NEC 690.8 is the conductor sizing section — and it requires three separate calculations stacked on top of each other. The 125% continuous load factor gets the most attention, but the rooftop temperature derating and conduit fill derating can be equally significant in hot climates.
The Three-Step 690.8 Calculation
Required ampacity = (Isc × 1.25) ÷ Temperature factor ÷ Fill factorStep 1: Isc × 1.25
Solar is a continuous load — it can produce near-maximum current for 3+ hours. NEC requires conductors serving continuous loads to be sized at 125% of actual current.
SurgePV’s solar design platform helps automate compliance documentation.
| Module Isc | × 1.25 Required Ampacity |
|---|---|
| 8.0A | 10.0A |
| 9.58A | 11.975A |
| 10.0A | 12.5A |
| 11.5A | 14.375A |
| 13.0A | 16.25A |
| 14.5A | 18.125A |
Step 2: Temperature Correction (NEC Table 310.15(B)(1)(1))
For rooftop conduit: effective temperature = maximum air temperature + 22°C
Temperature correction factors for 90°C (THWN-2) conductors:
| Effective Temperature | Correction Factor |
|---|---|
| ≤30°C (86°F) | 1.00 |
| 31–35°C (87–95°F) | 0.96 |
| 36–40°C (97–104°F) | 0.91 |
| 41–45°C (105–113°F) | 0.87 |
| 46–50°C (114–122°F) | 0.82 |
| 51–55°C (123–131°F) | 0.76 |
| 56–60°C (132–140°F) | 0.71 |
| 61–65°C (141–149°F) | 0.65 |
| 66–70°C (150–158°F) | 0.58 |
| 71–75°C (159–167°F) | 0.50 |
Rooftop conduit effective temperatures by city:
| City | Max Air Temp | + 22°C | Effective Temp | Correction Factor |
|---|---|---|---|---|
| Miami, FL | 33°C | + 22°C | 55°C | 0.76 |
| Los Angeles, CA | 38°C | + 22°C | 60°C | 0.71 |
| Phoenix, AZ | 43°C | + 22°C | 65°C | 0.65 |
| Las Vegas, NV | 42°C | + 22°C | 64°C | 0.65 |
| Atlanta, GA | 36°C | + 22°C | 58°C | 0.71 |
| Dallas, TX | 40°C | + 22°C | 62°C | 0.65 |
| Denver, CO | 36°C | + 22°C | 58°C | 0.71 |
| New York, NY | 33°C | + 22°C | 55°C | 0.76 |
| Boston, MA | 32°C | + 22°C | 54°C | 0.76 |
| Seattle, WA | 29°C | + 22°C | 51°C | 0.82 |
| Chicago, IL | 34°C | + 22°C | 56°C | 0.76 |
Step 3: Conduit Fill Derating (NEC Table 310.15(C)(1))
| Conductors in Conduit | Derating Factor |
|---|---|
| 1–3 | 1.00 (no derating) |
| 4–6 | 0.80 |
| 7–9 | 0.70 |
| 10–20 | 0.50 |
Both positive and negative conductors count. 2 strings in one conduit = 4 conductors → 0.80 derating.
Worked Examples
Single String, Phoenix Rooftop
Module Isc: 9.58A, Location: Phoenix, 1 string in conduit
Step 1: 9.58 × 1.25 = 11.975A
Step 2: 11.975 / 0.65 = 18.42A (conduit on Phoenix roof)
Step 3: 1.00 (single string = 2 conductors, no fill derating)
Required: 18.42AFrom NEC Table 310.16 (90°C column): 12 AWG = 30A ✓ (sufficient)
Two Strings, Same Phoenix Conduit
2 strings (4 conductors), same conduit, Phoenix
Step 1: 9.58 × 1.25 = 11.975A (per string)
Step 2: 11.975 / 0.65 = 18.42A
Step 3: 18.42 / 0.80 = 23.03A (fill derating for 4 conductors)
Required: 23.03A per string circuit conductor12 AWG (30A at 90°C table) — still sufficient.
High-Current Module, Miami Rooftop
Module Isc: 14.0A, Miami, 3 strings in one conduit (6 conductors)**
Step 1: 14.0 × 1.25 = 17.5A
Step 2: 17.5 / 0.76 = 23.03A (Miami rooftop effective 55°C)
Step 3: 23.03 / 0.80 = 28.79A (6 conductors, factor 0.80)
Required: 28.79AFrom NEC Table 310.16 (90°C): 10 AWG = 40A ✓. 12 AWG (30A) would fail.
Terminal Temperature Limitation
If inverter input terminals are only rated for 75°C (check the inverter spec sheet), you must use the 75°C column in NEC Table 310.16 even for THWN-2 (90°C rated) wire. 10 AWG at 75°C = 35A. This is a common oversight on Phoenix-area installations where the 90°C column barely passes but the 75°C column does not.
NEC Table 310.16 Reference (Copper, 90°C, in Conduit)
| AWG | Ampacity (90°C column) | Ampacity (75°C column) |
|---|---|---|
| 14 | 25A | 20A |
| 12 | 30A | 25A |
| 10 | 40A | 35A |
| 8 | 55A | 50A |
| 6 | 75A | 65A |
| 4 | 95A | 85A |
| 2 | 130A | 115A |
| 1/0 | 170A | 150A |
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Frequently Asked Questions
How do I size solar DC conductors under NEC 690.8?
Three steps: (1) Isc × 1.25. (2) Divide by temperature correction factor (add 22°C to air temp for rooftop conduit). (3) Divide by conduit fill derating factor (4–6 conductors = 0.80). Select conductor from NEC Table 310.16 that meets the result.
What is the 22°C rooftop adder?
NEC 310.15(B)(3)(c) requires adding 22°C to the maximum air temperature for conductors in conduit within 7 inches of the roof surface. Phoenix rooftop conduit, for example, sees an effective conductor temperature of 65°C, requiring a 0.65 correction factor that substantially reduces allowable ampacity.
Does fill derating apply to a single-string conduit?
No. Fill derating only applies when more than 3 current-carrying conductors share a conduit. A single-string source circuit has 2 conductors (positive and negative) — no fill derating required.
