Wire Sizing

Quick Facts

Wire Sizing Workflow

Step 1: Calculate maximum circuit current

I_circuit_max = 1.25 × Isc (NEC 690.8(A))
I_for_conductor = 1.25 × I_circuit_max = 1.5625 × Isc (NEC 690.8(B))

Step 2: Pick conductor type

• PV Wire: source circuits exposed outdoors.
• USE-2: source/output, direct burial OK.
• THHN/THWN-2: AC interior runs.
• XHHW-2: hot environments.

Source circuit conductors usually terminate at a combiner box, where string-level OCPD and grounding are consolidated before the feeder run to the inverter, the conductor type and ampacity selected here should match what the combiner box’s terminals are rated for.

Step 3: Look up base ampacity (NEC Table 310.16, 90°C column)

Step 4: Apply temperature derating (NEC Table 310.15(B)(2)(a))

Step 5: Apply conduit fill derating (NEC Table 310.15(B)(3)(a))

Step 6: Verify derated ampacity ≥ I_for_conductor

Step 7: Calculate voltage drop, upsize if > 2-3%

Worked Example

Project: 18-module string, Isc = 13.5 A.

• I_for_conductor = 1.5625 × 13.5 = 21.1 A.
• Ambient: 50°C. Conduit fill: 4 conductors.
• Use 10 AWG PV Wire (base ampacity 40 A at 90°C).
• Temp derate (50°C): × 0.82 = 32.8 A.
• Conduit derate (4 conductors): × 0.80 = 26.3 A.
• 26.3 A > 21.1 A → ✓ adequate.

Voltage drop check (round-trip 100 ft, R for 10 AWG = 1.0 ohm/kft):

Vd = 2 × 13.5 × 100 × 1.0 / 1000 = 2.7 V
Vd % = 2.7 / 600 = 0.45% → ✓ acceptable

NEC Ampacity Tables (90°C insulation)

Note: Final ampacity after derating per NEC 240.4 may not exceed 60°C or 75°C ampacity columns depending on terminal ratings.

EGC Sizing (NEC 250.122)

Common Mistakes

1. Using Imp instead of Isc.
2. Forgetting double 1.25 multiplier in 690.8.
3. Skipping temperature derating.
4. Ignoring conduit fill effect.
5. Voltage drop too high, undersizing for long runs.
6. EGC undersized.

Best Practices

• Show all derating calculations on the SLD.
• Use 90°C conductors (PV Wire, XHHW-2) for ampacity headroom.
• Target ≤ 2% DC voltage drop, ≤ 1% AC voltage drop.
• For long runs, increase string voltage rather than upsize wire.
• Verify terminal temperature rating limits final ampacity.

Standards & Certifications

• NEC 690.8, Solar conductor sizing.
• NEC 310, Conductor properties and derating tables.
• NEC 250.122, EGC sizing.
• UL 4703, PV Wire listing.
• UL 854, USE-2 listing.

Key Takeaways

• Wire sizing per NEC 690.8: 1.25 × Isc × 1.25 = 1.5625 × Isc minimum ampacity.
• Apply temperature and conduit-fill derating from NEC Table 310.15.
• Voltage drop ≤ 2% DC, ≤ 1% AC industry target.
• Use PV Wire outdoors, THHN-2 indoors, USE-2 underground.
• EGC sized per NEC 250.122 based on OCPD rating.

Related Reading

Wire sizing doesn’t happen in isolation, the Isc figure feeding the 1.5625 multiplier comes out of string sizing decisions made earlier in the design, and the derated ampacity and voltage-drop results feed straight into the single-line diagram and cable schedule. Qbitsenergy’s string sizing glossary entry walks through how that upstream Isc/Voc figure gets set, and its AC cable sizing reference covers the inverter-to-panel side of the run that this entry doesn’t. On the documentation side, Heaven Designs’ guide to solar SLD software explains how leading platforms auto-generate conductor schedules and derating calculations once these numbers are locked in, and SurgePV’s single-line diagram software comparison is a useful companion for evaluating tools built around that workflow.