Solar Wire & Cable Sizing Guide

Every conductor in a solar system has to pass two independent tests — one for safety, one for efficiency. Here is how to run both, why they often disagree, and why the answer is always the thicker wire.

Undersized wire is the quietest failure mode in a solar build. Panels still charge, the inverter still hums, and meanwhile a cable is running hot inside a conduit or a battery box, ageing its insulation and burning your harvest as heat. The fix is not guesswork or a forum rule of thumb — it is two simple, independent tests that every conductor must pass. Ampacity asks whether the wire can carry the current safely. Voltage drop asks whether it can carry it efficiently. The two tests frequently give different answers, and the rule is absolute: you install the thicker of the two.

Test 1: Ampacity — can the wire carry the current safely?

Ampacity is the current a conductor can carry continuously without its insulation exceeding its temperature rating. For PV source and output circuits, NEC-style rules stack two 125% factors. The first accounts for irradiance: on cold, clear days with reflection from snow or cloud edges, a module can briefly deliver more than its rated short-circuit current, so design current is Isc × 1.25. The second is the standard continuous-load factor — any circuit expected to run at full current for three hours or more is sized at 125% again. Multiply them and you get the number every solar wire calculator is built on:

A string with Isc of 11 A therefore needs a conductor and overcurrent device rated for at least 17.2 A — before derating. For non-PV circuits such as the battery-to-inverter run, use the continuous current × 1.25.

Derate for heat and bundling

Ampacity tables assume 30°C ambient. A rooftop conduit in full sun is nowhere near 30°C — add the rooftop temperature adder to the local design ambient and the correction factor for 90°C wire can fall to 0.82 or lower. Bundling costs you again: run 4–6 current-carrying conductors in one conduit and each is derated to 80% of table ampacity; 7–9 conductors drops it to 70%. These multiply. A 30 A-rated wire at 0.82 thermal correction and 0.8 fill adjustment is really a 19.7 A wire. If that is below your 1.56 × Isc design current, go up a size.

Test 2: Voltage drop — can the wire carry the current efficiently?

A wire that passes ampacity can still waste a painful fraction of your power as heat, because ampacity says nothing about length. Resistance grows with every foot of the round trip, and the power lost is current squared times that resistance. The usual targets are 3% maximum on any single run and 2% or less on the panel-to-controller leg, where drop directly costs harvest. The full physics, the formula and the worked examples live in our companion guide, Voltage Drop in Solar Systems, and you can test any run in seconds with the voltage drop calculator.

Here is the key point: the two tests disagree constantly. A short, fat battery jumper is governed by ampacity — drop over 18 inches is negligible. A 60-foot run from a ground-mount array is governed by voltage drop — the current may be modest, but the length forces a wire two or three sizes bigger than ampacity alone would require. Run both tests every time and install the larger gauge.

AWG and mm²: speak both languages

American Wire Gauge runs backwards — a smaller number means a bigger wire, and beyond 1 AWG the sizes continue as 1/0, 2/0, 3/0 and 4/0. Two mental anchors make the scale intuitive: every 3 gauge numbers roughly doubles the copper cross-section and halves the resistance, and every 6 gauge numbers doubles the diameter. Most of the world sizes cable in mm² instead, and datasheets mix the systems freely: 10 AWG is 5.26 mm² (buy 6 mm² metric), 6 AWG is 13.3 mm² (buy 16 mm²), 4 AWG is 21.2 mm² (buy 25 mm²), and 4/0 is 107 mm² (buy 120 mm²). When converting, always round up to the next standard metric size — our AWG to mm² converter does this automatically.

Use the right wire type for the location

• PV wire and USE-2 — the only conductors that belong in open air behind an array. Both are sunlight-resistant and wet-rated at 90°C; PV wire adds heavier insulation and is required where ungrounded (transformerless) inverters are involved.
• THHN / THWN-2 — the standard building wire for runs inside conduit. It is not UV-rated, so it must never be exposed; conduit also triggers the fill derates above.
• Battery cable — here lives the great welding-cable myth. Welding cable is wonderfully flexible thanks to fine stranding, and forums love it, but much of it carries no UL listing, its insulation is not rated for battery-compartment service, and fine strands need matched lugs and dies to crimp gas-tight. If an inspector or an insurer will ever look at your system, use UL-listed battery cable (UL 1426 marine cable is the gold standard) — you get the same flexibility with a listing behind it.

The battery-to-inverter run: the most underestimated cable in solar

Nothing else in an off-grid system moves current like this run, and nothing gets undersized more often. Watts divided by volts is amps, and at low battery voltage the numbers get violent. A 3,000 W inverter at 12 V draws 3,000 ÷ 12 = 250 A continuously — and more like 290 A once inverter efficiency is included, with surge beyond that. That is 4/0 territory, a cable as thick as a garden hose, with hydraulic-crimped lugs and a Class T fuse. The same 3,000 W at 48 V is 63 A — a tidy 6 AWG run. Same power, one-sixteenth the resistive loss, a tenth the copper cost. This single arithmetic is the strongest argument for higher battery voltage in any system above about 1,500 W; size the bank itself with the battery bank calculator.

Fuse the wire, not the load

A fuse or breaker exists to protect the conductor, not the appliance. Size the overcurrent device at or below the wire’s derated ampacity, and at or above 1.25 × the continuous load current. If those two bounds do not overlap, the wire is too small — fix the wire, never the fuse. Every battery positive lead needs its fuse within inches of the terminal, because the wire between battery and fuse is unprotected.

Lugs, crimps and torque: where systems actually fail

Most field failures are not mid-cable — they are at terminations. A loose or badly crimped lug adds milliohms of contact resistance exactly where hundreds of amps flow, and milliohms at 250 A means tens of watts of heat in a fitting the size of a thumb. Use a proper crimping tool matched to the lug and strand class, never solder-only joints on large cables (solder wicks, hardens the strands and cracks under vibration), and torque every terminal to the manufacturer’s spec with a torque wrench — then re-check after the first month of cycling. Adhesive-lined heat shrink over each lug keeps corrosion out.

Quick sizing table

Copper wire, 3% voltage drop target, 75–90°C insulation, one-way distance shown. Each cell is the larger of the ampacity and voltage-drop answers — notice how low voltage, not current, drives the monster gauges.

Common mistakes

• Sizing from a bare ampacity chart and ignoring voltage drop on long runs — the wire is safe but wastes 5–8% of the harvest.
• Forgetting the 1.56 factor on PV circuits and the temperature derate on hot rooftops.
• Using unlisted welding cable in a system that later needs inspection or an insurance claim.
• Sizing the battery-to-inverter run for average load instead of the inverter’s full continuous rating.
• Fusing at the load’s current instead of the wire’s ampacity — or skipping the battery-terminal fuse entirely.
• Perfect wire, terrible lugs: pliers-crimped terminals and hand-tight bolts undo everything the gauge bought you.
• Buying 4 mm² metric cable for a 12 AWG requirement without checking that the conversion actually rounds up.

Frequently asked questions

What size wire do I need for a 2,000 W inverter at 12 V?

About 167 A continuous, or roughly 195 A after inverter efficiency, so plan on 2/0 copper for short runs and 4/0 if the run exceeds a few feet — plus a Class T fuse sized to the cable. At 24 V the same inverter needs only about 2 AWG, which is why higher battery voltage is recommended above 1,500 W.

Can I use welding cable for battery connections?

It works electrically and is very flexible, but much welding cable is not UL-listed and its insulation is not rated for battery service, which matters for inspections and insurance. UL 1426 marine battery cable gives the same fine-strand flexibility with a proper listing, and its strand class matches standard battery lugs for reliable crimps.

Why multiply Isc by 1.56 for solar wires?

It is two stacked 125% factors from NEC-style rules: one because irradiance enhancement can push a module briefly above its rated short-circuit current, and one because PV circuits are continuous loads that run at full current for hours. Together, 1.25 times 1.25 equals 1.5625, so conductors and fuses are sized to at least 1.56 times Isc.

Does voltage drop matter on short cable runs?

Rarely — over a foot or two the resistance is tiny, and ampacity governs the size. Voltage drop takes over as the run gets longer or the voltage gets lower; a 60-foot array feed or any 12 V circuit will usually need a bigger wire for drop than for ampacity. Always run both tests and install the thicker answer.