What Size Wire for Solar Panels? PV Wire Sizing Made Simple
Use 10 AWG copper PV wire for solar panel wiring — it covers nearly every residential rooftop array. A modern 440W-class panel produces a design current of roughly 22 A under the NEC's 156% rule, and 10 AWG carries 40 A at its 90°C rating. Upsize to 8 or 6 AWG only for long ground-mount or low-voltage runs where voltage drop becomes the limit.
We sell wire, connectors, and everything they plug into every day, so here is the whole answer in plain English: the ampacity math, when distance forces a bigger conductor, and which insulation type goes where. One note up front — this is education, not engineering services. Your local inspector (the AHJ) has the final word, so verify before you pull wire.
Why 10 AWG Covers Almost Every Rooftop Array
Wire sizing starts with one number: the current the circuit can be asked to carry. For a single string of panels, that is the short-circuit current (Isc) on the panel label — typically around 14 A for today's 440–460W modules. Panels in series don't add current (only voltage), so a string of twelve panels still pushes the same ~14 A as one panel. If that surprises you, our series vs parallel explainer walks through it.
Compare that ~22 A design figure (more on the multiplier below) against standard copper ampacities and the answer falls out immediately:
| Copper Size | Ampacity (90°C) | Ampacity (75°C) | Where It Fits |
|---|---|---|---|
| 12 AWG | 30 A | 25 A | Small modules and very short runs — do the math before trusting it |
| 10 AWG | 40 A | 35 A | The default for module leads and string wiring on nearly every roof |
| 8 AWG | 55 A | 50 A | Longer home runs; combined output of paralleled strings |
| 6 AWG | 75 A | 65 A | Long ground-mount feeders and 48 V battery-side wiring |
Ampacities from NEC Table 310.16 (copper). Real circuits apply temperature and conduit-fill corrections, and terminations are usually limited to the 75°C column — your AHJ and the actual conditions of use govern.
Even on a hot roof where temperature corrections shave the 90°C rating down — a 45°C ambient correction takes 40 A to roughly 35 A — 10 AWG still clears a ~22 A design current with room to spare. That margin — plus the fact that factory module leads are already 10 or 12 AWG PV wire — is why 10 AWG is the industry answer.
The 156% Rule: Where That 22 A Comes From
Under the NEC's Article 690 rules for PV source circuits (690.8), you don't size wire to the panel's nameplate current — you size it to a padded design current. Two 125% multipliers stack:
- 125% for irradiance: real sunlight can exceed the lab's standard test conditions — think reflections off snow or cloud-edge focusing — so the code assumes the panel can beat its rated Isc.
- 125% for continuous duty: solar current flows for three-plus hours at a stretch, and the NEC pads every continuous load the same way.
Multiply them and you get the shorthand: Isc × 1.56. A panel with a 14 A short-circuit rating yields 14 × 1.56 ≈ 21.8 A of design current. That is the number your conductor's corrected ampacity has to beat — and it is why 12 AWG (30 A at 90°C, before derates) gets uncomfortably tight with modern high-current panels while 10 AWG stays comfortable. Again: educational framing of how the 2020 NEC approaches it, not a stamped design.
Voltage Drop: The Real Reason to Upsize
Ampacity keeps wire safe; voltage drop keeps it efficient. Every foot of copper has resistance, and the energy lost heating the wire is production you paid for and never sell. The usual design target is 2% or less on the DC side, roughly 3% total. Two things drive it: current and one-way distance.
Here is roughly how far each copper size can run before drop exceeds ~2% on a 240 V circuit — the situation you actually face on a microinverter trunk home-run or a long AC feeder from a ground mount:
| Copper Size | 15 A | 20 A | 30 A |
|---|---|---|---|
| 10 AWG | 130 ft | 95 ft | 65 ft |
| 8 AWG | 205 ft | 155 ft | 100 ft |
| 6 AWG | 325 ft | 245 ft | 160 ft |
| 4 AWG | 520 ft | 390 ft | 260 ft |
Maximum one-way run for ≈2% voltage drop at 240 V, copper conductors, rounded. For a 120 V circuit, halve these distances; for a 48 V battery circuit, divide by five. High-voltage DC strings (300–450 V) can go substantially farther.
The takeaways: on a rooftop grid-tied system with strings at 300–450 V DC, voltage drop almost never forces an upsize — a 400 V string at 14 A loses less than 1% through 100 ft of 10 AWG. Where people get burned is the 150-foot trench to a ground mount carrying 240 V AC at 25–30 A (step up to 8 or 6 AWG), and anything battery-side at 48 V, where even short runs want fat copper. If you're sizing an off-grid system, run the numbers in our off-grid sizing walkthrough before buying wire.
PV Wire vs USE-2 vs THHN: Which Insulation Goes Where
Gauge is only half the spec — the insulation listing decides where a conductor is allowed to live. Three types cover practically every residential solar project:
- PV wire — the purpose-built option: sunlight-resistant, 90°C wet-rated, typically listed to 1,000 or 2,000 V with extra-thick insulation and fine stranding that routes easily behind an array. NEC 690.31 permits it for exposed single-conductor wiring at the modules, and older code cycles required it on ungrounded arrays. It is what factory module leads are made of, and it is the default we recommend.
- USE-2 — underground service entrance cable, also sunlight-resistant and 90°C wet-rated, listed to 600 V. Also permitted for exposed PV source-circuit wiring and usually a little cheaper. Stiffer stranding, thinner jacket — fine work, less forgiving.
- THHN/THWN-2 — standard building wire for the conduit portion of the run: junction box to combiner, combiner to inverter, inverter to panel. Not listed for exposed single-conductor runs in free air, so it never substitutes for PV wire out at the modules.
A typical rooftop system uses all of these in sequence: factory PV-wire module leads, a short stretch of field-cut 10 AWG PV wire to a rooftop transition box, then THWN-2 in conduit down to the equipment. And remember rooftop DC conductors also carry rapid-shutdown obligations — our NEC 690.12 rapid shutdown explainer covers what that means for your wiring plan.
The Wire and Connectors We Actually Stock
For field wiring, we stock UL-listed Monroe PV wire in the exact spec this article recommends: 10 AWG, sunlight-resistant, rated for exposed runs. Grab a 250 ft spool in black for the negative conductor and a matching red spool for the positive ($165 each) — one pair covers a typical residential array with slack to spare. The rest of the small parts live in our wiring and accessories collection.
Two practical notes from the warehouse. First, if you're building with microinverters, most of your "wire sizing" is already done: the trunk cable is engineered for the system, and you buy it by the drop. You will still touch 10 AWG where trunk runs terminate — connectors like the APsystems 32A male AC connector ($16.24) are specifically rated for 10 AWG conductors. Second, when panel spacing outruns the factory leads, use listed MC4 extension cables rather than splicing — a 2-meter MC4 extension runs $9.91 and the 4-meter version $12.49, which is cheap insurance against a failed field crimp.
Planning a whole system rather than one cable run? Our complete DIY home solar parts checklist covers every component category — wire included — so nothing shows up missing on install day.
Prices are pulled live from the Pyra Solar catalog and refreshed hourly. Click any part to see current stock.
The 30-Second Cheat Sheet
- Rooftop module and string wiring: 10 AWG PV wire. Done.
- Conduit runs to the inverter or panel: THWN-2, sized to the same current with your electrician's derates.
- Ground mount 100+ ft away: run the voltage-drop numbers; expect 8 or 6 AWG on the AC side.
- 48 V battery circuits: throw the rooftop intuition out — size by amps and keep runs brutally short.
- Always: copper unless you have a specific reason, listed connectors instead of field splices, and a sign-off from your AHJ.
Frequently Asked Questions
What size wire do I need for solar panels?›
10 AWG copper PV wire handles nearly all residential rooftop module and string wiring. A modern 440W-class panel short-circuits at roughly 14 A; the NEC 690.8 design current works out to about 22 A, and 10 AWG carries 40 A at its 90°C rating — comfortable headroom even after heat derates. Upsize to 8 or 6 AWG only for long runs where voltage drop adds up.
Can I use 12 gauge wire for solar panels?›
Sometimes, but it is rarely worth it. 12 AWG copper is rated 30 A at 90°C, and a modern panel's ~22 A design current eats most of that before temperature derates. On small, older, or lower-current modules with short runs the math can work — but 10 AWG costs only slightly more and removes the question, which is why it is the industry default.
What is the difference between PV wire and USE-2?›
Both are sunlight-resistant, 90°C wet-rated single conductors permitted for exposed PV source-circuit wiring. PV wire has thicker, tougher insulation, is typically listed to 1,000 or 2,000 V instead of 600 V, and uses finer stranding that is easier to route behind an array. Older code cycles required PV wire on ungrounded arrays, and it remains the industry default for module wiring today.
Can I use THHN wire for solar panels?›
Only inside conduit. THHN (dual-rated THWN-2 for wet locations) is a building wire — it is not listed for exposed single-conductor runs in free air, so it cannot replace PV wire behind the modules. It is the normal, economical choice for the conduit run from a rooftop junction box down to the inverter or combiner.
How far can solar panels be from the house before wire size matters?›
On a grid-tied string inverter running 300–450 V DC, 10 AWG stays under 2% voltage drop for a couple hundred feet — distance is rarely the problem. The pain shows up on 240 V AC trunk runs past roughly 100 ft at 20–30 A, and especially on 48 V battery-based systems, where the same wire hits the same percentage drop in one-fifth the distance.
Does solar wire size depend on watts or amps?›
Amps. Wire only cares about current and length — a 5,000 W array at 400 V pushes about 12.5 A, while the same 5,000 W at 48 V pushes over 100 A and needs dramatically thicker copper. That is why high-voltage strings wire up with slim 10 AWG while off-grid battery banks use welding-cable-sized conductors.
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