Why 90°C for conductor derating?

1️⃣ Introduction

Derating a conductor is a process used to reduce the amount of current a conductor is allowed to carry based on the factors below:

① Number of conductors in a conduit

② Higher ambient temperatures

③ Internal design requirements for a facility

The goal is to protect conductors from overheating due to the extra heat produced by either of the conditions mentioned above.🙅

2️⃣ The code

As we have learned from our previous article, conductors for equipment rated for 600V and less are usually rated 75°C max.

However, according to the NEC section 110.14C, “Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.”

By beginning the derating process at the ampacity of the conductor based on the higher insulation value, you may not be required to upsize the conductor to compensate for the derating.

3️⃣ Derating requirements

When derating conductors, the following requirements have to be followed:

•The ampacity value determined after applying the derating factors must be equal to or less than the ampacity of the conductor based on the temperature limitations at its terminations.

•The derated ampacity becomes the allowable ampacity of the conductor, and the conductor must be protected against overcurrent in accordance with this allowable ampacity

4️⃣ How does derating work?

Let's consider the following example, where the ambient temperature is 40°C instead of 30°C and we have more than 3 current carrying conductors.

Here are the important considerations about this example:

NEC 310.15(b)(4)(c): Neutral must be considered to be a current-carrying conductor since a major portion of the load is a nonlinear load (electric discharge lighting).

NEC Table 310.15(B)(3)(a) Conductor ampacity reduction based on the number of conductors (greater than 3) in the raceway.

•NEC Table 310.15(B)(2)(a)Conductor ampacity adjustment based on the ambient temperature (When different than 30°C (86°C).

Let's consider two scenarios using the  75°C conductor (THWN) and the  90°C conductor (THHN).

⭕Scenario 1 ( 75°C  conductor THWN)

✦The load is 200A ➜ lets pick the 300 kcmil copper (285A at 75°C) ➜ Refer to table 310.15(B) (16) below.

✦4-conductor derating ➜ 285Ax0.80 =228A ➜ Refer to the table 310.15(B)(3)(a)

✦Ambient temp. derating ➜ 228Ax0.88 = 201A ➜ Refer to the table 310.15(B)(2)(a)

201 A is now the allowable ampacity of the 300 kcmil copper conductor for this circuit.

Had the derating factors for conduit fill and ambient not been required, a 3/0 copper conductor would have met the needs for this application.

🎯Scenario 2 ( 90°C  conductor THHN)

✦The load is 200A ➜ lets pick the 250 kcmil copper (290A at 75°C) ➜ Refer to table 310.15(B) (16) below.

✦4-conductor derating ➜ 290Ax0.80 =232A ➜ Refer to the table 310.15(B)(3)(a)

✦Ambient temp. derating ➜ 232Ax0.91 = 211A ➜ Refer to the table 310.15(B)(2)(a)

211 A is less than the 75 °C ampacity of a 250 kcmil copper conductor (255 A), so the 211 A would now be the allowable ampacity of the 250 kcmil conductor.

Had the calculation resulted in a number larger than the 75 °C ampacity, the actual 75 °C ampacity would have been used as the allowable ampacity of the conductor. This is critical since the terminations are rated at 75 °C.

Note that the conductor size was reduced by one size (300 kcmil to 250 kcmil) and still accommodated all of the required derating factors for the circuit.

This is the primary advantage of using 90 °C conductors.

If you liked the content, please share and leave us a comment.

If you are new here, please don't forget to follow the "The Ohm's Law" page so you never miss any of our articles and posts.

Author: Yves Zomebot, PE

Sources: NFPA 70

#cable #powersystem #theohmslaw #conductor #l#electricalengineering #derating