FREE ENGINEERING REFERENCE

NEC 690 Solar Conductor Sizing
Complete Reference Tables

DC circuit ampacity, temperature correction factors, conduit fill derating, and NEC 705 busbar rule tables for solar PV design. Bookmark this page.

📊 Table 1 — Copper Conductor Ampacity at 75°C (NEC Table 310.15)

These are base ampacity values at 75°C (167°F) terminal rating. Apply temperature correction and conduit fill derating factors from Tables 2 and 3 before final selection.

AWG / kcmil	THWN-2 (75°C) — Ampacity	USE-2 / PV Wire (90°C) — Ampacity	Typical Solar Use
#14 AWG	15 A	25 A	Low-power microinverter AC circuits
#12 AWG	20 A	30 A	Small string circuits, AC combiner
#10 AWG	30 A	40 A	Standard residential DC home-run runs
#8 AWG	50 A	55 A	Residential 2-string parallel combiner
#6 AWG	65 A	75 A	Commercial small array DC main
#4 AWG	85 A	95 A	Commercial mid-size DC combiner runs
#2 AWG	115 A	130 A	Commercial large array DC main
#1 AWG	130 A	145 A	Commercial high-current DC runs
#1/0 AWG	150 A	170 A	Commercial/utility scale DC main
#2/0 AWG	175 A	195 A	Large commercial DC main runs
#3/0 AWG	200 A	225 A	Utility-scale DC collection circuits
#4/0 AWG	230 A	260 A	Utility-scale DC collection circuits
250 kcmil	255 A	290 A	Utility-scale DC main feeders
350 kcmil	310 A	350 A	Utility-scale DC collection feeders
500 kcmil	380 A	430 A	Utility-scale DC main feeders

NEC 690.8 DC Conductor Sizing Formula: I_required = Isc × strings × 1.5625 Selected conductor ampacity ≥ I_required (After applying temperature & fill derating factors)

🌡️ Table 2 — Temperature Correction Factors (NEC Table 310.15(B)(1))

Multiply conductor ampacity by the correction factor for the ambient temperature at the conductor location (e.g. attic, rooftop conduit, direct burial).

Ambient Temp (°C)	Ambient Temp (°F)	60°C Rated Wire	75°C Rated Wire	90°C Rated Wire
10°C	50°F	1.29	1.20	1.15
15°C	59°F	1.22	1.15	1.12
20°C	68°F	1.15	1.11	1.08
25°C	77°F	1.00	1.00	1.00
30°C	86°F	0.91	0.94	0.96
35°C	95°F	0.82	0.88	0.91
40°C	104°F	0.71	0.82	0.87
45°C	113°F	0.58	0.75	0.82
50°C	122°F	0.41	0.67	0.77
55°C	131°F	—	0.58	0.71
60°C	140°F	—	0.47	0.65
70°C	158°F	—	—	0.50

⚠️ Rooftop conduit in direct sun can reach 60-70°C ambient. Always use 90°C rated USE-2 or PV Wire for solar DC circuits on rooftops.

🔌 Table 3 — Conduit Fill Derating Factors (NEC Table 310.15(C)(1))

Conductors in Same Conduit	Derating Factor	Common Scenario
1–3 Current-Carrying Conductors	1.00 (No Derating)	Standard 2-wire DC circuit
4–6 Current-Carrying Conductors	0.80 (80%)	Multiple string homeruns in one conduit
7–9 Current-Carrying Conductors	0.70 (70%)	Combiner-to-inverter trunk conduit
10–20 Current-Carrying Conductors	0.50 (50%)	Large commercial combiner conduit
21–30 Current-Carrying Conductors	0.45 (45%)	Utility-scale collection conduit
31–40 Current-Carrying Conductors	0.40 (40%)	Utility-scale collection conduit
41+ Current-Carrying Conductors	0.35 (35%)	Utility-scale multi-circuit conduit

💡 Note: Equipment grounding conductors (EGC) and grounded conductors (neutral) are NOT counted as current-carrying conductors for fill derating purposes.

⚡ Table 4 — 120% Busbar Rule Quick Reference (NEC 705.12)

Panel Bus Rating	Main Breaker	Max Solar Backfeed Breaker	Result
100A Bus	100A Main	(100×1.2)−100 = 20A max	Very limited — small system only
125A Bus	100A Main	(125×1.2)−100 = 50A max	Suitable for ~10kW system
150A Bus	150A Main	(150×1.2)−150 = 30A max	Limited — MPU often needed
200A Bus	200A Main	(200×1.2)−200 = 40A max	Standard residential — up to ~8-10kW
225A Bus	200A Main	(225×1.2)−200 = 70A max	Good capacity — up to ~14kW
200A Bus	150A Main	(200×1.2)−150 = 90A max	Excellent — large residential systems
400A Bus	400A Main	(400×1.2)−400 = 80A max	Commercial — up to ~20kW AC

Max Solar Breaker = (Bus Rating × 1.20) − Main Breaker Size If result is negative or too small → Main Panel Upgrade (MPU) Required per NEC 705.12(B)(2)(3)(b)

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