When dealing with conduit installed in areas that reach 110°F, it's crucial to account for the temperature's impact on conductor ampacity. Here's a breakdown of how you should approach this, combining code requirements, best practices, and considerations for safety and reliability

1. NEC (National Electrical Code) is the Authority:
NEC Article 310: Conductors for General Wiring

This article provides the fundamental requirements for conductor ampacity, temperature ratings, and adjustment/correction factors.

2. Key Considerations and Steps



A. Conductor Temperature Rating

The most critical factor.

Determine the conductor's insulation temperature rating (e.g., 60°C, 75°C, 90°C). This is marked on the conductor's insulation. Use conductors with a temperature rating suitable for the environment. Common types include THHN/THWN-2 (90°C in dry and wet locations), XHHW-2 (90°C in dry and wet locations). Important: You cannot exceed the lowest temperature rating of any connected termination or device. For example, if a breaker is rated for 75°C terminations, you must use the 75°C ampacity column, even if the conductor is rated for 90°C. B. Ambient Temperature Correction Factors



NEC Table 310.15(B)(1) or its equivalent

This table provides ampacity correction factors for ambient temperatures other than 30°C (86°F). This is where you adjust for the 110°F ambient temperature.



Use the maximum ambient temperature

For outdoor conduit on a building, absolutely use the maximum expected ambient temperature (110°F in your case) for your calculations. Do not make a "judgment call" to use a lower temperature. Consistent high temperatures have a cumulative effect on conductor insulation.

Convert to Celsius

You'll need to convert 110°F to Celsius: (110 - 32) 5/9 = ~43.3°C. You'll likely need to interpolate between values in the table.

Apply the Correction Factor

Find the correction factor corresponding to your conductor's temperature rating and the ambient temperature. Multiply the conductor's base ampacity (from the appropriate ampacity table - see below) by this correction factor. This is your adjusted ampacity.

C. Ampacity Tables



NEC Table 310.16 (or similar)

This table lists the ampacity of conductors based on their size, insulation type, and number of conductors in a raceway or cable. This is your starting point before any adjustments. You must use the appropriate table based on the conditions of the installation.

Number of Conductors

The number of current-carrying conductors in the conduit is a crucial factor. More conductors mean more heat, and therefore, a lower allowable ampacity for each conductor.

Raceway Fill

Ensure you comply with raceway fill requirements (NEC Chapter 9). Overfilling a conduit can impede heat dissipation, leading to overheating.

D. Adjustment Factors for Number of Conductors



NEC Table 310.15(C)(1) or equivalent

If you have more than three current-carrying conductors in a raceway or cable, you must apply adjustment factors to further reduce the ampacity. This is in addition to the ambient temperature correction factor.

Neutral Conductors

A neutral conductor is generally considered a current-carrying conductor unless it only carries the unbalanced current from other phase conductors in a balanced 3-phase system. In many scenarios, especially with non-linear loads, the neutral carries significant current and must be counted.

E. Voltage Drop

While not directly related to temperature, voltage drop is a crucial consideration, especially over longer distances. Excessive voltage drop can cause equipment malfunction and inefficiency. Calculate voltage drop to ensure it remains within acceptable limits (typically 3% for feeders and 5% total for feeders and branch circuits).

F. Enclosure Considerations

The type of enclosure (e.g., metal, PVC, size) can affect heat dissipation. Very small, tightly packed enclosures can trap heat. Consider using larger enclosures or enclosures with better ventilation in high-temperature environments.

G. Derating Examples

Example 1: A 10 AWG THHN copper conductor (90°C rating) in a conduit with 5 other current-carrying conductors, installed where the ambient temperature reaches 110°F (43.3°C). From NEC Table 310.16 (assuming not in free air): Ampacity of 10 AWG THHN is 40 amps. From NEC Table 310.15(C)(1): For 6 conductors, the adjustment factor is 80%. From NEC Table 310.15(B)(1): For 90°C conductor at 43.3°C ambient, interpolate the correction factor. (It will be somewhere around 0.87). Adjusted ampacity: 40 amps 0.80 0.87 = 27.84 amps. Therefore, this 10 AWG conductor, after derating for ambient temperature and number of conductors, can only safely carry 27.84 amps. Example 2: The same scenario as above, but you use a 8 AWG THHN copper conductor. From NEC Table 310.16: Ampacity of 8 AWG THHN is 55 amps. Adjusted ampacity: 55 amps 0.80 0.87 = 38.28 amps. In this case, you would need to use the larger 8 AWG conductor to carry more than 28 amps.

3. Practical Recommendations



Oversizing Conductors

Consider oversizing conductors to account for future load growth or unforeseen temperature increases. This can also help reduce voltage drop.

Using Higher Temperature Rated Conductors

Choosing 90°C rated conductors (like THHN/THWN-2 or XHHW-2) gives you more headroom with the temperature correction factors.

Conduit Type

Metallic conduit (e.g., EMT, RMC) generally dissipates heat better than PVC conduit. However, metallic conduit can also get hotter in direct sunlight. Consider the overall installation environment.

Sun Shields/Shading

If practical, shielding the conduit from direct sunlight can significantly reduce the ambient temperature.

Documentation

Meticulously document all calculations and assumptions made during the design process. This is important for future reference and troubleshooting.

Professional Consultation

For complex installations or if you are unsure about any aspect of the code requirements, consult with a qualified electrical engineer or licensed electrician.

4. Common Mistakes to Avoid



Ignoring Ambient Temperature

This is the most common and potentially dangerous mistake.

Using Incorrect Ampacity Tables

Ensure you are using the appropriate table based on the conductor type, insulation rating, and installation method.

Forgetting Conductor Adjustment Factors

Failing to adjust for the number of current-carrying conductors.

Exceeding Terminal Temperature Ratings

Using a 90°C conductor but connecting it to a device rated for only 75°C and using the 90°C ampacity.

Not accounting for Voltage Drop

Especially on long runs.

Using "Judgment Calls" Instead of Code Requirements

Always adhere to the NEC.

In Summary:

Always use the maximum expected ambient temperature (110°F in your case) to determine the appropriate temperature correction factors. Combine this with adjustments for the number of conductors in the conduit. Thoroughly review NEC Article 310 and consult with a qualified professional if needed. Safety and compliance are paramount. Flag for review