Wire Fill Chart for Solar PV Installations
Solar PV wire fill: 10 AWG PV wire on DC side, AC inverter output sized per inverter rating, EMT or PVC raceway. Governed by NEC Article 690.
Wire fill for solar PV installations is governed by NEC Article 690. Typical residential string PV uses 10 AWG USE-2/PV wire on the DC side from modules to combiner or inverter, then AC THHN/THWN-2 sized to the inverter's rated output on the AC side. Raceways are usually EMT indoors and PVC or EMT outdoors. Article 690 has unique conductor sizing rules: 156% of module short-circuit current for PV source circuits, plus rooftop temperature considerations.
Load characteristics
Residential PV systems are highly variable:
| System | DC Voltage range | DC current per string | AC output (240V) |
|---|---|---|---|
| 5 kW residential | 300-500V | 10A | 21A |
| 8 kW residential | 350-600V | 10-13A | 33A |
| 10 kW residential | 400-600V | 13-15A | 42A |
| 12 kW with battery | 400-600V | 15-18A | 50A |
The DC side is current-limited by the modules (Isc rarely exceeds 14A on modern panels). The AC side is set by the inverter nameplate continuous output current.
NEC code references
- NEC 690.7 — maximum DC voltage (Voc × cold-temperature multiplier)
- NEC 690.8 — circuit current sizing (1.25 × Isc, then 1.25 again for continuous)
- NEC 690.9 — overcurrent protection
- NEC 690.12 — rapid shutdown
- NEC 690.13/690.15 — DC and AC disconnects
- NEC 690.31 — wiring methods and conductor separation
- NEC 690.45-47 — equipment grounding and bonding
- NEC 705 — interconnection with utility (120% rule on bus)
Conductor sizing — DC side
PV source circuit conductor ampacity must equal or exceed 1.56 × Isc:
| Module Isc | 1.56 × Isc | Minimum copper (75°C) | Common gauge |
|---|---|---|---|
| 9A | 14.0A | 14 AWG (20A) | 10 AWG |
| 11A | 17.2A | 14 AWG | 10 AWG |
| 13A | 20.3A | 12 AWG (25A) | 10 AWG |
| 15A | 23.4A | 10 AWG (35A) | 10 AWG |
Most residential string installs use 10 AWG PV wire/USE-2 for source circuits because it withstands roof temperatures and gives a comfortable voltage-drop margin.
Conductor sizing — AC side
AC output of the inverter is a continuous load. Size at 125% of rated continuous output current per NEC 690.8(B):
| Inverter AC output | × 1.25 | Copper THHN | Aluminum |
|---|---|---|---|
| 24A (~6 kW) | 30A | 10 AWG | 8 AWG |
| 32A (~7.7 kW) | 40A | 8 AWG | 6 AWG |
| 40A (~9.6 kW) | 50A | 8 AWG | 6 AWG |
| 48A (~11.5 kW) | 60A | 6 AWG | 4 AWG |
| 64A (~15 kW) | 80A | 4 AWG | 2 AWG |
Conduit sizing
A typical DC home-run with 3 × 10 AWG PV wire (PV+, PV-, EGC) in EMT or PVC:
| Raceway | Conductors | Area | Fill % |
|---|---|---|---|
| 1/2" EMT | 3 × 10 PV wire (0.0260 in²) | 0.0780 in² | 39.6% (over 40%) |
| 3/4" EMT | 3 × 10 PV wire | 0.0780 in² | 16.5% |
| 1/2" PVC Sch 40 | 3 × 10 | 0.0780 in² | 26.1% |
| 3/4" PVC Sch 40 | 3 × 10 | 0.0780 in² | 14.7% |
Even though three 10 AWG just barely passes 40% in 1/2" EMT, most installers use 3/4" EMT for spare capacity and easier pulling on rooftop bends.
For larger inverters with 2 or more strings:
| Conductors | Raceway needed |
|---|---|
| 6 × 10 PV wire + 1 × 10 EGC | 1" EMT (~33% fill) |
| 8 × 10 PV wire + 1 × 10 EGC | 1-1/4" EMT |
AC side: 3 × 8 AWG + 1 × 10 EGC for 40A inverter — fits 3/4" EMT at ~28%.
Special requirements
- Rapid shutdown (NEC 690.12) — module-level or string-level rapid shutdown devices.
- DC disconnect within sight of inverter.
- AC disconnect typically a 60A non-fused disconnect between inverter and main panel.
- 120% rule (NEC 705.12) — inverter breaker + main breaker on a bus cannot exceed 120% of bus rating. Common backfed breaker at opposite end of bus from main.
- Equipment grounding — EGC sized per NEC 250.122 based on the overcurrent device, not the conductor.
Worked example — 8 kW PV system
System: 24 × 350W modules in 2 strings of 12. Inverter: 7.6 kW 240V single-phase, 32A continuous AC output. Module Isc = 11A.
- DC source circuits: 4 conductors (2 strings × +/-) of 10 AWG PV wire + 10 AWG EGC, run in 3/4" EMT from rooftop combiner down to inverter. Fill: 5 × 0.0260 = 0.130 in² in 3/4" EMT (0.213 in² 40% limit) = 24.4%, pass.
- AC output: 32A × 1.25 = 40A. Use 8 AWG copper THHN. 3 × 8 AWG + 1 × 10 EGC in 3/4" EMT to AC disconnect, then to a 40A backfed breaker.
- 120% check: 200A main panel bus, 200A main breaker + 40A PV breaker at opposite end = 240A. 200 × 1.2 = 240A. Pass (exactly at limit).
Use the Conduit Fill Calculator to verify.
Quick reference
| Component | Wire | Conduit |
|---|---|---|
| DC home-run (single string, 10A modules) | 10 AWG PV wire × 3 | 3/4" EMT |
| DC home-run (2 strings) | 10 AWG PV wire × 5 | 3/4-1" EMT |
| AC output (7.6 kW inverter) | 8 AWG THHN × 3 | 3/4" EMT |
| AC output (11.5 kW inverter) | 6 AWG THHN × 3 | 3/4-1" EMT |