Adding a 48 A EVSE to a 100 A Dwelling Service
A 1,800 sq ft single-family dwelling is served by a 120/240 V, single-phase, 100 A service. The cooking range is 12 kW electric, the clothes dryer is 5 kW electric, and space heating and water heating are gas. General lighting, small-appliance, and laundry circuits are typical for the size of the house. The owner wants a 48 A (11.5 kW) EVSE installed in the attached garage on a dedicated 60 A branch circuit. The helper says, "Nobody has ever tripped that main in twenty years, so you're fine." You have to determine the calculated demand load with and without the EVSE, then advise the owner on options grounded in Article 220 and Article 625 rather than in folklore.
1. Using the standard calculation method, compute the existing dwelling demand load in VA and amps, add the EVSE, and state whether the 100 A service is adequate.
Read the worked answer
Existing load (Part III of Article 220)
| Item | Basis | VA |
|---|---|---|
| General lighting | 1,800 ft² × 3 VA | 5,400 |
| Small-appliance | 2 × 1,500 VA (220.52(A)) | 3,000 |
| Laundry | 1,500 VA (220.52(B)) | 1,500 |
| Subtotal | 9,900 | |
| First 3,000 VA @ 100% | 220.42 | 3,000 |
| Remainder 6,900 VA @ 35% | 220.42 | 2,415 |
| Lighting/appliance demand | 5,415 | |
| Range, 12 kW | Table 220.55, Column C, one range ≤12 kW | 8,000 |
| Dryer, 5 kW | 220.54 (5,000 VA minimum) | 5,000 |
Existing demand = 5,415 + 8,000 + 5,000 = 18,415 VA → 76.7 A at 240 V.
Adding the EVSE
625.41 requires the branch circuit to be sized at 125% of the maximum load: 48 A × 1.25 = 60 A (a 60 A circuit, 14,400 VA). The EVSE load is continuous, so it is taken at 60 A in the service calculation.
New total = 18,415 + 14,400 = 32,815 VA → 136.7 A.
Conclusion: 136.7 A exceeds the 100 A service rating, so the existing service does not comply with the addition. The helper's "it never trips" reasoning is wrong - the main is not a load-measuring instrument, and the calculated load is what Article 220 requires you to use.
2. The contractor proposes to skip a service upgrade by recording 30 days of demand data. The recording shows a peak 15-minute demand of 38 A. Is that acceptable under 220.87, and what would the recorded demand have to be to pass?
Read the worked answer
220.87 permits using actual maximum demand for an existing service, but only with both conditions met:
- Demand data exists for a 1-year period, or (exception) a continuous 15-minute-interval recording over at least 30 days on the highest-loaded phase, and
- 125% of that maximum demand plus the new load does not exceed the rating of the service.
Apply condition 2:
- 38 A × 1.25 = 47.5 A
- 47.5 A + 60 A (EVSE circuit) = 107.5 A
- 107.5 A > 100 A → not acceptable.
Solving for the break-even demand: 1.25D + 60 ≤ 100 → D ≤ 32 A. A 38 A recording is too high; the measured peak would have to be 32 A or less to survive the 125% multiplier.
The lesson is that the 125% factor in 220.87 is what trips people up - a demand reading that looks comfortably under the service rating can still fail once the new continuous load is added.
3. The owner does not want a service upgrade. What code-compliant alternatives exist, and what is the trade-off of each?
Read the worked answer
Option 1 - Energy management system (625.42). An EMS that limits the EVSE's current can reduce the load used in the calculation. Back-solve for the largest EVSE circuit that still fits: 100 A service × 240 V = 24,000 VA capacity; existing 18,415 VA leaves 5,585 VA → 23.3 A of circuit. So an EVSE on a 20 A circuit (16 A output, 4,800 VA) yields 18,415 + 4,800 = 23,215 VA = 96.7 A, which complies. Trade-off: much slower charging, and the controller must be listed and set so the EVSE cannot draw more than the calculated allowance.
Option 2 - 220.87 with a qualifying recording. Only if the measured peak demand is 32 A or less, as shown above. Trade-off: requires a full year of data or a legitimate 30-day recording on the highest-loaded phase, plus owner permission and documentation the AHJ will accept.
Option 3 - Service upgrade to 200 A. The cleanest and most future-proof answer: 136.7 A fits a 200 A service with margin for a heat pump or second EV later. Trade-off: cost and utility coordination.
What is not an option is installing the 60 A EVSE circuit on the 100 A service on the assumption that the main will not trip. A 48 A continuous load on a 100 A service with an existing 76.7 A calculated demand is a foreseeable overload, not an inconvenience.