I am hoping this may simplify and help many of the people out there trying to do a few things for themselves with 12V wiring. Please be aware though that even 12V at high current is dangerous if not handled properly. For info my credentials say I am an Electronics Engineer - while most detailed theory is misplaced due to age, the basics of electricity are very solid.
If this is repeat information for some, forgive me, but I see a lot of questions on the forums asking about wire sizes and "what should I use for ..." and not many accurate answers - Collyn Rivers has put up some good articles in this area and how it can the operation and performance of some equipment.
I will put the facts first and waffle second for those who do not need to read past the facts. And I am happy to enter into discussion and correct myself anywhere as necessary.
*** SImple Bottom Line ***
There is a cost to doing it right and a cost saving in doing it only once. There "ain't no free lunch".
You need to know two things to choose the right cable - how much maximum current is going to be required in the load, and what is the minimum voltage required by the load to guarantee proper performance as per manufacturers design specification.
From this and the simple calculation below, you can then go to a retailer / supplier and get the right cable based on two specific parameters - the maximum continuous current and the resistance per unit measure.
Yes there are many variables to add to this, but we want to keep it simple and choosing cable over and above what you calculate at maximum possible requirements will allow for many of the variables we are not considering (such as losses through connections at batteries, fuse boxes, the load itself, in current, and duty cycles).
** Simple Calculation ***
You need to know how far the cable is going to travel from the 12V source and the maximum current the load (one or more devices) need.
Determine cable required by calculating the Maximum Resistance the cable can have to achieve the maximum voltage drop allowed over the distance required. I will use ohms per 1000m for this formula.
Max Resistance = (Max Volt Drop x 1000) divided by (Max continuous Current x Distance in metres)
So for 30A over 5 metres with a maximum 0.36V drop
Max Resistance = (0.36 x 1000) / (30 x 5) = 2.4 ohm per 1000 m
So whatever cable you are looking to buy (solid, low or high strand cable) it MUST simply have a resistance of less that 2.4 ohms per 1000 metre AND be rated for 30Amps continuous current to suit this example.
*** Good References ***
The following link which is a very handy guide as to what size cable is required for what current over what distance - note this has nothing to do with how much current a cable can carry!!
http://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html
The following links are for cable sizes and equivalents in various standards - note B&S is the same as AWG.
http://www.engineeringtoolbox.com/wire-gauges-d_419.htmlhttp://www.dave-cushman.net/elect/wiregauge.html
*** Waffle ***
Forget looking a a retail spec of (for example) 8B&S 100Amp 7.9mm2 cable - this is meaningless for what you are trying to achieve. It might carry 100Amp for 10 seconds before is self destructs. More realistic spec for this cable is that 8B&S should be near 8.3mm2 (not 7.9mm2) which relates to resistance of the cable, and would carry 30Amp over 15 feet, or only 15A over 30 feet ao as to maintain a low loss of voltage over the length of cable.
The only real meaningful specifications are maximum Continuous Current rating (Amps) and resistance per unit measure (ohms per metre). The cable has to be capable of carrying the amount of current you need continuously (or based on duty cycles of high current / no current which gets far more complex) so that it does not break down due to heat, and has to have a low enough resistance such that you do not lose a lot of voltage over the length of the cable.
If they cannot supply this information, don't buy the cable - and if they can supply the information even then you have to trust the manufacturer's specification is accurate so the reputability and country of origin of the manufacturer also comes into question. If it looks thinner and costs less it is probably not up to spec regardless of what they tell you.
You need to stay within the maximum continuous current rating of the cable as determined by the load you are supplying, and you want the voltage at the load (not the source) to be no less than 11.64V (based on 3% loss of voltage over the length of cable as being the general maximum loss for 12V circuits). This does of course depend on the device - globes will care a lot less than fridges or TV's and manufacturers will each have their own design specifications and minimum voltage for reliable operations.
Note that maximum continuous current ratings of cables are also affected by the way they are run based on heat dissipation - as in free air, inside insulation materials, in conduit, in ground, and combinations of these - and you are not likely to get the individual specifications of the cable. Retailers will always tend to promote the maximum values to make the product sound good - eg. the 100A rating on the 8B&S cable I have seen.
The 2 most important factors in your finished product are capability to carry the current required and a low voltage drop from the source to the load. Quality of cable and connectors, and clean, tight fitting connections from souce to destination is paramount. This is far more critical at 12V operations than 240V operations.
The source might not be the battery - you can have a number of intermediate points at time such as fuses, distribution panels, terminal blocks, etc. Like a
water reticulation system, you have the biggest pipes at the start and they reduce down as you start to distribute the
water to the individual destinations and the required
water flow becomes less.
If in doubt or the cable and connector size is an issue, run 2 or more runs of smaller cable from the source to the destination instead of one larger cable and use terminal blocks (both Pos and Neg) as necessary.
Use distribution points (terminal blocks or fuse boxes) to feed devices independently rather than try and feed a lot of devices from one cable run. This will make calculations less complex.
The reason you use high strand count cables as opposed to single solid conductors or low strand count household electrical cable is simply for flexibility of the cable and thus resistance to fracturing at joints due to constant vibration and movement in a motor vehicle. Nothing to do with current carrying capacity!!
A general rule is that (for a 12V circuit) the voltage drop should be less than 0.36V (3% of 12V) from source to load. This means you need a minimum of 11.64V at the load - obviously some devices (light globes) will care less than others (fridges, TV's) in terms of operations, and every manufacturer will have their own minimum specification).
Voltage drop over cable is determined by current flowing and resistance of the cable - again nothing to do with how much current a cable can carry!! Hence the important specifications being the continuous current rating of a cable and its resistance per unit measure.
This is based on a 12V source - and of course battery voltage (source) will probably start out at more tha 12V but will reduce to less than 12V untill it reaches the point where no matter what you have done in terms of cabling the load will not work (or not work properly).
Ken