What is the NEC rooftop conduit temperature adder?

Under NEC Table 310.15(B)(2) (and prior NEC 310.15(B)(3)(c)), conduit exposed to direct sunlight on or above rooftops experiences severe internal temperature rise. If conduit is mounted less than 0.5 inches from the roof surface, a +33°C (+60°F) temperature adder must be added to the ambient design temperature. For conduit mounted 0.5 to 3.5 inches above the roof, a +22°C (+40°F) adder applies. This elevated internal temperature drastically derates conductor ampacity.

How does conductor bundling in conduit reduce ampacity?

When multiple current-carrying conductors are routed together in a raceway, heat cannot dissipate efficiently into surrounding air. Under NEC Table 310.15(C)(1), bundling 4 to 6 conductors applies a 0.80 (80%) adjustment factor, 7 to 9 conductors applies a 0.70 (70%) factor, and 10 to 20 conductors cuts ampacity in half to 0.50 (50%).

Why is 90°C rated wire (THHN/XHHW-2/PV Wire) required for rooftop solar?

Rooftop solar conduits frequently reach internal operating temperatures between 55°C and 70°C. Standard 75°C wire would be derated to near-zero usable ampacity. Using 90°C rated insulation (such as UL 4703 PV Wire, XHHW-2, or dual-rated THHN) provides sufficient thermal headroom so that after temperature and bundling deratings, the conductor still safely carries the string current.

How do you satisfy NEC 690.8 for continuous solar current?

NEC 690.8 requires two independent sizing checks: first, the derated ampacity of the conductor must be equal to or greater than the maximum circuit current (1.25 * Isc). Second, the un-derated conductor base ampacity must be equal to or greater than the 125% continuous load design requirement (1.25 * 1.25 * Isc = 1.56 * Isc).

Rooftop Solar PV Conduit Thermal Ampacity Derater

Calculate derated solar DC cable ampacity by compounding outdoor ambient design temperatures (NEC 310.15(B)(1)), rooftop conduit distance thermal adders (NEC Table 310.15(B)(2)), and conductor bundling fill factors.

Quick Engineering Standard: Compounded Rooftop Thermal Derating

NEC 310.15 • NEC 310.16 • NEC 690.8

Rooftop solar DC conductors experience extreme thermal stress where outdoor air temperature, radiant roof heat, and bundled electrical resistance combine. Under NEC Table 310.15(B)(2), conduit installed within 0.5 inches of a dark roof incurs a +33°C (+60°F) adder, elevating internal conduit temperatures past 65°C to 75°C on a standard summer day. When compounded with a 0.80 bundling factor for 4 to 6 conductors, a 10 AWG 90°C copper wire rated for 40A in free air is derated by over 50% to just 18.5A to 20.8A. Elevating the conduit at least 3.5 inches above the roof surface saves up to 15°C of heat, often eliminating the need to upsize to 8 AWG wire.

Raceway & String Design Inputs

Conductor Gauge (Copper 90°C THHN / PV Wire)

Base ampacity from NEC Table 310.16 (90°C column).

Outdoor Ambient Design Temp (°C)

ASHRAE 2% summer design dry bulb temperature (e.g. 35°C to 42°C).

Conduit Height Above Roof Surface

NEC Table 310.15(B)(2) rooftop distance thermal adder.

Current-Carrying Conductors in Conduit

NEC Table 310.15(C)(1) conduit fill bundling factor.

PV String Short-Circuit Current (I_sc in Amps)

Panel nameplate short-circuit current (Isc).

Derated Ampacity & NEC 690 Compliance

Effective Conduit Temperature

-- °C

Ambient + rooftop height adder

Combined Derating Factor

-- %

Temp factor × bundling factor

Allowable Derated Ampacity

-- A

Net continuous carrying capacity

NEC 690.8 Max Circuit Current

-- A

1.25 × Isc required threshold

NEC 690.8 Compliance & Installation Assessment

Select parameters and click Calculate to evaluate rooftop thermal derating.

Governing NEC Rooftop Derating Formulas

Rooftop raceway sizing requires compounding two distinct derating multipliers against base ampacity:

Effective Internal Conduit Temperature (T_effective)

T_effective = T_{ambient_design} + ΔT_{rooftop_adder}

ΔT_{rooftop_adder} is derived from NEC Table 310.15(B)(2). A 38°C summer day with conduit within 0.5" to 3.5" of the roof (+22°C adder) results in a 60°C operating environment inside the raceway.

Net Derated Allowable Ampacity (I_allowable)

I_allowable = I_{base_table310.16} × CF_temp × CF_bundling

Where CF_temp is from NEC Table 310.15(B)(1) (e.g. 0.71 at 60°C for 90°C wire) and CF_bundling is from NEC Table 310.15(C)(1) (e.g. 0.80 for 4 to 6 conductors).

NEC 690.8 Conductor Sizing Criteria

Condition 1: I_{allowable_derated} ≥ 1.25 × I_sc | Condition 2: I_{base_underated} ≥ 1.25 × 1.25 × I_sc

The derated conductor must carry the 125% maximum circuit current, while the base un-derated conductor must carry the 125% continuous duty multiplier.

Empirical NEC Rooftop Distance & Thermal Adder Matrix

Reference table showing the thermal adders, temperature correction factors (90°C rated wire), and resulting derated ampacity for 10 AWG copper wire across various roof elevations:

Conduit Height Above Roof Surface	NEC Thermal Adder	Eff. Temp @ 35°C Ambient	Temp Factor (90°C Wire)	4–6 Cond. Factor	10 AWG Allowable Ampacity (Base 40A)
On Roof / < 0.5 in (< 13 mm)	+33°C (+60°F)	68°C (154°F)	0.58	0.80	18.5 Amperes (-53.8%)
0.5 in to 3.5 in (13 mm to 90 mm)	+22°C (+40°F)	57°C (135°F)	0.71	0.80	22.7 Amperes (-43.2%)
3.5 in to 12 in (90 mm to 300 mm)	+17°C (+30°F)	52°C (126°F)	0.76	0.80	24.3 Amperes (-39.2%)
12 in to 36 in (300 mm to 900 mm)	+14°C (+25°F)	49°C (120°F)	0.82	0.80	26.2 Amperes (-34.5%)
> 36 in / Ground-Mounted	0°C (0°F)	35°C (95°F)	0.96	0.80	30.7 Amperes (-23.2%)

Worked Engineering Walkthrough: 2-String 10 AWG Rooftop Run

Determine Base Conductor Ampacity & Maximum Current:

Two parallel strings of PV modules with $I_{sc} = 11.5\text{A}$ are routed through a single rooftop EMT conduit using 10 AWG THHN copper wire. Base ampacity from NEC Table 310.16 (90°C column) is $40.0\text{ Amperes}$. Maximum circuit current per NEC 690.8 is:

I_max = 1.25 * 11.5A = 14.38 Amperes.

Calculate Compounded Deratings (Temperature + Rooftop + Bundling):

The outdoor summer design temperature is $38^\circ\text{C}$. The EMT conduit is mounted on rooftop strut blocks $1.5\text{ inches}$ above the shingles ($+22^\circ\text{C}$ adder per NEC Table 310.15(B)(2)). Total effective internal temperature is:

T_effective = 38°C + 22°C = 60°C.

At 60°C, the temperature correction factor for 90°C rated wire is $CF_{temp} = 0.71$. Because the two strings require 4 current-carrying conductors (2 positive, 2 negative), the bundling adjustment factor from NEC Table 310.15(C)(1) is $CF_{bundle} = 0.80$. The net allowable derated ampacity is:

I_allowable = 40.0A * 0.71 * 0.80 = 22.72 Amperes.

Verify NEC 690.8 Compliance:

Condition 1: $I_{allowable} = 22.72\text{A} \ge I_{max} = 14.38\text{A}$ (PASSED with a 58% safety margin).

Condition 2: $I_{base} = 40.0\text{A} \ge 1.5625 \times 11.5\text{A} = 17.97\text{A}$ (PASSED). 10 AWG wire satisfies all governing NEC requirements.

Related Solar PV & Raceway Sizing Calculators

Ensure conduit and solar string compliance with these complementary tools:

Conduit Fill Percentage Calculator — Calculating raceway fill for EMT, PVC, and liquidtight conduit.
Solar Combiner Box & String Fuse Sizer — Sizing string fuses per NEC 690.9 for parallel solar arrays.
Bifacial Solar Ground Albedo Gain Calculator — Calculating rear-side current boost and overcurrent sizing.
Solar PV Array Cable Sizer — Voltage drop and thermal ampacity calculations for string conductors.