In last month’s column, I said that the subject of electric vehicles and the size of a corresponding electrical service required for new and existing homes is fairly complex. Here, I continue explaining the calculation and service requirements. The details of charging an EV depend on the size and type of battery. Probably the best method of charging will be a system installed at home in the garage, since the charging time is usually based on an overnight period. This is often the least expensive method of charging the car at about one-third the rate of a conventional commercial electric charging.

Three major issues

Three major factors must be understood with vehicle chargers. We must determine how much current is available at the electric service. Is there power in the garage or in the area where the car can be parked outside for additional power, such as adjacent to the electrical service? The number of EVs and how much charge is required for each vehicle must be examined when determining potential power usage.

Many smaller homes only have a 100A, 120/240V single-phase service, as is required as a minimum by 230.79(C) in the NEC. Several larger homes have 200A, 120/240V single-phase services, a smaller number have 400A, 120/240V single-phase services, with very few having larger single-­phase or three-phase services of 600A or greater. For many dwelling unit services, existing electrical loads for air conditioning, heating, water heaters, washers and dryers can consume much of the existing power with little left over for EV charging. Changing the electrical service out and installing a larger service to handle one, two or three EV chargers, plus the charger’s cost, must be budgeted and planned for before purchasing an EV.

Turning to the NEC

There are a few new sections in the 2023 NEC and some older ones that can help determine new or existing feeder or service loads. Section 220.57 is new in the 2023 NEC and states that EV supply equipment must be calculated (for feeders or services) at either 7,200W (volt-amperes) or the nameplate rating of the charging system, whichever is larger. Noncoincident loads, located in existing 220.60, could also be used where one or more of the larger existing loads, such as the air conditioning system or dryer, could be locked out while the single or multiple vehicle charging is accomplished. The text states, “if it is unlikely that two or more noncoincident loads will be in use simultaneously, using only the largest load(s) that will be used at one time for calculating the total load of a feeder or service shall be permitted.”

Energy management systems

Installing a new energy management system, as permitted in Article 750, is an option. New Section 220.70 could be helpful and states “If an energy management system (EMS) is used to limit the current to a feeder or service in accordance with 750.30, a single value equal to the maximum ampere set point of the EMS is permitted to be used in load calculations for the feeder or service. The setpoint value of the EMS must be considered a continuous load for the purposes of load calculations.”

An EMS can be programmed for load shedding and can have a set point, as mentioned, so it will not permit a branch circuit, feeder or service to be overloaded. Take care, however, where the EMS has an override function. An alternate power source, such as a generator, small wind turbine or photovoltaic panels can also be installed to provide power to the vehicle charger, but that adds considerable cost to the installation.

Section 220.83 can be used to determine a calculation for adding new loads to existing ones at a service for a dwelling unit. In addition, where the utility has installed a peak demand meter, it can be contacted and will provide the peak demand registered on the meter for the past year or two. Section 220.87 permits the calculation of a feeder or service load for existing installations with permission to use the actual maximum demand from the meter to determine the existing load under all of the following conditions: the maximum demand data is available for a one-year period, the maximum demand is calculated at 125% and the new load does not exceed the feeder ampacity or service rating.

There are a considerable number of methods that can be used to add an EV charging system to an existing electrical service, but most of them will require considerable expense.

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