sorry folks....but the term "Voltage Drop" is the correct one and it accuratly describes exactly what is going on.

This issue is very much poorly understood and as people have testified, very often underestimated.

All wire has resistance.
When current flows in that wire the voltage will differ from one end to the other...ALWAYS...may be a little may be a lot.
There is a "voltage Drop" over the length of the wire......every wire carrying current.

Voltage drop is the correct term, it is used consistently in all the text books and in all the standards and specifications.

Voltage drop can be accuratelly calculated and and easily measured.

However all to often we do not know accurately what is happening.....in the case where the voltage measured is the same before and after heavier cabling has been installed It is clear to me that the measurement was not taken when the full load was not present....IE the fridge motor was not running.

NOW
Back to what we need to know.
In low( extra low to be trechnically correct) Voltage DC situations we have only 12 or sometimes 24 volts...if one or two volts go missing....we have a problem.

On 240V, like in our homes if a volt or two goes missing.....its no biggie...we can loose 12 whole volts and most appliances will still run just fine.



The problem is compounded, because DC equipment draws a hell of a lot of current ( that causes voltage drop) and often suffers badly from reduced voltage.

All too often people underestimate voltage drop and its effects and install cable that is far too small.

In fact in a lot of situations the "correct" or an "adequate" cable may look rediculously large.

In 12 volt situations we should always be using cable heavier than it's required rated current unless we are going very short distances....eg...15 amp cable is only any good for 15 amps if its going a couple of feet..because of voltage drop......

The further you go and the higher the current.....the size of the cable required to minimise voltage drop gets rapidly...very rapidly larger.......easily to the point that it can look rediculoulsy large for the accessory connected.


This is why all the heavy current draw devices fitted standard to a car are very close to the battery.

If we are to run high current demand items in the rear of the vehicle....the cables will need to be large....they may seem rediculoulsy large.

Even many of the "professionals" don't grasp how heavy cables need to be.
Almost every car comming out of the factories is wired in cable as light as it possibly could be and still work.......I hear full ticket 240v sparkies horified when they find out what is normal in how much current is put thru quite small automotive wiring

cheers

All well and good. Voltage verses current reminds me of the chicken and egg conundrum. Some like the "voltage drop" approach and others prefer a "limited current" scenario. All fine as long as we get the right wiring done properly.

But the thread is stuck on the "loss" issue. What about the fire that we had! That is life threatening and I would think, much more important.

Questions:
What, if any, fire extinguisher would you carry?
What type and what size?
And if so how big and where would you store it?
Have you ever shown any passengers or even shown the family how to use it?
What could the layman look for that could cause an electrical fire or short?
How many can very quickly put their hand on some gloves and a toolbox? (to disconnect the boiling battery etc)

What have I missed? Feel free to add to the points above.

I (naturally being humble) have some answers but I would like to hear what others say. Maybe even what people have found done "professionally".

This will be telling a lot to suck eggs. But we all need a refresher some times. And maybe learn from our mates. I am open to new ideas.

I have emails and thanks from both new and old 4WDers. So this thread is doing what I wanted. Discussion and education. No darn whinging or complaining. Excellent.

Phil

PS: Guess who can't sleep!!