Adding an EV Charger to a 100 A Dwelling Service
A 2,400 sq ft single-family dwelling is served by a 120/240 V, single-phase, 100 A overhead service with 2 AWG aluminum service-entrance conductors. The owner is finishing a remodel and wants four loads energized at once: a 9.6 kW free-standing range, a 4.5 kW storage-type water heater, a 5.0 kW clothes dryer, and a 48 A (11.5 kW) Level 2 EVSE in the attached garage. The existing 24-space load center has four spare spaces. There is no central air conditioning; the only fixed heating is a gas furnace with a 1/8 hp blower. Two 20 A small-appliance circuits and one laundry circuit exist. The owner's plan is to 'just add a quad breaker' and keep the existing service.
1. Using the NEC Article 220 optional method, calculate the service load and state whether the existing 100 A service is adequate.
Read the worked answer
General load - 220.82(B)(1)
- Lighting/general-use: 2,400 sq ft × 3 VA = 7,200 VA
- Two small-appliance circuits: 3,000 VA
- Laundry circuit: 1,500 VA
- Subtotal: 11,700 VA
Apply the optional-method demand factor: 100% of the first 10,000 VA + 40% of the remainder.
10,000 + (1,700 × 0.40) = 10,680 VA
Other loads - 220.82(B)(2), at nameplate
- Range: 9,600 VA
- Water heater: 4,500 VA
- Dryer: 5,000 VA
- EVSE: 11,520 VA × 1.25 (continuous load, 625.41) = 14,400 VA
- Subtotal: 33,500 VA
No air-conditioning load to add under 220.82(C).
Total = 10,680 + 33,500 = 44,180 VA 44,180 ÷ 240 V = 184 A
A 100 A service is not adequate by calculation. The calculated load exceeds the service rating by more than 80%, so the service must be upgraded (a 200 A service is the practical answer) or the load must be reduced/managed and verified by another recognized method.
2. The owner produces 12 months of utility interval data showing a peak demand of 7.0 kW. Does that change the answer? Show the calculation and state the conditions attached to that method.
Read the worked answer
Yes - this invokes 220.87 (determining existing loads), which is the realistic route for an existing dwelling whose actual demand is far below a worst-case calculation.
- Existing demand × 125%: 7.0 kW × 1.25 = 8.75 kW
- New load (EVSE at 125%): 11.52 kW × 1.25 = 14.4 kW
- Total: 23.15 kW ÷ 240 V = 96.5 A - within the 100 A service rating.
Conditions that must be satisfied before relying on this:
- The demand data must cover at least a 1-year period for the existing load (a 30-day recording is allowed only where the facility has not been occupied for a full year), and it must be the maximum demand - not an average.
- The new load is added at 125% where it is continuous, as the EVSE is.
- The existing service equipment must be in good condition and listed for the application; the 2 AWG aluminum service conductors remain acceptable for a 100 A dwelling service only through the 310.15(B)(7) 83% allowance (83 A minimum vs. 90 A ampacity at 75 °C).
- The panel must have the physical spaces and the bus must be rated for the added breaker; a quad breaker must not violate the panel's listing.
If the data cannot be produced, the 220.82 answer (184 A) governs and the service is upgraded.