Solar panel to charge controller wire size
Use the string's operating voltage (Vmp) and current (Imp) for voltage drop. Panels wired in series raise the voltage and keep the current, which is why longer roof runs often use a series string. The ampacity check uses a different current: NEC 690.8 takes 1.25 x the short-circuit current (Isc) as the maximum, and asks for conductors rated for 1.25 x that, so 1.56 x Isc in total.
Use 12 AWG (3.31 mm²) copper or larger.
Voltage drop sets the size. 14 AWG would drop 4.21%, over your 3% limit. Ampacity alone would allow 18 AWG.
Check: 12 AWG (3.31 mm²)
| Load current | 10 A |
| Wire in the circuit (out and back) | 2 x 30 ft = 60 ft |
| Resistance of 12 AWG | 1.588 ohm per 1000 ft; 0.09528 ohm for this circuit |
| Voltage drop | 0.953 V = 2.65% within 3% |
| Voltage at the load | 35.05 V |
| Power lost in the wire | 9.53 W |
| Ampacity of 12 AWG | 25 A (NEC 310.16, 75 C) ok |
Vd = I x 2L x R / 1000 = 10 x 2 x 30 x 1.588 / 1000 = 0.953 V, and 0.953 / 36 = 2.65%. Length L in feet, R in ohms per 1000 ft.
| AWG | mm² | Drop (V) | Drop (%) | At load (V) | Rated (A) | Result |
|---|---|---|---|---|---|---|
| 18 | 0.82 | 3.831 | 10.64 | 32.17 | 10* | too much drop |
| 16 | 1.31 | 2.41 | 6.69 | 33.59 | 13* | too much drop |
| 14 | 2.08 | 1.515 | 4.21 | 34.48 | 20 | too much drop |
| 12 | 3.31 | 0.953 | 2.65 | 35.05 | 25 | smallest that passes |
| 10 | 5.26 | 0.599 | 1.66 | 35.4 | 35 | passes |
| 8 | 8.37 | 0.377 | 1.05 | 35.62 | 50 | passes |
| 6 | 13.3 | 0.237 | 0.66 | 35.76 | 65 | passes |
| 4 | 21.2 | 0.149 | 0.41 | 35.85 | 85 | passes |
| 3 | 26.7 | 0.118 | 0.33 | 35.88 | 100 | passes |
| 2 | 33.6 | 0.094 | 0.26 | 35.91 | 115 | passes |
| 1 | 42.4 | 0.074 | 0.21 | 35.93 | 130 | passes |
| 1/0 | 53.5 | 0.059 | 0.16 | 35.94 | 150 | passes |
| 2/0 | 67.4 | 0.047 | 0.13 | 35.95 | 175 | passes |
| 3/0 | 85 | 0.037 | 0.1 | 35.96 | 200 | passes |
| 4/0 | 107 | 0.029 | 0.08 | 35.97 | 230 | passes |
* chassis-wiring guidance; 14 AWG and up: NEC Table 310.16, copper, 75 C column.
Runs in your browser; nothing you type is sent anywhere. Copper conductors only.
Wire size for 10 A from the panels, by length and string voltage
| One-way length | 18 V Vmp, 3% | 36 V Vmp, 3% | 72 V Vmp, 3% |
|---|---|---|---|
| 10 ft (3 m) | 14 | 16 | 18 |
| 20 ft (6.1 m) | 10 | 14 | 16 |
| 30 ft (9.1 m) | 8 | 12 | 14 |
| 40 ft (12.2 m) | 8 | 10 | 14 |
| 50 ft (15.2 m) | 6 | 10 | 12 |
| 75 ft (22.9 m) | 4 | 8 | 10 |
| 100 ft (30.5 m) | 4 | 6 | 10 |
Voltage drop only, at 10 A Imp. Then check ampacity: for a panel with Isc 11 A, 1.56 x 11 = 17.2 A, which needs at least 14 AWG (20 A) before any derating for heat on a roof. Use whichever of the two is thicker. One-way length; the math uses twice that.
Worked example
10 A at 36 V, 30 ft one way, 3% drop allowed. The current goes out and comes back,
so the circuit is 2 x 30 = 60 ft of wire, and the allowed drop is 36 x 3 / 100 =
1.08 V. Voltage drop is Vd = I x 2L x R / 1000, with R in ohms per 1000 ft:
- 14 AWG:
10 x 60 x 2.525 / 1000 = 1.515 V, which is 4.21% of 36 V; over the 3% limit. - 12 AWG:
10 x 60 x 1.588 / 1000 = 0.953 V, which is 2.65% of 36 V; passes both checks.
Answer: 12 AWG (3.31 mm²), rated 25 A, the smallest size that passes both.
Wire for the location
Conductors on a roof or in sunlight need insulation rated for it (PV wire or USE-2 in the NEC), and roof heat lowers the current a conductor may carry. The calculator's ampacity figures assume 30 C ambient.
Before you buy wire
- Ampacity figures for 14 AWG and larger are from NEC (NFPA 70) Table 310.16, copper, 75 C column, which assumes 30 C ambient and no more than three current-carrying conductors bundled together. Hot spaces, bundles and conduit lower them. 18 and 16 AWG figures are chassis-wiring guidance, not NEC ratings.
- On boats, follow ABYC E-11 for DC wiring. Engine rooms are hot, and conductors there are allowed less current than the same wire elsewhere.
- Use wire rated for the location: heat, oil, water, sunlight and abrasion all matter. Stranded copper is the norm in vehicles and boats, where wire vibrates.
- Fuse every positive run close to the source, sized to protect the wire. NEC 240.4(D) limits the overcurrent device on 14, 12 and 10 AWG to 15, 20 and 30 A.
- This calculator does arithmetic. It does not certify an installation; check your work against the standards and the equipment makers' instructions, or have it checked.
Questions
Which voltage do I enter for a solar string?
The string voltage at maximum power, Vmp, which is the sum of the panel Vmp figures for panels in series. Use the string current, Imp, as the load.
Why does a 3% drop matter on the panel side?
Every volt lost in the wire is power the panels make but the controller never sees. At 3% you lose about 3% of the string power as heat in the wire.