Let's break down derating and those conductor ampacity tables, focusing on common THHN wire.

What is Derating?

Derating, in the context of electrical wiring, is the reduction of the ampacity (current-carrying capacity) of a conductor below its initially rated ampacity. It's necessary to protect the conductor's insulation from overheating, which can lead to insulation breakdown, short circuits, and fires. Think of it like this: A conductor can only handle so much heat before its insulation starts to degrade. Derating makes sure you don't exceed that temperature limit.

Why Do We Need to Derate?

Several factors can cause a conductor to overheat beyond its rated temperature, necessitating derating: High Ambient Temperatures

If the surrounding air is already hot, the conductor has a harder time dissipating its own heat. Think of it like running a car engine in the desert.

More Than Three Current-Carrying Conductors in a Raceway or Cable

When multiple conductors are bundled together (e.g., in a conduit, cable tray, or direct burial), they all generate heat. Each conductor's heat affects the others, raising the overall temperature within the enclosure. This is the most common reason for derating.

Continuous Loads

If a circuit is expected to operate at its maximum load for 3 hours or more, it's considered a "continuous load." Continuous loads create sustained heating, so we need to ensure the conductors can handle it.

Voltage Drop

Although not strictly "derating," excessive voltage drop is related. While not a direct heat issue, high voltage drop can cause equipment malfunction and inefficiency. Undersized conductors lead to increased voltage drop, thus the need to ensure adequate ampacity.

Table 310.15(B)(16): Ampacity of Insulated Conductors

This table is the foundation for determining the ampacity of conductors under specific conditions. It provides the allowable ampacity of conductors, based on: Insulation Type

The type of insulation used on the conductor (e.g., THHN, THWN, XHHW).

Conductor Size

The gauge of the wire (e.g., 14 AWG, 12 AWG, 10 AWG).

Temperature Rating

The insulation's maximum temperature rating (60°C, 75°C, or 90°C).

Installation Conditions

Specifically, that the conductors are installed in a manner that allows for reasonable heat dissipation.

The 60°C, 75°C, and 90°C Columns

These columns represent the maximum temperature rating of the conductor's insulation. Here's how they relate to real-world use: 60°C (140°F) Column

Often associated with older equipment or terminations. Some older circuit breakers, panels, and other devices are only rated for 60°C terminations.

75°C (167°F) Column

The most common temperature rating for modern equipment and terminations. Most circuit breakers, panels, and equipment are rated for 75°C terminations. You'll often see this listed on the equipment itself.

90°C (194°F) Column

This column is typically only used for derating purposes. While THHN/THWN-2 can be rated for 90°C, you usually can't connect it directly to a breaker or device that is not rated for 90°C. You use the 90°C ampacity figure for derating calculations, and then reduce the ampacity to the 75°C rating for final sizing and termination.

Important Considerations


Termination Temperature Rating

This is crucial. The weakest link determines the maximum ampacity you can use. Even if your wire is rated for 90°C, if your breaker is only rated for 75°C, you must use the 75°C ampacity values. This is often overlooked!2.

THHN and THWN-2

THHN is often dual-rated as THWN-2. The "2" means it's suitable for wet locations and has a 90°C rating in dry and wet locations.

How to Use Table 310.15(B)(16) and Derating: A Step-by-Step Example

Let's say you need to run a circuit to a motor that draws 20 amps continuously. You're planning to use THHN wire in conduit, and you have four current-carrying conductors (hot, neutral, and two hots in a multiwire branch circuit) in the same conduit. Your circuit breaker is rated for 75°C terminations.1. Determine the Minimum Required Ampacity (Before Derating)

For a continuous load, you need to size the conductors to handle 125% of the continuous load (NEC 210.19(A)(1) and 215.2(A)(1)). 20 amps 1.25 = 25 amps minimum.
2.

Consult Table 310.15(B)(16)

Go to the table and look at the 75°C column. Find the smallest wire gauge that has an ampacity greater than or equal to your required 25 amps. Looking at the table: 12 AWG is rated for 25 amps at 75°C. 10 AWG is rated for 30 amps at 75°C.You might be tempted to use 12 AWG since it meets the minimum 25A. However, we must check for other derating factors.3.

Determine Derating Factors



Number of Conductors

You have four current-carrying conductors in the same conduit. Consult Table 310.15(C)(1). It will give you a derating factor based on the number of conductors. For 4-6 conductors, the derating factor is 80% (0.8).

Ambient Temperature

Let's assume the ambient temperature is normal, so we don't need to apply a temperature correction.4.

Apply Derating

To use the 12 AWG wire, you must verify it will still carry at least 25A after derating.

Check 12 AWG after derating

25 amps (from the 75°C column) 0.8 (derating factor) = 20 amps.The 12 AWG will only safely handle 20A after derating. It is not sufficient.

Check 10 AWG after derating

30 amps (from the 75°C column) 0.8 (derating factor) = 24 amps.The 10 AWG still handles 24A after derating. This is closer, but still not quite sufficient.To make sure we meet the requirement of 25A after derating, we must consult the 90°C column to perform calculations. The 90°C column shows that 12 AWG would be rated at 30A. However, we cannot use 12 AWG. We can only use the 90°C rating for calculation, but we must terminate to 75°C.

Check 12 AWG using the 90°C Column for Derating, then reverting back to 75°C for terminations: 30 amps (from the 90°C column) 0.8 (derating factor) = 24 amps.We can see it still does not meet the 25A minimum. Check 10 AWG using the 90°C Column for Derating, then reverting back to 75°C for terminations: 35 amps (from the 90°C column) 0.8 (derating factor) = 28 amps.The 10 AWG handles 28A after derating and we can terminate at 75°C, so this will satisfy the calculation.
5. Select the Correct Wire Size

You must use 10 AWG THHN wire in this scenario. It is large enough to carry the continuous load plus derating factor AND can be terminated on a 75°C breaker.

Key Takeaways Always consider termination temperature ratings. This is the most frequent mistake. Derating is cumulative. If you have multiple factors (ambient temperature and number of conductors), multiply all the derating factors together. When in doubt, upsize the wire. It's better to err on the side of caution and use a larger conductor.* Always consult the latest edition of the NEC (National Electrical Code). Electrical codes are constantly updated. The code is the final authority, and my explanation here is for educational purposes only.This explanation should give you a solid understanding of derating and how to use Table 310.15(B)(16). If you have further questions, don't hesitate to ask! Working with electricity can be dangerous, so always consult with a qualified electrician for any actual installations. Flag for review