Wiring guide for 24V systems

By choosing a 24V system over a 12V system, you have saved yourself a great deal of copper! Because to transmit a certain power, a cable need be only half as heavy as its 12V counterpart.

What cable to use

Tri-rated cables

A selection of tri-rated multi-strand cables

When installing wiring in boats and vehicles, it is important that a multi-stranded cable is used. Household wiring with a single or a few stiff strands is not suitable as it is liable to break when exposed to vibration.

It is also wise to stick to the standard colours of red (positive) and black (negative) when installing DC wiring, to avoid confusion with any AC wiring.

Standard or slimline automotive wiring can be used, but a tri-rated cable will tolerate higher temperatures without burning - a good idea if your cables are running through insulation or in tight bundles.

Our sister company, Midsummer Energy, sells a complete range of cables and connectors ideal for all internal wiring on 24V DC systems.

Cables installed outdoors should be resistant to ultraviolet light and abrasion; the excellent FlexSolar cable is perfect for the job.

How thick?

To work out what thickness of cable you need for a particular job, first you need to know the maximum current it will need to carry in amps. Often, this will be written on the appliance somewhere. If you can only find a power rating in watts, simply divide it by 24.

Next, you need to measure the length of the cable. Longer cables need to be thicker to transmit same power because longer cables tend to suffer voltage drop - where the cable itself uses up some of the power. You can measure the length of either the positive or negative cable, you don't need to add them together.

Now just check the chart below to select the appropriate cable thickness. This will tell you the lightest cable you can get away with, while losing no more than 5% of the voltage in the cable. There never any harm in using a heavier cable.

Minimum cable size in mm2 (for max 5% volt drop on 24V DC wiring)
AmpsCable length (one way, metres)
 1251015202530
10.750.750.750.750.750.750.751.5
20.750.750.750.751.51.51.52.5
30.750.750.751.51.52.52.52.5
40.750.750.751.52.52.544
50.750.750.751.52.5446
7.51.51.51.52.54688
101.51.51.5466816
152.52.52.5816161616
202.52.52.5616161625
25444816162525
304461616252535
406661625253535
60881625255050 
80161616253550  
1001616163550   
12525252550    

If you want to be a bit more accurate, or if you have cable sizes other than the standard tri-rated ones listed above, you can calculate the minimum cable size yourself.

To ensure a voltage drop of no more than 5% (1.2V), the formula is:

Minimum cable area (in mm2) = 0.0273 x current (in A) x length (in m)

Again, the length here is the distance from the battery to the appliance only, and not all the way back again.

Fuses

Automotive fuses

Various automotive fuses CC Havarhen

That's not the end of the story - fuses are an essential safety feature of all 24V wiring (compulsory on boats). The fuse can be anywhere on the positive side of the circuit, but it makes sense to put them all in a convenient place by using some sort of switchboard. They can be fitted directly to cables using an appropriate in-line fuse holder.

Fuses protect your cables not your appliances. Cables get hot when carrying currents close to their design limits, and if a fault somewhere causes an even higher current they can start a fire! Fitting a correctly-rated fuse ensures the circuit is broken before the fault starts a fire.

Always fit a fuse with a equal or lower rating than the cable. These ratings are a little on the conservative side, to allow for cables that might get usually hot, for example when in contact with insulation:

Maximum fuse ratings
Cable thickness (mm2)Fuse rating (amps)
0.755
1.510
2.520
430
640
850
1680
25100
35150
50200

The one place where a fuse should not be installed is the engine starter cable. Starter motors can draw very high currents that will blow even the largest fuses, but since they are only switched on for a few seconds at a time the risk of overheating is minimal. This is why you should not crank your engine for more than about 20 seconds at a time, and allow a few minutes for everything to cool down between attempts.

Circuit breakers make a good alternative to fuses. Just ensure you use breakers that are rated for DC wiring not just AC, or they can be damaged each time they trip.

Making good connections

Crimp-on terminals come in all shapes and sizes. There's little point in investing in good quality, thick cables unless you make a sound, strong, low-resistance connection at the ends!

Some devices, such as our MorningStar 24-volt solar regulators, can connect directly to a bare wire but most of the time you need a suitable terminal.

Battery terminal clamps generally require a ring terminal with an 8mm hole. Regulators and solar panels often take a fork terminal, and switchboards, fuses and some appliances often require spade terminals.

Use the correct terminal for your cable gauge. Crimps with red sleeves fit 0.5 to 1.5mm2 cables, blue terminals fit 1.5 to 2.5mm2 and yellow 4 to 6mm2. Larger cables have specific terminals, generally uninsulated.

If possible use the proper tools to attach them, if not a vice will do a reasonable job. The larger uninsulated terminals can also be soldered on, but take care not to let the solder run down the cable as this can create an inflexible weak spot.

For that professional finish, cover the join with some heat-shrink insulation.