Level 2 EVSE on a 100 A Dwelling Service
A 2,000 sq ft single-family dwelling in Connecticut has a 120/240 V, single-phase, 100 A overhead service. The owner wants to add a hardwired Level 2 EVSE in the attached garage, 15 ft from the panel. The unit is capable of 48 A output but will be set to 40 A continuous. Existing loads are a 12 kW range, 4,500 W water heater, 5,500 W dryer, 1,500 W dishwasher, 1,000 W disposal, and 6,000 VA central air conditioning; there is no electric space heat. The panel has no spare spaces, and the utility transformer is shared with two neighbors. The homeowner asks whether the existing service can stay and what the least expensive compliant path is.
1. Using the optional method of Article 220, calculate the dwelling demand load both without and with the EVSE, and state whether the existing 100 A service is adequate.
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Step 1 - General loads (220.82(B)):
- General lighting: 2,000 ft² × 3 VA/ft² = 6,000 VA
- Small appliance: 2 × 1,500 = 3,000 VA
- Laundry: 1,500 VA
- Subtotal = 10,500 VA; first 10,000 VA at 100%, remainder at 40%
- Demand = 10,000 + (0.40 × 500) = 10,200 VA
Step 2 - Appliances at nameplate (220.82(B)): 12,000 (range) + 5,500 (dryer) + 4,500 (water heater) + 1,500 (dishwasher) + 1,000 (disposal) = 24,500 VA
Step 3 - Largest of heating/cooling (220.82(C)): A/C at 100% = 6,000 VA (no electric heat to compare).
Step 4 - Subtotal before EVSE: 10,200 + 24,500 + 6,000 = 40,700 VA ÷ 240 V = 169.6 A
Step 5 - Add the EVSE: EVSE is a continuous load and is added at 100% of nameplate with no 40% reduction: 40 A × 240 V = 9,600 VA. Total = 40,700 + 9,600 = 50,300 VA ÷ 240 V = 209.6 A
Conclusion: The existing 100 A service is not adequate even before the EVSE is added - the calculated load is already roughly 170 A. Adding the charger pushes it to about 210 A, so a service upgrade to at least 225 A (or one of the alternatives below) is required. Do not rely on the fact that the house 'has never tripped' - the calculation governs, and 220.87 load-study data is the only recognized way to substitute measured demand for calculated load.
2. The homeowner does not want to pay for a service upgrade. What options exist, and how would you rank them?
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NEC 220.87 measured-load study (best first step). Record the actual maximum demand over a representative period (typically one year of utility data, or 30 days with the utility's consent), then take 125% of the maximum demand and add the new EVSE load. Many homes of this vintage measure far below their calculated load, and this can sometimes keep the 100 A service.
EV energy management system, 625.42(B). An EMS that sheds or limits the EVSE so the total service load cannot exceed the service rating allows the charger to be installed without an upgrade. This is usually the least expensive compliant answer, but it must be listed for the purpose, set so the charger is curtailed, and accepted by the AHJ.
Reduce the EVSE setting. Dropping from 40 A to 24 A or 16 A continuous reduces the added load proportionally and may fit within the measured demand. The vehicle still charges overnight in most cases.
Service upgrade. 209.6 A requires the next standard rating, 225 A, with service conductors sized per Table 310.16 and the 310.12 allowance for 120/240 V single-phase dwellings (83% of the service rating, next higher standard OCPD permitted). This is the most expensive option.
Judgment: Never present the upgrade as the only option without first running the 220.87 study - but never sign off on the existing service on the basis of 'it has always worked,' either.
3. The homeowner later decides to run the unit at its full 48 A output. What changes in the branch circuit, and what code rules govern the wiring method and protection?
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Branch circuit sizing (625.42, 210.19(A)(1)): EVSE is a continuous load, so the branch-circuit conductors and overcurrent device are sized at 125% of the maximum output.
- 48 A × 1.25 = 60 A branch circuit - 60 A OCPD and conductors with at least 60 A ampacity: 6 AWG copper at 75 °C (65 A) or 4 AWG aluminum.
- At the original 40 A setting: 40 × 1.25 = 50 A circuit; 8 AWG copper at 75 °C (50 A) would suffice.
Other requirements:
- 625.40 requires an individual branch circuit - no sharing with garage receptacles.
- 625.17 governs conductor and equipment sizing; the equipment grounding conductor must be run with the circuit conductors.
- 625.54 requires GFCI protection where the EVSE is cord-and-plug connected to a receptacle. A hardwired unit does not need a GFCI receptacle, because the listed EVSE provides its own personnel protection (CCID).
- The disconnecting means and instructions must be per the listing (625.23/625.26); the nameplate rating and the setting must match.
Practical note: If the service was kept at 100 A with an EMS, raising the EVSE to 48 A will require re-checking the EMS setpoint and the 220.87 calculation - the higher setting may blow the budget the EMS was designed around.