What wire gauge do I need for a 20-amp solar circuit?

For 20 amps on a 48V system with a 30-foot round-trip run at 2% drop, 12 AWG is typically sufficient. For 12V systems or longer runs, you may need 8 AWG or larger. Use this calculator for the exact recommendation for your specific voltage and distance.

What is the maximum voltage drop for solar wiring?

The NEC recommends 3% for branch circuits, but solar best practice is 2% or less for DC circuits to minimize energy losses. For critical battery-to-inverter cables, aim for 1% or less since the high current magnifies any losses.

Why does wire gauge use larger numbers for thinner wire?

AWG (American Wire Gauge) is based on the number of drawing steps needed to make the wire — more steps creates thinner wire with a higher number. So 18 AWG is thinner than 12 AWG, and 2/0 AWG is much thicker than 4 AWG. This is the opposite of metric mm² sizing, where a larger number means a thicker cable.

Wire Gauge Calculator

Enter current, distance, voltage, and acceptable drop — get the right AWG gauge for your solar or electrical circuit.

Circuit parameters

Current (amps)?

One-way distance?

System voltage?

Max voltage drop?

Distance unit

Recommended wire gauge

4/0 AWG AWG

Metric equivalent107.2 mm²

Actual voltage drop0.03V (0.06%)

Power loss0.6 W

Wire cost estimate~$345

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How to Use This Calculator

Enter current and one-way distance

Input the maximum continuous current in amps and the one-way distance from your source to your load. The calculator automatically uses 2× the one-way distance for the complete circuit (wire goes out and returns). For solar strings, use panel Isc × 1.25. For inverter cables, use the inverter's rated continuous current.

Set system voltage and acceptable voltage drop

Enter the circuit voltage — 12V, 24V, or 48V for DC solar circuits, or 120V/240V for AC. Select your maximum acceptable voltage drop. The NEC recommends 3% for branch circuits, but solar best practice is 2% or less for DC circuits, since losses compound: a 2% drop in your PV wire + 2% in your battery wire = 4% total system loss.

Use scenario buttons for common solar circuits

The preset scenarios cover the three most common solar wiring runs: panels to charge controller, charge controller to battery bank, and inverter to battery bank. Each uses appropriate current and distance defaults.

The Formula

Total circuit length = one-way distance × 2 (round trip) Max resistance = (System voltage × drop%) × 1000 / (total circuit feet × amps) (in ohms/1000 ft) Select the thinnest AWG whose: 1. Resistance ≤ max resistance (voltage drop constraint) 2. Ampacity ≥ load amps (current capacity constraint) Actual voltage drop = (total circuit ft × wire resistance × amps) / 1000 Power loss = voltage drop × amps

Two constraints must both be satisfied: the wire must have low enough resistance to keep voltage drop within your limit, AND it must have enough ampacity to safely carry the current without overheating. The voltage drop constraint usually dominates for long runs at low voltage; the ampacity constraint dominates for short, high-current runs (like battery-to-inverter cables).

Notice that low voltage systems are much more sensitive to wire resistance. A 1-ohm resistance causes 1V drop at 1A — which is 8.3% of 12V but only 2.1% of 48V. This is why 12V systems need much thicker wire than 48V systems for the same power transfer.

Example

PV string to MPPT controller — 30ft run

A solar array produces 8.5A (Isc). The panels are mounted 15 feet from the charge controller. The system runs at 48V. Maximum acceptable voltage drop is 2%.

Current (Isc × 1.25)8.5 × 1.25 = 10.6A

Total circuit length15ft × 2 = 30ft

Max voltage drop48V × 2% = 0.96V

Max resistance0.96V × 1000 / (30ft × 10.6A) = 3.02 Ω/kft

Result

Recommended AWG12 AWG (resistance 1.588 Ω/kft)

Actual voltage drop30 × 1.588 × 10.6 / 1000 = 0.50V (1.05%)

Power loss0.50V × 10.6A = 5.3W

Wire cost estimate~$11

12 AWG easily handles this run. If this were a 12V system instead of 48V, the same calculation yields a 2% max drop of only 0.24V — requiring 8 AWG or larger to stay within the limit.

FAQ

Why is voltage drop worse on 12V systems? ▼

Because voltage drop is absolute, but your tolerance is percentage-based. A 0.5V drop is 4.2% of 12V but only 1.0% of 48V. For the same power (say, 200W), a 12V system draws 16.7A while a 48V system draws only 4.2A. Higher current through the same resistance creates more drop. The result: to transfer the same power over the same distance with the same percentage drop, a 12V system needs wire with 16× lower resistance than a 48V system — roughly 4 AWG sizes thicker. This is the biggest reason experienced off-grid installers use 24V or 48V systems.

What wire type should I use for solar installations? ▼

For the solar array (outdoor, UV-exposed): use USE-2 or PV Wire — rated for direct sunlight, UV resistant, and suitable for outdoor conduit or direct burial. For runs inside conduit or in protected indoor/battery areas: THWN-2 or THHN copper wire is the standard choice. For battery cables (high current, short runs): use flexible welding cable or purpose-made battery cable — it's much easier to route around batteries than stiff building wire. Avoid stranded speaker wire or automotive wire for permanent solar installations — they're not rated for the application.

Should I go bigger than the minimum recommended gauge? ▼

Often yes, especially for permanent installations. Going one gauge size bigger (e.g., 8 AWG instead of 10 AWG) reduces voltage drop by ~37%, cuts resistive losses, generates less heat, and gives you headroom for future load increases. The cost difference for a typical solar run is often just $20–50, but the efficiency benefit compounds over years. For battery-to-inverter cables in particular, err on the side of larger — the current is high, the run is short, and undersized cable here causes voltage sag under load.

Is aluminum wire acceptable for solar installations? ▼

Aluminum is used in utility-scale and large commercial solar installations for home runs (the wire from the array to the inverter building), where the long distances and large wire gauges make copper prohibitively expensive. Aluminum has about 61% the conductivity of copper, so you need to go 2 AWG sizes larger (e.g., 2/0 Al instead of 2 Cu). For residential and small off-grid systems, stick with copper — aluminum connections require anti-oxidant compound and special fittings, and connection integrity at terminals is more demanding.