Correcting Conductor Ampacities: Code rules and calculations for applied ampacities

Receptacles and Conductors Photo Credit: Shutterstock / Constantine Pankin
Shutterstock / Constantine Pankin
Published On
Jul 15, 2021

If a large quantity of current-carrying conductors is installed in a raceway outside in an ambient temperature that can reach 125°F, what are the Code requirements regarding applied ampacities? These conductors supply continuous-duty motor loads, all three-phase and no neutrals loads. The raceway fill does not exceed 40% of the cross-sectional area of the raceway, as indicated in Chapter 9, Table 1 and the associated notes.

Here are the specific details: A 2-inch EMT is installed outdoors, and it contains (36) 10 AWG copper XHHW conductors. The installation is in Arizona and mostly in the shade; however, the outside temperatures have been recorded to 125°F. Each of the 12 motors draws 16.7A at 208V. What is the allowable ampacity for the 10 AWG conductors?

First let’s look at the applicable Code rules. Section 310.15 and Table 310.16 need to be applied together to determine the adjusted ampacities. There are two adjustments needed, one for the ambient temperature and one for the number of current-carrying conductors installed in the raceway. A review of Table 310.16 reveals that an XHHW copper conductor sized at 10 AWG is rated at 40A at 90°C/194°F. The terminations are rated at 75°C/167°F, which limits the final conductor ampacity to the 75°C column ampacity, but ampacity adjustments can start at the 90°C column value of 40A.

Looking back at Section 310.15(B)(1), it refers to Table 310.15(B)(1) as applicable based on the ambient temperature of 30°C/86°F specified in Table 310.16. So, for an ambient temperature of 52°C/125°F, a multiplier of 0.76 must be applied. Multiplying the starting ampacity of 40A by 0.76 gives us a resulting ampacity of 30.4A. This is the adjusted conductor ampacity after applying the temperature correction, according to 310.15(B)(1). There are (36) 10 AWG current-carrying conductors in the same run of EMT.

Section 310.15(C) indicates that the second ampacity adjustment is required based on the values shown in Table 310.15(C)(1) when the number of current-carrying conductors exceeds three. With 36 conductors installed, the adjustment factor is 40%. This factor has to be applied on top of the resulting ampacity of 30.4A from the first ampacity adjustment for ambient temperature. That said, 30.4 multiplied by 0.40 leaves 12.6A worth of conductor remaining. This is the adjusted ampacity after the correction factors for ambient temperature and the number of current-carrying conductors have been applied.

Each motor draws 16.7A, so the calculated, adjusted value of 12.6A is inadequate for the motors. Also, there is additional work to be done to determine the branch circuit conductor size to comply with the requirements in Article 430 for motors. These are continuous-duty motor loads. Section 430.22(A) indicates that conductors supplying a single motor used in a continuous-duty application shall have an ampacity not less than 125% of the motor’s full load current rating in Table 430.250, which is 16.7 for a five-horsepower, three-phase, 208V motor. Take the value 16.7 and multiply it by 125%, and the result is 20.87A required for each motor. The result is that the 10 XHHW copper conductors will not have adequate ampacity to meet the requirements of 430.22(A), especially after the ampacity adjustments for ambient temperature and number of current-carrying conductors in the single 2-inch EMT.

Looking at Table 310.16 for the next-larger size XHHW conductor (8 AWG), it shows a value of 55A can be used for the starting point at 90°C. This value will still be too small after the adjusted ampacity calculations. Checking an XHHW (6 AWG) ampacity shows 75A as the starting point at 90°C. Using the same calculation methods indicated above, the resulting ampacity for these motor circuit conductors is 22.8A. These ampacity adjustments satisfy the requirements in Section 310.15 using the ampacity values provided in Table 310.16. Using 6 AWG copper XHHW conductors instead of 10 AWG copper conductors also satisfies the requirements in 430.22(A), after the correction factors have been applied.

Now after all of those calculations to determine current-carrying capacities, there still lies the question of whether a 2-inch EMT will still be Code -compliant for the (36) 6 AWG XHHW copper conductors it contains. Referring to Table C.1, it looks like the maximum number is 22 for a 2-inch EMT. In this case, the EMT size must be adjusted to 2½ inches (trade size 63). Hopefully, the installation is only in the planning stages and the appropriate adjustments can be made for NEC compliance. Obviously, there are options that can reduce the ampacity correction requirements, such as using multiple raceways to supply these motors, which could also result in smaller conductors being required after ampacity corrections.

About the Author

Michael Johnston

Executive Director of Standards and Safety, NECA

Michael Johnston is NECA’s executive director of standards and safety. He is a member of the NEC Correlating Committee, NFPA Standards Council, IBEW, UL Electrical Council and NFPA’s Electrical Section. Reach him at mj@necanet.org.

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