Adding a 48 A EV Charger to a 125 A Service Using the Existing-Demand Method
A homeowner in a 1970s single-family home with a 125 A, 120/240 V overhead service wants a 48 A Level 2 EV charger installed in the carport. The panel is a 125 A main-breaker panel with 24 spaces, serving a 40 A range, a 30 A dryer, a 4,500 W water heater, and a 3-ton split system air conditioner. The serving utility has provided 15-minute interval demand data for the past 12 months; the maximum recorded demand is 78 A. The electrician wants to use the existing-demand method rather than a full Article 220 calculation so the service does not have to be upgraded, and the homeowner has said plainly that a service change is not in the budget.
1. Using the existing-demand method, determine whether the 48 A charger can be added to this service. Show the arithmetic and identify the rule you are applying.
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The method is 220.87, which permits the existing load to be based on measured maximum demand rather than a calculated load when utility demand data exists. The test in 220.87(2) is: 125% of the maximum demand plus the new load must not exceed the service rating.
- 78 A x 1.25 = 97.5 A
- New EVSE load at nameplate = 48 A → 97.5 + 48 = 145.5 A
- Treating the EVSE as a continuous load (125% per 625.41) gives 60 A → 97.5 + 60 = 157.5 A
Either way the result far exceeds the 125 A service rating. The 48 A charger cannot be added on the existing service. Note also that 220.87 requires the demand data to cover a one-year period (a 30-day recording is allowed only in the exception where a year of data is unavailable) and requires that the load not be increased afterward without re-evaluation.
2. What code-compliant options remain short of a full service upgrade, and what is the largest continuous EVSE current that still passes the 220.87 test?
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Solve for the allowance left: 125 A - 97.5 A = 27.5 A of new load. Dividing by the 1.25 continuous factor gives a maximum continuous charging current of about 22 A. An EVSE configured for 20 A continuous (25 A branch circuit) yields 97.5 + 25 = 122.5 A, which passes. A 24 A setting would need 30 A and fail at 127.5 A.
Other compliant paths:
- Reduce the charger setting in accordance with the manufacturer's instructions and 625.42; the branch circuit and breaker must match the setting (for 20 A continuous: 25 A circuit, 25 A OCPD).
- Energy management system (625.42) - a listed EVEMS that limits the EVSE based on the service load lets the charger stay at 48 A while the service stays within rating. This requires AHJ acceptance and often utility concurrence, and it must be documented in the permit.
- Service upgrade to 200 A, which triggers grounding/bonding review of the existing service.
Avoid the temptation to simply install a 60 A breaker on the 125 A panel with the charger turned down in software only; if the load calculation is not satisfied and the setting is not fixed and documented, the installation is not compliant.
3. If the electrician wants to rely on the utility interval data, what must be verified and given to the inspector?
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Three things make or break the 220.87 submittal:
Authenticity and period. The data must be the utility's 15-minute (or shorter) interval demand record covering a full year, showing the maximum demand. A screenshot of a smart-meter app is not a utility record. If a full year is genuinely unavailable, the exception permits a minimum 30-day continuous recording, but that recording must be made with a recording ammeter/analyzer and must capture the true peak - which means spanning the summer air-conditioning peak.
What the number represents. Confirm the 78 A is a demand value (average over the interval), not a momentary peak and not a reading taken at one conductor. For a 120/240 V service, both legs are read and the larger leg governs, not the sum.
Documentation of the conclusion. Provide the arithmetic (125% of maximum demand + new load vs. service rating), the EVSE model and its configured current, and a statement that no other load is being added. The calculation should appear on the permit documents, because the inspector is being asked to accept a measured load in place of a calculated one, and the authority to do that is discretionary, not automatic.