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SolarCalcWorks

Solar Panel to Charge Controller Wire Size Calculator

The array run is the one circuit on a solar system with its own NEC article. It carries the 690.8(A) 125 % factor on short-circuit current, it is often exposed to sunlight in free air, and on a roof it takes a 33 °C temperature adder that halves what the wire can carry.

  • NEC 690.8 PV source circuit rules
  • Free-air PV wire ampacity
  • Rooftop temperature adder
  • Series vs parallel guidance

Solar Panel to Charge Controller Wire Size Calculator

12 AWG2.38% drop
Inputs3 % target
System voltage
Conductor material

3% of 100 V = 3 V of budget across the whole round trip.

Installation conditionsdefaults are code-safe

Already have wire? Check the drop on a size you own.

Conductor3.3 mm²

12AWG · Cu · 90 °C

Ampacity and voltage drop agree

12 AWG is simultaneously the smallest conductor that satisfies NEC ampacity and the smallest that holds the drop at or below 3.0%. No margin to give back on either.

Voltage drop
2.38%
2.38 V lost
At the load
97.62 V
from 100 V
Amps at terminals
35 A
needs 31.25 A
Heat in the wire
48 W
at 20 A
  • NoteThe 125 % factor on Isc (NEC 690.8(A)(1)) accounts for irradiance above 1000 W/m², which happens with snow reflection, cloud edge effects and at altitude. It is not a safety margin you can skip.
  • NoteVoltage drop was calculated at Isc, which is conservative. Panels operate near Imp, typically 3–8 % below Isc, so the real drop in service will be slightly lower.
Calculation

1Maximum circuit current

20 A Isc × 1.25 = 25 A

NEC 690.8(A)(1): a PV source circuit maximum current is 125 % of rated Isc, because irradiance can exceed the 1000 W/m² test condition.

2Test A — 125 % at the terminals

25 A × 1.25 = 31.25 A required
12 AWG @ 75 °C = 35 A ✓

Continuous loads need 125 % headroom, and NEC 110.14(C) caps the usable column at the lowest-rated termination. No derating applies to this test.

3Test B — derated ampacity

40 A @ 90 °C = 40 A
must be ≥ 25 A ✓

30 °C ambient falls in the 26–30 °C band of Table 310.15(B)(1).

4Test C — can a device protect it?

device needed = 35 A (NEC 240.6(A))
12 AWG may be protected at up to 35 A ✓

NEC 240.4(B) permits rounding up to the next standard rating; 240.4(D) then caps 14, 12 and 10 AWG regardless — except on PV circuit conductors, which 240.4(G) exempts. A conductor with adequate ampacity can still fail here.

5Voltage drop

2 × 30.0 ft × 20 A × 1.9800 Ω/kft ÷ 1000
= 2.38 V = 2.38% of 100 V

The run is doubled because current returns on the second conductor. Resistance from NEC Chapter 9, Table 8. Voltage drop is a design target, not a code requirement — NEC 210.19(A) Informational Note 4 recommends 3 % or less.

6The answer

Ampacity needs 12 AWG, protection coordination needs 12 AWG, and the 3.0% drop target needs 12 AWG. The conductor has to satisfy all three, so the answer is the largest: 12 AWG.

Reference 690.8(A)(1) · 690.8(B) · 690.9(B) · 110.14(C) · 310.15 · 240.4 · Ch.9 T.8 — NEC 2023. Sources and limitations.

Overcurrent protection35 A

Calculated minimum 31.25 A, rounded up to the 35 A standard rating in NEC 240.6(A).

NEC 690.9(B): the device rating must be at least 125 % of the maximum circuit current from 690.8(A). PV circuit conductors are protected under 690.8 and 690.9 rather than the small-conductor limits of 240.4(D) — see 240.4(G).

NextSize the fuse or breaker for this circuitA conductor is only protected once a device is sized to it.
Nearby sizes20 A · 30 ft
Conductor sizes compared by ampacity margin and voltage drop
SizeAmps spareDropVerdict
14 AWG-1.253.8%Under ampacity
12 AWGpick3.752.4%Meets all
10 AWG18.751.5%Meets all
8 AWG38.750.9%Meets all
6 AWG63.750.6%Meets all

Amps spare is the margin on the tighter of the two NEC ampacity tests.

The array run has its own rulebook

Most DC circuits in a solar system are sized under Article 210 like any branch circuit. The run from the modules is different: NEC Article 690 governs it, and three things follow from that.

The 125 % on Isc

NEC 690.8(A)(1) defines the maximum current of a PV source circuit as 125 % of the sum of parallel module short-circuit currents. This is not a safety margin you can decide to skip. Real irradiance exceeds the 1000 W/m² standard test condition regularly — snow reflection, cloud-edge focusing and altitude all push modules above their rated Isc.

NEC 690.8(B) then applies a second 125 % as the continuous-duty factor, exactly as it does elsewhere. The combined multiplier on nameplate Isc is 1.5625.

Free air, not conduit

PV wire and USE-2 run openly between modules and racking are single conductors in free air, covered by NEC Table 310.17. Those ampacities are meaningfully higher than the Table 310.16 figures for conductors in a raceway — 8 AWG copper at 90 °C is 80 A in free air against 55 A in conduit.

The moment the run enters conduit, Table 310.16 governs that portion, along with the conductor fill adjustment if there are four or more current-carrying conductors in it.

The rooftop adder

Series wiring is what makes this run cheap

Voltage drop is proportional to current, and current is what series wiring leaves alone.

Take twelve 400 W modules with Isc of 10.3 A and Vmp of 41.5 V, 100 ft from the controller:

  • All parallel at 12 modules: 124 A of Isc at 41.5 V. That needs very large conductors.
  • Four series, three parallel: 31 A at 166 V. Dramatically smaller.
  • Six series, two parallel: 21 A at 249 V. Smaller again.

Same array, same energy, wildly different cable bills. The limit is your charge controller’s maximum input voltage — checked at your coldest expected temperature, because module open-circuit voltage rises as temperature falls under NEC 690.7(A)(1). That check is the job of the charge controller calculator, and it is what stops this optimisation destroying the equipment.

Choosing the conductor type

Where the run is Type Why
Exposed under the array PV Wire or USE-2 Sunlight and moisture resistant, 90 °C wet, and required for exposed single-conductor runs under NEC 690.31(C).
In conduit outdoors or buried THWN-2 or XHHW-2 90 °C wet rating. Plain THHN is dry-location only.
Inside the building THHN in conduit 90 °C dry, the standard interior choice.

Ordinary building wire outside conduit in sunlight is not acceptable here: UV degrades the insulation and the conductor is not listed for the exposure.

Fusing the array

Two or fewer parallel strings usually need no string fuses at all — with only one other string able to back-feed a fault, the fault current cannot exceed what the conductors already carry. Three or more parallel strings generally do need a fuse per string, and that fuse must not exceed the module’s maximum series fuse rating from the datasheet. The fuse calculator checks that limit explicitly, because NEC 690.9(B) sets a minimum device size and the module rating sets a maximum the code does not license you to exceed.

What to calculate next

NextCheck the controller can take this arrayCold-weather Voc is what actually destroys controllers.

Frequently asked questions

Do I enter Isc or Imp for the array run?

Enter Isc, the short-circuit current from the module datasheet, summed across every string wired in parallel on this run. NEC 690.8(A)(1) defines the maximum circuit current of a PV source circuit as 125 % of that figure, because irradiance can exceed the 1000 W/m² test condition. Voltage drop in service will be slightly lower than the calculator shows, because panels actually operate near Imp — the tool tells you when it has been conservative in this way.

What voltage do I enter for the array run — panel or battery?

The string's operating voltage, not the battery voltage. This is the single most common error on this circuit. A three-panel series string operating around 120 V is a 120 V circuit for voltage drop purposes, even if it charges a 12 V bank through an MPPT controller. Entering 12 V instead would overstate the required conductor by roughly a factor of a hundred.

Why does the calculator default to free air rather than conduit?

Because PV wire and USE-2 run openly between modules and racking are single conductors in free air, which NEC Table 310.17 covers and which permits noticeably higher ampacity than Table 310.16. If your run enters conduit — as it must once it leaves the array under NEC 690.31 in many cases — switch the wiring method, because the conduit portion is what governs.

What is the rooftop temperature adder?

NEC 310.15(B)(2) adds 33 °C to the outdoor ambient for raceways and cables exposed to direct sunlight on or above a roof where the gap to the roof surface is less than 3/4 in. On a 40 °C day that gives 73 °C, where even 90 °C conductors lose roughly half their ampacity. Mounting the raceway on standoffs that lift it clear removes the adder entirely, and is usually far cheaper than the extra copper.

Does series or parallel wiring change the wire size?

Substantially. Series adds voltage and leaves current unchanged, so a long series string needs much less copper for the same power. Parallel adds current, which drives both ampacity and voltage drop up. This is why long array runs are wired in series wherever the charge controller's maximum input voltage allows — and why that voltage limit, checked at your coldest temperature, is what constrains the design.

Last reviewed 2026-08-19. Calculations reference NFPA 70 (NEC) 2023 where a code section applies. Sources, and what these tools deliberately do not model.