Cable Sizing Explained

Choosing the right cable size is one of the most important parts of safe, reliable 12V wiring.

If your cable is too thin, it can:

• overheat

• cause appliances to malfunction

• create fire risks

• reduce battery efficiency

If your cable is too thick, the only downside is cost.
So our mission is simple: use cable that’s thick enough for the job — every time.

1. Why Cable Size Matters

12V systems are especially sensitive to voltage drop.
Because the voltage is low, even a small drop can stop things working properly:

• Fridges fail to start

• Diesel heaters lock out

• LED lights flicker

• Pumps lose pressure

• Inverters shut down

Correct cable sizing prevents these problems before they happen.

2. Three Things Affect Cable Size

There are only three factors you need to think about:

① Current (Amps)

How much power the appliance draws.
More amps = thicker cable.

Examples:

• LED lights: 1–3A

• Diesel heater: 8–10A startup, 1A running

• Fridge: 4–6A average

• Water pump: 4–8A

• Roof fan: 2–3A

If you don’t know the amps, use our tool — it calculates them automatically.

② Cable Length (There + Back)

Every 12V circuit has two wires:

• positive out

• negative return

This means your true cable length is double the distance.

Example:
A 4-metre run from the battery to the fridge = 8 metres of cable in the circuit.

Longer cable = more resistance = more voltage drop.

③ Voltage Drop (%)

Voltage drop is the loss of voltage along the cable.
In van conversions, the general rule is:

• Up to 3% drop → excellent

• Up to 5% drop → acceptable for lighting

• Above 5% → avoid

Sensitive appliances (fridges, heaters) MUST stay under 3%.

3. The Easy Way: Use Our Cable Sizing Tool

We’ve built a calculator that does everything for you — no maths needed.

👉 Use our Cable Sizing Tool to instantly get the correct cable size. to instantly get the correct cable size.

It takes into account:
✔ Amp load
✔ Voltage
✔ Circuit length
✔ Voltage-drop rules
✔ Special cases like fridges & diesel heaters (minimum 6 mm²)

4. Typical Cable Sizes Used in Van Builds

To keep things simple, here are the sizes most van builders use:

5. Understanding Manufacturer Requirements

Some appliances (especially fridges) specify a fixed minimum cable size, normally:

• 6 mm² for 12V compressor fridges

• 6 mm² for diesel heaters

Even if the cable run is short, always follow the manufacturer’s minimum.

6. When Should You Upgrade to a Thicker Cable?

Use a larger size if:

• The run is long (over 5m one-way)

• The appliance is sensitive to low voltage

• You’re adding multiple devices to the same circuit

• The wire will be inside insulation or hot areas

• You want maximum efficiency

Bigger cable = less power wasted = longer battery life.

7. Quick Reference: Resistance of Common Cable Sizes

8. Still Not Sure?

Choosing cable sizes can be confusing at first – but you only need to get it right once.

Use the calculator to avoid mistakes:

👉 Use our Cable Sizing Tool to instantly get the correct cable size.