ChargePoint Express 250 - Page 40

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A 3/0 copper conductor has an ampacity of 200 A at 75°C, which is the ampacity column required
for equipment rated 100 A or greater per NEC 110.14(C)(1)(b). If, following the derating from the
90°C column, the resultant ampacity of the 3/0, 90°C wire is equal to or greater than the ampacity
rating of the same size conductor in the 75°C, the conductor is permitted.
From NEC table 310.15(B)(16) in 90°C column, after the derating, the 3/0 conductor ampacity is 216
A which is greater than the minimum required 200 A. Thus the permitted copper conductor size is
3/0.
The permission to use the 90°C ampacity for ambient temperature adjustment comes from the
general requirement in 110.14(C); Conductors with temperature ratings higher than specified for
terminations shall be permitted to be used for ampacity adjustment, correction, or both.
AC Mains Input Wiring, Example Calculation: Phoenix, AZ
Assumptions:
• Breaker and equipment terminal rating minimum 75°C
• Continuous duty equipment
• Maximum 50°C ambient rating
• Installation location: Phoenix, AZ, USA
• 90°C rated wire required
• 480 VAC Input, 3 phase, no neutral
• Maximum station AC input rating: 80 A
• Three current carrying conductors in conduit
The continuous duty derating per 625.41 is 80 x 1.25, or 100 A.
From the Appendix D ASHRAE table, the summer design temperature is 107°F for Phoenix, AZ.
Temperature derating from 2017 NEC Table 310.15(B)(2)(a) based on 30°C the derating factor for
107°F and 90°C wire is 0.87 (from the 105-113°F row).
The allowable ampacity for a #3 AWG 90°C copper conductor per NEC Table 310.15(B)(16) is 115.
Applying the temperature derating factor from Table 310.15(B)(2)(a), 115 X .87 = 100.05 A.
100 is the minimum ampacity this conductor must have per the NEC to prevent potential insulation
damage and provide the ability of the conductor to dissipate heat caused by the current flow. After
the derating is applied, the resulting ampacity of 100.05 A is greater than the 100 A required.
A #3 AWG copper conductor is rated at 100 A at 75°C, which is the ampacity column required for
equipment rated 100 A or greater per NEC 110.14(C)(1)(b). However in this case, the equipment is
only rated to 80 A. Since the equipment is listed and identified with a 75°C termination rating, we
can use NEC Section 110.14(C)(1)(a)(3). As long as after derating from the 90°C column of table
310.15(B)(16) the resultant ampacity is equal to or greater than the ampacity rating of the conductor
in the 75°C column, the #3 AWG conductor is permitted.
From NEC table 310.15(B)(16) in 90°C column, after the derating, the conductor ampacity is