Wire Sizing for Off-Grid Solar: Voltage Drop, Ampacity, and Fusing

Cables are where many DIY off-grid systems lose efficiency and, in the worst cases, start fires. Low-voltage DC systems carry high currents, so a cable that would be generous for a household outlet can be dangerously small between a battery and an inverter. Two independent checks decide the cable size: voltage drop and ampacity. Always use the larger result.

Check 1: Voltage drop

Every cable has resistance, so some voltage is lost as heat along it. Too much voltage drop wastes energy, makes inverters trip on low voltage, and stops controllers from charging properly. Common design targets are:

• 3% or less for solar array to charge controller runs.
• 1–2% for battery to inverter and charge controller to battery.
• 3–5% for DC loads such as lights and pumps, depending on the device.

For copper cable, the required cross-section is:

Area (mm²) = (2 × Length (m) × Current (A) × 0.0172) ÷ Allowed voltage drop (V)

The length is the one-way distance; the factor of 2 accounts for the positive and negative conductors. 0.0172 Ω·mm²/m is the resistivity of copper at 20 °C. The allowed voltage drop in volts is the system voltage multiplied by the target percentage.

Example 1: Why series strings save copper

Three 400 W panels, 15 m from the charge controller, with a 2% voltage drop target. Each panel has a Vmp of about 38 V and an Imp of about 10.5 A.

The same power over the same distance needs about six times less copper in series, because tripling the voltage cuts the current to a third, and voltage drop depends on current while the allowed drop grows with voltage. Before choosing series, check the controller's maximum input voltage at your coldest temperature. The MPPT guide explains how.

Example 2: Battery to inverter

A 3,000 W inverter on a 24 V battery bank, with a 1.5 m one-way cable run and a 2% target. At full load and a low battery voltage of about 22 V, the DC current is roughly 3,000 ÷ (22 × 0.9) ≈ 152 A.

Area = (2 × 1.5 × 152 × 0.0172) ÷ 0.48 ≈ 16.3 mm². By voltage drop alone, 16 mm² looks almost enough. But it is far too small to carry 152 A continuously without overheating. This is where the second check comes in.

Check 2: Ampacity

Ampacity is the maximum continuous current a cable can carry without exceeding its insulation temperature rating. It depends on the insulation material (for example 75 °C vs 105 °C), whether cables are bundled, run in conduit, or in free air, and the surrounding temperature. Ampacity tables in the NEC, IEC 60364-5-52, ABYC E-11 (for boats and many vans), and manufacturers' datasheets give the limits for your situation.

For the 152 A inverter feed above, typical tables point to around 35 mm² (about 2 AWG) or larger depending on insulation and installation. That is more than double the voltage-drop result. Short, high-current battery cables are almost always limited by ampacity; long, low-current solar runs are almost always limited by voltage drop.

Follow the inverter manufacturer's minimum cable size too, because it accounts for surge currents and terminal sizes.

Fusing and overcurrent protection

A fuse protects the cable, not the appliance. Its rating must be no higher than the cable's ampacity and at least about 1.25 times the normal continuous current, so it does not blow during normal use.

• Fuse at the battery. Every positive cable leaving the battery should have a fuse or breaker as close to the terminal as practical (ABYC guidance is within about 18 cm / 7 in). A short circuit in an unfused battery cable can melt it within seconds.
• Use high interrupt-capacity fuses on lithium banks. LiFePO4 batteries can deliver many thousands of amps into a short circuit. Class T fuses, or fuses with a similarly high interrupt rating, are commonly recommended for the main battery fuse.
• Fuse parallel PV strings when there are three or more in parallel, so a faulted string cannot be back-fed by the others.
• Provide a DC disconnect for the array and for the battery, so the system can be safely isolated for maintenance.

Practical installation tips

• Use fine-stranded, tinned copper cable in vans and boats, where vibration and moisture are present.
• Crimp lugs with the correct hydraulic or ratchet tool and cover them with adhesive heat-shrink. A poor crimp has more resistance than metres of cable.
• Use UV-rated, double-insulated solar cable (for example H1Z2Z2-K or PV wire) for array wiring.
• Keep positive and negative battery cables the same length when paralleling batteries, so they share current evenly.
• Torque terminals to the manufacturer's specification and re-check them after the first few weeks.

If these examples make 12 V look difficult for larger systems, that is the point: moving to 24 V or 48 V halves or quarters every current in this guide. The inverter sizing guide shows the effect on battery current.