Existing 100 A Dwelling Service With a New 48 A EVSE
A 2,400 ft² single-family dwelling is supplied by a 120/240 V, single-phase, 100 A service. The calculated general lighting and receptacle load is based on 3 VA/ft², two 20 A small-appliance branch circuits, and one 20 A laundry circuit. The dwelling has a 12 kW electric range, a 5.0 kW electric dryer, and a 4.8 kVA central air conditioner; space heating and water heating are gas. The owner wants a 48 A Level 2 EVSE installed in the attached garage on a 60 A branch circuit. The utility transformer and meter socket are existing. The contractor proposes to keep the 100 A service and add the EVSE because the panel has spare spaces. The local inspector has asked for the load calculation before approving the permit.
1. Using the standard method, what is the calculated demand load before the EVSE, and what minimum service rating does it require? Show the steps.
Read the worked answer
General lighting and receptacles
- General lighting: 2,400 ft² × 3 VA/ft² = 7,200 VA
- Small appliance: 2 × 1,500 VA = 3,000 VA
- Laundry: 1,500 VA
- Subtotal = 11,700 VA
Demand factor, NEC 220.42
- First 3,000 VA at 100% = 3,000 VA
- Remainder: 11,700 − 3,000 = 8,700 VA at 35% = 3,045 VA
- General lighting demand = 6,045 VA
Major appliances
- 12 kW range, Table 220.55, one range: 8,000 VA
- 5.0 kW dryer, Table 220.54: 5,000 VA
- Central A/C: 4,800 VA
Total calculated load
6,045 + 8,000 + 5,000 + 4,800 = 23,845 VA
23,845 VA ÷ 240 V = 99.4 A
The minimum standard service rating is 100 A. The existing 100 A service is therefore at its calculated limit before the EVSE, with essentially no spare capacity.
2. What minimum standard service rating is required after adding the 48 A EVSE, and what branch-circuit conductors and overcurrent device are required? Explain the continuous-load treatment.
Read the worked answer
The EVSE is a continuous load under NEC 625.41, so the service and branch-circuit calculation must use 125% of the EVSE current.
- EVSE load: 48 A × 240 V = 11,520 VA
- Continuous-load value: 11,520 VA × 1.25 = 14,400 VA
- New total: 23,845 + 14,400 = 38,245 VA
- 38,245 VA ÷ 240 V = 159.4 A
The next standard service rating is 200 A. A 150 A service is not large enough because 159.4 A exceeds 150 A. The contractor's plan to keep the 100 A service fails the calculation even before considering spare breaker spaces.
Branch circuit
- Required ampacity: 48 A × 1.25 = 60 A
- Overcurrent device: 60 A
- Copper conductors with at least 60 A ampacity at the termination temperature: 6 AWG copper, 75°C column, 65 A
- Equipment grounding conductor: 10 AWG copper
If the EVSE is cord-and-plug connected, GFCI protection is required for the receptacle under NEC 625.54. The branch-circuit conductors, breaker, and service must all be evaluated, not just the panel spaces.
3. If the utility will not upgrade the service, what code-recognized approaches could allow the EVSE, and what data would you need?
Read the worked answer
There are two practical approaches.
Existing-load determination under NEC 220.87. Record the existing maximum demand over a 30-day period with a demand meter, then add the new EVSE load. The calculation must show that the existing service can carry the recorded demand plus the new continuous load. You need the utility or a qualified party to provide the 30-day maximum-demand data, and the calculation must account for the EVSE at 125% because it is continuous.
Energy management under NEC 625.42. A listed energy-management system can monitor the service and limit the EVSE so the total service load does not exceed the service rating. The system must be listed for the purpose, set to the calculated limit, and not user-adjustable above that limit. This is not the same as simply turning the EVSE down manually.
Neither approach makes the 100 A service adequate by itself. The first depends on measured data; the second depends on a listed control system. The inspector will require documentation for whichever method is used.