Definition
Wire sizing for solar PV systems determines conductor type, AWG/kcmil size, and length to safely carry the maximum circuit current under NEC 690.8, with acceptable voltage drop (typically ≤ 2–3%) and temperature/conduit-fill derating.
Quick Facts
| Field | Detail |
|---|---|
| Term | Wire Sizing |
| Category | Solar Engineering |
| Engineering Discipline | Electrical Design |
| Relevant Codes | NEC 690.8, NEC 215, NEC 310 |
| Software | Bluebeam plugins, Excel, NREL Wire Sizer |
| Difficulty Level | Intermediate |
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.
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% → ✓ acceptableNEC Ampacity Tables (90°C insulation)
| AWG/kcmil | Ampacity (90°C, NEC 310.16) |
|---|---|
| 14 | 30 A |
| 12 | 40 A |
| 10 | 55 A |
| 8 | 75 A |
| 6 | 105 A |
| 4 | 140 A |
| 2 | 190 A |
| 1/0 | 260 A |
| 4/0 | 405 A |
| 250 kcmil | 455 A |
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)
| OCPD (A) | Copper EGC (AWG) |
|---|---|
| 15 | 14 |
| 20 | 12 |
| 30–60 | 10 |
| 100 | 8 |
| 200 | 6 |
| 400 | 3 |
Common Mistakes
- Using Imp instead of Isc.
- Forgetting double 1.25 multiplier in 690.8.
- Skipping temperature derating.
- Ignoring conduit fill effect.
- Voltage drop too high — undersizing for long runs.
- 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.
Frequently Asked Questions
10 commonly searched questions about Wire Sizing.
How do I size PV wire?
Apply NEC 690.8: I_circuit = 1.25 × Isc, then I_continuous = 1.25 × I_circuit. Conductor ampacity (from NEC Table 310.16/310.15(B)) after temperature and conduit-fill derating must ≥ I_continuous. Then verify voltage drop ≤ 2-3%.
What conductor types are used in solar PV?
PV Wire (source circuit, exposed outdoors): single-conductor, sunlight-resistant. USE-2 (source/output): underground. THHN/THWN-2: AC and interior runs. XHWN-2: AC, high-temperature.
What is voltage drop limit?
NEC 215 recommends ≤ 3% for branch circuits, ≤ 5% combined. Solar industry typically targets ≤ 2% DC and ≤ 1% AC for system efficiency.
How do I calculate voltage drop?
Vd = 2 × I × L × R_ohms_per_kft / 1000, where I is current (A), L is length (ft, one-way for AC; round-trip is 2×L for DC), R is conductor resistance from NEC Chapter 9 Table 8.
What's the difference between PV Wire and USE-2?
PV Wire is sunlight-resistant for exposed outdoor source circuits. USE-2 is rated for direct burial but not for exposed installations above ground. Both can be used in solar source circuits depending on location.
How does temperature derating work?
NEC Table 310.15(B)(2)(a) provides correction factors based on ambient temperature. At 50°C ambient, 90°C conductors derate by 0.82. At 70°C ambient, derate by 0.45.
What is conduit fill derating?
NEC 310.15(B)(3)(a) requires derating when >3 current-carrying conductors share a conduit. 4–6 conductors: 80%. 7–9 conductors: 70%. 10+ conductors: progressively lower.
Can I use 12 AWG PV Wire on most residential systems?
Often yes. 12 AWG PV Wire at 90°C has 30 A ampacity. After typical derating (50°C ambient × 0.82 = 24.6 A), it suffices for module Isc < 16 A (1.56 × Isc constraint). Verify on a project-by-project basis.
How long is too long for a PV wire run?
Voltage drop limits practical length. A 10 AWG wire carrying 10 A: 1% drop at 60 ft, 3% drop at 180 ft. For longer runs, upsize the wire or use higher string voltage.
Do I need separate ground conductors?
Yes, per NEC 690.45. Size EGC per NEC 250.122 based on OCPD rating. For 20 A OCPD: 12 AWG copper EGC minimum.
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