12v cable sizing

[Colin K]

Hello,
I wondered if someone could point me towards a plain English guide or give me some advise from experience on what size cable to use from my domestic bank to the fuse board?
I have not had to worry too much in the past as my whole system was 24v and so the equation less critical and the batteries very close to the fuse board/ engine.
My big amp eaters are the radar when on transmit and the Waeco compressor all the rest are small so I think the maximum drain at worst case sinario would be @ 10 amps but I guess I have to think about the charge from the alternator the other way??
The distance from my proposed battery position to the fuse board is going to be 6 meters.

Thanks for any advise,
Cheers, Colin.

 

[VicS]

Here's one ... (note you have to double the distance to get the total length of wire)

http://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html

Here's another ( It automatically doubles the distance for you)

http://www.freesunpower.com/wire_calc.php
Really for solar panels but still applies




Personally I usually calculate from the cable resistances quoted here

http://www.engineeringtoolbox.com/wire-gauges-d_419.html

Usual to allow a max of 3% volts drop but you don't want to drop 3% between battery and panel then another 3% between panel and equipment

 

[GrahamM376]

There's a useful calculator here for varying loads and distances - http://www.bulkwire.com/wireresistance.asp

 

[VicS]

There's a useful calculator here for varying loads and distances - http://www.bulkwire.com/wireresistance.asp

What that does not tell you is that you need to enter twice the distance, ie length of positive plus length of negative wire, or the "there and back" distance.

 

[GrahamM376]

What that does not tell you is that you need to enter twice the distance, ie length of positive plus length of negative wire, or the "there and back" distance.

Strangely, nor do others I've just looked at. Wonder if they've already taken the return into account when user has to input the cable run? No doubt someone will do the calcs to check.

 

[VicS]

Strangely, nor do others I've just looked at. Wonder if they've already taken the return into account when user has to input the cable run? No doubt someone will do the calcs to check.

The two in my first reply do state whether or not its taken into account. Thats why I always refer people to one or other of those two.
The first says, " calculate the total length of the wire from the source to the device and back again".
The second says, "Step 3 - This is the distance in feet from your solar panels to the charge controller and battery bank location. Even though you will actually be running 2 wires, one negative & one positive, do NOT double the distance. The Wire Size Calculator assumes this and does it for you in the calculation."

The problem is that some take it into account, some dont. Some state which, some dont.

 

[jon711]

Try this

http://www.bluesea.com/resources/535/Voltage_Drop_in_Conductor_-_Wire_Sizing_Chart

I would always aim for 3% or less voltage drop...

Jon

 

[Colin K]

Hello,
Thanks for all the replies chaps. I estimate the load to be 10 amps but I have doubled it to 20A. Put in the length which is the distance total of the positive and negative and it says I need 8 gauge or just over 8mm cable. I must confess I thought I would need monster cable and was looking at 25 or 40 mm but I think your standard 16mm stuff off flea bay will be ok?
Hope I have it about right and thanks again.

Cheers, Colin.