12 AWG to mm²
12 AWG (4 mm²) is the workhorse of 15–25 A DC circuits. It is the gauge most camper builders pick for the water pump, vent fan, and USB charger branches — heavier than the 14 AWG lighting circuit, lighter than the 10 AWG fridge feed. In 12V automotive applications it is the standard for accessory sockets, electric awnings, dual-purpose Maxxair fans, and small inverter feeds up to roughly 300 W. The metric pair is 4 mm², stocked in every auto-electrical shop in Europe.
All values on this page assume copper conductors at 20°C ambient. Aluminum has ~60% higher resistance for the same cross-section and is generally not recommended for low-voltage DC.
Calculate for your specific cable run
The tables below assume 12 AWG (4 mm²) copper. Use the calculator to confirm this gauge is correct for your exact load and length, or get a different gauge recommendation if needed.
= 60W
= 16.4 ft (one way, round-trip calculated automatically)
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Download Cable for iOS12 AWG Voltage Drop at 12V DC
Round-trip voltage drop for 12 AWG (4 mm²) copper wire at 20°C ambient. Cells highlighted in yellow exceed the 3% target; red cells exceed 5% and are not recommended.
| Current | 1 m / 3 ft | 3 m / 10 ft | 5 m / 16 ft | 7 m / 23 ft | 10 m / 33 ft |
|---|---|---|---|---|---|
| 5 A | 0.045 V (0.37%) | 0.13 V (1.11%) | 0.22 V (1.86%) | 0.31 V (2.60%) | 0.45 V (3.72%) |
| 10 A | 0.089 V (0.74%) | 0.27 V (2.23%) | 0.45 V (3.72%) | 0.62 V (5.20%) | 0.89 V (7.43%) |
| 15 A | 0.13 V (1.11%) | 0.40 V (3.35%) | 0.67 V (5.57%) | 0.94 V (7.80%) | 1.34 V (11.15%) |
| 20 A | 0.18 V (1.49%) | 0.54 V (4.46%) | 0.89 V (7.43%) | 1.25 V (10.41%) | 1.78 V (14.87%) |
| 25 A | 0.22 V (1.86%) | 0.67 V (5.57%) | 1.11 V (9.29%) | 1.56 V (13.01%) | 2.23 V (18.58%) |
12 AWG Voltage Drop at 24V DC
Round-trip voltage drop for 12 AWG (4 mm²) copper wire at 20°C ambient. Cells highlighted in yellow exceed the 3% target; red cells exceed 5% and are not recommended.
| Current | 1 m / 3 ft | 3 m / 10 ft | 5 m / 16 ft | 7 m / 23 ft | 10 m / 33 ft |
|---|---|---|---|---|---|
| 5 A | 0.045 V (0.19%) | 0.13 V (0.56%) | 0.22 V (0.93%) | 0.31 V (1.30%) | 0.45 V (1.86%) |
| 10 A | 0.089 V (0.37%) | 0.27 V (1.11%) | 0.45 V (1.86%) | 0.62 V (2.60%) | 0.89 V (3.72%) |
| 15 A | 0.13 V (0.56%) | 0.40 V (1.67%) | 0.67 V (2.79%) | 0.94 V (3.90%) | 1.34 V (5.57%) |
| 20 A | 0.18 V (0.74%) | 0.54 V (2.23%) | 0.89 V (3.72%) | 1.25 V (5.20%) | 1.78 V (7.43%) |
| 25 A | 0.22 V (0.93%) | 0.67 V (2.79%) | 1.11 V (4.65%) | 1.56 V (6.50%) | 2.23 V (9.29%) |
What is 12 AWG (4 mm²) used for?
Camper / RV applications
- Water pumps (Shurflo 4008, Whale Universal, 7–10 A continuous, 15–20 A surge)
- 12V cigarette sockets and 12V accessory outlets
- Maxxair / Fiamma roof vents on high speed (8–10 A)
- Small inverter feeds up to 300 W at 12V (~25 A)
- Compressor cooling fan branches
Boat / marine applications
- Electric heads (toilet macerators, 12–20 A surge)
- Anchor windlass remote and accessory feeds
- High-output cabin LED arrays
- VHF radio and chartplotter combined feeds
- Engine-room ventilation fans
Off-grid / solar applications
- Branch circuits in tiny homes (lighting + outlets combined)
- DC water pumps for off-grid plumbing
- Garden shed and workshop sub-feeds
- Solar charge controller to battery on 15–25 A systems
- Sub-panel branch feeders
When to step up
- Continuous current above 20 A — within 20% of ampacity
- Inverter feeds above 300 W at 12V (~25 A)
- Cable runs longer than 4 m at 12V with currents above 15 A
- Motor loads with inrush above 30 A (use 10 AWG to absorb the spike)
Fuse size for 12 AWG (4 mm²)
The fuse must protect the wire, not the load. For 12 AWG copper (4 mm²) the maximum fuse rating is 25 A. Below this ceiling, size the fuse at 125% of your continuous load current and round up to the next standard fuse size.
| Continuous Load | 125% Calculation | Recommended Fuse | Fuse Type |
|---|---|---|---|
| 2 A | 2.5 A | 5 A | Blade (ATC) |
| 5 A | 6.25 A | 7.5 A | Blade (ATC) |
| 8 A | 10 A | 10 A | Blade (ATC) |
| 10 A | 12.5 A | 15 A | Blade (ATC) |
| 21 A | 26.25 A | 25 A | Blade (ATC) |
Related conversions
Frequently Asked Questions
What is 12 AWG in mm²?
12 AWG is precisely 3.31 mm² by the IEC formula. The standard metric pairing in international cable tables is 4 mm² — the closest commonly stocked size. The VoltPlan calculator uses 4 mm² (25 A ampacity) as the metric equivalent of 12 AWG.
How many amps can 12 AWG wire handle?
12 AWG copper (4 mm²) is rated for 25 A continuous at 20°C in chassis wiring — the standard for RV, marine, and off-grid DC systems. This is enough for water pumps, USB outlet branches, vent fans, and small inverter feeds up to about 300 W at 12V.
What size fuse for 12 AWG wire on a 12V system?
Size the fuse at 125% of the continuous load and round up to the next standard fuse, capped at 25 A (the wire ampacity). Examples: 10 A load → 15 A fuse, 15 A load → 20 A fuse, 20 A load → 25 A fuse. Loads above 20 A continuous push 12 AWG close to its limit — consider 10 AWG instead.
Can I use 12 AWG for a 12V water pump?
Yes, 12 AWG is the recommended gauge for most camper water pumps. Typical pumps (Shurflo 4008, Whale Universal) draw 7–10 A continuous and 15–20 A at startup inrush; 12 AWG comfortably handles both. Use a 15 A or 20 A blade fuse. For larger pumps over 12 A continuous, step up to 10 AWG.
Is 12 AWG enough for a 12V inverter?
Only for small inverters up to ~300 W at 12V (~25 A continuous). A 500 W inverter draws ~42 A and needs 10 AWG minimum. A 1000 W inverter needs 6 AWG or 4 AWG. Match the wire to the inverter manufacturer spec — the rated input current on the inverter label is what determines wire size.