Why does NEC use 125% of Isc for solar conductors?

Solar PV systems are treated as continuous loads under the NEC — they can produce current at or near their rated output for 3 or more hours at a time. For continuous loads, the NEC requires conductors to be sized at 125% of the actual load current. The module Isc represents the theoretical maximum DC current from the source circuit.

Do I need to derate for temperature on rooftop conductors?

Yes. Conductors in conduit on a rooftop are subject to elevated ambient temperatures that significantly reduce their allowable ampacity. NEC Table 310.15(B)(2) provides ambient temperature correction factors. Rooftop conduit can reach 60–80°C in summer, requiring substantial ampacity derating — often reducing allowable ampacity by 20–40%.

What is conduit fill derating and when does it apply?

When more than 3 current-carrying conductors occupy a conduit, NEC Table 310.15(C)(1) requires derating the allowable ampacity. For 4–6 conductors, the derating factor is 0.80 (80%). For 7–9 conductors, it is 0.70 (70%). This applies to both AC and DC circuits. Plan conduit layouts to limit conductor counts per conduit.

What wire types are allowed for PV source circuits?

NEC 690.31 specifies approved wiring methods: PV wire (listed specifically for PV use, rated 600V or 1000V, sunlight resistant), USE-2 cable, conductors in conduit (THWN-2 or equivalent), or listed PV cable assemblies. PV wire is the most common choice for exposed module wiring. Conductors inside conduit can use THWN-2 or XHHW-2.

NEC 690.8 Circuit Sizing: Conductor Ampacity Rules for Solar PV Systems

Q: What does NEC 690.8 require for conductor sizing?

NEC 690.8 requires that conductors in PV source and output circuits be rated at a minimum of 125% of the module rated short-circuit current (Isc). This 125% continuous current multiplier is applied first, and then standard NEC ampacity derating factors for temperature and conduit fill are applied on top.

Every solar conductor must be sized to carry current safely under the worst-case conditions the installation will ever experience. NEC 690.8 defines how to do that calculation for PV source and output circuits — and the answer is almost never the same as a standard building wiring calculation.

The key difference: solar is treated as a continuous load, which means the 125% rule applies before any other derating. Then ambient temperature correction and conduit fill derating stack on top. Miss any one of these and the permit fails plan review.

The 690.8 Calculation Framework

NEC 690.8 establishes a three-step minimum sizing requirement:

Step 1: Start with the module rated Isc
Step 2: Multiply by 1.25 (continuous load factor)
Step 3: Apply standard NEC derating factors (temperature, conduit fill)

The conductor must carry the result of Step 3 under all conditions. This is the minimum required ampacity — you then select the smallest standard conductor from NEC Table 310.16 that meets or exceeds it.

Source Circuit vs. Output Circuit

A PV source circuit connects modules within a string to the combiner box or inverter input. A PV output circuit connects the combiner box output to the inverter. Source circuit current = single string Isc. Output circuit current = sum of all parallel string Isc values feeding that circuit.

Step 1: Identify Circuit Isc

Series Strings (Source Circuit)

Current does not add in series. A string of 20 modules in series has the same current as a single module:

Source circuit Isc = Module Isc (regardless of string length)

Parallel Strings (Output Circuit)

Current adds in parallel. If 4 strings (each with Isc = 10A) feed a combiner, the output circuit Isc is:

Output circuit Isc = N_strings × Module Isc = 4 × 10A = 40A

Step 2: Apply the 125% Continuous Load Factor

NEC 690.8(A) requires that the minimum conductor ampacity equal at least 125% of the maximum circuit current:

Minimum ampacity (before derating) = Isc × 1.25

Module Isc	×1.25 Minimum Ampacity
8.0 A	10.0 A
9.5 A	11.9 A
10.0 A	12.5 A
11.0 A	13.75 A
12.5 A	15.6 A
14.0 A	17.5 A

This is the floor. Everything after this point only makes the required ampacity larger.

Step 3A: Ambient Temperature Derating

Conductor ampacity decreases as ambient temperature increases. The NEC tables are based on a 30°C (86°F) reference temperature for THWN-2 conductors. In hotter environments, you must derate.

Rooftop Conduit: The 22°C Adder

NEC 310.15(B)(3)(c) requires adding 22°C (40°F) to the local design maximum ambient air temperature when conductors are in conduit installed on or above a rooftop and within 7 inches of the roof surface.

Example: Phoenix, AZ has a design maximum air temperature of 43°C (109°F). Conduit on roof: 43°C + 22°C = 65°C effective ambient

At 65°C, the temperature correction factor for 90°C-rated conductors (THWN-2) from NEC Table 310.15(B)(1)(1) is 0.58.

Phoenix Example Is Not an Edge Case

In Phoenix, the rooftop conduit temperature correction factor of 0.58 means you need a conductor rated for nearly twice the current at table ampacity compared to what the conductor would need at standard conditions. A 10 AWG THWN-2 rated at 40A (table) is only allowed to carry 40A × 0.58 = 23.2A in a Phoenix rooftop conduit — before conduit fill derating.

Temperature Correction Factors for 90°C Conductors (NEC Table 310.15(B)(1)(1))

Effective Ambient Temperature	Correction Factor
21–25°C (70–77°F)	1.04
26–30°C (78–86°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 (113–122°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

Step 3B: Conduit Fill Derating

When more than 3 current-carrying conductors share a conduit, NEC Table 310.15(C)(1) requires derating:

Number of Conductors	Derating Factor
1–3	1.00 (no derating)
4–6	0.80
7–9	0.70
10–20	0.50
21–30	0.45
31–40	0.40
41+	0.35

Both positive and negative conductors count as current-carrying. A conduit carrying 3 source circuits has 6 current-carrying conductors (3 positive, 3 negative) → 0.80 derating factor.

Keep Conduit Runs Short and Separate

The single most effective way to manage ampacity derating is to limit source circuit conductors per conduit. Two conduits with 3 circuits each (factor: 1.00) always allows smaller conductors than one conduit with 6 circuits (factor: 0.80), because neither conduit requires conduit fill derating.

Complete Worked Example

Project: 8.64 kW residential, Dallas, TX Module: 360W, Isc = 9.58A, 90°C THWN-2 in conduit String configuration: 2 strings × 12 modules, both strings in same conduit Local maximum air temperature: 40°C (104°F), rooftop conduit

Step 1 — Source circuit Isc:

Isc = 9.58A (single string)

Step 2 — Apply 125% factor:

Minimum ampacity = 9.58 × 1.25 = 11.98A

Step 3A — Temperature correction:

Effective ambient = 40°C + 22°C (rooftop adder) = 62°C
Correction factor at 62°C for 90°C conductor = 0.65
Required ampacity after temp derating = 11.98 / 0.65 = 18.43A

Step 3B — Conduit fill:

Conductors in conduit = 4 (2 positive + 2 negative for 2 source circuits)
Derating factor = 0.80
Required ampacity after fill derating = 18.43 / 0.80 = 23.04A

Step 4 — Select wire size:

From NEC Table 310.16 (90°C column):

12 AWG THWN-2: 30A → sufficient (23.04A required)
10 AWG THWN-2: 40A → provides more margin

With terminal temperature limitations, if the inverter terminals are only rated for 75°C, use the 75°C ampacity column: 12 AWG = 25A. This still covers the 23.04A requirement.

Result: 12 AWG THWN-2 is the minimum for this installation’s source circuits.

Common 690.8 Mistakes

Mistake	Consequence	Fix
Skipping temperature derating for “mild climate” installations	Undersized conductors that overheat — AHJ reject + fire risk	Always calculate — even San Diego rooftops add 22°C
Forgetting the 22°C rooftop adder	Underestimated conductor temperature	Add 22°C when conduit is within 7 inches of roof surface
Counting only one conductor per circuit	Underestimates fill, misses conduit derating	Count both positive and negative conductors
Using 60°C terminal limitation for 75°C-rated equipment	Undersizes conductor	Check inverter terminal temperature rating from spec sheet
Using wire ampacity from Table 310.17 (free air) instead of 310.16 (conduit)	Overstates allowable ampacity for conduit installations	Confirm wiring method before selecting table

Documentation for Permit Packages

AHJ plan reviewers checking 690.8 compliance look for:

Source circuit Isc clearly labeled on one-line diagram
125% calculation shown explicitly
Ambient temperature documented with source (ASHRAE or local weather data)
22°C rooftop adder applied (if applicable) with rooftop conduit noted
Conductor fill count with total current-carrying conductors per conduit
Final required ampacity and selected wire size
Wire size consistent between calculation, one-line diagram, and wire schedule

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Frequently Asked Questions

What does NEC 690.8 require for conductor sizing?

Conductors in PV source and output circuits must be rated at a minimum of 125% of the module rated short-circuit current (Isc), then derated for ambient temperature and conduit fill per standard NEC ampacity tables.

Why 125% and not 100% of Isc?

Solar PV is classified as a continuous load — it can output near-maximum current for 3 or more hours. The NEC requires continuous loads to be served by conductors (and overcurrent devices) sized at 125% of the actual load.

What’s the biggest factor affecting conductor size on rooftop installations?

Ambient temperature derating, particularly the mandatory 22°C adder for conduit within 7 inches of a rooftop surface. This single factor can reduce allowable conductor ampacity by 30–40% and force upsizing from 12 AWG to 10 AWG or larger.