You're dealing with a common and complex situation. Running wires in PVC-coated rigid steel conduit (RGS) on a roof presents unique derating challenges due to solar heating. Here's a breakdown of how to handle it

1. Understanding the Problem
Solar Heating

Conduit on a roof absorbs significant heat from the sun, especially on sunny days. This heat is transferred to the wires inside, increasing their temperature.

Ampacity Reduction

Higher wire temperatures reduce the wire's ampacity (current-carrying capacity). This is because increased heat can damage the insulation and potentially create a fire hazard.

NEC Requirements

The National Electrical Code (NEC) mandates derating to account for these conditions.

2. The Process and Relevant NEC Articles1. Determine Ambient Temperature



NEC 310.15(A)

Wires must be chosen and protected against overcurrent conditions based on the ambient temperature. This is determined from Table 310.15(B)(1)

NEC 310.15(B)(3)(c)

Where conductors or cables are installed in raceways exposed to direct sunlight on or above a rooftop, the ambient temperature correction factors in Table 310.15(B)(1) shall be based on the increased ambient temperature shown in Table 310.15(B)(3)(c).| Distance Above Roof to Bottom of Raceway | Temperature Additions

:--------------------------------------- | :--------------------

Less than 1/2 inch | 30°C (54°F)

1/2 inch to less than 3 1/2 inches | 22°C (40°F)

3 1/2 inches to less than 12 inches | 17°C (30°F) || 12 inches or more | 14°C (25°F) |

Local Climate Data

Obtain the highest recorded ambient temperature for your location. Add the appropriate temperature from Table 310.15(B)(3)(c) to this value. This is your adjusted ambient temperature.2.

Determine the base Ampacity of the Conductor



NEC 310.16

Look up the ampacity of your chosen wire size and insulation type in NEC Table 310.16 based on its temperature rating (usually 60°C, 75°C, or 90°C). Select the correct column based on the terminal rating of the equipment it is connected to.

Important

You must use the correct temperature rating. Most terminals are rated for 75°C, but check the equipment specifications. Don't automatically assume you can use the 90°C ampacity column.3.

Apply Ambient Temperature Correction Factor



NEC 310.15(B)(1)

Find the correction factor from Table 310.15(B)(1) that corresponds to your wire's temperature rating and the adjusted ambient temperature you calculated in step 1.

Multiply

Multiply the base ampacity (from step 2) by the correction factor.4.

Apply Conduit Fill Derating (if applicable)



NEC 310.15(B)(3)(a)

If you have more than three current-carrying conductors in the conduit, you need to apply an additional derating factor based on the number of conductors. Refer to Table 310.15(B)(3)(a).

Important

Neutral conductors may be considered current-carrying in certain situations (e.g., unbalanced 3-phase systems, harmonics). Grounding conductors are not considered current-carrying.

Multiply

Multiply the ampacity (after ambient temperature correction) by the appropriate conduit fill derating factor.

3. Addressing the Partial Roof Run

This is the tricky part where you need to make a judgement call and document your reasoning. There are a few approaches, and the best one depends on the specific installation and the AHJ (Authority Having Jurisdiction): Conservative Approach (Treat Entire Run as Rooftop)

The simplest and safest approach is to assume the entire conduit run is exposed to rooftop conditions and apply the full derating factors based on the rooftop ambient temperature. This is the most conservative approach and generally the easiest to justify to an inspector.

Weighted Average Approach (NEC Isn't Clear on This)

Some engineers may try to calculate a weighted average derating factor based on the percentage of the run exposed to the roof versus the percentage in a cooler environment. This is more complex and requires careful documentation. I do not recommend this approach as it opens the door to interpretation and potential issues. Determine the length of the conduit on the roof (L_roof) and the total length of the conduit run (L_total). Calculate the percentage of the run on the roof: P_roof = (L_roof / L_total) 100 Calculate the percentage of the run not on the roof: P_not_roof = 100 - P_roof Find ambient temperature correction factor for roof run (CF_roof). Find ambient temperature correction factor for non-roof run (CF_not_roof). Weighted Average Correction Factor: CF_weighted = (P_roof/100 CF_roof) + (P_not_roof/100 CF_not_roof) Multiply the base ampacity (from step 2) by the weighted average correction factor. Then, apply conduit fill derating.



Segregated Ampacity (Not Recommended)

This involves calculating the ampacity separately for the rooftop section and the non-rooftop section and then ensuring the entire circuit operates below the lower of the two calculated ampacities. This is difficult to enforce and not generally recommended.

4. Mitigation Strategies (Alternatives to Derating)Instead of relying solely on derating, consider these strategies to reduce heat buildup and potentially minimize derating requirements:

Increase Clearance

Elevate the conduit above the roof surface as much as possible (as indicated in Table 310.15(B)(3)(c)). Higher clearances significantly reduce the temperature addition.

Shielding

Install a shield or sunshade over the conduit. This will reduce the amount of direct sunlight it absorbs.

Larger Conduit Size

Using a larger conduit can improve airflow and reduce heat buildup.

Oversized Conductors

Install a larger conductor size to compensate for the derating.

Conduit Routing

If possible, route the conduit along shaded areas of the roof or inside the building.

White or Light-Colored Coating

Apply a reflective coating to the conduit to reduce solar absorption.

5. ExampleLet's say you're running #12 AWG THHN conductors in PVC-coated RGS on a roof in Phoenix, Arizona. You have 4 current-carrying conductors in the conduit.1.

Ambient Temperature

The highest recorded ambient temperature in Phoenix is 45°C (113°F). The conduit is less than 1/2 inch above the roof, so the temperature addition is 30°C. Adjusted Ambient Temperature: 45°C + 30°C = 75°C (167°F)2.

Base Ampacity

#12 AWG THHN is rated for 30 amps at 90°C, but since the equipment terminals are rated for 75C, we must use the 75C column, so the ampacity is 25 amps.3.

Ambient Temperature Correction

From Table 310.15(B)(1), the correction factor for 75°C-rated conductors at 75°C is 0.
58. Corrected Ampacity: 25 amps 0.58 = 14.5 amps4.

Conduit Fill Derating

With 4 current-carrying conductors, the derating factor from Table 310.15(B)(3)(a) is 80%. Final Ampacity: 14.5 amps 0.80 = 11.6 ampsTherefore, even though #12 AWG THHN is normally rated for 30 amps, in this scenario, you're limited to only 11.6 amps.

6. Documentation Keep thorough records of all calculations, ambient temperature data, and derating factors used. Provide this documentation to the inspector and explain your reasoning.

Important Considerations



AHJ Approval

Always consult with the local Authority Having Jurisdiction (AHJ) before finalizing your design. They may have specific requirements or interpretations of the NEC.

Professional Engineer

For complex installations, consider consulting with a qualified electrical engineer. They can perform detailed calculations and ensure the installation meets all applicable codes and standards.

Safety First

Always prioritize safety. Err on the side of caution when calculating ampacity and applying derating factors.

Ongoing Monitoring: Consider installing temperature sensors in the conduit to monitor the actual wire temperature. This can provide valuable data and help you optimize your design.By following these steps and carefully considering all relevant factors, you can ensure a safe and compliant electrical installation in PVC-coated RGS conduit on a roof. Remember, derating is crucial to protect the wiring and prevent potential hazards. Flag for review