why is bigger better

Yeah I can understand where you're confusion is comming from. But it's all about length. It's just like using a garden hose sprayer. The further your spray the water the more it disperses. If you hold the hose right up to your pavers it'll clean em' good, if you try to clean them from 10m away you will have lost pressure.

The further the voltage has to go down the wire the more "loss" you have. When you start taking a substantial load off that long wire (ie a fridge at 4 amps) then you get what is called "voltage drop" because of the shear distance the cable is unable to provide the power correctly to the appliance because too much of it is getting dispersed along the way.
A fuse is only a 1 cm long, the voltage drop over that distance would be minimal. As long as that peice of wire can handle the current being drawn through it it should make very little difference to the overal picture.
Hope that makes sense...

Damn good analogy!!

Now, the fuse is what............ 15 - 20 mm. long of wire? That is the toughest part for the power to go thru.

The rest of the wire will let the power just sort of dawdle thru, without getting all squashed up.

This may make it easier.............

You have BIG power stations in the Yallourn Valley, and your light switch at home. You flick the switch to ON. Now, contrary to popular belief, all those little electrons just don't go hurtling down from Yallourn to your house. They dance around, like a moving sea of electrons could take up to a year to get to your house!! You'll get the bill quicker than that!!

So, the electrons need HEAPS of room to FREELY MOVE AROUND, and they are not like water, and come out the end of the hose all sequential like.

Hope this helps.

Cheers

Wolfie

just to add more confusion if you heat the wire anyway it will cause more resistance and more voltage drop
lol

steve

Diamond,

I started typing out a reply but it all got too complicated, so here is my version of a simple answer.

It is a bit like driving along on a muddy track, there is a bit more resistance than on the bitumen but you can manage it easily. Think of this part as the heavy guage wire going to your fridge.

OK now the going gets a bit heavier and you hit a bog about 1' deep but only 2' long. You feel it but it really doesn't slow you down that much. This is the situation with the fuse, more resistance but short enough that it doesn't lose too much voltage across it.

Taking it one step further if your wire going to the fridge was the same size as your fuse, can you guess how long that 1' deep bog is now??? Lead wellie time....big time voltage drop.

The difference between your wellie position and your forward progress represents the resistance of the road surface, a bit like the resistance to voltage along a length of cable, the thinner the cable (deeper the bog) the more resistance to forward progress (voltage) there is.

Before all the physics professors get on my case I know it is not stictly a good example, just a representation in a way that may make a bit more sense to those with no electronics background.

Jim don't worry about it, Just accept that this is what happens and you need a big wire, the bigger the better.

GaryInOz, a good reply and definitely a different way of putting it.
Cheers

That was a well described analogy to explain resistance. Now, can someone answer this.....if you were to take a 6mm multistrand wire and a 6mm single strand wire, which one will carry current more efficiently, with less resistance?
OK. I have it on good authority it is the multistrand, but I have no idea why?
Anyone????

Neil.

Smoke.
Most people think electricity is a flow of electrons in a piece of wire which cause things to work by their electromagnetic properties (electric motors) or the effect of heat due to resistance (Light bulb).
This is not in fact the case.
Electricity is in fact smoke. All those wires you see around the places are actually little pipes carrying smoke of various colours.
When all is good there are no leaks and all the smoke stays in the wire (pipe) and things work properly.
When something goes wrong, however, the pipe gets broken and the smoke escapes.
The amount of smoke is directly proportional to the cost of the repair. A small amount of clean white nearly odorless smoke will be relatively cheap to fix, whereas a large amount of dirty brown smoke, usually accompanied by what is technically called a “dirty brown smell”, will usually cost you a second mortgage to rectify.
So make sure that all your taps (terminals) are tight and clean, your pipes (wires) are in good condition and free from cuts and corrosion, and have a smoke free day.
Cheers.....

Well all,

The diameter of the wire determines the resistance of the wire per meter. A thick diameter has less resistance per meter.

Then look at the equation of current (I) = voltage(E)/resistance(R).

From this the less resistance in the circuit the more current will be available for the fridge.

Example:

Average Waeco draws 3.5 amps (approx) so has a resistance of R=E/I = 3.4 ohms
If the resistance of your wiring was 1 ohm it would mean the fridge would only have a supply voltage of 9.27 volts with a battery if 12 volts

AND

8.88 volts when the battery falls to 11.5 volts.

I realise that the resistance of the wiring is less than one ohm but hope this explains why you require the thicker wiring when considering the voltage of a battery can decline when not being charged.

Redeye

This thread may hopefully solve a number of electrical problems - particularly relating to 12 volt fridges.

Materials such as copper, that allow electricity to flow, are not perfect conductors. To varying degrees they oppose that flow. This opposition causes that conductor to heat up and that energy is lost.

The smaller the cross sectional area of the conductor, and the greater its length, the larger the loss.

A fuse has very small cross-sectional area, but of such short length, that the loss is not that great. The older-type glass fuses however can cause problems as the contact between the fuse and the fuseholder tends to corrode.

The major concern however is that raised by Klaus and others - and that relates to cable 'rating' standards.

Electrical appliance makers worldwide (except the USA) specify cable in terms of its cross sectional area (in sq. mm.). But when you set out to buy the (say) 4.0 or 6.0 sq mm cable called for, what you will almost certainly be sold is 4.0 or 6.0 mm auto cable.

For reasons that defy sanity, auto cable is sold with similar 'numbering' (eg. 3.0 mm. 4.0 mm, 6.0 mm) but that measurement is of its overall diameter - including the insulation.

The appliance maker logically specifies the cross sectional area of the copper that carries the current.

The auto cable maker specifies the size of the hole the cable can be pushed through - and uses the same bloody numbering!

This is not an error made by auto parts staff. It is the way the stuff is rated by its makers. I have however yet to find a single person in an auto parats store who is even vaguely aware of this matter - let alone its consequences.

Is those consequences serious?

You bet they are!

Because the ratio of copper/insulation varies from auto cable maker to auto cable maker there is no such thing as a universal comparison table.

Roughly though (and using the mosty common sizes available in Australia):

3.00 mm auto cable is about 1. sq mm.

4.00 mm auto cable is 1.8-2.0 sq mm.

6.00 mm auto cable is typically a tad under 4.6 sq mm.

As a very commonly used size is 4 mm, a HUGE number of fridges (probably most) are connected via cable that is, at best 50% undersized, and in many cases close to 60% undersized.

Using the correct size cable makes an enormous difference to performance.

Be aware also that the so-called 'current rating' (like 30 amp) is simply a measure of the current that cable can carry before the insulation melts. It bears no relationship to voltage drop.

The above affects a huge number of RV electrical installations including many done professionally.

I first raised this issue in an early edition of my book 'Motorhome Electrics' - I thought then that it was of concern, but only limited concern. It has turned out be a huge issue worldwide (except in the USA where a different cable rating system is used) - and a huge number of RVs are affected.

I am currently finishing a 'Guide to Solar' for publication on this Forum - and I have covered this issue also in this.

Re single/multi-strand cable: multi-strand is necessary for anything that moves as it is very much more flexible. The current carrying capacity at dc and low frequency ac (such as mains voltage) is virtually identical, but at radio type frequencies and above, the ac current tends to travel on the outer surface of the copper hence multi-strand is used because of it massively larger surface area.
Trust this helps
Collyn Rivers

GT

There is no auto cable that approximates 6.0 sq mm. The closest is so-called '8 gauge'. This is an unusual size in that it's about the same cross-sectional area in virtually all rating systems!

You may find it called 8 B&S (and I bought 60 metres of it labelled exactly that from Rheen Auto Electrics in Broome, yesterday). It's a tad over 8 sq mm. The American AWG 8 gauge is virtually identical. And a few auto parts stores know it as 8 mm auto cable.

It's a bit overkill - but for fridge wiring etc nothing succeeds like excess!
Most auto-electricians will sell you a length (about $2-$3 a metre but it's worth every cent).

Trust this helps
Collyn Rivers