The basics of Amp / Current Draw ...

[RoldIT]

I need to clarify a couple of questions I have been considering lately.


1) Amp ratings of a piece of wire are for what purpose?

Is it the amount of current a wire will ONLY carry or is it the amount of current the wire will handle before frying?

ie - A device that draws 50amps on a wire that is rated to carry 25amp, will it fry the wire while trying to draw 50amp down the line or will the device only run at 1/2 capacity being that the wire is only supplying 25amps?

I currently believe it to be "the current a wire can safely carry before damage" but places like Piranha lead us to believe (ie with their Super Looms) that underated wiring, specifically in lighting installations, provides a lower output rather than actual failure.

This is why I'm confused.


2) ... and to follow-up, why do we fuse the line?

Is it to stop the device drawing more current than the wire will handle or to stop more current being sent to the device (ie spike/surge) to protect the device from damge?


Any clarification from budding AutoElecs would be appreciated.

Cheers.

[murcod]

Here's my explanation:

When a wire is rated at a certain current, it is only a recommendation of what it is capable of carrying without too many voltage losses from resistance. And also, of course, without frying itself! Generally the more cross sectional copper in the wire, then the less resistance it will have and the more current carrying capacity.

It's the resistance within the wire that will limit it's current capacity. Resistance combined with a high current will result in a large voltage drop across the wire and it will heat up (and even burn out if the current is high enough). That's why fuses are used- they consist of a small piece of wire with a small crossectional area (ie high resistance compared to the wiring they protect), so when the current rises they will be the first part to overheat and burn out. Fuses are to protect both the wiring and the devices hanging off that supply line.

Aftermarket looms work on the theory that manufacturuers use the smallest gauge wire possible to save money and weight. It will therefore have a larger voltage drop across it (due to the resistance thing again) and the headlight globes will not get the full battery voltage across them- thus being a bit duller.

A formula : voltage = current x resistance

From that formula you can see if a wire has more resistance (ie is smaller) it will get more voltage dropped across it.

[RoldIT]

OK, that probably clears a few things up.

Another example I was thinking of were winches. Although only hooked up with 200amp or less cable usually, most winches will double that current draw at full load but it still works. So theorically even though it drawing more current down the cable than it's rated to, most everthing will still work but probably not as good as it could, and will make a nice warm cable to boot.

This brings me to my next question, when working out current draw is it a direct mathematical calculation? (ie 2 seperate 50amp loads will run happily on 100amp cable or 4 seperate 25amp loads and 100amp cable, etc)

[murcod]

This brings me to my next question, when working out current draw is it a direct mathematical calculation? (ie 2 seperate 50amp loads will run happily on 100amp cable or 4 seperate 25amp loads and 100amp cable, etc)

Yes, add all the loads together and you'll get the cable capacity required.

[RoldIT]

Cool, thanks heaps for the good info.

Cheers.

[Bluey]

This brings me to my next question, when working out current draw is it a direct mathematical calculation? (ie 2 seperate 50amp loads will run happily on 100amp cable or 4 seperate 25amp loads and 100amp cable, etc)

Yes, add all the loads together and you'll get the cable capacity required.

wot he said. add total of all current draw devices you are going to run to work out what size wire/fuse/relay to use. should tho add percentage, say 25% to total, for determining wire size. that way shouldnt have any reliability issues

[r0ck_m0nkey]

To extend on what people have already said.

Basically if you draw to much current down a cable, most things have been covered already, but the cable will get extremely hot and lead to the potential of the insulation melting and causing fires. If you have ever had an under dash fire you will know what a pain in the arse this can be.

Now to the Fuses. Their sole purpose is for protection of the circuit. If you lower the resistance of a cicuit current increases. So if the resistance drops below what the circuit is built for, i.e. in a short circuit situation the fuse is designed to blow at a given Current Draw which in turn will stop the current from flowing.

Placement of the fuse is extremely important, you want it as close to the power source as possible. In the case of a negative earthed vehicle you want it on the Positive side aswell. The reason for this is fairly simple, seeing as the vehicle is negative earth, any shorts between the negative wire and the body will not cause any problems, but a short between a positive wire and the body will, so this is the side you want to be isolated once the fuse has blown.

If you have more then one device working off a given circuit you want a fuse at the source of the power which is big enough to supply all the devices on the circuit. It's purpose is only to protect the circuit, not any of the devices. When the devices are wire off the circuit they should have their own seperate fuse which is to protect the device. Reasons for this is if you just had a big fuse protecting the circuit and all it's devices together. There is the potential that you may have a device turned off, a fault occurs on another device causing a higher current draw on it's part of the circuit. The main Fuse won't see this problem, but the smaller wiring for the device will and again their is the potential of over powering the wire causing the earlier mentioned problems. Of course it will lead to the device causing more damage to itself and again have a potential for a fire.


-----

Now for a really good thing for yourself to do, either read books or search around the internet for sites covering Basic DC Electrical Principles, covering Ohm's Law, and what happens with voltages, currents' resistances etc when wired in parrallel and in series. It only requires basic maths to understand and will be quite useful. Lastly, with Ohm's Law, it calculates things from "perfect" conditions, throw in things such as heat etc. which happen in electrical circuits and it all goes out a little.

[-Scott-]

Lastly, with Ohm's Law, it calculates things from "perfect" conditions, throw in things such as heat etc. which happen in electrical circuits and it all goes out a little.

I agree 100% with everything else written, but this I can't let go. For a purely resistive DC circuit Ohm's Law is a Law. The resistance of a circuit changes with temperature changes, and maybe this is what r0ck_m0nkey is trying to say, but at any given instance V=IxR. If you can't make all your numbers agree check your measuring equipment - Ohm's Law isn't wrong!

Scott

[r0ck_m0nkey]

Lastly, with Ohm's Law, it calculates things from "perfect" conditions, throw in things such as heat etc. which happen in electrical circuits and it all goes out a little.

I agree 100% with everything else written, but this I can't let go. For a purely resistive DC circuit Ohm's Law is a Law. The resistance of a circuit changes with temperature changes, and maybe this is what r0ck_m0nkey is trying to say, but at any given instance V=IxR. If you can't make all your numbers agree check your measuring equipment - Ohm's Law isn't wrong!

Scott

Had to get overly technical didn't you

Thats what i was getting at though, simply working it out on paper then using that information etc. won't work due to other factors like heat which will effect everything. So then need to compensate.

[murcod]

You'd have to be doing very accurate measurements before heat would be a concern.

[-Scott-]

Had to get overly technical didn't you

It's a genetic defect. Who'd become an engineer by choice?

Scott

[Mechman]

Had to get overly technical didn't you

It's a genetic defect. Who'd become an engineer by choice?

Scott

i agree.. but at least im a mechanical engineer...none of this black magic voodoo stuff....lol

i just brought a kit from jaycar for an LED battery charge indicator as my FJ40 has a wonky pointy thing..

hats off to you guys.. i had problems remembering what side of the CB to solder the bits too...hahahahahaha

[HeathGQ]

Had to get overly technical didn't you

It's a genetic defect. Who'd become an engineer by choice?

Scott

Hey hey hey... thats enough of that. I'm an engineer by choice! But geotechnical.

[-Scott-]

Hey hey hey... thats enough of that. I'm an engineer by choice! But geotechnical.

Well, you think it was your choice...

[RoldIT]

Thanks guys.

I have a bit of an amateur passion for auto-electrics and what you've all said pretty much confirms what was in my mind, I've just neven investigated the technical principles behind it.

Nothing pi$$es me off more than dodgy wiring in a car so I thought I'd better get it right the first time.

Cheers.

[MissDrew]

A fuse or circuit breaker protects the cables.


So the way you work it out is

use the cable that will carry the load required.
then use a fuse that is the same or lower than what the cable will carry.

Then you have derating factors to concider, like insulation, bunching of cables, distance bla bla bla.

[murcod]

A fuse or circuit breaker protects the cables.

The fuse is also there to protect the electrical gear hanging off the end of the cable. You dont want your CD player catching on fire if something shorts inside it causing the PCB tracks to overheat!

[MissDrew]

A fuse or circuit breaker protects the cables.

The fuse is also there to protect the electrical gear hanging off the end of the cable. You dont want your CD player catching on fire if something shorts inside it causing the PCB tracks to overheat!

Fault currents are huge, so it don`t matter if you have a 100a fuse on a radio it is still going to blow. If a cd player gets a short in it it is going to burn out inside before even a 1a fuse blows.

A CD player should be built to take its normal operating amps and the cable you use suits that and the fuse you use suits the cable you use.

[murcod]

A fuse or circuit breaker protects the cables.

The fuse is also there to protect the electrical gear hanging off the end of the cable. You dont want your CD player catching on fire if something shorts inside it causing the PCB tracks to overheat!

Fault currents are huge, so it don`t matter if you have a 100a fuse on a radio it is still going to blow. If a cd player gets a short in it it is going to burn out inside before even a 1a fuse blows.

A CD player should be built to take its normal operating amps and the cable you use suits that and the fuse you use suits the cable you use.

I'm an Electronics technician by trade- trust me PCB tracks will burn at a lot less than 100Amps (10 amps is usually enough)- and the 100Amp fuse will happily continue supplying the current without blowing. Say a filter capacitor in your stereo power supply goes short circuit, the track will heat up and burn until it becomes the fuse and goes open circuit (the capacitor could also blow). If you're lucky nothing else will catch on fire, but by this stage acrid toxic smoke will have filled the cabin. A smaller fuse would have blown well before anything started smoking.

Most factory fuse boxes run about a 7.5A fuse for the stereo wiring- but the cable could handle at least three times that current. The reason is to prevent a short inside the stereo from causing the above situation.

Have a look under a car dash- different equipment has different value fuses but often will use the same gauge wire for the supply. It's all got to do with supplying enough current for the equipment to operate under all normal given condtions, but as soon as the current increases beyond this limit the fuse blows as something has gone wrong.

The fuse value is selected to suit the current draw of the equipment it is supplying, not the cable's rated maximum current.

[MissDrew]

For starters the 100a fuse was just a eg.




You haven`t read what I have said, cable is selected at the correct size for the load, vd, etc and the fuse/cb is selected to suit the cable. Unless you are dumb you an`t going to use a cable or fuse that is 100 times to big or small.

[MissDrew]

The fuse value is selected to suit the current draw of the equipment it is supplying, not the cable's rated maximum current.

To selelct a cable you take the load need say 10 amps then you use a cable that will carry 10amps, then you get a 10a fuse. Or if vd comes into it you take the 10a x distance x the vc rating for that sized cable and divide all that by 1000 and that gives you your vd for that load. Now lets just say all that workouts ot a normally 15a rated cable but because of the load and the distance its now only rated at 10a, you still only use a 10a fuse. Commensence tells you you can`t use a 9a fuse and the cable with its new rating because of the distance will not allow you to use a 15a fuse.

But I`m just a dumb arse so what would I know

[murcod]

Or if vd comes into it you take the 10a x distance x the vc rating for that sized cable and divide all that by 1000 and that gives you your vd for that load. Now lets just say all that workouts ot a normally 15a rated cable but because of the load and the distance its now only rated at 10a, you still only use a 10a fuse. Commensence tells you you can`t use a 9a fuse and the cable with its new rating because of the distance will not allow you to use a 15a fuse.

vd= voltage drop (?)

vc=???

A cable's current rating does not change with the length of the cable; but there will be an increase in resistance and more voltage dropped. If the voltage drop is too great then the wrong guage wire has been used.

The point I'm trying to get across is the fuse size is related to the load presented by the device on the end of the wire. Yes, the wire guage will have been chosen to suit the application in the design stage- but the fuse size isn't dictated by the maximum current capabilty of the wire.

[MissDrew]

vd= voltage drop (?)

vc=???

vd = voltage drop

vc = the voltage drop over the route length of circuit for variuos conductors, in millivolts per ampere metre.




eg for this we will use 6mm cable (which is fairly big for cars)
lets say the length of cable used is 25mts (I know thats not going to happen on a car but it will make it easy for the eg)
the amps needed is 32amps.
the vc for 6mm is 7.5 (its a given amount)

So we go 25 x 32 x 7.5 / 1000 = 6volts voltage drop.
Or if we go up to 25mm cable the vc is only 1.4 so that works out to only 1.12volts voltage drop.

Now if you don`t want anymore than 1.12volts of voltage drop at the other end of the cable when you have a load of 32amps you have to use 25mm cable, which if its only say 2mts it will carry up to 100amps if the conditions are perfect.
Now if we use the same 25mm cable but up its amps to say 64a then it will have a voltage drop of 2.24volts.

So yes a cables max current carrying capacity does change with length if you want the correct voltage at the end of it.

Now with houses etc you can have up to 5% vd.
Just rang one of my mates who is an auto leck and up to 5% is also ok with cars, but it does change depending on the situation.

[Bluey]

hey Guts, do you know what the Vc (average as assume would be slight differences between manufacturers) values are for common wire sizes used in auto applications? like 1mm, 2, 4, 6 plus any others

or some company that provides this info?



Cheers
Lance

[murcod]

OK, when Guts is talking Vc he is talking about the resistance of the wire.

All wire has some resistance- there is no such thing as the perfect wire with zero Ohms resistance. Wire manufacturers usually specify this as an Ohms/ foot or metre rating. This resistance will depend on the crossectional area of the conductor, conductor material (eg plain copper or oxygen free copper), temperature etc.

A cable rated at say 80 Amps will still be be perfectly safe for 80 Amps over any distance, however the longer the cable the higher the resistance and the greater the voltage drop across the cable length.

Ohm's law comes into play:

V (Voltage)= I (Current) x R (Resistance).

SO the more current you draw through any wire , the more voltage drop you will across it. However, most electrical systems in a car are very tolerant to supply voltage changes and will happily operate anywhere from 11-15 Volts. Using a car radio again as an example, most of them will continue to operate down to at least 10V with no problems. Also the cabling runs in cars are usually no longer than 5-6 metres max and as such there shouldn't any need to worry about voltage losses, and you can simply use the manufacturers rating on the cable (if there is one...)

As an example Jaycar sell 8 guage wire (WH3060 ) which is an oygen free copper wire rated at 56 Amps. It has a resistance of 0.00252ohms/Metre
at 20 degrees celcius. So if you're drawing the maximum 56 amps through it over a five metre length (longer than most cars/ 4WDs) you'll have 0.7V dropped across it. ( 0.00252 x 5 x 56 ). Remember this example is running the cable at it maximum rating which wouldn't normally happen, you'd use some larger 6 guage wire if you were at the upper limits.

But this has gone way off track- the original statement

A fuse or circuit breaker protects the cables.


So the way you work it out is

use the cable that will carry the load required.
then use a fuse that is the same or lower than what the cable will carry.

isn't correct.

I use heavy duty wire in my vehicle (a lot larger then really needed for the current being drawn), but the fuse value doesn't get increased because I've used higher capacity wire- it stays the same value because it's related to the amount of current expected to be drawn by the electrical load. Sure, the fuse will protect the cable as well, but it's rating is derived from the load at the end of the cable- not the capacity of the cable itself.

[MissDrew]

I have never said that the fuse or CB doesn`t stay releative to the load.


You have a load
then you use cable to suit then fuse/cb to suit.


Only a complete dickhead would use a 50a fuse when the load is only 2a

[murcod]

Didn't you? I must have misread this then?!

Fault currents are huge, so it don`t matter if you have a 100a fuse on a radio it is still going to blow.

A CD player should be built to take its normal operating amps and the cable you use suits that and the fuse you use suits the cable you use.

[MissDrew]

yep you missed read it.


All I said is fault currents are huge.

[murcod]

Re read what I wrote about the possible short inside a stereo- you do not need huge currents to cause a fire, PCB tracks cannot cope with large amounts of current and will overheat and burn...... If a cable gets shorted to earth then you will get a huge current- but an internal short inside equipment does not always result in such a situation- and if the fuse is too big the equipment will start to smoke/ burn.

It's up to you how you wire your rig, but don't encourage other people (through misinformation) to use fuses that are way too high for the load. What you said about how to choose a fuse is simply .... WRONG!

A friend had a car amp start smoking once- why, because he'd used too large a fuse and the internals of his amp were going up in smoke. It didn't stop until he'd pulled over and removed the supply wire. The problem was a diode had gone short circuit between the +12V and earth lines inside- the proper fuse would have blown and not smoked his amp.....

[carts]

The point is you are both right. From the info provided I assume that Guts is a sparky of the domestic/commercial/industrial side of things seeing as though he is quoting figures from AS3000 and AS3008 (i may be wrong). When selecting protection devices for the general wiring applications, your aim is to protect the cables. Sparkies don't give a crap about the connected load, as long as the house doesn't burn down from overcurrent or short circuit current on the wiring. They dont always know what the connected load is. For example, you may install 40 lighting points on a circuit for a new house. The circuit has 1mm square cable and is protected by a 10amp breaker. As the sparky, you don't know what wattage globes the new owner wants to use, all you care about is if the tool wants to put 40 100watt globes in, the circuit breaker will trip before the actual cabling in the circuit is damaged.

When it comes to car wiring, the principles are the same generally speaking, however, when the car is manufactured, the manufacturer knows exactly what the connected load is, so they can select cabling and fuses that will protect both the wiring and the equipment at the other end. The only real unknown is what devices the car owner is going to stick in his/her cigarette lighter, in which case the fuse is most definately selected on protecting the fixed wiring supplying the power outlet/cigarette lighter.