Solar Conductor Temperature Derating: NEC 310.16

Conductor temperature derating is one of the quieter cost drivers in hot-climate solar design. The same inverter output can need a larger conductor in Phoenix, Riyadh or Dubai than it does in a cool climate. The load is the same. What changes is the stack of ambient correction, conduit fill adjustment and, on rooftops, a possible temperature adder that compounds when sun, roofing and tight raceways meet a 50°C afternoon.

Last updated: November 2026

This guide walks through the correction chain step by step: the ambient correction table, the conduit fill adjustment, how the two stack, how the rooftop adder has changed between code editions, how to choose between THWN-2, XHHW-2, USE-2 and PV Wire, and two worked examples with stated assumptions. Table values below were checked against reproductions of the 2020 NEC tables on Elliott Electric Supply and Conduit.site. This is a design-method guide, not a substitute for the code text. Section and table numbers move between NEC editions, so confirm every value against the edition your authority having jurisdiction (AHJ) has adopted.

In this guide:

• The three temperatures that drive solar conductor ampacity
• The ambient correction table and how to read it
• Conduit fill adjustment and how it stacks with ambient correction
• How the rooftop temperature adder changed between editions
• THWN-2, XHHW-2, USE-2 and PV Wire compared for hot climates
• Two worked examples with stated assumptions: an inverter output circuit and a combiner run
• Common derating mistakes and how to document the math

Quick Reference: Hot-Climate Conductor Derating

The working method is four steps. Take the 90°C column ampacity from Table 310.16 for the conductor size. Multiply by the ambient correction factor for the design temperature. Multiply by the adjustment factor if more than three current-carrying conductors share a raceway. Then compare the result with the ampacity the equipment terminals allow, which is set by the lowest temperature rating of any termination under NEC 110.14(C). Check the temperature marking on your actual equipment, because the terminal rating decides the final limit.

The table below applies a 0.82 ambient factor (90°C column, 46 to 50°C) and a 0.80 adjustment factor (4 to 6 conductors) to copper conductors. The 90°C and 75°C ampacities are from Table 310.16 as reproduced by Conduit.site, which states its tables follow the 2020 NEC.

For voltage drop, conductor length and short-circuit questions, the companion guide on solar cable sizing calculation covers what temperature derating does not. For NEC-specific wire selection logic, read solar wire sizing under NEC 690.8.

Latest Updates: Conductor Derating Code in 2026

Adoption of a new NEC edition varies by state and AHJ, and it changes over time. This guide does not publish an adoption table because adoption status is set by each jurisdiction and was not verified for this update. Ask the AHJ which edition it enforces before you start the calculation. For a summary of the newest edition’s solar changes, see our NEC 2026 solar changes post.

What Changed Between Editions

Three points matter for conductor derating.

• Ambient correction factors. The 2020 reproductions linked above give 0.82 for a 90°C conductor at 46 to 50°C. Check whether your edition’s table differs.
• Table numbering. The adjustment table for more than three current-carrying conductors is numbered 310.15(C)(1) in the 2020 reproduction, and it was formerly 310.15(B)(3)(a). A design note that cites an old number can confuse a plan reviewer.
• Rooftop adder. The 2014 NEC used a graduated adder table. The 2017 NEC deleted it and kept a single 33°C adder for raceways within 7/8 inch of the roof, as described by Jade Learning. The threshold distance and any exceptions differ in later editions, so read the adopted text.

Why Hot-Climate Conductors Need Extra Derating

Solar conductors live in three thermal stress zones. The first is the air around the wire. The second is the conduit surface, which can run hotter than the air when exposed to sun. The third is the inside of the raceway when several current-carrying conductors share it. Each zone raises the conductor’s operating temperature, and the NEC treats each one separately.

The module current does not change between Stuttgart and Dubai. What changes is the temperature of the wire carrying it. In this illustration, a #10 AWG THWN-2 conductor starts at 40 A from the 90°C column at the 30°C reference. At 50°C ambient the 0.82 factor gives 32.8 A. Put six conductors in the conduit and the 0.80 adjustment gives 26.2 A. Add a 33°C rooftop adder to a 50°C ambient and the effective ambient becomes 83°C, where the 90°C factor is 0.29, so the same wire is down to about 9 A once the 0.80 adjustment is applied. The 33°C adder applies only where your edition requires it, for example for raceways within 7/8 inch of the roof under the 2017 text.

That is the same wire, the same insulation and the same load, with three different ampacities. The designer’s job is to find the worst combination along the full length of each conductor and size for that hot spot.

The cost of getting this wrong is not only a failed inspection. Sustained operation above a conductor’s insulation rating shortens insulation life, which can show up later as insulation faults. Our hot climate solar installation challenges guide covers the wider thermal picture.

The Three Temperatures That Affect Conductor Ampacity

Track three temperatures for every conductor: outdoor ambient, conduit surface, and internal raceway temperature from bundled conductors. The NEC applies a separate correction to each.

Outdoor Ambient Temperature

The first input is the outdoor ambient: the air temperature in the shade at the location. The table reference point is 30°C, and the table gives factors for other temperatures. The design ambient should be a documented choice, not a guess. Common inputs are climate design data such as ASHRAE climatic data, a local AHJ requirement, or a project engineering standard. Some designers add a margin on top of the climate value. Whatever you choose, write it down with its source.

Conduit Surface Temperature

The second input is the conduit surface temperature. Conduit on an open roof in direct sun runs hotter than the surrounding air, which is why older and some current editions add a temperature adder for raceways close to the roof. The history of that rule is covered in the rooftop section below. Non-metallic raceways also warm in sun, so ask the AHJ whether it applies the same adder to them.

Internal Raceway Temperature from Bundling

The third input is the heat produced by the conductors themselves. Each current-carrying conductor generates I²R losses. When more than three share a raceway, heat builds faster than it dissipates, and an adjustment factor applies. The 2020 Table 310.15(C)(1) reproduction on Conduit.site lists these factors.

Whether a neutral or an equipment grounding conductor counts depends on the circuit and the code text, so check the counting rules in your edition. On a DC string circuit, the positive and negative conductor of each string both carry current. A combiner conduit with six DC strings has twelve current-carrying conductors and an adjustment factor of 0.50. Our glossary entry on ampacity defines the term, and the equipment grounding conductor entry explains why grounding conductors are treated differently.

The three corrections combine by multiplication. Take a conductor with a 78°C effective ambient (45°C plus a 33°C adder), where the 90°C factor is 0.41, in a conduit of twelve current-carrying conductors at 0.50. The combined factor is 0.41 × 0.50 = 0.205. A #10 AWG at 40 A falls to about 8 A. That is why module-level conductors on hot rooftops are often sized well above what the same current needs at sea level in a mild climate.

Ambient Temperature Correction Factors

The ambient correction table is the workhorse of conductor temperature derating. It gives a multiplier for each ambient band and each insulation temperature rating. The values below are from Table 310.15(B)(1) as reproduced on Conduit.site and Elliott Electric Supply, both citing the 2020 NEC, and the two reproductions agree for the rows shown.

Correction Factors for Ambient Other Than 30°C

Three points matter for hot-climate solar. First, the 60°C column falls steeply above 45°C and has no listed value above 55°C. A 60°C-rated conductor is a poor choice for a hot outdoor run. Second, the 75°C column runs out of listed values above 70°C. Third, only the 90°C column has listed values at the highest temperatures, which is why solar conductors are specified with 90°C wet-rated insulation such as THWN-2, XHHW-2, USE-2 or PV Wire.

Choosing Between Insulation Ratings in Hot Climates

The insulation’s wet rating decides which column you can use in a raceway that may see moisture. Check the conductor’s marking and listing, because a conductor with only a dry 90°C rating cannot use the 90°C column in a wet location.

For the home run between a rooftop combiner and a ground-level inverter, USE-2 or THWN-2 in conduit are both used, depending on the wiring method and the listing. For exposed string wiring between modules and the first junction point, use a cable listed for PV use. Article 690 sets the rule, so check the section number in your edition.

The terminal limit is the second constraint. Even if the 90°C column allows a high ampacity, the breaker, lug or inverter terminal may be rated lower, and NEC 110.14(C) limits the usable ampacity to the lowest temperature rating of any termination. The final allowable ampacity is the smaller of the corrected value and the ampacity from the terminal’s temperature column. Read the marking on your actual equipment.

Conduit Fill Derating Stacking

The conduit fill adjustment is simple in principle: more current-carrying conductors in one raceway means more heat, so the ampacity drops. It gets harder when it stacks with ambient correction, and when you have to decide which conductors count.

For DC solar circuits, count both conductors of each string. A six-string combiner conduit feeding a string inverter carries twelve current-carrying conductors and takes the 0.50 factor from the table above. For three-phase AC inverter output, the three phase conductors count. Whether a neutral counts depends on the circuit and the code text, so check your edition.

Stacking Math

Ambient correction and conduit fill adjustment combine by multiplication. Do not take the lower of the two. Worked example for an 8-conductor combiner conduit at 50°C ambient with 90°C THWN-2:

• Base ampacity for #8 AWG at 90°C = 55 A
• Ambient correction at 46 to 50°C = 0.82
• Conduit fill for 8 conductors = 0.70
• Corrected ampacity = 55 × 0.82 × 0.70 = 31.6 A
• 75°C column ampacity for #8 AWG = 50 A, which does not limit because the corrected value is lower
• Usable ampacity = 31.6 A

At the 30°C reference with three conductors, the same wire would be limited by the 50 A terminal column. The hot-climate derating therefore leaves 31.6 / 50 = 63.2% of that figure. A circuit that needs 22 A still works on #8 AWG. A circuit that needs 30 A is marginal, so a one-size increase to #6 AWG, where 75 × 0.82 × 0.70 = 43.05 A, gives headroom.

On long runs, voltage drop often drives an upsize before temperature derating is even applied. The method is in our companion post on the solar DC cable length calculator.

Rooftop Conduit: How the Adder Has Changed

Rooftop conduit in direct sun is a harsh thermal environment. Because the rule has changed across editions, the first step is to identify which text your AHJ enforces.

The 2014 Graduated Table and the 2017 Single Adder

According to Jade Learning, the 2014 NEC used Table 310.15(B)(3)(c) with graduated adders by distance above the roof: 60°F for raceways under 1/2 inch, 40°F from 1/2 inch to 3 1/2 inches, 30°F from 3 1/2 to 12 inches, and 25°F from 12 inches to 3 feet. The same source reports that the 2017 NEC deleted the graduated table and kept one adder, 33°C (60°F), for raceways and cables installed within 7/8 inch of the rooftop surface.

Older articles, including earlier versions of this guide, show the 2014 graduated table as if it were current. If your AHJ enforces the 2017 text or a later one, follow that edition’s wording and any exceptions it lists.

The adder is applied to the ambient before the correction factor is looked up. A 45°C design ambient with conduit inside the threshold distance becomes 45 + 33 = 78°C effective ambient, where the 90°C factor is 0.41. That is a large step from the 0.87 factor at 45°C.

How to Handle the Adder in Practice

Raise the conduit above the threshold distance where the roof design allows it, and confirm with the AHJ before relying on the clearance. Where the conduit must sit close to the roof, apply the full adder and select 90°C insulation. Some editions list an exception for particular insulation types, so read the exception before using it. Some reviewers may apply the adder regardless, so document your reasoning in the submittal.

Attic Heat

For US residential systems where conductors pass through the attic between the inverter and the service panel, attic heat is a separate concern. Use the ambient temperature of the space the cable passes through, not the outdoor air, and check the wiring-method article for any attic-specific limits on cable types. For solar inverter output circuits in attics:

• Use 90°C wet-rated insulation in conduit rather than a cable type that has attic limits
• Apply the ambient correction at the attic design temperature, which you should document
• Check the conduit fill adjustment when several branch circuits share the raceway
• Record the attic ambient assumption in the design notes

For a complete walkthrough of inverter output sizing, see solar wire sizing under NEC 690.8. The matching guide for the AC disconnect on the same circuit is AC disconnect sizing for solar.

Conductor Insulation Selection: THWN-2, XHHW-2, USE-2 and PV Wire

The insulation rating sets the upper limit of what the derating math can deliver. Four choices dominate hot-climate solar work.

THWN-2: Building Wire in Raceway

THWN-2 is a 90°C wet-rated building wire used inside conduit. It is widely stocked and pulls easily. The wet rating makes it eligible for the 90°C column. For inverter output, AC disconnects and feeders from the inverter to the service panel, THWN-2 in conduit is a common choice in the US.

XHHW-2: Cross-Linked Polyethylene

XHHW-2 uses cross-linked polyethylene insulation and is also marked for 90°C wet locations. Designers sometimes specify it in place of THWN-2 for commercial and utility projects. Check the insulation datasheet for the temperature and aging data you need, because this guide does not publish a manufacturer comparison.

USE-2: Underground Service Entrance

USE-2 is listed for direct burial and outdoor exposure. A cable that carries both USE-2 and PV Wire markings can cover both the in-conduit exterior portion and an exposed module-to-combiner portion of a circuit, provided its listing covers each use.

PV Wire

PV Wire is the module-level DC conductor. Article 690 requires cable that is suitable for exposed array wiring where conductors run in free air between modules and the first junction point, and the section number varies by edition. PV Wire is listed to the UL 4703 standard for photovoltaic wire. Its ratings are printed on the cable and in its datasheet, so read both before you assume a voltage or temperature rating. Do not use the dry temperature rating as an ampacity reference unless the code permits it.

Building wire in module-level free-air runs is a problem because THWN-2 is not listed for that exposure. Once conductors enter a junction box, combiner or raceway, the wiring-method rules for the raceway apply. Many designs transition from PV Wire to THWN-2 or USE-2 at the combiner, provided the listing and the wet rating suit the new location.

For module wiring discussion including connectors, see our guides on MC4 connector crimping best practices and solar panel stringing and wiring.

Why THHN Alone Is a Poor Fit for Wet Locations

THHN is dry-rated only. Conduit exposed to weather, and many underground raceways, are treated as wet locations under the NEC, so a THHN-only conductor is limited to a lower temperature column there. Many manufacturers dual-mark THHN/THWN-2 conductors. Specify THWN-2 by name in solar conductor notes so a substitution cannot slip through.

Worked Example 1: Inverter Output Circuit at 45°C Ambient

This is a hypothetical single-phase 240 V residential inverter output circuit. Every input is an assumption for teaching, not data from a real project, and the 1.25 multiplier below is an assumed input. Take the actual multiplier from NEC 690.8 in your edition.

Project Parameters

• Inverter continuous output current: 32 A (assumed)
• Required ampacity: 32 × 1.25 = 40 A (assumed multiplier)
• Design ambient: 45°C (assumed)
• Conductors: two current-carrying conductors plus a grounding conductor, so no conduit fill adjustment
• Conductor: copper THWN-2, 90°C wet rating
• Terminal rating at inverter and disconnect: 75°C (assumed, read the real nameplate)

Case A: Conduit Above the Adder Threshold

With no adder, the effective ambient is 45°C and the 90°C factor is 0.87.

• #10 AWG: 40 × 0.87 = 34.8 A, which is below 40 A
• #8 AWG: 55 × 0.87 = 47.9 A, which meets 40 A
• 75°C column for #8 AWG is 50 A, which meets 40 A

Result: #8 AWG THWN-2.

Case B: Conduit Inside the Adder Threshold

With a 33°C adder, the effective ambient is 78°C and the 90°C factor is 0.41.

• #6 AWG: 75 × 0.41 = 30.8 A, below 40 A
• #4 AWG: 95 × 0.41 = 39.0 A, still below 40 A
• #3 AWG: 115 × 0.41 = 47.2 A, meets 40 A
• 75°C column for #3 AWG is 100 A, which meets 40 A

Result: #3 AWG THWN-2. The conduit height alone moved the answer by several AWG sizes.

Step 6: Check Voltage Drop Separately

Voltage drop is a separate check. Round-trip length multiplied by current multiplied by the conductor’s resistance per unit length gives the volt drop, and you divide by the system voltage for a percentage. Take resistance from NEC Chapter 9 Table 8 or the cable datasheet, not from this guide. Compare the result with the project’s voltage-drop target.

Price the Difference From Quotes

A larger conductor costs more, but this guide publishes no copper prices. Get current quotes for each size and multiply by the run length. Compare that cost with the cost of a failed plan review. The thermal correction math belongs in the engineering review of every hot-climate bid, not only the load math.

Worked Example 2: Combiner Conduit in Direct Sun

This second example is also hypothetical, a commercial rooftop with a denser conduit. It does not describe a real project.

Project Parameters

• Array: 6 strings of 22 modules, 1500 V DC (assumed)
• Per-string Isc: 14.1 A (assumed)
• Source-circuit multiplier: 1.56 (assumed input; take the real factor from NEC 690.8 in your edition)
• Required ampacity: 14.1 × 1.56 = 22.0 A
• Design ambient: 48°C (assumed)
• Conductors in conduit: 6 strings × 2 = 12 current-carrying, so the 0.50 factor
• Conductor: copper PV Wire or USE-2, 90°C wet rating

Case A: No Adder

At 48°C the 90°C factor is 0.82. Combined with the 0.50 fill factor, the multiplier is 0.41.

• #10 AWG: 40 × 0.41 = 16.4 A, below 22.0 A
• #8 AWG: 55 × 0.41 = 22.6 A, meets 22.0 A

Result: #8 AWG.

Case B: 33°C Adder

Effective ambient is 48 + 33 = 81°C, where the 90°C factor is 0.29. With the 0.50 fill factor, the multiplier is 0.145.

• #4 AWG: 95 × 0.29 × 0.50 = 13.8 A, below 22.0 A
• #2 AWG: 130 × 0.29 × 0.50 = 18.9 A, below 22.0 A
• #1 AWG: 145 × 0.29 × 0.50 = 21.0 A, below 22.0 A
• 1/0 AWG: 170 × 0.29 × 0.50 = 24.7 A, meets 22.0 A
• 75°C column for 1/0 AWG is 150 A, far above the requirement

Result: 1/0 AWG. Thermal derating drives the size, not voltage drop. In this hypothetical, the adder and the 12-conductor bundle together move the answer from #8 AWG to 1/0 AWG.

Voltage Drop Check

Using an assumed resistance of 0.122 ohm per 1000 ft for 1/0 copper (verify in Chapter 9 Table 8), a 30 m run is about 98 ft one way and 197 ft round trip. At the assumed 13.2 A per string, the drop is 13.2 × 0.122 × 197 / 1000 = 0.32 V. On an assumed string voltage of about 1100 V, that is under 0.03%. The thermal case governs.

Local Review Notes

Utility and design-review practice vary by project, and this guide does not publish any utility’s review habits. Ask the reviewing authority how it applies the rooftop adder and which edition it cites, and put the answer in writing in the design basis.

PV Wire Versus Building Wire for Module-Level Conductors

Conductors in free air between modules and the first junction box, combiner or inverter input face sun, rain, abrasion against module frames and heat from the module backsheet. For that reason Article 690 calls for cable suited to exposed array wiring, and PV Wire or a dual-listed USE-2 and PV Wire cable is the usual answer.

What PV Wire Is Listed For

PV Wire is listed to UL 4703. A typical datasheet lists a conductor material, an insulation compound, a jacket, voltage ratings and temperature ratings. Read the datasheet for the cable you buy, because ratings differ by product. Do not assume a 600 V, 1000 V or 1500 V rating from the cable type alone.

When Building Wire Is Acceptable

Once conductors enter a junction box, combiner or raceway, the rules for that wiring method apply. Inside conduit, THWN-2, XHHW-2 or USE-2 can all be acceptable for the same circuit if the wet rating and listing suit the location. Many designs transition from PV Wire to THWN-2 at the rooftop combiner to reduce material cost on the longer combiner-to-inverter run.

Hot-Climate Considerations

Module backsheets and connector bodies run hot in direct sun, and the listing of the cable and connector pair sets the limits. This guide does not publish measured backsheet or connector temperatures. Use the datasheet temperature ratings and the manufacturer’s installation instructions, and keep conductors off hot roof surfaces where the design allows.

For a deeper look at how module-level power electronics change the conductor picture, see microinverters vs string inverters vs optimizers and the solar string design guide.

Common Hot-Climate Derating Mistakes

Eight mistakes cause most conductor derating problems on hot-climate projects. Avoiding them helps a design pass its first AHJ review.

Mistake 1: Using Table Ampacity Without Correction

The most common error is reading the Table 310.16 ampacity at face value. A designer who sizes a 32 A inverter output to #10 AWG because the 90°C column reads 40 A has skipped the derating step. At 50°C ambient the corrected value is 40 × 0.82 = 32.8 A, which is already below a 40 A requirement.

The fix is procedural. Show three numbers for each conductor in the design notes: the base 90°C ampacity, the corrected ampacity after all derating, and the terminal-column ampacity. The smallest governs.

Mistake 2: Ignoring the Rooftop Adder

Designers who learned in temperate climates often skip the adder. A 45°C design ambient gets used as 45°C even when the conduit sits close to a dark roof. If the adopted edition requires a 33°C adder, the effective ambient is 78°C and the 90°C factor drops from 0.87 to 0.41, less than half.

The fix is to identify which adder rule your edition has, apply it to every rooftop run inside the threshold distance, and have a responsible engineer sign off on any reliance on an exception.

Mistake 3: Forgetting Conduit Fill Stacking

The ambient correction and the adjustment factor stack multiplicatively. Designers who apply only the larger correction undersize the conductor. In a spreadsheet or software, corrected ampacity should always equal base ampacity × ambient factor × fill factor.

Mistake 4: Mixing Up Insulation Ratings

Specifying THHN where the application is wet, or THWN where THWN-2 is needed, drops the eligible column from 90°C to 75°C. At 46 to 50°C that is the difference between 0.82 and 0.75 in the factor, and the base ampacity also differs between columns. The fix is to standardize on wet-rated 90°C insulation for every solar conductor and name it in the specification.

Mistake 5: Skipping the Terminal Check

The corrected 90°C ampacity limits conductor heating. The terminal’s temperature rating limits terminal heating. The final usable ampacity is the smaller of the two. A calculation can pass the thermal check and still fail the terminal limit, especially for smaller conductors where the 75°C and 90°C ampacities differ by 5 A or more. For #10 AWG the gap is 5 A (35 A versus 40 A).

Mistake 6: Wrong Conductor Count in the Fill Adjustment

Counting only strings, or only AC phases, undercounts the current-carrying conductors. A 6-string DC combiner conduit has 12 current-carrying conductors, not 6.

Mistake 7: Using an Old Edition’s Section Numbers

Section and table numbers move between editions. For example, the adjustment table for more than three conductors is Table 310.15(C)(1) in the 2020 reproduction and was formerly 310.15(B)(3)(a). A designer reading an old edition will find correction factors, the rooftop adder and its exceptions in different places. Write the adopted edition into the design specification.

Mistake 8: Not Documenting the Derating Math

A design can derate correctly and still be sent back if the reviewer cannot see the work. Publish one row per circuit in the submittal: base ampacity, ambient correction, fill factor, corrected ampacity, terminal limit, final ampacity, required ampacity, AWG and insulation. That gives a reviewer a pass or fail at a glance.

Software and Derating: When Hand Calculations Fail

Manual derating works for a single residential string. For a commercial array with many inverters and string circuits, spreadsheet errors multiply. A design platform that supports electrical workflow and bill-of-materials output can help, but the engineer still owns the result. SurgePV’s outputs support design and documentation workflows and do not replace approval by the responsible engineer or the AHJ. Features worth looking for in any tool:

• Per-circuit ambient temperature input, not one global value
• A conduit-height field that can drive the adder
• A conduit fill counter that tracks current-carrying conductors per raceway
• A terminal rating field on every connected device
• Final ampacity checked against the minimum required
• Voltage drop computed on the same selected conductor
• A bill of materials with conductor gauge, insulation type and total length

When the tool handles the lookups, the engineer can spend time on conduit routing and on whether a home run can be combined. For the financial side of the same projects, see the generation and financial tool. For solar PV design software that covers layout and bill-of-materials output, start with the feature page.

Climate Data Sources for Conductor Derating

Accurate temperature derating starts with documented ambient data. Four inputs are common.

ASHRAE Climatic Data

ASHRAE publishes climatic design data for building design standards. Designers often use a high-percentile design dry-bulb temperature for the location. Which percentile to use is an engineering decision for the project, so state the percentile and the data year in the calculation.

NREL National Solar Radiation Database

The NREL NSRDB provides solar and meteorological data, including typical meteorological year files, that modeling tools such as pvlib can read. For conductor derating, a high-percentile or maximum hourly ambient from a data file can support a documented design ambient.

Local AHJ Requirements

Some AHJs publish their own design temperature requirements. Ask for the written requirement and keep it with the calculation.

Project Engineering Standards

Many owners and EPCs have an internal margin, such as adding a fixed number of degrees for any conductor in direct sun. If you use one, record it as a project assumption and apply it consistently.

The Cost of Getting Derating Wrong

Conductor derating has a financial side, and three failure modes account for most of it. This guide publishes no market cost data for re-pulls, insurance or production loss, so the examples below are formulas with assumed inputs.

Mode 1: Inspection Fail and Re-Pull

A failed inspection means pulling new conductors, buying material and rescheduling a crew, with a schedule delay while the new cable is procured. The cost of upsizing at design time is a material difference. The cost of a re-pull adds labor, schedule and rework on top of material. Get quotes for both on your own project rather than relying on a generic ratio.

Mode 2: Insulation Aging

Even when a conductor passes inspection, one running near its thermal limit ages faster. Check with your insurer whether it requires thermal imaging or other inspections of commercial arrays, and what a hot junction box finding would trigger.

Mode 3: Soft Failure and Production Loss

A conductor near its thermal limit may not fail outright, but repeated nuisance trips take an array offline. As arithmetic only, assume a site producing 160 MWh a year at $0.10 per kWh loses 1.5% of annual production to trips. That is 160,000 kWh × 1.5% = 2,400 kWh, or $240 a year. Replace each input with your own data. The figure is small for one site and can add up across a portfolio.

For the broader financial framing of design decisions, see our generation and financial tool page.

Conclusion: Three Action Items for Hot-Climate Solar Projects

Conductor temperature derating is not optional in hot climates. Three steps for any project with a high design ambient:

• Use 90°C wet-rated insulation everywhere. THWN-2, XHHW-2, USE-2 or PV Wire, named in the specification. Avoid THHN or THWN alone.
• Compute and document every multiplier per circuit. Base 90°C ampacity × ambient correction × conduit fill = corrected ampacity. Compare against the terminal column. Publish the table in the submittal.
• Identify and apply the rooftop adder your edition requires. If the edition has a 33°C adder inside a threshold distance, a 45°C ambient becomes 78°C and the factor falls from 0.87 to 0.41.

Conductors are a small line on the bill of materials and a large line on the risk register. Treat the temperature math with the same rigor as structural and module-level design, and keep the result on file. To see how conductor assumptions can sit beside layout and bill-of-materials output, book a SurgePV demo.

Frequently Asked Questions

What is temperature derating for solar conductors?

Temperature derating is reducing a conductor’s allowable ampacity when it runs hotter than the 30°C table reference. You multiply the table ampacity by an ambient correction factor and, for more than three current-carrying conductors in a raceway, an adjustment factor. Then check the result against the ampacity your equipment terminals allow.

What is the NEC ambient temperature correction factor at 50°C?

In the 2020 NEC table as reproduced by Conduit.site and Elliott Electric Supply, the factor at 46 to 50°C is 0.82 for 90°C insulation, 0.75 for 75°C insulation and 0.58 for 60°C insulation. Multiply it by the ampacity from the matching column of Table 310.16. Check the table in the edition your AHJ enforces.

How much does rooftop conduit add to ambient temperature for solar?

It depends on the NEC edition. Per Jade Learning, the 2014 NEC used a graduated table, and the 2017 NEC kept one 33°C adder for raceways within 7/8 inch of the roof. Later editions may differ, so read the adopted text and ask the AHJ how it applies the adder.

Do I need to derate PV wire for temperature?

Yes, PV wire is corrected for ambient temperature and conduit fill like other conductors. Its ratings are printed on the cable and datasheet, and the ampacity table that applies depends on the wiring method. Check the listing and the code text instead of using a dry temperature rating as an ampacity.

Can I use the 90°C ampacity column for the final conductor rating?

No, not as the final rating. The 90°C column is the starting value for derating, and the final usable ampacity is limited by the lowest temperature rating of any termination under NEC 110.14(C). Read the temperature marking on your breaker, lug and inverter terminals.

How does conduit fill derating stack with temperature correction?

The two factors multiply. For 4 to 6 current-carrying conductors at 46 to 50°C with a 90°C conductor, 0.82 × 0.80 = 0.656, so a #6 AWG with 75 A at 90°C drops to about 49.2 A. Then compare that value with the terminal limit.

Which NEC edition should I use for solar conductor derating?

Use the edition your AHJ has adopted, because table numbers and the rooftop adder rule differ between editions. The tables checked for this guide are 2020 NEC reproductions. Write the edition and the AHJ name at the top of your calculation sheet.