Free Online Calculators
Wire size, voltage drop & fuse for 12/24/48 V DC systems
This calculator determines the right wire size, voltage drop and recommended fuse rating for 12V, 24V or 48V DC systems, such as in RVs, boats or solar setups. It uses current, cable length and allowed voltage drop based on Ohm's law.
Enter the one-way run — the return conductor is already in the formula.
Start from the current, the one-way cable length and the voltage drop you're willing to accept, then work out the cross-section that keeps the drop within that limit. In DC circuits the current flows out and back, so the resistance of both conductors counts — enter load, system voltage, length and target voltage drop and compare the result with the ampacity the wire is rated for.
At that current the cable is usually sized in the heavy welding-cable range, and the exact size depends on the run length and the voltage drop you allow. Enter the current, the one-way distance and your voltage-drop limit; on long runs the drop, not the ampacity, decides the cross-section.
It depends on the insulation temperature rating, whether the conductor is bundled or free in air, and the ambient temperature — those factors change the rating considerably. For a DC circuit you also have to check the voltage drop over the run, which often limits 16 AWG to less current than its thermal rating suggests.
There's no single gauge — it follows from the load current, the cable length and the acceptable voltage drop at your system voltage. Low-voltage DC systems need noticeably thicker conductors than mains wiring for the same power, because the same wattage means much higher current.
That depends mainly on the run length, because a low system voltage leaves very little room for voltage drop. Short runs get by with a moderate gauge, while longer runs need a distinctly thicker conductor to stay inside the usual drop limit — calculate it with your actual one-way length.
The ampacity of 12 gauge depends on the insulation rating, installation method and ambient temperature, so it isn't a single fixed figure. For DC use, check the voltage drop over your run as well, since that frequently sets a lower practical limit than heat does.
10 gauge carries more current than 12 gauge, but the exact limit follows from the insulation temperature rating, ambient temperature and whether the cable is bundled or in free air. Add a voltage-drop check for the length of the run before you settle on it.
The gauge follows from the run length and the voltage-drop limit, not from the current alone. Pick a conductor whose ampacity rating covers the current under your installation conditions, then increase the size if the calculated drop over the cable length is too high.
The safe current for 14 gauge depends on insulation temperature rating, ambient temperature and how the cable is installed, so the figure varies with the conditions. In DC circuits the voltage drop over the total out-and-back length usually limits it further.
That depends on the conductor's temperature rating and the installation conditions — bundled cables in warm surroundings carry less than a single cable in free air. Also compare the result against the voltage drop for your run length before you finalise the size.
Not in general — 14 gauge is normally protected at a lower current than that, and overloading it raises the conductor temperature beyond what the insulation is rated for. Use the next larger gauge and check the voltage drop for your run.
No — that is well above the usual ampacity of 14 AWG, and the overcurrent protection must match the smallest conductor in the circuit. Choose a larger cross-section sized for the current and the run length.
The distance is limited by voltage drop rather than by the breaker, so it depends on the actual load current and the drop you accept. Calculate with the one-way run length and your load; the lower the load, the further the same cable can go.
A DC Wire Size Calculator helps you choose the correct cable cross-section for 12V, 24V or 48V electrical systems commonly found in campervans, boats, trucks, RVs and off-grid solar setups. Unlike AC mains wiring, direct current systems operate at much lower voltages, which means the same power draw pulls dramatically higher current. This makes voltage drop — not just wire ampacity — the dominant design constraint. Get the wire size wrong and you risk dimming lights, tripped inverters, overheating cables, or even fire. This tool calculates the required cross-section in both millimeters squared (mm²) and American Wire Gauge (AWG), checks the actual voltage drop at your chosen size, and recommends a properly rated fuse.
The core relationship starts with basic power math to find current draw:
I = P / U (Current = Power ÷ Voltage)
This single equation explains why low-voltage DC systems need thick cables. A 1000 W load draws only 4.3 A at 230 V AC, but the same 1000 W at 12 V DC draws a whopping 83 A. That's nearly 20 times more current, and current — not power — is what determines wire size.
Once you know the current, the required cross-section is calculated as:
A = (2 × L × I) / (κ × ΔU_allowed)
Notice the factor 2 in the numerator. This accounts for the return conductor — current has to travel to the load AND back to the battery, doubling the effective cable length. This is, by far, the single most common mistake in DC wiring: entering the total round-trip distance instead of the one-way run. If your battery-to-inverter cable run is 3 meters one-way, you enter L = 3, not L = 6. The formula already doubles it for you.
Once a cable size is selected (either the calculated minimum or the next standard size up), the calculator also reports the actual expected drop:
ΔU = (2 × L × I) / (κ × A)
There's no power factor, no cos-φ, and no √3 anywhere in these equations — this is direct current, so the math stays refreshingly simple compared to three-phase AC calculations.
Once the current draw is known, the fuse rating should be at least:
Fuse ≥ 1.25 × continuous current
Critically, the cable itself must be rated to carry the fuse's rating — not just the load's normal current. The fuse protects the cable from a fault condition, so if the cable can't handle the fuse rating, you've created a fire hazard rather than a safety device.
Example 1: 2000 W Inverter at 12V vs. 24V
A 2000 W inverter running from a battery 3 meters away draws about 167 A at 12V. To keep voltage drop under 3%, you'd need roughly 95 mm² cable — that's AWG 3/0, an extremely thick and expensive cable. Simply doubling the system voltage to 24V halves the current to about 83 A, and the required cross-section drops to around 35 mm² (AWG 2). Doubling system voltage is genuinely the cheapest and most effective fix available to any DIY electrical builder — it shrinks cable cost, weight, and voltage drop all at once.
Example 2: Bilge Pump Circuit
A 5 A bilge pump runs 4 meters (one-way) from a 12V battery. This is a critical safety circuit, so you apply the ABYC 3% drop standard. ΔU_allowed = 12 × 0.03 = 0.36 V. Plugging into the formula: A = (2 × 4 × 5) / (56 × 0.36) ≈ 1.98 mm², rounding up to the next standard size of 2.5 mm² (AWG 14). The fuse should be sized at 1.25 × 5 A = 6.25 A, so a 7.5 A fuse is appropriate — and the 2.5 mm² cable is rated for 20 A continuous, easily covering that fuse.
Example 3: Cabin Lighting
A string of LED lights drawing 3 A runs 6 meters one-way from a 12V system. As non-critical lighting, a 10% drop is acceptable: ΔU_allowed = 1.2 V. A = (2 × 6 × 3) / (56 × 1.2) ≈ 0.54 mm², which rounds up to the smallest practical size, 1.5 mm² (AWG 16) — comfortably rated for 15 A continuous.
| Cross-section (mm²) | AWG | Max continuous current (30°C, free air) |
|---|---|---|
| 1.5 | 16 | 15 A |
| 2.5 | 14 | 20 A |
| 4 | 12 | 30 A |
| 6 | 10 | 40 A |
| 10 | 8 | 55 A |
| 16 | 6 | 75 A |
| 25 | 4 | 100 A |
| 35 | 2 | 125 A |
| 50 | 1/0 | 150 A |
| 70 | 2/0 | 190 A |
At low voltages, even a small voltage loss represents a large percentage of the total. A 0.5 V drop is negligible at 230 V but catastrophic at 12V, causing dimming lights, inverter low-voltage shutdowns, and inefficient charging. Because DC systems also carry much higher current for the same power, cables sized purely for ampacity often end up far too thin once voltage drop is factored in — which is why the drop calculation, not the ampacity table, usually determines the final wire size.
Yes. The formula already multiplies your length by 2 to account for the return path back to the battery or busbar. If you measure and enter the full round-trip distance, you'll double the calculated cross-section unnecessarily, wasting money on oversized cable — or worse, if you make the opposite mistake and undersize, you'll end up with dangerous voltage drop and heat buildup.
Lithium iron phosphate (LiFePO4) batteries have extremely low internal resistance, which means their short-circuit current can be enormous — far exceeding the interrupting capacity of standard blade or ANL fuses. In a real short circuit, an underrated fuse may fail to open safely, leading to arcing or fire. This is why marine and RV standards increasingly recommend Class T fuses for LiFePO4 battery banks: they're rated for the very high fault currents these batteries can deliver.