Examples of Express 250 Paired Wire Sizing

The required DC wire gauge varies based on the specific site. Use these example scenarios to help you determine the correct wire gauge for your site.

These scenarios are only examples, and are not intended to replace an assessment by a local electrician. Always follow all applicable local and national codes and requirements. A site drawing should be engineered for your specific site to reduce installation costs and ensure compliance with local codes.

DC Interconnection, Example Calculation: Newark, NJ

Assumptions:

  • Breaker and equipment terminal rating minimum 75 °C

  • Electric Vehicle Charging Equipment rated for continuous duty per Article 625.41

  • Maximum 50 °C ambient rating

  • Installation location: Newark, NJ, USA

  • 90 °C rated wire required

  • Maximum station DC output/input current: 160 A

  • Four conductors in conduit, only two current carrying conductors

The continuous duty derating per 625.41 is 160 x 1.25, or 200 A.

Temperature derating from 2017 NECClosed National Electric Code Table 310.15(B)(2)(a) based on 30 °C the derating factor for 91 °F and 90 °C rated wire is 0.96 (87-95 ˚F row).

From the 90 °C column of NECClosed National Electric Code Table 310.15(B)(16), a 3/0 copper conductor has an ampacity of 225 A.

Applying the temperature derating factor, 225*0.96 = 216 A

200 A is the minimum rated ampacity this conductor must have per the NECClosed National Electric Code to prevent potential insulation damage and provide the ability of the conductor to dissipate heat caused by the current flow. After the temperature derating calculation, the resulting 216 A is greater than the 200 A required.

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 NECClosed National Electric Code 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 NECClosed National Electric Code 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.

Temperature derating from 2017 NECClosed National Electric Code 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 AWGClosed American Wire Gauge 90°C copper conductor per NECClosed National Electric Code 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 NECClosed National Electric Code 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 AWGClosed American Wire Gauge copper conductor is rated at 100 A at 75 °C, which is the ampacity column required for equipment rated 100 A or greater per NECClosed National Electric Code 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 NECClosed National Electric Code 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 AWGClosed American Wire Gauge conductor is permitted.

From NECClosed National Electric Code table 310.15(B)(16) in 90 °C column, after the derating, the conductor ampacity is 100.05 A, which is greater than the minimum required 100 A. Thus the permitted conductor size is 3 AWGClosed American Wire Gauge.

The permission to use the 90 °C ampacity for ambient temperature 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.