The owners of an office building have decided to install six, Level 2 fixed electric vehicle charging stations (EVSE) due to an increase in workers that drive electric vehicles. Each station has two charging cords and draws a maximum input current of 24 amperes when both cords are being used at the same time. The voltage range of each station is 208-240V, single phase. Since the office building only has 480Y/277-Volt and 208Y/120-Volt systems, the charging station branch circuits will consist of 2 ungrounded conductors with a voltage of 208V between them. Also, due to the location of these stations, the raceway containing the branch circuits will need to be directionally bored under the parking lot and therefore, all circuits will occupy the same underground PVC conduit from the building to a junction box in the ground near the charging stations. What is the minimum size ungrounded, copper conductor using THHW insulation that is permitted by the NEC® for the circuit conductors run to the junction box? Assume that volt drop is not an issue and that all terminations are rated for 75°C.

Answer: 6 AWG

First, let’s examine the given parameters for each branch circuit:

208V

24 amperes maximum current

six branch circuits

Single underground PVC conduit

THHW insulation

Wet Location

75°C terminations

Voltage drop is not an issue

Second, let’s fill in any missing information that may be needed such as what size overcurrent device will supply each branch circuit. Section 625.41 gives requirements for sizing the OCPD for electric vehicle supply equipment (EVSE). This equipment must be treated as a continuous load and therefore the OCPD shall have a rating not less than 125% of the maximum load of the equipment. This results in an OCPD rating of 30A.

Next, start the process of sizing the ungrounded conductors. Section 210.19(A)(1) contains the general conductor sizing requirements for branch circuits under 600V. There are two methods listed for sizing conductors that must be compared to determine which method requires the larger conductor size. The first method is to determine the conductor size based on the continuous nature of the load. Since EVSE is generally expected to be used for longer periods of time than three hours, it must be considered a continuous load and the conductor ampacity must be based on 125% of the load. This results in a minimum conductor ampacity of 30A. Referring to Table 310.15(B)(16) it is determined that THHW is listed in two columns of the table. This should raise a red flag and it is necessary to review to Table 310.104(A) for applications of conductors. Since the inside of the underground conduit is considered a wet location per 300.5, it is dtermined that the 75°C rating of our insulation must be used. Therefore, the minimum size conductor is selected from the 75°C column in Table 310.15(B)(16), and this results in a 10 AWG conductor.

The second condition requires that the conductor have sufficient ampacity for the load to be served after application of any adjustment or correction factors. So let’s determine what, if any correction or adjustment factors might be applicable. Ambient temperature above 86 degrees F is not an issue as the conductors are being installed underground. The other factor would be the number of current carrying conductors in the raceway. Since there are 12 current carrying conductors, the adjustment factor is 50%. Now it is necessary to select a conductor size from the 75°C column of Table 310.16(B)(16) that after a 50% adjustment, still has adequate ampacity to serve the load of 24A. An 8 AWG conductor after an adjustment of 50% has an ampacity of 25 A and this is sufficient to serve the load. The ampacity adjustment cannot be made from the 90°C column as the THHW insulation is limited to 75°C when installed in a wet location.

The second condition requires a larger conductor size in order to serve the load. However, there is one last step in determining our conductor size. Is the conductor properly protected by the OCPD? As determined earlier, the minimum permitted OCPD is a 30A. The ampacity of the conductor based on the conditions of use is 25A. Section 240.4 states that conductors must be protected from overcurrent in accordance with their ampacity. Since the OCPD is rated more than the ampacity and use of the next higher rating is not permitted since 25A corresponds to a standard size OCPD. Therefore, it is necessary to increase the size of the conductor to a 6 AWG so that the conductor is protected in accordance with its ampacity per 240.4 (B).