The basics of Amp / Current Draw ...

Joined: 28 Sep 2003 Location: in the shower..

Also, generally speaking, in a car, the fuse at the beginning of the circuit closest to the power source (battery) is there to protect the cable and any faults that may occur on that cable between the power source and the appliance. A smaller inline fuse may be used at the actual appliance rated to protect only that appliance. You may have more than one appliance attached to the once 12volt circuit, in which case you wouldn't want one faulty appliance to take out 4 other ones on the same circuit. This is known as discrimination of protective devices. Whether it is a car or a house, the principles are the same.

Joined: 30 Dec 2002 Location: Adelaide

Yes, that's right.

There's usally large fusible wires (or heavy duty fuses) located in the engine bay as close as possible to the positive terminal of the battery. The fuses in there are really high values (eg 40Amp - 80Amp range) and protect the large feeder cables that go from the battery through out the vehicle.

One or more of these cables will feed into the cabin and into the fuse box located inside. There the cable will feed into multiple smaller fuses of various values (5 Amp - 20 Amp are common) and feed off to the electrical loads inside the vehicle- your heater fan, radio, interior lights, central locking, electric windows etc. The fuse values are selected to suit the load expected to be drawn by each device (with a safety margin added in for current surges when devices are turned on.)

Joined: 21 Oct 2002 Location: Brisbane

murcod wrote:

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!

I can tell you now that my rig is wired up perfect.

I an`t telling people to use bigger fuses, just you findout the load, select the cable to suit, then a fuse to suit. Only a dummy would go, 20a load, 35a cable and 30a fuse.

You find the load, then the cable, then the fuse. Its that simpal. each relats to the last.

And yes I am a sparky, domestic, commercail and a small amount of industrail.

Joined: 11 Jan 2004 Location: Adelaide

Sorry Guts, too much time on my hands...

Guts wrote:

I can tell you now that my rig is wired up perfect.

That sounds like a challenge to me. Twisted Evil

Guts wrote:

I an`t telling people to use bigger fuses, just you findout the load, select the cable to suit, then a fuse to suit. Only a dummy would go, 20a load, 35a cable and 30a fuse.

But you weighed into this thread with:

Guts wrote:

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.

So your statement

Guts wrote:

You find the load, then the cable, then the fuse. Its that simpal. each relats to the last.

contradicts your earlier statement.

So, it was the Butler in the Library with the Knife! armsup

Guts wrote:

And yes I am a sparky, domestic, commercail and a small amount of industrail.

And looking at the formula in your example, vc is the cable resistance in milliohms per metre. This is the form more likely to be found by people buying cable for automotive applications (or ohms per metre, which equals milliohms per meter / 1000)

Cheers,

Scott

Joined: 26 Jul 2003 Location: Brisney Land

ahh i love nerd talk Very Happy

honestly - i think you guys are great, and i'm sure everyone appreciates the tips you freely give in here. armsup

Joined: 24 Dec 2003 Location: Brisbane, Australia

Guts wrote:

murcod wrote:

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!

I can tell you now that my rig is wired up perfect.

I an`t telling people to use bigger fuses, just you findout the load, select the cable to suit, then a fuse to suit. Only a dummy would go, 20a load, 35a cable and 30a fuse.

You find the load, then the cable, then the fuse. Its that simpal. each relats to the last.

And yes I am a sparky, domestic, commercail and a small amount of industrail.

No such thing as perfect Very Happy

35Amp cable & 30 amp fuse may be exactly what you want to feed 20A worth of spotlights due to voltage drop, Halogen lamps can loose 30% of their light output with a 10% voltage drop. A 20A fuse would be silly because fuses "wear" especially when bounced around in a 4x4 whilst at full load. 25A fuse would be OK. There's also the possibility of bigger lamps later. Running 20A cable at 20A will work but possible create voltage drop.

Fuses are only designed to protect the cable, look inside any appliance and you will find wiring that could melt down at way under the 16A fuse or 20A circuit breaker feeding it. England actually had fused plugs on appliances to suit the individual appliance. A 20A fuse will protect the cars internal wiring up to the cigarette socket. If you plug a 3A light on a 5A lead then put a 10A bulb in, the cable would get uncomfortably warm after a minute and start to smell and break down, a 20A bulb would see the 5A cable glowing hot melting it's insulation and the wire burning itself out, but still no blown fuse, But if that glowing wire was to end up shorting out against the seat base it may then create a 100A fault current it will blow the 20A immediately.

Your right about select the load, then cable, then fuse. Actually there's usually no selection n the load, more like, "this is load what you need to make work", rarely you can say "can you use a smaller load?"

I'm also a Sparky, with more industrial & commercial experience than domestic.

Joined: 21 Oct 2002 Location: Brisbane

Rainbow Warrior wrote:

Your right about select the load, then cable, then fuse. Actually there's usually no selection n the load, more like, "this is load what you need to make work", rarely you can say "can you use a smaller load?"

I'm also a Sparky, with more industrial & commercial experience than domestic.

Thank you, and select the load is just a typo Rolling Eyes

but you knew what I meant.

Ya I am more domestic, shit I look after about $600,000.00 of houses (thats just electrical) a year and 6 blokes as my boss doesn`t want to know about it anymore as he is retiring in 18 months time.

Joined: 24 Dec 2003 Location: Brisbane, Australia

Guts wrote:

Rainbow Warrior wrote:

Your right about select the load, then cable, then fuse. Actually there's usually no selection n the load, more like, "this is load what you need to make work", rarely you can say "can you use a smaller load?"

I'm also a Sparky, with more industrial & commercial experience than domestic.

Thank you, and select the load is just a typo Rolling Eyes

but you knew what I meant.

Ya I am more domestic, shit I look after about $600,000.00 of houses (thats just electrical) a year and 6 blokes as my boss doesn`t want to know about it anymore as he is retiring in 18 months time.

I just got to supervise 4 fitters, a boilermaker and toolmaker, and look after & upgrade about $5million worth of machinery mechanically, electrically & financially, whilst keeping it running 2 shifts per day, 6 days a week.
Luckily I only do breakdown work now, got my old boss & contracting company working for me now for installations Very Happy