WORKING OUT WHAT YOU NEED

1 Study wiring diagram

Find a wiring diagram that corresponds exactly to your car or is as close as possible to its original arrangement. Make a copy of it so you can tick off wires as you fit them. Identify anything that’s irrelevant (eg, optional extras that your car has lost or never had). The diagram may specify the diameter of each wire. If not, you’ll have to work out the current in each wire and buy accordingly.

2 Calculate current

To work out the current, divide the wattage by 12 for a 12V car or 6 for a 6V car. For example: 12V brake light bulbs are 21W each, so each uses a little under 2A. There are two, so the switch handles 4A.

Constant demand

Use wire specified somewhat above the required current. For something in continuous use, like headlamps, use wire rated 1.5 times the current.

Intermittent use

For items in intermittent use, like the brake lights, horn and indicators, you can be less generous. A brake light could use 2A wire and the pair be supplied with a 5A wire.

4 Wire colours

It may not be possible to obtain all the wire colours needed, especially if striped wire is involved. If you have to use an alternative colour, then make a note of it on the diagram as you fit the wire.

5 Length

You’ll have to estimate the lengths needed – perhaps by dissecting the old loom or by doing a bit of measuring on the car. Remember to allow plenty of extra length for convenient routing.

6 Connectors

Survey the connectors you’ll need, both at each end of the wire and at any junctions in between. Buy good-quality connectors. The ones in cheap selection boxes don’t usually fall into this category…

7 Modular looms

Modern classics have modular looms made of plug-together sections. Buy new plugs or terminals that will fit into the old ones. Do you actually need to replace the full loom – or just some modules?