Solar PV Wire Sizing Service Guide
// PLAN PV SOURCE CIRCUITS, OUTPUT CIRCUITS, INVERTER FEEDERS, AND ROOFTOP CONDUIT RUNS WITH AMPACITY, DERATING, AND VOLTAGE-DROP CHECKS BEFORE INSTALLATION. //
Solar PV wiring is a current, temperature, and distance problem before it is an AWG lookup. Start with module Isc, string count, inverter input, circuit voltage, conductor insulation, rooftop ambient temperature, raceway grouping, one-way length, and the adopted NEC or IEC rule set before selecting PV wire, THWN-2, USE-2, or metric cable.
NEC 690 current basis
PV source and output circuits commonly start with module short-circuit current and NEC 690.8 continuous-current multipliers before ampacity correction.
Rooftop temperature
Conductors in sun-heated roof raceways can lose usable ampacity after ambient correction and conduit grouping under NEC 310.
IEC cross-check
IEC 60364-7-712 and IEC 60364-5-52 review PV installation method, cable current capacity, protection, and voltage drop.
TL;DR
- Do not size PV wire from inverter watts alone
- Start with module Isc, parallel strings, max circuit current, temperature, and one-way distance
- Check NEC 690.8, NEC 690.31, NEC 310, grounding, and rapid-shutdown requirements
- Use 2% DC drop and 3% AC drop as common design targets when the project does not specify tighter limits
- Document whether the run uses PV wire, USE-2, THWN-2 in conduit, or IEC 60228 metric cable
Key definitions
PV source circuit
A PV source circuit is the DC conductor path from one module string to a combiner or inverter input.
PV output circuit
A PV output circuit is the conductor path that carries combined DC output from source circuits toward inverter equipment.
Voltage drop
Voltage drop is the conductor voltage loss caused by current, resistance, material, temperature, and circuit length.
Sizing workflow
Step 1 - collect PV data
Record module Isc, Voc, strings in parallel, inverter input current, DC voltage, AC output current, conductor material, insulation type, and the farthest one-way route.
Step 2 - calculate circuit current
Apply NEC 690.8 current logic or the IEC project basis before ampacity correction. For example, 11.2A Isc x 1.25 = 14A before conductor derating review.
Step 3 - apply conditions
Check rooftop ambient, raceway fill, number of current-carrying conductors, terminal temperature, wet-location listing, sunlight resistance, and copper or aluminum terminations.
Step 4 - check voltage drop
Run DC source, DC output, and AC inverter feeder segments separately. Long 600V or 1000V DC strings may tolerate smaller percentage loss than 120V or 240V AC branches.
Step 5 - coordinate inspection details
Verify disconnects, overcurrent protection, equipment grounding, bonding, labeling, rapid shutdown, conduit fill, and AHJ documentation before ordering cable.
Worked examples
Residential string to inverter
Two strings in parallel, module Isc 11.2A each, 450V DC operating voltage, 95 ft one-way copper PV wire route.
11.2A x 2 x 1.25 = 28A design current before correction factors. 10 AWG copper may pass ampacity, but the voltage-drop tool should compare 10 AWG and 8 AWG when the owner wants about 2% DC loss.
Rooftop combiner output
Four 12A strings combined to 48A, rooftop conduit section at high ambient, 160 ft one-way to inverter equipment.
48A x 1.25 = 60A current basis before derating. 6 AWG copper may be a starting point, while 4 AWG can be reviewed when temperature correction and 2% drop are both tight.
400V IEC inverter feeder
25A AC inverter output, 42 m one-way copper multicore cable in tray, grouped with other renewable-energy circuits.
6 mm2 may pass in some IEC methods, but 10 mm2 is often checked when grouping and IEC 60364-5-52 Clause 525 voltage drop are included.
PV circuit comparison
| Circuit | Sizing basis | Likely conductor check | Voltage-drop risk | Code note |
|---|---|---|---|---|
| Single string source circuit | Module Isc x NEC 690.8 basis | 10-12 AWG copper PV wire common range | Low to medium unless route is long | NEC 690.8 and 690.31 |
| Combined DC output circuit | Parallel string current after combiner | 8-4 AWG copper depending strings and distance | High beyond 100-200 ft | NEC 690, 310, conduit fill |
| AC inverter feeder | Inverter continuous output current | Breaker and conductor at 125% where required | Medium on remote inverters | NEC 705, 215, 310 |
| Rooftop raceway run | Corrected ampacity after heat and grouping | Upsize after NEC 310 correction factors | Medium because heat also raises resistance | NEC 310 ambient and adjustment |
| IEC PV cable route | Design current and installation method | 4-10 mm2 depending method and drop | Medium on 30-60 m routes | IEC 60364-7-712 and 5-52 |
Code checkpoints
Use these references as design checkpoints, then confirm the adopted code edition, inverter listing, module instructions, and local AHJ interpretation.
NEC 690.8
Defines PV circuit current calculations used before conductor ampacity and overcurrent checks.
NEC 690.31
Covers PV wiring methods, conductor types, routing, and installation requirements for photovoltaic systems.
NEC 310 and Chapter 9
Check conductor ampacity, correction and adjustment factors, terminal temperature, and conduit fill together.
NEC 250 and 705
Review grounding, bonding, interconnected power production sources, inverter output, and service coordination.
IEC 60364-7-712
Provides IEC requirements for photovoltaic power supply systems and PV-specific installation checks.
IEC 60364-5-52
Reviews cable current-carrying capacity, installation method, grouping, ambient temperature, and Clause 525 voltage drop.
Field checklist
- Use module Isc and string count, not only array watts.
- Separate source-circuit, output-circuit, and inverter-feeder calculations.
- Check rooftop temperature and current-carrying conductor count before trusting an ampacity table.
- Enter one-way distance for each DC and AC segment in the calculator.
- Verify PV wire, USE-2, or THWN-2 suitability for sunlight, wet locations, and conduit routing.
- Confirm rapid shutdown, disconnects, labels, grounding, bonding, and equipment listings.
- Keep the calculation with the inverter datasheet, string map, conduit schedule, and inspection package.
Solar PV wire sizing FAQ
What wire size is common for a residential PV string?
Many residential source circuits use 10 AWG or 12 AWG copper PV wire, but the final size depends on module Isc, strings in parallel, temperature correction, and voltage drop.
Why does NEC 690.8 use short-circuit current?
PV modules can produce current near Isc under strong sun, so NEC 690.8 starts from short-circuit current and applies multipliers before conductor protection is finalized.
Is 2% voltage drop required for solar DC wiring?
Not usually as a universal code rule. Many designers use about 2% DC loss as a performance target and about 3% on AC inverter feeders unless the owner or IEC specification says otherwise.
Can THHN be used for outdoor PV circuits?
THHN alone is not the same as a listed PV conductor for exposed rooftop DC wiring. Verify PV wire, USE-2, THWN-2 in conduit, wet-location, sunlight-resistant, and equipment listing requirements.
How do I size wire from combiner to inverter?
Add the parallel string currents, apply NEC 690.8 or IEC project factors, correct ampacity for rooftop conditions, then run voltage drop for the full one-way route to the inverter.
How does IEC solar cable sizing differ from AWG?
IEC projects use mm2 conductor areas, installation methods, grouping factors, protective-device coordination, and IEC 60364 voltage-drop checks instead of AWG table selection alone.
Check the PV run before cable is ordered
Use the solar, voltage-drop, and ampacity tools to compare the minimum conductor with the practical size that limits energy loss and keeps the inspection package clear.