What is the 22°C rooftop temperature adder?

NEC 310.15(B)(3)(c) requires adding 22°C (40°F) to the maximum ambient air temperature for conductors in conduit on or above a rooftop when the conduit is within 7 inches (175mm) of the roof surface. This accounts for heat gain from the roof absorbing solar radiation. The 22°C adder can significantly reduce allowable conductor ampacity in hot climates.

What is the conduit fill derating rule?

When more than 3 current-carrying conductors are in a conduit, NEC Table 310.15(C)(1) requires derating. 4–6 conductors: multiply allowable ampacity by 0.80. 7–9 conductors: multiply by 0.70. Both positive and negative conductors count as current-carrying in DC circuits. Derating is applied after the temperature correction derating.

What AWG wire do I need for a 10A solar source circuit?

Start with 125% × 10A = 12.5A minimum ampacity. For 10 AWG THWN-2 in a Phoenix rooftop conduit (effective temperature ~65°C), the derated ampacity is 40A × 0.58 = 23.2A — sufficient for a single-string conduit. In cooler climates (Boston, Denver), 12 AWG may be sufficient depending on the temperature correction result. Always calculate for the specific climate.

Does the 125% factor apply to both source and output circuits?

Yes. NEC 690.8 applies the 125% continuous load factor to all PV source circuits (one string's Isc) and PV output circuits (sum of all parallel strings' Isc). For output circuits combining multiple strings, the starting Isc is the sum of all string Isc values, and the 125% factor applies to that combined value.

Solar Conductor Sizing Calculator: NEC 690.8 Ampacity Guide

Q: How do I size solar DC conductors under NEC 690.8?

Three-step process: (1) Multiply the module short-circuit current (Isc) by 1.25 (continuous load factor). (2) Apply ambient temperature correction using the correction factor from NEC Table 310.15(B)(1)(1) for the effective conductor temperature — add 22°C to the maximum air temperature for rooftop conduit within 7 inches of the roof. (3) Apply conduit fill derating from NEC Table 310.15(C)(1) if more than 3 current-carrying conductors share a conduit. Select the smallest conductor from NEC Table 310.16 that meets the derated requirement.

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 factor

Step 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.

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: 18.42A

From 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 conductor

12 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: 28.79A

From 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.