another wire question

Current rating of a wire is only an indication of what the wire will handle before the insulation degrades. Your single piece of wire will have a certain resistance (arguments sake 0.1 ohm). Three runs of that same cable over the same length with the same current drawn, will be 1/3 of the resistance, ie 0.033333 ohm.

The bit that people really miss is that it doesn't really matter how much the "current capacity" of the cable is, but it is the resistance of that cable that is the main factor when wiring things up.

In you example if you had a 15 amp draw using my 0.1 ohm resistance Then by V= IR =15 x 0.1= 1.5volts, then you would develop a voltage drop along the length of the cable of 1.5 volts, meaning you put 12 volts in one end and at the other end (with the load applied) you will see 10.5 volts.

+12-----------load---ground
............^
.....1.5v drop here

this is obviously not good if you like your coldies in the bush (fridge cuts out way to early despite battery being fine at its terminals)

The solution as you alluded to is to run bigger cable, or do multiple runs of the same cable hooked up in parallel. In this situation (of our example) you end up with V = IR = 15 x 0.033333 = 0.5 volts lost along the cable length. Go celebrate with a coldie.....you fridge still works as it is still getting 11.5 volts.

Coldies, the universal education tool...............lol!

Chips,
Voltage drop depends on the resistance per metre of the cable.

Calculate using E= I * R.

Subtract this result from the applied voltage.

Yes you could draw 300amps if the cables are in parallel.
Therefore the resistance of the three in parallel would be less than one cable.
This is because you have three times the cross sectional area of cable.

This is mathematically proved by the following

Total resistance of parallel resistances equals

1/R = 1/r1 + 1/r2 + 1/r3

The total resistance is less than the value of the smallest resistor is the easy way to remember parallel circuits.

Hope this helps

Garth

Hi Chips59

8 guage cable is rated at around 60a with resistance approx .0025/m and so your 10m of cable at 15amps would drop around 0.37v or 1/3 of a volt approx.

3 in parallel does give 3 times the rating and 1/3rd the volts drop.

Robin Miller

i am running two 60 ltre everkools in the camper. I started with main battery then red ark to aus. battery then 3x 8awg cables x 300 mm to 60 amp auto circuit breaker then 3 x 8awg 4mtres to anderson plug (175amp) then 2mtres of 3x 8awg to 2nd 60 amp auto breaker then 3x8awgx 300mm to two 100amp hour deep cycles in parallel. then one 8awg 2.5mtres to one fridge and another 8awg 2.5 mtres to the other fridge. both with 20 amp auto circuit breakers. the reason i am using 8 awg is because i have a few rolls of the stuff.