Solar Wire Sizing: NEC 690.8 Worked Example

Solar conductors are sized with two 1.25 multipliers, then checked against temperature, conduit fill, and the lowest terminal rating in the circuit. Skip any one of those steps and a calculation that looks fine on paper can leave a conductor hotter than the code allows. That is why Article 690 has its own conductor sizing rules, separate from the branch circuit rules most electricians learn first.

Last updated: November 2026

This guide covers every step: the two 1.25 multipliers, temperature correction for rooftop conduit, conduit fill derating, OCPD sizing under NEC 690.9, and a complete worked example for a 3-string system in Dallas, TX. The module values in the example are illustrative datasheet numbers, so use your own module’s datasheet. Table numbers and some rules change between NEC editions, so verify each step against the edition your AHJ has adopted. The code itself is NFPA 70, the National Electrical Code.

For related reading, see solar cable sizing calculation. Mayfield Renewables walks through the same two-method structure in its NEC 690.8(B) wire size article.

In this guide:

• Why solar wiring uses two separate 1.25 multipliers, and the reasoning behind each
• How to read NEC Table 310.16 for THWN-2 and USE-2 conductors
• Temperature correction for rooftop conduit, with a city reference table
• Conduit fill derating when multiple strings share a raceway
• OCPD sizing under NEC 690.9: floor, ceiling, and DC-listing requirements
• A step-by-step worked example for a 3-string, 400 W module system
• The most common AHJ rejection triggers and how to prevent them

Why Solar Wiring Has Its Own Code Section

Standard branch circuit wiring follows NEC Article 310. PV systems have their own rules in Article 690 because a PV source circuit behaves differently from a typical AC load circuit in two ways that affect conductor sizing.

First, a branch circuit serving a lighting load draws current intermittently. A PV string produces current continuously whenever sunlight is available, often for several hours at a time. That continuous duty is why the code applies a 125% factor to conductor sizing, in the same spirit as the continuous load rule in NEC 210.20(A).

Second, solar irradiance can briefly exceed the Standard Test Condition of 1,000 W/m2 due to cloud edge reflection, high-elevation sites, or albedo from adjacent surfaces. NEC 690.8(A)(1) accounts for this with its first 1.25 multiplier.

These two factors stack: NEC 690.8 requires conductors sized for irradiance spikes and for continuous operation, not just one.

The NEC 690.8 Formula: Both Multipliers Explained

Step 1: Maximum Circuit Current, NEC 690.8(A)(1)

The first calculation converts module Isc (from the datasheet at STC) to the maximum circuit current the NEC uses for downstream sizing:

Maximum Circuit Current = Isc x 1.25

The 1.25 factor is the NEC’s code-standard irradiance margin. Standard Test Conditions assume 1,000 W/m2 and 25C cell temperature. The NEC uses 1.25x as a standard margin and does not require site-specific irradiance studies.

Step 2: Minimum Conductor Ampacity, NEC 690.8(B)(1), Method A

The second 1.25 converts maximum circuit current into minimum conductor ampacity before any adjustment or correction factors:

Minimum Ampacity (Method A) = Maximum Circuit Current x 1.25 = Isc x 1.5625

Combined: 1.25 x 1.25 = 1.5625, typically rounded to 1.56 and called the “156% rule.”

Method A vs Method B

The NEC gives designers two checks for minimum conductor ampacity, and the conductor must satisfy the larger requirement:

The rule: calculate both. In hot climates or with many conductors in a conduit, Method B often takes the larger conductor, and in mild conditions Method A does.

Five-Step Calculation Summary

NEC Table 310.16: Reading Ampacity for Solar Conductors

Wire Types Used in PV Systems

USE-2 and PV Wire (UL 4703) are specifically listed for PV systems under Article 690. THWN-2 is acceptable inside conduit. THHN is not acceptable in outdoor or wet locations, a common reason for AHJ comments.

NEC Table 310.16: Copper Conductor Ampacity (30C Ambient)

Values from NEC Table 310.16, copper conductors. Verify against your adopted edition.

The Terminal Temperature Constraint: NEC 110.14(C)

The most overlooked constraint in solar wire sizing: the ampacity of the circuit is limited by the lowest temperature-rated termination in that circuit.

Most residential and small commercial inverters have terminals rated at 75C. THWN-2 wire is rated at 90C. The 90C column is useful for starting a derating calculation, but the final result cannot be higher than the 75C column ampacity of the same conductor at a 75C terminal.

In practice that gives two checks:

1. Method A: Isc x 1.5625 must not exceed the 75C column ampacity, with no temperature or fill factor applied.
2. Method B: Isc x 1.25 must not exceed the 90C column ampacity multiplied by the temperature and fill factors.

A third check protects the conductor: the overcurrent device must not exceed the conductor’s derated ampacity, rounded up to the next standard size where NEC 240.4(B) allows it.

Temperature Correction: The Factor Most Installers Miss

NEC Table 310.16 ampacity values assume an ambient of 30C (86F). Installations above that temperature need a correction factor that reduces effective ampacity.

Base Ambient Temperature Correction

Values from the NEC ambient temperature correction table (Table 310.15(B)(1) in recent editions). Verify against your adopted edition.

Rooftop Conduit Temperature Adder

For raceways or cables exposed to direct sunlight on or above a rooftop, earlier NEC editions add a temperature increment to the ambient before applying correction factors. Whether your adopted edition applies this adder, and how, depends on the edition and the distance above the roof, so confirm it with your AHJ.

A standard 1 1/4 in EMT conduit on standoffs places the conduit bottom roughly 1 to 2 in above the roof, which falls in the +22C category where the adder applies.

City Reference Table: Effective Temperature for Rooftop Conduit

This table uses the +22C adder and shows the 90C wire correction factor. The ambient values are rounded illustrative numbers, so use the design temperature your AHJ or the NEC edition specifies.

Mini-example: Phoenix derating of #10 AWG THWN-2

• 90C column base ampacity: 40 A
• Temperature correction at 65C: 0.65
• Derated ampacity: 40 x 0.65 = 26 A
• The 75C terminal limit applies to the unadjusted 1.5625 x Isc requirement, which #10 AWG meets up to 35 A / 1.5625 = 22.4 A of Isc

A #10 AWG conductor with a 20 A string fuse has about a 6 A margin over the fuse rating in Phoenix. Many designers in hot climates move to #8 AWG for homeruns for the extra margin.

Conduit Fill Derating: When Multiple Strings Share a Raceway

More than 3 current-carrying conductors in a single conduit require an additional ampacity reduction. Conductors bundled together cannot shed heat as effectively as isolated ones.

NEC 310.15(C)(1) Adjustment Factors

Values from the NEC adjustment factor table for more than three current-carrying conductors. Verify against your adopted edition.

What Counts as Current-Carrying in a PV Circuit

Count toward fill:

• All ungrounded DC conductors (positive and negative of each string)

Do not count:

• Equipment Grounding Conductors (EGC), bare copper or green insulation
• Spare conductors not energized
• Balanced neutral conductors in 3-phase AC circuits

A combiner box with 4 parallel strings has 8 current-carrying conductors in the pre-combiner conduit (4 positive + 4 negative). Adjustment factor: 0.80.

If those strings run in separate conduits to the combiner, each conduit holds only 2 current-carrying conductors. Factor: 1.00, with no fill adjustment.

Temperature and fill derating stack:

Combined Derate = Temp Correction Factor x Fill Adjustment Factor

In Dallas with 6 conductors in one conduit: 0.65 x 0.80 = 0.52. A #10 AWG (40 A at 90C) derates to 40 x 0.52 = 20.8 A. That clears a 12.73 A maximum circuit current, but a 20 A OCPD sits right at the derated ampacity, so any further stress leaves no margin.

Step-by-Step Worked Example: 3-String System in Dallas, TX

This example uses an illustrative 400 W module to demonstrate the NEC 690.8 calculation for string conductors and the homerun conductor. The datasheet values are examples. Replace them with the values from your module’s datasheet.

Module and System Specifications

System configuration:

Part 1: String Conductor Sizing

Step 1: Maximum Circuit Current, NEC 690.8(A)

10.18 A x 1.25 = 12.73 A

Step 2: Method A Minimum Ampacity, NEC 690.8(B)(1)

12.73 A x 1.25 = 15.91 A

Check against the 75C column (inverter terminal constraint), with no temperature or fill factor:

• #12 AWG: 25 A, which is above 15.91 A, so it passes

Step 3: Method B with Corrections, NEC 690.8(B)(2)

Effective temperature:

• Illustrative design ambient: 40C
• Rooftop conduit adder (about 2 in above roof): +22C
• Effective ambient: 62C

From the temperature correction table at 62C for 90C wire: factor = 0.65 (the 61 to 65C row).

String conduit carries 2 current-carrying conductors: fill factor = 1.00.

Check the derated ampacity against the maximum circuit current:

• #12 AWG THWN-2 at the 90C column: 30 A
• Derated: 30 x 0.65 x 1.00 = 19.5 A
• Required: 12.73 A, so it passes with 6.77 A of margin (53%)

Step 4: Compare Methods

Both methods clear #12 AWG. #10 AWG would give 40 x 0.65 = 26 A derated, which is 104% above the requirement.

Design decision: #12 AWG is compliant for this module in the Dallas example, but #10 AWG is common practice in hot climates, particularly because many modules have higher Isc where the margin shrinks.

Step 5: OCPD Sizing, NEC 690.9

String-level fuses are needed here: 3 parallel strings create a backfeed path.

Minimum OCPD = 10.18 A x 1.56 = 15.88 A

Next standard size per NEC 240.6(A): 20 A

• 20 A is no higher than the module maximum series fuse rating (20 A), at the ceiling and acceptable
• #12 AWG derated ampacity is 19.5 A. A 20 A fuse is the next standard size above it, which NEC 240.4(B) generally permits. Confirm with your AHJ.

Step 6: Voltage Drop Check (String)

String voltage: 411 V. Operating current: 12.73 A (a conservative value). #12 AWG resistance: about 1.98 ohm per 1,000 ft (NEC Chapter 9, Table 8). One-way run: 80 ft.

VD% = (2 x 80 x 12.73 x 1.98) / (411 x 1,000) x 100 = 0.98%, which is within the 2% target

Part 2: Homerun Conductor Sizing

The homerun carries the combined output of all 3 strings from the combiner box to the inverter.

Step 1: Total Short-Circuit Current

3 x 10.18 A = 30.54 A

Step 2: Maximum Circuit Current, NEC 690.8(A)

30.54 A x 1.25 = 38.18 A

Step 3: Method A Minimum Ampacity

38.18 A x 1.25 = 47.72 A

At the 75C column, #6 AWG = 65 A, which passes.

Step 4: Method B with Corrections

The combiner merges all strings internally. The homerun conduit carries 2 current-carrying conductors. Fill factor: 1.00. Temperature: same 62C effective, factor = 0.65.

#6 AWG at 90C: 75 A x 0.65 = 48.75 A Required: 38.18 A, so it passes with 10.57 A of margin (28%)

Governing homerun conductor: #6 AWG

Step 5: Voltage Drop Check (Homerun)

#6 AWG copper resistance: about 0.491 ohm per 1,000 ft (NEC Chapter 9, Table 8). Operating current: 38.18 A. One-way: 80 ft.

VD% = (2 x 80 x 38.18 x 0.491) / (411 x 1,000) x 100 = 0.73%, which is within the 2% target

Summary: Dallas 3-String System

OCPD Sizing Under NEC 690.9: Floor, Ceiling, and DC Requirements

Overcurrent protection in a PV DC circuit differs from a standard branch circuit breaker in three ways: there is a minimum size floor, a maximum size ceiling, and a DC listing requirement.

When OCPD Is Not Required

NEC 690.9(A) exempts PV circuits from OCPD when:

1. Conductors have ampacity equal to or exceeding the maximum circuit current, AND
2. Current from all sources cannot exceed the device rating on the module or power converter

A single-string system with no backfeed path, where the inverter is the only path for current, typically does not need string-level fusing. The inverter’s own input protection covers it.

For 3 or more parallel strings, string-level fusing is typically needed. A fault in one string creates a current path from healthy strings back through the faulted conductor. That combined current can exceed the conductor’s rated ampacity.

OCPD Floor: Minimum Size

Minimum OCPD = Isc x 1.56, rounded UP to the next standard size

Standard sizes from NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100…

For Isc = 10.18 A: 10.18 x 1.56 = 15.88 A, so the next standard size is 20 A.

OCPD Ceiling: Maximum Size

Maximum OCPD is no more than the module’s maximum series fuse rating (from the nameplate)

This rating is the highest OCPD the module can withstand during a reverse-current fault without damage. It is on the module nameplate and in the datasheet.

If the calculated floor exceeds the ceiling, the module cannot be used in that parallel string count. Options are to reduce the number of parallel strings, select a module with a higher fuse rating, or redesign the array.

DC-Rated Equipment Only

AC circuit breakers are not designed to interrupt DC current. A DC arc does not self-extinguish at a current zero crossing the way an AC arc does.

All OCPDs in PV DC circuits must be:

• Listed for DC service, such as photovoltaic fuses covered by UL 248-19, or DC-rated circuit breakers
• Voltage-rated at or above the maximum system DC voltage (calculated per NEC 690.7)

For string sizing against Voc limits, see our solar string design guide.

AWG 10 vs AWG 12: String Wire Selection Guide

The most common field question: does a residential string need #10 or #12 AWG? Run the three checks: Method A at the 75C column, Method B at the derated 90C ampacity, and the OCPD against the derated ampacity.

Dallas Scenario Table (62C effective, 2 conductors per conduit, #12 AWG THWN-2)

For #12 AWG, the 75C column is 25 A and the derated 90C ampacity is 30 x 0.65 = 19.5 A.

With 4 to 6 conductors in the conduit (factor 0.80), the derated ampacity of #12 AWG falls to 30 x 0.65 x 0.80 = 15.6 A. That fails the Method B check for any Isc above 12.5 A and leaves little room below it, so #10 AWG becomes the practical choice.

When #12 AWG Is Sufficient

• Module Isc is moderate and the three checks above pass with margin
• Two current-carrying conductors per conduit
• Conduit not exposed to direct sunlight, or a mild climate

When #10 AWG Is the Better Choice

• The OCPD floor is above 20 A (Isc above about 12.8 A)
• Derated ampacity of #12 AWG falls near or below the OCPD rating
• Four or more current-carrying conductors share a conduit in a hot climate
• You want extra inspection margin

In hot climates with rooftop conduit, #10 AWG is widely used for margin. A solar design suite that lets you set derating inputs helps catch marginal cases before they reach inspection.

Common Mistakes and AHJ Rejection Triggers

These are common wiring errors found during permit review and field inspection.

Each of these mistakes is a recalculation, or a permit rejection, that is better caught before drawings are submitted. A checklist or design tool that applies these checks as you design helps.

The Calculation Sequence: Start to Finish

When sizing a system’s conductors, the sequence is:

1. Pull module Isc from the datasheet (STC value, not NOCT)
2. Identify installation climate and conduit height above roof
3. Calculate maximum circuit current: Isc x 1.25
4. Run Method A: x 1.25, check against the 75C column
5. Run Method B: divide maximum circuit current by temperature factor x fill factor, check against the 90C ampacity
6. Check the OCPD against the derated conductor ampacity
7. Select a conductor that passes all checks
8. Size OCPD: Isc x 1.56, round up, verify against the module max fuse rating
9. Check voltage drop for each run, target 2% or less
10. Repeat for the homerun with combined string current

A residential 3-string system has at minimum 2 distinct conductor calculations. A commercial system with multiple combiner boxes can have many. SurgePV’s solar proposal software builds proposals from your design, while the electrical sizing itself should be checked against the code and signed off by the responsible engineer. SurgePV outputs support design and documentation workflows and do not replace that approval. To see the design workflow, book a SurgePV demo.

Frequently Asked Questions

What size wire do I need for solar panels?

It depends on module short-circuit current (Isc), temperature, and conduit fill. Under NEC 690.8 the conductor must carry at least Isc x 1.25 x 1.25 before derating, and Isc x 1.25 after derating. For a module with Isc of 10.18 A, that is 15.9 A before derating, which #12 AWG THWN-2 meets in the guide’s Dallas example.

What is the 1.56 rule in NEC 690.8?

The 1.56 rule combines two 1.25 multipliers: 1.25 x 1.25 = 1.5625. The first converts module Isc to maximum circuit current under NEC 690.8(A)(1). The second applies 125% to that current for conductor sizing under NEC 690.8(B)(1). The same 1.56 factor sets the minimum size of the overcurrent device.

Can I use AWG 12 wire for residential solar panels?

Often yes, but check it. #12 AWG THWN-2 passed in the guide’s Dallas example for Isc up to about 12.8 A with two conductors per conduit, after temperature derating and with a 20 A fuse. More conductors in the conduit, a higher Isc, or a larger fuse can change the answer, and many designers choose #10 AWG for margin.

What is the difference between NEC 690.8(B)(1) and (B)(2)?

Method (B)(1) uses 125% of maximum circuit current before adjustment and correction factors, checked against the 75C terminal column. Method (B)(2) uses maximum circuit current after temperature and conduit fill factors, checked against the 90C column. Calculate both, and the conductor must satisfy both checks.

Do I need a fuse for every solar string?

Not always. NEC 690.9(A) allows omitting overcurrent protection when conductors can carry the maximum current and no source can supply more than the module’s rating. With three or more parallel strings, string-level protection is typically needed. The device must be at least Isc x 1.56 and no higher than the module’s maximum series fuse rating.

Can I use THHN wire for solar panel wiring?

Not outdoors. THHN is rated for dry and damp locations, while rooftop and outdoor runs are exposed to weather. Use THWN-2 (wet and dry rated, 90C) inside conduit, USE-2 for outdoor or direct-burial runs, or UL 4703 PV Wire for array wiring outside conduit. Check the details with your AHJ.