Solar Wire Size Calculator

How the Calculator Works (The Math)

Sizing a wire correctly means passing two independent tests at once, then choosing the smallest gauge that survives both:

1. Ampacity: the wire must be rated to carry your current without overheating. This calculator sizes to the conservative NEC Table 310.16 60°C column; systems whose terminals and devices are all listed for 75°C may use the higher 75°C ratings, but the 60°C column keeps the result safe for the typical DIY build (see NEC 310.16).
2. Voltage drop: the wire must not lose more than your chosen percentage (1%, 2%, or 3%) of system voltage to resistance. The off-grid industry standard ceiling is 3%; 2% or better is ideal on charge-controller and battery circuits.

The voltage drop on a DC circuit is governed by a single equation:

VD = 2 × L × I × R_per_ft

  VD        = voltage dropped across the run (volts)
  L         = one-way run length (feet)
  I         = current (amps)
  R_per_ft  = conductor resistance (ohms per foot)
  2         = round trip: out on positive, back on negative

The factor of 2 is the part beginners forget. Current has to travel to the load and return, so a panel 20 feet away involves 40 feet of conductor. Enter the one-way distance; the formula doubles it for you. To turn volts into a percentage, divide by the nominal system voltage and multiply by 100:

VD% = (VD / V_system) × 100

The resistance figures this tool uses come from the NEC Chapter 9, Table 8 DC-resistance values for stranded uncoated copper and stranded aluminum, expressed in ohms per 1,000 feet and divided by 1,000 to get per-foot. Aluminum uses its own Table 8 column (roughly 64% higher resistance than copper) and its own, lower NEC 310.16 ampacity column. The recommended-fuse output applies the NEC continuous-load rule: multiply current by 1.25 and round up to the next standard overcurrent-device size, never exceeding the wire's own ampacity — the conductor is always sized to cover both the 125% current and the breaker that protects it.

Why higher voltage needs thinner wire: wire is sized by current, not power. A 1,000W load draws 83A at 12V but only 21A at 48V. Lower current means a quarter the voltage drop, and the allowable window (3% of 48V is 1.44V versus 0.36V at 12V) is four times wider. That is why serious off-grid builds move to 48V.

Worked Example: 30A at 24V, 20-Foot Run, Copper, 3% Drop

This is the default loaded in the calculator above. Let us size it by hand and confirm the tool agrees.

Step 1 — set the voltage-drop budget. 3% of 24V is 0.72V. That is the maximum we will allow.

Step 2 — test candidate gauges using VD = 2 × L × I × R, with L = 20 ft and I = 30A. Copper resistance per 1,000 ft (the calculator's NEC Ch. 9 Table 8 DC figures, stranded): 10 AWG = 1.24Ω, 8 AWG = 0.778Ω, 6 AWG = 0.491Ω.

10 AWG:  VD = 2 × 20 × 30 × (1.24/1000)  = 1.49 V  = 6.20%   FAIL (drop)
 8 AWG:  VD = 2 × 20 × 30 × (0.778/1000) = 0.93 V  = 3.89%   FAIL (drop)
 6 AWG:  VD = 2 × 20 × 30 × (0.491/1000) = 0.59 V  = 2.46%   PASS

Step 3 — check ampacity. 6 AWG copper is rated 55A at the conservative NEC 60°C column (65A if every terminal in the circuit is listed for 75°C). Our load is 30A, so even at 55A we have 25A of headroom — comfortably clear.

Step 4 — size the overcurrent device. 30A × 1.25 = 37.5A. The next standard fuse/breaker size up is 40A, and 40A is under the 55A ampacity of 6 AWG, so the wire is protected.

Result: 6 AWG copper, 0.59V drop (2.46%), 55A ampacity (25A margin), 40A fuse. That matches the calculator exactly. Notice that on a 12V system the same 30A over 20 feet would need far heavier wire, because 3% of 12V is only 0.36V — half the budget.

AWG Reference Table (14 → 4/0)

Resistance is DC, stranded, ohms per 1,000 ft (NEC Ch. 9 Table 8 — the same constants the calculator computes with; NEC does not list 14 AWG aluminum building wire). Ampacity is the conservative NEC 310.16 60°C column — the same column the calculator sizes to (a 75°C-listed system may use the higher 75°C values). The last column is the largest standard NEC 240.6 breaker this tool will ever pair with the wire: for 14/12/10 AWG that is the NEC 240.4(D) cap, and for every gauge the breaker never exceeds the copper wire's 60°C ampacity — the same rule the calculator enforces.

Fuse and Breaker Sizing (The 125% Rule)

A fuse protects the wire, not the device. Its job is to open before the conductor can overheat. For continuous loads — anything running more than three hours, which describes a charge controller or inverter in steady use — the NEC requires the overcurrent device to be rated at least 125% of the continuous current. In practice:

1. Multiply your continuous current by 1.25.
2. Round up to the next standard size: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400A.
3. Confirm that size does not exceed the ampacity of the wire it protects. If it does, go up a wire gauge.

Example: a 60A charge controller output. 60 × 1.25 = 75A, round up to 80A. The protected wire must be rated at least 80A — that is 3 AWG copper (85A at the 60°C column) or larger. The calculator runs this whole chain for you and reports the device size in the readout and the BOM.

Copper vs Aluminum

Copper is the default for solar: lower resistance, easier terminations, no special compounds. Aluminum (usually as AA-8000 alloy or copper-clad) becomes attractive on large, expensive runs — battery feeders, sub-panel feeds — because it costs far less per foot at big gauges. The trade-off is roughly 64% higher resistance than copper, so for the same voltage drop you typically jump one to two gauge sizes. Aluminum terminations also require antialox antioxidant compound and lugs listed for aluminum (marked AL or AL/CU), torqued to spec. Toggle the conductor selector to compare; the calculator re-derives the gauge, drop, resistance, and ampacity from that metal's own NEC columns — aluminum is never rated with copper's numbers.

Avoid copper-clad aluminum (CCA) "marine" cable for anything that matters — its resistance is closer to aluminum than copper, but it is often sold as if it were copper, so your real voltage drop ends up well above what a copper chart predicts.

Sizing Each Circuit Type

The demo buttons above the calculator load representative numbers, but the principle differs by circuit:

• Panel → charge controller: current is the array's short-circuit current (Isc) summed across parallel strings, multiplied by 1.25 for irradiance enhancement (NEC 690.8(A)). Enter that figure with the continuous-load box checked and the tool applies the second 1.25, giving the code's full 1.56 × Isc sizing basis. Voltage for the drop calc is the array operating voltage (often higher than nominal in series strings, which helps).
• Charge controller → battery: the controller outputs at battery voltage, so current is higher than on the panel side. Size to the controller's rated output amps and keep this run short — ideally under 3 feet. See our charge controller reference.
• Battery → inverter: the highest-current run in the system. A 3,000W inverter at 12V can pull 275A continuous plus surge. Use fine-stranded battery cable, fuse close to the battery, and keep it as short as possible.
• Branch DC load: lights, pumps, fridges. Size to the device's running current with margin for inrush on motor loads.

For diagrams of where each of these runs lives in a complete system, see the complete wiring diagrams guide, and for connecting solar into a load center, the solar breaker panel guide.

Common Mistakes This Calculator Prevents

• Sizing for ampacity only. A wire can be "rated" for the current and still bleed 8% of your power to voltage drop on a long run. This tool tests both.
• Using one-way distance in a round-trip formula (or vice versa). Enter one-way; the factor of 2 is built in.
• Forgetting the 125% fuse rule and protecting a 30A circuit with a 30A breaker that nuisance-trips, or a 60A wire with a 100A fuse that never protects it.
• Ignoring temperature. NEC ampacities assume a 30°C ambient. In a 50°C attic the 60°C-column ampacity this tool reports must be multiplied by 0.58 (NEC 310.15(B)(1)) — a ~42% derate — which can push a current-limited run up a gauge. Apply that factor by hand to the ampacity before comparing it to your load.
• Assuming aluminum behaves like copper. It needs heavier gauge and listed terminations.

Frequently Asked Questions

Related references

• Solar Wire Sizing Guide — full AWG charts for every voltage and distance.
• Solar Wiring Diagrams: Complete Guide — where each run lives in the system.
• How to Wire Solar to a Breaker Panel — connecting solar into a load center safely.