Size your 12V / 24V / 120V electrical installation (AWG)
Learn in just a few minutes how to size your electrical wiring for your van build, RV or boat. We walk you through the tool step by step and show you how to read the AWG results.
This free tool lets you precisely size your electrical wiring for any van conversion, RV or vessel. Whether you're building the electrical system for your van build, wiring your RV, outfitting your camper or installing the electrical system on your boat, our calculator instantly determines the correct AWG wire gauge, the right fuse rating and checks the voltage drop on every circuit.
Wire sizing is governed by two fundamental constraints: the thermal limit (the wire must not overheat) and the voltage drop (limited to 3% to ensure your devices work correctly). Our calculator applies both rules automatically and selects the more conservative gauge.
The basic formula is I = P / V (amps = watts / volts). A 60W fridge on 12V draws 5A. A 2000W inverter on 12V draws 167A. The higher the amperage, the larger the wire gauge needs to be (lower AWG number).
In a 12V system, distance matters enormously. Current flows through both the positive and negative wires: for a device 10 ft from the battery, the actual electrical path is 20 ft. That's why our formula uses 2 × L in the wire size calculation.
The calculator determines the minimum wire gauge meeting both constraints, then recommends a fuse rated at 125% of the nominal amperage. The fuse protects the wire (not the device): it must always be below the maximum thermal capacity of the chosen wire.
Here are the recommended wire gauges for the most common equipment in a van build, RV or boat (copper wire, max 3% voltage drop, 10 ft run):
| Equipment | Power | Amps (12V) | Min. gauge (10 ft) | Fuse |
|---|---|---|---|---|
| LED lighting | 20W | 1.7A | 18 AWG (0.75 mm²) | 5A |
| Water pump | 40W | 3.3A | 16 AWG (1.5 mm²) | 5A |
| 12V fridge | 60W | 5A | 14 AWG (2.5 mm²) | 10A |
| MaxxFan vent | 35W | 2.9A | 16 AWG (1.5 mm²) | 5A |
| Diesel heater | 120W | 10A | 12 AWG (4 mm²) | 15A |
| 1000W inverter | 1000W | 83A | 4 AWG (25 mm²) | 100A |
| 2000W inverter | 2000W | 167A | 1/0 AWG (50 mm²) | 200A |
| Anchor windlass | 1500W | 125A | 2 AWG (35 mm²) | 150A |
| Bow thruster | 3000W | 250A | 2/0 AWG (70 mm²) | 300A |
These values are for a 10-foot wire run. Use the calculator above to get a precise result for your actual wire length.
A van build electrical system is typically 12V with one or two house batteries (AGM, gel or lithium LiFePO4). Common equipment includes: LED lighting, a Dometic or Iceco fridge, a Shurflo water pump, a diesel heater, a MaxxFan roof vent, and a 12V to 120V inverter for AC outlets. Average wire runs range from 6 to 20 feet.
RV wiring can be 12V or 24V depending on the chassis. Installations are often more complex than a van: solar panels, MPPT charge controller, DC-DC charger, high-capacity lithium battery bank, and an energy management system. Wire runs are longer (up to 25-30 ft), requiring heavier gauge wire.
Boat wiring follows the same electrical principles but with additional constraints: saltwater environment, vibration, and ABYC standards. Always use tinned marine-grade wire for corrosion resistance. Boats typically run 12V (small boats) or 24V (vessels over 40 ft). Specialized equipment includes: anchor windlass, bow thruster, watermaker, navigation lights.
The gauge depends on the amperage (watts / volts) and the wire length. For a 12V fridge (60W) with 10 ft of wire, you need a minimum of 14 AWG. For a 2000W inverter, you'll need at least 1/0 AWG. Our calculator accounts for voltage drop (max 3%) and thermal limits.
The formula calculates the minimum cross-section based on amperage, wire length, copper conductivity, and allowable voltage drop (3%). At 12V, the max drop is 0.36V. Our calculator handles this automatically and displays results in both AWG and mm².
The fuse should be rated at 125% of the nominal amperage. For example, a 10A device requires a 15A fuse. Standard ratings: 5A, 10A, 15A, 20A, 25A, 30A, 40A, 50A, 60A, 80A, 100A. The fuse protects the wire, not the device.
At 24V, the amperage is halved for the same wattage. This allows smaller wire gauges and reduces power losses. 24V is recommended for large RV installations with high power demands or long wire runs.
The sizing follows the same principles as a van build. In a marine environment, always use tinned copper wire for corrosion resistance, and allow an additional 20-30% safety margin. ABYC standards require more conservative minimum wire sizes.
Voltage drop is the loss of voltage along a wire due to its resistance. The longer or thinner the wire, the greater the drop. It's limited to 3% (0.36V at 12V) so devices receive sufficient voltage. Beyond this, a fridge might not start or an inverter may shut down.
AWG (American Wire Gauge) is the US standard — lower numbers mean thicker wire. mm² is the metric standard used in Europe. Common equivalents: 14 AWG = 2.5 mm², 10 AWG = 6 mm², 4 AWG = 25 mm², 1/0 AWG = 50 mm².
Both protect wires, but in a 12V van build, fuses are more common (cheaper, compact). Use an ANL fuse holder for heavy wires (battery to inverter) and a blade fuse box for distribution. Circuit breakers are useful on 120V AC circuits.
⚠️ Important disclaimer — This calculator is a sizing assistance tool designed to estimate wire gauge and fuse ratings. It does not replace the advice of a licensed electrician or a professional inspection of your installation. All electrical work should be performed in accordance with applicable codes (NEC / ABYC) and verified by a certified professional before use.