Wire Size Calculator – Voltage Drop, AWG & mm² (12V to 240V)

Check voltage drop over a cable run and find the smallest wire gauge that keeps it within limits — for cars, RVs, boats, solar and home circuits.

This free wire size calculator works out the voltage drop for any cable run and finds the right wire gauge in AWG or mm². Use it for 12 V and 24 V DC wiring in cars, RVs, vans, boats and solar systems, or for 120 V, 240 V and three-phase AC circuits. Enter the voltage, current and one-way distance to see the voltage drop, the voltage at the load and the smallest wire that stays within your limit.

Voltage drop formula

DC / single-phase: Vdrop = 2 × L × I × R
Three-phase: Vdrop = √3 × L × I × R
R = resistivity ÷ cross-section area (Ω per metre)

L is the one-way length and I is the current. Resistivity is taken at 20 °C: 1.724 × 10⁻⁸ Ω·m for copper and 2.826 × 10⁻⁸ Ω·m for aluminium. Warm wires have higher resistance (about 0.4% more per °C), so leave some margin.

Worked example: 12 V fridge in a camper

A 20 A load 15 ft (4.57 m) from the battery on 10 AWG copper (5.26 mm², 3.28 mΩ/m): 2 × 4.57 × 20 × 0.00328 = 0.60 V, which is 5.0% of 12 V — too much. Moving up to 8 AWG cuts it to 3.1%, and 6 AWG to 2.0%.

12V wire size chart (3% voltage drop)

Smallest copper wire (AWG) that keeps the voltage drop at or below 3% on a 12 V DC circuit, by current and one-way length. For 24 V you can usually go about three sizes thinner.

Current	5 ft (1.5 m)	10 ft (3 m)	15 ft (4.6 m)	20 ft (6.1 m)	30 ft (9.1 m)
5 A	18	14	12	12	10
10 A	14	12	10	8	6
15 A	12	10	8	6	6
20 A	12	8	6	6	4
30 A	10	6	6	4	3
40 A	8	6	4	3	1
50 A	8	4	3	2	1/0

Voltage drop only. The wire must also be rated for the current (ampacity) in your installation, and every circuit needs a fuse or breaker sized to protect the wire.

AWG to mm² and resistance (copper, 20 °C)

AWG	mm²	Ω per 1000 ft	Ω per km
18	0.82	6.39	21.0
16	1.31	4.02	13.2
14	2.08	2.53	8.28
12	3.31	1.59	5.21
10	5.26	1.00	3.28
8	8.37	0.628	2.06
6	13.3	0.395	1.30
4	21.2	0.249	0.815
2	33.6	0.156	0.513
1/0	53.5	0.098	0.322

Related calculations

Find the current a load draws from its watts with the Ohm's law calculator (current = watts ÷ volts).
Wiring a battery bank or power station? See how long it lasts with the battery runtime calculator.
Sizing solar panels before running the cables? Use the solar panel calculator.

Frequently asked questions

What size wire do I need for 30 amps at 12 V?

For a 3% voltage drop, 30 A at 12 V needs 10 AWG copper for a 5 ft run, 6 AWG for 10 ft and 4 AWG for 20 ft (one-way length). Always check the wire's ampacity and protect it with a correctly sized fuse.

How far can I run 12 AWG wire on 12 V?

With 3% maximum voltage drop, 12 AWG copper carries 10 A about 11 ft one way and 20 A only about 6 ft. On a 120 V circuit the same wire can carry 20 A about 57 ft, because the allowed drop is ten times larger.

How do I calculate voltage drop?

Multiply the round-trip wire length by the current and by the wire resistance per metre: Vdrop = 2 × L × I × R for DC and single-phase AC. Divide by the supply voltage and multiply by 100 for the percentage. The calculator does this for every AWG size and picks the smallest that meets your limit.

What is an acceptable voltage drop?

A common guideline (from the US National Electrical Code informational notes) is no more than 3% on a branch circuit and 5% total from the service to the load. For 12 V systems in cars, RVs and boats, 3% is a good target for most loads and 10% for non-critical lighting.

Why is voltage drop worse on 12 V systems?

For the same power, a 12 V circuit carries ten times the current of a 120 V circuit, and the voltage lost in the wire rises with current. The same 1 V drop is under 1% at 120 V but over 8% at 12 V — which is why low-voltage DC runs need much thicker cable.

Should I use the one-way or round-trip length?

Enter the one-way distance from the source to the load. The calculator doubles it for DC and single-phase circuits, because current flows out on one conductor and back on the other.

Does this calculator check if the wire is safe for the current?

No. It only checks voltage drop. Every wire also has a maximum current rating (ampacity) that depends on insulation, temperature and installation method. Always confirm the size against your local electrical code or a qualified electrician; for mains wiring, the code minimum may be larger than the voltage-drop result.

What is the difference between AWG and mm²?

AWG (American Wire Gauge) is used mainly in North America; smaller numbers mean thicker wire. Most other countries specify cross-sectional area in mm². The calculator shows the mm² equivalent of each AWG size.

Wire Size & Voltage Drop Calculator