Adding a 48 A EVSE to an Existing 200 A Dwelling Service
A 2,150 sq ft single-family dwelling has a 240/120 V, single-phase 200 A overhead service with a 200 A panelboard and a 200 A main breaker. Existing major loads are a 4.5 kW water heater, a 3-ton air conditioner with 10 kW of electric strip auxiliary heat, a 12 kW range, and a 5.5 kW dryer. The owner wants a hardwired 48 A (11.5 kW) EVSE in the attached garage, about 25 ft from the panel. To avoid a full Article 220 calculation, the contractor proposes to justify the addition under 220.87 using a 30-day demand recording made in April: the meter logged 15-minute intervals and the maximum demand was 118 A. April weather was mild; the strip heat never operated and the air conditioner cycled lightly.
1. Is the April 30-day demand recording an acceptable basis for the 220.87 calculation? Explain what the method requires and whether this data qualifies.
Read the worked answer
What 220.87 requires
- Maximum demand data from a recording over a 1-year period, or, where 1-year data is not available, a recording of at least 30 days using a demand meter that records at 15-minute intervals or less.
- The recorded maximum demand must represent the maximum usage of the dwelling.
- Calculated load = 125% of the recorded maximum demand + the new load.
- The method is subject to AHJ acceptance.
Application here
The 30-day duration and 15-minute interval are technically satisfied, but the content of the data is not. The April recording never captured the 10 kW strip auxiliary heat, which draws about 10,000 VA ÷ 240 V ≈ 41.7 A on its own and is almost certainly the governing peak in a northwest Arkansas winter. The recorded 118 A therefore does not represent maximum usage.
Conclusion: the recording is not acceptable as submitted. Either re-record across the heating season (or a full year), or add the unrecorded heating load back into the calculation. Confirm with the AHJ and verify the meter is a true demand recorder, not an averaging logger.
2. Using the 118 A figure, does the 200 A service have enough capacity for the 48 A EVSE? Show the arithmetic and state your conclusion.
Read the worked answer
220.87 arithmetic
- 125% of recorded demand: 118 A × 1.25 = 147.5 A
- New EVSE load: EVSE charging is a continuous load, so the load calc carries it at 125%: 48 A × 1.25 = 60 A
- Total: 147.5 A + 60 A = 207.5 A > 200 A → fails
Even if the AHJ allowed the EVSE to be added at its nameplate 48 A (147.5 + 48 = 195.5 A), the result passes only because the winter heating load was never in the record. Adding the 41.7 A strip heat (not a continuous load, so at 100%) puts the total far above 200 A.
Cross-check with 220.82 (optional method)
- General lighting/appliances: 2,150 sq ft × 3 VA = 6,450; 2 small appliance + 1 laundry = 4,500 → 10,950 VA → 10,000 + (950 × 40%) = 10,380 VA
- Range 12 kW (Table 220.55) = 8,000 VA; dryer = 5,500 VA; water heater = 4,500 VA
- Heating (larger of heat vs. A/C) = 10,000 VA
- EVSE = 48 A × 240 V = 11,520 VA
- Total ≈ 49,900 VA ÷ 240 V ≈ 208 A → still over 200 A
Conclusion: the 200 A service is not adequate for a full-rate 48 A EVSE without load management.
3. The service cannot carry the EVSE at full rate. Identify two or more code-compliant alternatives and the trade-offs of each.
Read the worked answer
Option 1 - Energy management (625.42/750.30)
Use a listed energy management system (EVEMS) that limits the EVSE's maximum current, or a load-management system complying with Article 750. If the EVSE is limited to, say, 24–30 A:
- 30 A setting → 30 × 1.25 = 37.5 A → 147.5 + 37.5 = 185 A, within 200 A.
- The branch circuit, breaker, and conductor are then sized to the managed current, not the 48 A nameplate, provided the equipment is identified for that use.
- Trade-off: slower charging; requires listing/instructions, AHJ acceptance, and correct commissioning so the limit can't be defeated.
Option 2 - Service upgrade
Replace with a 320/400 A meter-main and 400 A panel (or a 400 A service with a 200 A feeder to the existing panel plus a dedicated EVSE panel). Requires utility coordination, new grounding electrode conductor sizing per 250.66, load-side tap or double-lug considerations, and significantly more cost.
Option 3 - Re-record and re-analyze
Install a demand meter for a full year (or at least through the heating season) and redo 220.87 with the true peak. This usually reduces available capacity once strip heat is captured, so it is unlikely to rescue this project - but it is the honest path if the contractor insists the April data was representative.
Note on the branch circuit itself: a full-rate 48 A EVSE still needs a 60 A circuit with conductors of at least 60 A ampacity (for example 6 AWG Cu with 75°C terminations, 10 AWG minimum grounding per 250.122) and a disconnecting means per 625.43 where applicable.