How the solar wire size calculator works
This tool finds the minimum cable size for a solar circuit — panel to charge controller, controller to battery, or battery to inverter — based on the current it will carry, the system voltage, the one-way cable length, and how much voltage drop you're willing to accept.
You enter:
- Current (A) — the maximum current the circuit will carry
- System voltage (V)
- One-way cable length, with a toggle between metres and feet
- Acceptable voltage drop — default 3 percent for DC solar runs, 2 percent for battery-to-inverter cabling
- Conductor material — copper (default) or aluminium
- Circuit type — DC or AC single-phase (round-trip factor of 2), or AC three-phase (factor of 1.732)
The math, in plain terms
The core formula is:
Minimum cross-section (mm²) = (factor × resistivity × one-way length in metres × current) ÷ (voltage × drop fraction)
Where:
- factor is 2 for DC or single-phase AC (current travels out and back), or 1.732 (√3) for three-phase AC
- resistivity is 0.0175 ohm·mm²/m for copper, or 0.0282 ohm·mm²/m for aluminium
- drop fraction is your acceptable percentage expressed as a decimal (3 percent = 0.03)
Once the minimum cross-section is known, the calculator rounds up to:
- The next standard IEC metric size: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, or 120 mm²
- The next standard AWG size: 14, 12, 10, 8, 6, 4, 2, 1/0, 2/0, 3/0, or 4/0
It then cross-checks that size against a simplified ampacity table to confirm it can safely carry 125 percent of your entered current, accounting for the continuous-duty derating solar circuits need since they can run near full output for hours. Finally, it reports the actual voltage drop and power loss (in watts) for the selected wire.
Why voltage drop matters more at low voltage
Voltage drop is driven by current, not power, and current rises sharply as system voltage falls for the same wattage. A 1,000 W load draws about 83 A at 12 V but only about 21 A at 48 V — four times less. A fixed percentage drop at 12 V is a smaller absolute voltage cushion (0.36 V at 3 percent) than at 48 V (1.44 V at 3 percent), so low-voltage systems need noticeably thicker cable, shorter runs, or both to stay within the same percentage loss.
The 3 percent rule of thumb
Three percent is a widely used target for DC solar wiring because it balances cable cost against energy loss. Tighter than that means oversized, expensive cable for little practical gain, while looser than 3 percent noticeably reduces charging efficiency and can cause controllers or inverters to misread battery voltage. Battery-to-inverter runs often use a tighter 2 percent limit since they carry the highest current in the system.
AWG to mm² conversion
| AWG | Approx. mm² | AWG | Approx. mm² |
|---|---|---|---|
| 14 | 2.5 | 4 | 25 |
| 12 | 4 | 2 | 35 |
| 10 | 6 | 1/0 | 50 |
| 8 | 10 | 2/0 | 70 |
| 6 | 16 | 3/0 | 95 |
These are close approximations for comparing standards — check the exact ampacity on a manufacturer's chart before finalizing.
Typical wire sizes for common runs
| Run | Typical voltage | Typical current | Common wire size range |
|---|---|---|---|
| Panel to charge controller | 12–48 V | 5–15 A | 4–10 mm² (AWG 12–8) |
| Controller to battery (short run) | 12 V | 20–40 A | 10–16 mm² (AWG 8–4) |
| Controller to battery (short run) | 48 V | 20–40 A | 4–6 mm² (AWG 10–8) |
| Battery to inverter | 12 V | 80–150 A | 35–70 mm² (AWG 2–2/0) |
| Battery to inverter | 48 V | 20–40 A | 6–16 mm² (AWG 8–4) |
Exact figures depend on your actual current draw and cable length — run your own numbers through the calculator rather than relying on this table alone.
Ampacity and voltage drop both have to pass
These are two separate checks, and a wire has to satisfy both. Ampacity is about heat — a wire carrying more current than it's rated for gets hot enough to damage insulation or start a fire. Voltage drop is about efficiency and equipment behaviour — a wire that's ampacity-safe can still be too thin if the run is long, wasting power and starving equipment of voltage. On short runs, ampacity is usually the limiting factor. On long runs, especially in 12 V systems, voltage drop often forces a larger size than ampacity alone would require.
Fuse and breaker sizing
The fuse or breaker's job is to protect the wire, not the device at the end of it. Size overcurrent protection at about 125 percent of the expected continuous current, and never larger than the wire's rated ampacity. If you upsize the wire for voltage drop, keep the fuse or breaker sized to the actual current draw, not to the wire's higher ampacity capability.
PV wire types
Outdoor solar cable is usually a different product from standard indoor building wire — look for cable rated for UV exposure, temperature extremes, and often double insulation, commonly labelled PV1-F or H1Z2Z2-K in Europe, or USE-2 / PV wire in North America. Indoor runs can generally use standard building or battery cable rated for the environment and current involved.
Local codes take precedence
This calculator gives an engineering estimate, not a compliance check. Electrical codes vary by country — NEC in the United States, BS 7671 in the United Kingdom, AS/NZS 3000 in Australia, and national IEC adaptations elsewhere — and can mandate specific minimum sizes, derating factors, conduit fill limits, or grounding requirements beyond this simplified calculation. Confirm your final wire choice against the code in your location, and consult a licensed electrician for anything permanent or safety-critical.