Choose the Right Wire Size for the Current

Why this happens

Every wire has resistance, so current through it causes heat and a voltage drop. Too thin a wire runs hot, which can melt insulation and start a fire, and drops so much voltage that accessories underperform. The safe current a wire can carry depends on its cross-section, insulation temperature rating, length and whether it is bundled with others, so a proper choice considers all of these.

What you need

Step by step

1. 1

   Find the circuit current

   Read the accessory's label or datasheet for its power or current. If it lists power, divide by the supply voltage: a 24 W accessory on 12 V draws about 2 A. Add up all accessories on the same wire. If in doubt, measure the actual current with a clamp meter or a meter in current mode on a bench supply, following the meter's instructions.

2. 2

   Add a safety margin

   Some loads draw much more current when they start, such as motors, pumps and lamps with inrush. Allow a margin above the steady current, and plan for the worst case. Write down the expected maximum continuous current for the circuit.

3. 3

   Choose the fuse rating

   Pick a fuse rated a little above the normal maximum current so it does not nuisance-blow, but no higher than the wire can safely carry. The fuse protects the wire, so the wire must be chosen to handle at least the fuse rating continuously. Place the fuse close to the supply point.

4. 4

   Check the wire maker's rating

   Look up the wire's current rating in its manufacturer's datasheet, noting its insulation temperature rating and any reduction for bundling or high ambient temperature. Choose a size whose rating is at or above the fuse rating in your conditions. Wire under a scooter's footboard can get warm, so pick a size with margin.

5. 5

   Calculate the voltage drop

   Resistance of copper wire is about 0.0172 ohm for one metre of 1 mm² cross-section, and it scales with length divided by cross-section. Remember to count the length there and back. For example, 3 m total of 0.75 mm² is about 0.069 ohm; at 4 A that drops about 0.28 V. Decide what drop your accessory can tolerate and size up if it is too much.

6. 6

   Convert between AWG and mm²

   Wire is sold in AWG and in mm². Smaller AWG numbers mean thicker wire, while larger mm² means thicker wire. Use the wire maker's table to convert rather than guessing, and check the actual copper cross-section, because some cheap wire is mostly insulation with thin copper inside. Prefer stranded, flexible wire for anything on a vehicle.

7. 7

   Record and label your choice

   Note the current, fuse rating, wire size, length and calculated drop for each circuit in your wiring diagram. After installation, run the circuit at full load and check that the wire and connectors stay near ambient temperature with an infrared thermometer. A warm wire is a sign it is undersized.

How to tell it worked

Each circuit has a fuse matched to its wire, the calculated voltage drop is acceptable, and under full load the wire stays close to ambient temperature while accessories work at full brightness or speed.

Questions