6.3 Conductor Sizing and Ampacity Derating

Conductor Sizing and Ampacity Derating

Conductors in photovoltaic power systems are subjected to extreme environmental stresses rarely encountered in standard commercial or residential wiring. Outdoor PV array wiring operates under prolonged direct ultraviolet (UV) radiation, severe ambient temperatures, thermal cycling, and continuous maximum DC electrical loading. Sizing these conductors requires balancing two distinct engineering requirements: providing adequate current-carrying capacity (ampacity) to prevent insulation degradation and conductor overheating, while complying with equipment terminal temperature limits under National Electrical Code (NEC) Article 110.14(C).

Failure to properly size and derate conductors leads to insulation brittleness, dangerous hot spots, nuisance overcurrent device tripping, and elevated risk of DC arc faults. Solar professionals must master the standardized two-step sizing method to ensure every conductor satisfies code requirements under both normal and extreme operating conditions.

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1. Conductor Types and Insulation Ratings in PV Systems

Wiring methods in photovoltaic arrays vary depending on whether conductors are exposed to outdoor weather or enclosed within protective raceways.

PV Wire (UL 4703) vs. USE-2 (UL 854)

• PV Wire (Photovoltaic Wire): Specifically developed for solar installations. PV Wire features an extra-heavy, sunlight-resistant cross-linked polyethylene (XLPE) or thermoset insulation jacket rated for $90^\circ\text{C}$ wet and dry conditions. It carries listings for $600\text{ V}$, $1000\text{ V}$, and $2000\text{ V}$, exhibits exceptional low-temperature flexibility (down to $-40^\circ\text{C}$), passes stringent flame tests, and is listed for direct burial. The 2017 NEC 690.31(C)(1) permits PV wire or USE-2 as exposed single conductors in PV source circuits within the array.
• USE-2 (Underground Service Entrance): A service-entrance cable type rated $90^\circ\text{C}$ wet and dry and sunlight resistant. The 2017 NEC permits it for exposed PV source circuits within the array, but it is typically rated only 600 V and has thinner insulation than PV wire, so it cannot serve 1000 V or 1500 V arrays. (The 2014 NEC required PV wire on ungrounded arrays; the 2017 NEC deleted that rule.)

Conduit Conductors: THHN/THWN-2 and XHHW-2

Once PV circuits transition from module interconnects into raceways (such as electrical metallic tubing [EMT], rigid PVC, or fiberglass conduit), standard dual-rated building wire is used:

• THWN-2: Thermoplastic insulation with a nylon jacket, rated for $90^\circ\text{C}$ in both dry and wet locations. (THHN alone is a dry-location rating, and dual-rated THHN/THWN is only $75^\circ\text{C}$ when wet. The interior of a raceway in a wet location above grade is itself a wet location (NEC 300.9), so use a $90^\circ\text{C}$ wet-rated conductor such as THWN-2 or XHHW-2 to keep the $90^\circ\text{C}$ column available for derating.)
• XHHW-2: Thermoset cross-linked polyethylene insulation, rated $90^\circ\text{C}$ wet and dry. XHHW-2 provides superior chemical, moisture, and abrasion resistance compared to THWN-2 and is widely favored in commercial and industrial rooftop applications.

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2. Terminal Temperature Limitations (NEC 110.14(C))

One of the most widely misunderstood concepts in electrical sizing is the distinction between conductor insulation rating and equipment terminal rating.

The 75°C Equipment Ceiling

Most commercial and residential electrical equipment—including circuit breakers, fused disconnects, combiner box busbars, and inverter terminals—is tested and listed under UL standards for use with conductors operating at $75^\circ\text{C}$ (or $60^\circ\text{C}$ for devices rated 100 amperes or less with #14 through #1 AWG conductors, though modern solar components are universally rated for $75^\circ\text{C}$).

If a $90^\circ\text{C}$-rated conductor (such as #10 AWG THWN-2) carries current up to its full $90^\circ\text{C}$ capacity ($40\text{ A}$), the metal conductor itself will heat up toward $90^\circ\text{C}$. When this hot copper is bolted to a terminal lug rated for only $75^\circ\text{C}$, the conducted heat will transfer into the terminal, overheating internal springs, thermal trip elements, and sensing electronics.

Therefore, NEC 110.14(C) dictates that regardless of conductor insulation temperature ratings ($90^\circ\text{C}$ or higher):

The ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device.

The Role of the 90°C Column

Although a conductor terminated at a $75^\circ\text{C}$ lug cannot be operated above its $75^\circ\text{C}$ ampacity table value under continuous load, NEC 110.14(C) allows the $90^\circ\text{C}$ column ampacity to be used as the starting point for derating adjustments (ambient temperature correction and conduit fill). The final derated ampacity must simply be verified against the design current.

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3. The Standard Two-Step Conductor Sizing Method

To ensure full compliance with NEC 690.8, the ampacity table, and 110.14(C), solar designers must apply the Two-Step Conductor Sizing Method.

Two-Step Conductor Sizing Criteria:
  Step 1 (Terminal Limit):  Ampacity_75C >= I_sc * 1.5625  (Continuous load without derating)
  Step 2 (Derated Limit):   Ampacity_90C * F_temp * F_fill >= I_sc * 1.25 (Under conditions of use)

Step 1: Minimum Ampacity Before Derating (Terminal Protection)

The conductor must have an ampacity before derating sufficient to carry $125\%$ of the maximum circuit current ($1.25 \times 1.25 \times I_{sc} = 1.5625 \times I_{sc}$) based on the terminal temperature rating (the $75^\circ\text{C}$ column in NEC Table 310.15(B)(16)):

$\text{Table Ampacity}_{75^\circ\text{C}} \ge I_{sc} \times 1.5625$

Step 2: Derated Ampacity Under Conditions of Use

The conductor's ampacity adjusted for environmental conditions of use (ambient temperature and conduit fill) must be equal to or greater than the maximum circuit current ($1.25 \times I_{sc}$):

$\text{Ampacity}_{derated} = \text{Table Ampacity}_{90^\circ\text{C}} \times F_{temp} \times F_{fill} \ge I_{sc} \times 1.25$

Where:

• $\text{Table Ampacity}_{90^\circ\text{C}}$ is the unadjusted conductor ampacity from the $90^\circ\text{C}$ column of NEC Table 310.15(B)(16).
• $F_{temp}$ is the ambient temperature correction factor from NEC Table 310.15(B)(2)(a) or Table 310.15(B)(16).
• $F_{fill}$ is the conduit fill adjustment factor from NEC Table 310.15(B)(3)(a).

(Note: If the circuit is protected by an overcurrent protective device, the derated ampacity must also be equal to or greater than the OCPD rating, unless the small conductor rules or next-size-up exceptions of NEC 240.4 apply).

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4. Temperature Correction and Conduit Fill Adjustment Factors

Ambient Temperature Correction ($F_{temp}$)

NEC Table 310.15(B)(16) establishes base ampacities referenced to an ambient temperature of $30^\circ\text{C}$ ($86^\circ\text{F}$). When conductors operate in warmer environments, heat dissipation from the conduit slows, requiring ampacity to be derated.

Conduit Fill Adjustment ($F_{fill}$)

When multiple current-carrying conductors are bundled together in a raceway or cable tray, the mutual heating between conductors reduces thermal dissipation. NEC Table 310.15(B)(3)(a) specifies the adjustment factors based on the total number of current-carrying conductors:

Important Grounding Distinction: Equipment Grounding Conductors (EGC) carry current only during abnormal ground-fault events. Under normal operating conditions, the EGC is non-current-carrying and is never counted when determining conduit fill adjustment factors.

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5. Rooftop Raceway Height and Temperature Adders

Rooftop surfaces absorb solar radiation and create an intense thermal microclimate directly above the roof deck. Conduits mounted flat against a dark roofing membrane can experience internal temperatures $30^\circ\text{C}$ to $40^\circ\text{C}$ hotter than ambient air.

Historical NEC Rule vs. the Current 7/8-Inch Rule

• Historical Editions (2008–2014 NEC): Contained a tiered table (Table 310.15(B)(2)(c) in 2008; Table 310.15(B)(3)(c) in 2011 and 2014) assigning temperature adders based on elevation above the roof (e.g., $+33^\circ\text{C}$ for $\le 0.5\text{ in.}$, $+22^\circ\text{C}$ for $0.5 - 3.5\text{ in.}$, $+17^\circ\text{C}$ for $3.5 - 12\text{ in.}$, and $+14^\circ\text{C}$ for $12 - 36\text{ in.}$). Commercial testing demonstrated that once conduit is elevated slightly off the roof, convective airflow significantly reduces solar heat gain.
• 2017, 2020, and 2023 NEC: Simplified to the $7/8\text{ inch}$ ($23\text{ mm}$) rule (2017 NEC 310.15(B)(3)(c); renumbered 310.15(B)(2) in 2020 and 2023):
  • Raceways installed on or above rooftops exposed to direct sunlight where the distance from the bottom of the raceway to the roof surface is less than $7/8\text{ inch}$ ($23\text{ mm}$) must add a temperature adder of $33^\circ\text{C}$ ($60^\circ\text{F}$) to the ambient design temperature.
  • Raceways supported at least $7/8\text{ inch}$ ($23\text{ mm}$) above the rooftop deck (e.g., using standard strut blocks, pillow blocks, or roof pipe stands) require zero temperature adder.
  • XHHW-2 Exception: Type XHHW-2 insulated conductors are not subject to the rooftop temperature adder.

Solar installation professionals universally mount rooftop conduits on supports at least $7/8\text{ inch}$ (typically $2\text{ to }4\text{ inches}$) above the roof surface to avoid the severe $33^\circ\text{C}$ penalty.

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6. Comprehensive Step-by-Step Worked Sizing Calculation

A solar installation team is installing a commercial rooftop PV system. Three source circuits are routed in a single continuous electrical metallic tubing (EMT) raceway from the array to a combiner box.

System Parameters

• Module Short-Circuit Current ($I_{sc}$): $13.6\text{ A}$
• Number of Source Circuits in Conduit: 3 circuits (each circuit consists of 1 positive and 1 negative conductor, totaling 6 current-carrying conductors).
• Equipment Grounding Conductor: One #8 AWG bare copper wire (non-current-carrying).
• Conductor Insulation: Copper THWN-2 ($90^\circ\text{C}$ rating).
• Terminal Rating: Inverter and combiner terminals are rated for $75^\circ\text{C}$.
• Site Ambient Design Temperature: $38^\circ\text{C}$ ($100^\circ\text{F}$, ASHRAE 2% design high dry bulb).
• Raceway Mounting: Elevated on rubber rooftop support blocks at an elevation of $3.5\text{ inches}$ above the white TPO membrane roof ($> 7/8\text{ inch}$, so no rooftop temperature adder applies).
• String Overcurrent Protection: Each string is protected by a $25\text{ A}$ fuse.

Step 1: Terminal Ampacity Evaluation (NEC 110.14(C) / 75°C Column)

Calculate the required continuous design current before derating:

$I_{continuous} = I_{sc} \times 1.5625 = 13.6\text{ A} \times 1.5625 = 21.25\text{ A}$

Consult NEC Table 310.15(B)(16) under the $75^\circ\text{C}$ Copper column:

• #14 AWG Copper: Rated $20\text{ A}$ at $75^\circ\text{C}$. ($20\text{ A} < 21.25\text{ A}$ — Fails Step 1).
• #12 AWG Copper: Rated $25\text{ A}$ at $75^\circ\text{C}$. ($25\text{ A} \ge 21.25\text{ A}$ — Passes Step 1).
• #10 AWG Copper: Rated $35\text{ A}$ at $75^\circ\text{C}$. ($35\text{ A} \ge 21.25\text{ A}$ — Passes Step 1).

Step 1 establishes that the conductor must be at least #12 AWG Copper to satisfy terminal temperature limitations.

Step 2: Derating for Ambient Temperature and Conduit Fill (90°C Column)

Determine the adjustment factors:

1. Ambient Temperature Correction ($F_{temp}$): At an ambient temperature of $38^\circ\text{C}$ (within the $36^\circ\text{C to } 40^\circ\text{C}$ range), NEC Table 310.15(B)(2)(a) specifies a correction factor of $0.91$ for $90^\circ\text{C}$ wire.
2. Conduit Fill Adjustment ($F_{fill}$): For 6 current-carrying conductors in a single raceway, NEC Table 310.15(B)(3)(a) specifies an adjustment factor of $0.80$ (80%).
3. Combined Derating Factor: $\text{Total Derate} = F_{temp} \times F_{fill} = 0.91 \times 0.80 = 0.728$

Now, test candidate conductors using their unadjusted $90^\circ\text{C}$ table ampacities from NEC Table 310.15(B)(16):

Testing #12 AWG Copper THWN-2

• Base $90^\circ\text{C}$ ampacity = $30\text{ A}$
• Derated ampacity: $\text{Ampacity}_{derated} = 30\text{ A} \times 0.728 = 21.84\text{ A}$
• Verify against maximum circuit current ($I_{max} = 13.6\text{ A} \times 1.25 = 17.0\text{ A}$): $21.84\text{ A} \ge 17.0\text{ A} \quad \text{(Passes circuit current)}$
• Check Overcurrent Protection Compliance: The string is protected by a $25\text{ A}$ fuse. Under NEC 240.4(D), the maximum overcurrent protection for #12 AWG copper is strictly capped at $20\text{ A}$. Furthermore, the derated ampacity of $21.84\text{ A}$ cannot protect a $25\text{ A}$ fuse. Therefore, #12 AWG is disqualified by overcurrent protection rules.

Testing #10 AWG Copper THWN-2

• Base $90^\circ\text{C}$ ampacity = $40\text{ A}$
• Derated ampacity: $\text{Ampacity}_{derated} = 40\text{ A} \times 0.728 = 29.12\text{ A}$
• Verify against continuous circuit current: $29.12\text{ A} \ge 17.0\text{ A} \quad \text{(Passes with substantial margin)}$
• Verify against overcurrent protection: Under NEC 240.4(D), the maximum overcurrent protection for #10 AWG copper is $30\text{ A}$. The derated ampacity of $29.12\text{ A}$ safely exceeds the $25\text{ A}$ string fuse rating ($29.12\text{ A} \ge 25\text{ A}$).

Conclusion

#10 AWG Copper THWN-2 is the minimum code-compliant conductor size. It satisfies the $75^\circ\text{C}$ terminal requirement before derating ($35\text{ A} \ge 21.25\text{ A}$), provides an adjusted ampacity under conditions of use exceeding circuit current ($29.12\text{ A} \ge 17.0\text{ A}$), and coordinates with the $25\text{ A}$ string fuse.