10 kW Residential Rooftop Solar Design
Design a 10 kW residential solar PV system — module count, string configuration, inverter selection, and NEC 690/705 compliance.
Scenario Overview
Design a 10 kW-DC rooftop solar PV system for a single-family home with a 200A main panel. Includes module layout, string sizing, inverter selection, and interconnection per NEC 690 and 705.
Given Information
- •Target array size: 10 kW-DC
- •Modules: 400W, Voc=37.2V, Vmpp=31.4V, Isc=13.9A
- •Inverter: string inverter, MPPT range 180-500V, max input 550V
- •Main panel: 200A, 120/240V
- •Location: moderate climate (min temp −10°C, max 45°C)
- •Roof: south-facing, 30° tilt, minimal shading
Calculation Steps
Determine Module Count
10,000W ÷ 400W/module = 25 modules.
Total array: 25 × 400W = 10.0 kW-DC.
Result: 25 × 400W modules = 10.0 kW-DC
String Sizing — Maximum Modules
Correct Voc for coldest temperature: Voc × [1 + Tc_Voc × (Tmin − 25)].
Assume Tc_Voc = −0.27%/°C.
Voc_cold = 37.2 × [1 + (−0.0027 × (−10 − 25))] = 37.2 × 1.0945 = 40.7V.
Max modules per string = 550V / 40.7V = 13.5 → 13 modules max.
Result: 13 modules max per string
String Sizing — Minimum Modules
Correct Vmpp for hottest temperature:
Vmpp_hot = 31.4 × [1 + (−0.0027 × (45 − 25))] = 31.4 × 0.946 = 29.7V.
Min modules = 180V / 29.7V = 6.06 → 7 modules minimum.
Result: 7 modules min per string
Final String Configuration
25 modules / valid range 7-13 per string.
Option A: 2 strings of 13 + 1 string of 12 = 38 modules (too many).
Option B: 2 strings of 13 modules = 26 modules (close, +1 module).
Use: 2 strings × 13 modules = 26 modules (10.4 kW-DC). Or 3 strings × 8 = 24 (9.6 kW).
Best fit: 2 strings of 13 modules = 10.4 kW-DC.
Result: 2 strings × 13 modules = 10.4 kW-DC
Inverter Selection and DC/AC Ratio
Array: 10.4 kW-DC. Select 7.6 kW-AC inverter.
DC/AC ratio: 10.4 / 7.6 = 1.37 (slightly high but acceptable for moderate climate).
Inverter output current: 7,600 / 240 = 31.7A.
Result: 7.6 kW string inverter, DC/AC ratio 1.37
Interconnection — 120% Rule (NEC 705.12)
200A main panel: 200 × 1.20 = 240A max total.
PV backfeed breaker: 31.7A × 1.25 = 39.6A → 40A breaker.
Total: 200A main + 40A PV = 240A. Exactly at 120% limit — compliant.
PV breaker must be at opposite end of bus from main breaker.
Result: 40A PV breaker — at 120% limit ✓
Final Answer
10.4 kW-DC array (26 × 400W modules in 2 strings of 13) with a 7.6 kW-AC string inverter. Interconnected via 40A backfeed breaker in 200A panel per NEC 705.12 120% rule. Rapid shutdown compliant with DC optimizers.
Key Takeaways
- •Always temperature-correct Voc for the coldest expected temperature — modules produce more voltage in cold weather
- •The 120% rule limits PV system size on load-side connections — this system is at the exact limit
- •DC/AC ratios up to 1.3-1.4 are acceptable and save inverter cost with minimal clipping losses
- •Rapid shutdown (NEC 690.12) requires module-level power electronics for residential installations
Calculators Used
NEC References
- NEC 690 — Solar Photovoltaic Systems
- NEC 690.7 — Maximum Voltage
- NEC 690.12 — Rapid Shutdown
- NEC 705.12 — Interconnection 120% Rule