Solar Wire Size Calculator — Free Online Calculator
Size wire for solar panel installations. Accounts for NEC 690 requirements including 125% continuous load factor.
How to Use This Calculator
Enter the module short-circuit current (Isc from the panel datasheet), system voltage, wire run distance, and conductor material. The calculator applies NEC 690 factors (1.25 × 1.25 = 1.5625) and limits voltage drop to 2%.
The Formula Explained
NEC 690.8 requires solar conductors be sized for 125% of Isc for continuous duty, and 690.9 requires overcurrent protection at 125% of that value, resulting in a total factor of 1.5625× Isc. Wire must also keep voltage drop under 2% for DC circuits to maintain inverter efficiency.
Solar PV Wire Sizing Is Different
Solar PV systems have unique wiring rules because of their unique operating conditions. Modules are rated at STC (Standard Test Conditions: 1000 W/m², 25°C cell temperature), but real-world conditions can produce higher current on cold sunny days. NEC 690.8(A)(1) requires sizing PV source circuit conductors at 156% of the module short-circuit current (Isc) — 125% for continuous load plus another 125% for enhanced irradiance margin. This is why solar wire is often oversized compared to what the nameplate suggests.
The other distinctive feature: most PV wire runs outdoors, exposed to UV and extreme temperature swings. Standard THHN wire PVC insulation degrades rapidly under direct sunlight. PV Wire and USE-2 are the two common choices — PV Wire for exposed rooftop runs and USE-2 for conduit or buried runs. Module-to-module jumpers use pre-made MC4 terminated cables, but the home-run from the last module back to the combiner or inverter is the installer responsibility.
Worked Example: Residential 10 kW Rooftop System
A 10 kW residential PV system with 25 modules at 400W each. Each module has Isc = 11.5A, Voc = 45V, operating current Imp = 10.8A, operating voltage Vmp = 37V. System configured as 2 strings of 12 modules plus some balance-of-system — typical strings are 12 or 13 modules. String Voc summed: 13 × 45 = 585V DC (within most inverter limits of 600V) and 12 × 45 = 540V.
PV source circuit conductor sizing: 156% × 11.5A Isc = 17.9A minimum ampacity. 10 AWG THWN-2 handles 40A at 90°C (derating for rooftop temperature may bring this down to 27A) — plenty of margin. Most installers use 10 AWG PV Wire for the home runs from string combiner to inverter, even though 12 AWG would meet code, because the voltage drop on longer runs becomes meaningful.
Worked Example: Ground-Mount System Wire
A ground-mount system 200 feet from the house, 8 kW total (20 × 400W modules in 2 strings of 10). Each string Voc = 450V, Isc = 11.5A. The DC home run from combiner to inverter goes 200 feet through buried conduit.
Ampacity check: 156% × 11.5A = 17.9A. 12 AWG copper handles this easily. Voltage drop check: (2 × 10.37 × 11.5 × 200) / 6,530 = 47,702 / 6,530 = 7.3V drop on 450V = 1.6%. Acceptable but borderline. Bump to 10 AWG: (2 × 10.37 × 11.5 × 200) / 10,380 = 4.6V drop = 1.02%. Better.
For 200-foot DC runs, most installers use 10 AWG or even 8 AWG to minimize production loss. Over 25 years of operation, a 1% production loss costs a meaningful sum in lifetime value at residential electricity rates — easily worth the extra copper cost.
Temperature Corrections for Rooftop Installs
Rooftop PV modules can operate at 70°C on a hot summer day, and the wire right next to them experiences similar temperatures. NEC 310.15(B) requires ambient temperature correction, and for rooftop installations NEC 310.15(B)(3)(c) historically required an additional rooftop adder because temperatures above the rooftop can exceed standard ambient assumptions. The 2017 and later NEC removed the mandatory rooftop adder but still requires realistic ambient temperature consideration.
For a 75°C wire (THWN) rated for 30A ambient, operating at 60°C ambient reduces ampacity by a factor of about 0.58 — so 10 AWG with 40A at 30°C is really 23A at 60°C. Still OK for a 17.9A circuit, but the margin gets thin. Use 90°C wire (PV Wire is always 90°C rated) for rooftop applications to preserve margin.
Common Solar Wiring Mistakes
1. Using Isc directly instead of the 156% factor. Miss the multiplier and you undersize by 60%. The code requires 156%, not Isc.
2. Running THHN on exposed rooftop. Within two years the UV destroys the insulation. Use PV Wire for anything exposed to sunlight.
3. Forgetting DC disconnect requirements. NEC 690.13 requires a DC disconnect within 10 feet of the array or at the inverter. The wire between the disconnect and the inverter is subject to the same sizing rules.
4. Ignoring string fusing requirements. NEC 690.9 requires overcurrent protection on PV source circuits in most configurations (exceptions for 2-string systems where the module is rated for backfeed current). Miss this and a short in one string can be backfed from parallel strings.
5. Using aluminum on the DC side. Aluminum is code-legal but connectors are problematic — most MC4 and similar solar connectors are copper-only. Use copper throughout the DC side, save aluminum for the AC feeder if cost matters.
Professional Solar Wiring Practices
Size for production, not just code. A 12 AWG wire might meet code minimum but lose 2% of your production every sunny day. Over 25 years that adds up. Upsizing to 10 AWG is almost free compared to the production loss.
Use rail-mount or parallel-to-array wire runs when possible. Mechanical protection against rodents and UV exposure is critical. Wire management clips along the array rails are fast to install and keep wires off the roof surface.
Label everything. DC+ and DC-, string numbers, combiner box positions. A future service tech (who might be you) will thank you.
Install DC-rated fuses at the combiner. Do not use AC-rated fuses even if they physically fit — they can fail catastrophically on DC arcs.
Ground the module frames and racking per NEC 690.43. Use WEEBs (washer electrical equipment bonds) or equivalent listed hardware, not just looks-grounded-enough. Module frame grounding is a common inspection fail.
NEC Article 690 Highlights
690.8 — circuit current sizing including the 156% rule for PV source circuits. 690.9 — overcurrent protection requirements for PV source and output circuits. 690.13 — DC disconnect requirements. 690.31 — wiring methods including the requirement for MC or metallic raceway in accessible locations.
690.41 — system grounding requirements for PV systems. 690.43 — equipment grounding (the WEEB requirement for module frames). 690.47 — grounding electrode requirements for PV systems, which may require supplemental grounding electrodes beyond the existing building system. 690.56 — identification and labeling requirements including the DC arc-fault device and rapid shutdown labeling at the service.
Solar PV wire sizing: NEC Article 690 conductor rules
PV wire sizing is harder than standard branch-circuit sizing because of three factors: aggressive temperature derating (rooftop conduit hits 70 C on a hot day), higher continuous-load multipliers, and the irradiance correction that bumps current 25 percent above the module nameplate Isc.
NEC 690.8 sets the maximum circuit current at 156 percent of Isc (125 percent for continuous x 125 percent for irradiance). That number then has to fit within the conductor ampacity after temperature and bundling derating. The result is that PV conductors are usually one to two AWG larger than the same current in a fixed-load circuit.
The formula and what it does
Isc is the module short-circuit current from the nameplate. Multiply by 1.25 for continuous, then 1.25 again for irradiance correction (sun events can briefly exceed STC). The conductor selected must carry this current under the actual roof temperature, which often hits 70 C ambient inside conduit, derating standard 90 C THWN by 41 percent.
Worked example
Scenario: 12-module string, modules rated Isc = 11 A. Conduit on dark roof, 35 C summer ambient, conductor run through conduit on top of roof membrane.
I_max = 1.56 x 11 = 17.16 A. Effective ambient: 35 C ambient + 33 C rooftop adder (NEC 310.15(B)(2)) = 68 C. At 68 C using 90 C insulation: ampacity correction factor is 0.41. So conductor 75 C ampacity must be at least 17.16 / 0.41 = 41.85 A. From NEC 310.16: 10 AWG (35 A) fails, 8 AWG (50 A) passes. So 8 AWG PV wire minimum for a circuit that would only be 11 A in fixed installation.
Code references and standards
NEC 690.8(A)(1) maximum circuit current = 1.25 x Isc for continuous loads (an Isc is treated as the circuit current for sizing).
NEC 690.8(B) conductors must be rated 125 percent of max circuit current after applying ampacity adjustments. That is the second 1.25, totaling 1.56 x Isc.
NEC 690.9 requires overcurrent protection at the same point; OCPD also sized to 1.56 x Isc.
NEC 310.15(B)(2) rooftop conduit temperature adder. 17-33 C above ambient depending on conduit height above roof. Solar conduit on top of membrane gets the full 33 C.
NEC 690.31(B)(2) DC wiring methods: PV wire (USE-2, RHW-2, PV) is required outside the building for sunlight-resistance and wet-location ratings.
Common mistakes to avoid
Sizing for nameplate Isc only. Skipping the 1.56 multiplier is the most common PV mistake on Reddit installer threads. The conductor melts insulation in summer, the inspector fails the job, and the install has to be redone.
Forgetting the rooftop adder. 35 C ambient feels manageable until you add 33 C for conduit on the roof. Then the ampacity correction goes from 1.04 (35 C) to 0.41 (68 C), and your wire size jumps two AWG.
Using THHN inside conduit on a roof. THHN is rated wet/dry but not sunlight-resistant for exposed runs. Roof conduits exposed to sun need PV wire (USE-2, PV-style) or sunlight-resistant cable per NEC 310.10(D).
Frequently asked questions
Why is solar wire so much larger than the load suggests?
Three derating factors stack: 1.25x for continuous, 1.25x for irradiance, and the rooftop ambient correction. Combined, a 10 A nameplate current becomes a 42 A ampacity requirement on a hot roof.
Does the inverter output side use the same rules?
NEC 690.8(A)(2) treats inverter output (AC side) at 1.25x continuous, not the full 1.56x. But you still derate for temperature on the AC side. AC conductor between inverter and main panel is usually one size smaller than DC string conductor for the same kW rating.
Can I parallel small conductors instead of using one big PV conductor?
Below 1/0 AWG, no, per 310.10(G). Most residential PV strings fit in 6-10 AWG single conductors so paralleling rarely comes up. Commercial systems with 100+ A combiner outputs sometimes parallel 1/0 or larger.
What does the rapid-shutdown requirement do to wire sizing?
Rapid shutdown (NEC 690.12) does not change conductor sizing directly. It limits voltage in the conductors leaving the array within 30 seconds of an emergency. The equipment that performs the shutdown (module-level optimizers or rapid-shutdown devices) is what changes; conductors are sized normally.
What is the difference between PV wire and USE-2?
Both are sunlight-resistant and wet-location rated. USE-2 is dual-rated for service-entrance use too. PV wire is specifically listed under UL 4703 for photovoltaic systems. Functionally interchangeable; use whichever the inspector prefers in your area.
Do micro-inverters and string inverters need different conductors?
Yes. Microinverter trunk cables are AC at lower current per module, often 12 AWG. String inverters have higher DC current per string, often needing 10 AWG or 8 AWG. The calculator handles both if you pick the right configuration.