No, simply bundling the wires with zip ties every 6 feet, without them being in a raceway or conduit, does not automatically qualify as "free air" in the context of NEC ampacity calculations, and yes, it is still subject to the bundling derating factors in NEC Table 310.15(C)(1).Here's why: "Free Air" Definition is Specific

The NEC uses "free air" to describe conductors that are adequately spaced and exposed to ambient air so heat can dissipate effectively. This usually means supported by insulators at intervals that allow for significant air circulation around the conductors. Simply being outside a raceway isn't enough.

Bundling Negates Free Air Cooling

When conductors are bundled together, even with occasional zip ties, they impede heat dissipation. The heat generated by each conductor affects the temperature of the others, leading to a need for ampacity derating. The zip ties help bundle the conductors, and the conductors may still touch each other causing the heat to build up.

NEC 310.15(C)(1) Applies

Because the conductors are bundled, the derating factors in Table 310.15(C)(1) apply. You need to count the total number of current-carrying conductors in the bundle to determine the appropriate derating factor.

PV Applications and Special Considerations

In PV applications, you also need to consider other factors, such as:

Temperature Correction

NEC 310.15(A) addresses temperature correction factors based on the ambient temperature where the conductors are located. PV systems often operate in high-temperature environments.

NEC 690.8(B)

For PV source circuits and PV output circuits, you must use specific calculations outlined in NEC 690.8(B) to determine the maximum current, which then affects the required ampacity of the conductors. This calculation often requires multiplying the short-circuit current (Isc) or maximum power point current (Imp) by a factor (typically 1.25 or 1.56) depending on the specific circuit type.

Wet Location

THHN is rated for wet locations; however, always ensure terminations are also rated.

Spacing

NEC 310.15(C) (Free Air) has specific requirements for spacing, and unless these requirements are met, it is considered bundled.

In Summary


Derating is Required

The conductors are bundled and require derating per NEC Table 310.15(C)(1)2.

Calculate Current

Determine the maximum current for the PV circuit per NEC 690.8(B).
3.

Determine Ambient Temperature

Determine the maximum ambient temperature where the conductors are installed.4.

Apply Temperature Correction

Use NEC Table 310.15(A) to determine the temperature correction factor for THHN wire based on the ambient temperature and the wire's insulation temperature rating (typically 90°C for THHN).
5.

Count Current-Carrying Conductors

Count the number of current-carrying conductors in the bundle. (Neutral conductors in a balanced system generally don't count, but in PV systems with DC circuits, all conductors typically do.)6.

Apply Bundling Derating

Use NEC Table 310.15(C)(1) to find the derating factor for the number of current-carrying conductors.
7.

Calculate Adjusted Ampacity

Multiply the base ampacity of #10 AWG THHN (check NEC Table 310.16, typically 30 amps at 30°C for the 90°C column) by both the temperature correction factor and the bundling derating factor. This is the adjusted ampacity of the wire.
8.

Ensure Ampacity is Sufficient

The adjusted ampacity must be equal to or greater than the maximum current calculated in step 2.

Example (Simplified)

Let's say: Maximum PV circuit current (after NEC 690.8(B) calculation): 20 amps Ambient temperature: 40°C Number of current-carrying conductors in the bundle: 41.

Temperature Correction

From NEC Table 310.15(A), the correction factor for THHN at 40°C might be approximately 0.
91. (This varies by table, and your jurisdiction may vary)2.

Bundling Derating

From NEC Table 310.15(C)(1), the derating factor for 4 current-carrying conductors is 80%.
3.

Adjusted Ampacity

30 amps (base) 0.91 (temp correction) * 0.8 (bundling) = 21.84 ampsIn this example, the adjusted ampacity (21.84 amps) is greater than the maximum circuit current (20 amps), so the #10 AWG THHN would be acceptable (provided all other requirements are met). However, if the ambient temperature were higher, or the number of conductors greater, the adjusted ampacity could fall below 20 amps, requiring a larger wire size.

Crucial Recommendation:

Always consult with a qualified electrical engineer or licensed electrician experienced in PV systems to ensure your wiring is compliant with all applicable codes and regulations. Incorrect wiring can lead to overheating, fire hazards, and system malfunction. Flag for review