Case study: How to save money on EV charging equipment

A university-wide electric vehicle (EV) deployment plan included the installation of more than 100 EV chargers of various types. The university is currently in the process of transitioning its maintenance fleet vehicles to electric. It hired SmithGroup to study feasibility, provide recommendations, complete a cost estimate and complete the construction documents for the project. The plan was to provide 30 amp (A), Level 2 chargers for parking spaces throughout various parking structures and lots, typically in groups of 10. In addition, 50 A, Level 2 chargers and 200 kilowatt (kW), Level 3 chargers were to be installed in a few select locations specifically for the charging of fleet vehicles.

The client was procuring the chargers themselves and working with the local utility company for rebates. During the study phase, historical meter data was analyzed for all existing electrical services near the lots and loading calculations were conducted to determine if there was capacity for the new chargers.

Where capacity allowed, surveys were conducted and EV parking spaces were identified, keeping in mind that at least one at each lot needed to be designed to Americans With Disabilities Act (ADA) standard per local codes. The client was very averse to losing any parking spots, so it was important to locate spots such that ADA striping and spacing requirements could be met with minimal impact to the existing number of spaces.

Additionally, the existing slopes were analyzed to ensure ADA spaces met the current grade requirements. Surveying also included examining the condition of available electrical equipment that would serve the EV chargers, determining conduit routes and determining placement of new electrical equipment as required.

Figure 1: One-line diagram of power distribution to electric vehicle chargers. Courtesy: SmithGroup

The engineering team collected all this information to develop a schematic design for the EV installation and power distribution. Once the plan for each site was approved by the client, a cost estimate was developed. Various scenarios were tested to determine what would be the most economical installation.

Example: The client wanted 12 50 A, Level 2 chargers for fleet vehicles at one of the locations.

Load calculation: 12 chargers × 208 volts (V) × 50 A = 124.8 kW

Historical metering showed that the building service transformer and main distribution panel could support the load.

The site survey revealed the chargers would be over 300 feet from the building. This meant that due to voltage drop, it would be better to use a 480 V feeder and then step down to 208 V locally near the charging equipment in lieu of a 208 V feeder. The cost of wiring was the biggest driver to making this decision.

For example, using a 208 V feeder to supply the EV charging equipment:

124.8 kW ÷ 208 V ÷ √3 = 336 A

Due to voltage drop, a 300-foot, 208 V, 336 A, 3-phase feeder would require two sets of 250 kcmil (thousand circular mils) conductors. Per RSMeans estimating software, the material and install costs for just the wire would be:

300 feet × 2 sets × (4 wires)/set × $10.81/foot = $25,944

By contrast, using a 3-phase 480 V feeder for the above calculated load would result in:

124.8 kW ÷ 480 V ÷ √3 = 150 A

Accounting for voltage drop, the 480 V feeder would also be a single set of #1/0 AWG wire. The material and install costs for just the wire would be:

300 feet × 1 set × (3 wires)/set × $5.42/foot = $4,878

The savings in just the cost of wire is $21,116. Also, the 208 V option would require an additional 300 feet of buried conduit due to the two sets of wires needed. A 480 Y-208/120 V, 150 kilovolt-amperes dry type transformer runs under $15,000. Even with a National Electrical Manufacturers Association 3R rated enclosure the savings justify the added transformer. This cost exercise was one of the drivers for using the 480 V supply even though the chargers themselves are 208 V.

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