12 Volt Frequently Asked Questions (FAQs)

In this section are answers to many of the commonly asked technical questions related to 12 volt wiring. We hope you find this useful as a quick reference to save you time when deciding which parts to select for your project.


Cables & Wiring

Q Is your cable specified by the outside diameter of the cable, including insulation?

No, all our cable is specified by the cross-sectional area of the copper conductor (in mm2) which is related to the current carrying capacity. Cable insulation thickness changes by manufacturer and cable type so you should never rely on outside diameter alone.

Q What do the red / blue / yellow colours of pre-insulated crimp terminals mean?

This relates to the cable size that should be used with them.  Red suits 0.5-1.5mm2 cable, blue suits 1.5-2.5mm2 cable, yellow suits 3.0-6.0mm2 cable.

Q I have cable larger than 6mm2 so what terminals should I use for the ends?

For cable over 6mm2 you will need to use copper tube terminals.

Q How do I fit copper tube terminals to larger cable?

You will need to use a heavier duty crimping tool such as this one (10mm2 max cable.) or this one (120mm2 max. cable).

Q Can I solder copper tube terminals rather than crimp them?

You can, but it is not recommended in automotive applications due to potential cracking of the solder with vibrations over time. Crimping is also neater, cleaner and less risky than soldering (care must be taken when soldering not to set the insulation alight when heating the terminal).

Q My inverter is rated at 1000W but I how do I know what cable size to use to connect it to the battery?
A For a 12V system divide 1000 (W) by 12(V) to give you 83 (A) (Current = Power/Volts). Now choose a cable that is rated to at least this, plus a margin of safety  - we would recommend 25mm2 (170A) in this case to allow for surges.

Q I have heard the voltage drop also needs to be taken into account when selecting cable, but how do you calculate this?

See our voltage drop calculator

Q My battery is 120Ah, so do I need a 120A cable to connect it?

No. The capacity of a battery in Ah is unrelated to the cable size you need to use to connect it to a charging source or load. You should rate the cable based upon the maximum charge or discharge current that you expect to put through the cable, plus a margin of safety. This also applies to cables used to inter-connect batteries.

Q How do I connect two 12V batteries in parallel?

Connect the positive terminals together and the negative terminals together. This keeps the system voltage at 12V but doubles the capacity in Ah.

Q How do I wire my lighting circuit so that I can switch it on and off from 2 different locations

For a 2-way lighting circuit you will need 2x On/On switches. Each switch has a common terminal (normally the centre terminal) and 2 switched terminals (normally the outside terminals).  Connect your +12V supply to the common terminal of one of the switches, then connect the common terminal of the other switch to the +12V input to your lights.  You then connect the switched terminals of the switches together (imagine a train track running between the terminals).

Q What cable size should I use for my compressor fridge?

The motors driving the compressors in these fridges tend to be quite sensitive to voltage drop and too high a drop can prevent them starting-up.  The fridge manufacturer's handbook will usually recommend a cable size for a given cable run length (from battery to fridge) and this can be much larger than you would expect (perhaps between 6 and 10 mm2). This large size is not because a high current will be drawn (these fridges tend to draw only around 2-3A); it is to prevent voltage drop being an issue.

Q How do I connect two different sized cables together (e.g. small cable to much larger cable)?

Connecting two dissimilar sized cables can be tricky as most connectors are designed to connect similar sized cable. One area where this problem is often encountered is in the above example of extending the cables on a compressor fridge where the flying leads from the fridge are usually quite small gauge but you need to connect to larger gauge cable to prevent voltage drop. The best option here is to terminate the cable ends with suitable ring terminals (normally insulated terminals for cable up to 6mm2 and copper tube terminals for larger cable) and then use a terminal block. This provides a professional, secure connection.

Another option if you are connecting two dissimilar cables together that are 6mm2 or smaller is to use a splice connector. The cable ends can be terminated with suitable 6.3mm wide female blade terminals and then joined to the splice connector.



Q Should I fuse my circuits and, if so, which cable should I fuse?

Yes, we would always recommend fusing the positive cable of each circuit. The fuse should be located as close to the power source (e.g. battery) as possible.

Q How do I know what value fuse to use?

You should protect a cable with a fuse that is lower than the cable's rated value in Amps (A). We recommend fusing at around 70-80% of the cable's rating.


LED Lighting

Q Can I dim LED lights?

It is possible to dim some LED lights, however, manufacturers normally state that their lights are not suitable for dimming unless this is specifically stated in the specification, so we cannot guarantee that any of our lights are dimmable.


Q Which wire is positive and which is negative?

If it's not clear which is positive and which is negative try connecting both ways around. You can't damage the lights as they are all protected against reverse polarity.

Q How can I tell how bright an LED light will be?

LED lights cannot be compared directly with traditional incandescent bulbs (which are described as having an outputs in Watts) because they draw so little power in comparison. Their output is normally specified in 'effective lumens' and we have created a guide to allow you to compare this with the outputs of other types of incandescent bulbs.