Dwelling Service Upgrade With 48 A EVSE
A 1,800 sq ft single-family dwelling has a 120/240 V, single-phase, 100 A service. The owner is adding a 48 A EVSE and replacing a gas range with a 9.6 kW electric range. Existing loads include a 5.5 kW dryer, 4.5 kW water heater, 1.2 kW dishwasher, 0.75 kW disposal, and a 3-ton AC with nameplate 18 A at 240 V. General lighting, small-appliance, and laundry loads are wired per the NEC. The electrician proposes to keep the 100 A service and add a 60 A breaker for the EVSE. The AHJ asks for a load calculation using the standard method and, separately, the optional method for an existing dwelling. Neither calculation may ignore the EVSE continuous-load requirement. The service conductors are copper and the panel has no spare capacity.
1. Using the standard method, determine the calculated service load in volt-amperes and amperes. Is the existing 100 A service adequate?
Read the worked answer
Use NEC Article 220 Part III. General lighting: 1,800 sq ft × 3 VA = 5,400 VA. Small appliance: 2 × 1,500 = 3,000 VA. Laundry: 1,500 VA. General subtotal = 9,900 VA. Apply the general demand factor: first 3,000 VA at 100% plus the remainder at 35%: 3,000 + 0.35 × 6,900 = 5,415 VA.
Range: 9.6 kW nameplate, one range not over 12 kW, Table 220.55 demand = 8,000 VA. Dryer = 5,500 VA. Water heater = 4,500 VA. Dishwasher = 1,200 VA. Disposal = 750 VA. EVSE: 48 A × 240 V = 11,520 VA; as a continuous load use 125% = 14,400 VA. AC: 18 A × 240 V = 4,320 VA.
Total = 5,415 + 8,000 + 5,500 + 4,500 + 1,200 + 750 + 14,400 + 4,320 = 44,085 VA. Amps = 44,085 ÷ 240 = 183.7 A. The 100 A service is not adequate; the standard calculation requires a 200 A service.
2. Using the optional method of NEC 220.82, determine the calculated load and the minimum standard service rating. Which result should the electrician present, and why?
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Under 220.82, include the general loads and appliances at nameplate: lighting 5,400 + small appliance 3,000 + laundry 1,500 + range demand 8,000 + dryer 5,500 + water heater 4,500 + dishwasher 1,200 + disposal 750 + EVSE 11,520 = 41,370 VA. Apply the optional demand: first 10,000 VA at 100%, remainder at 40%: 10,000 + 0.4 × 31,370 = 22,548 VA. Add the AC at 100%: 22,548 + 4,320 = 26,868 VA. Amps = 26,868 ÷ 240 = 112 A, so the minimum standard service is 125 A.
The optional method is permitted for an existing dwelling, but both results should be shown because the standard method gives 184 A and the optional method gives 112 A. The EVSE is a large continuous load, and 625.42 still requires its branch circuit and overcurrent device to be sized at 125% of the 48 A load. A 125 A service meets the optional calculation, but a 200 A service is the practical recommendation for a 48 A EVSE and future load; the AHJ should confirm the method accepted for the permit.
3. What minimum branch-circuit rating and copper conductor size are required for the 48 A EVSE if the terminals are 75°C and the wiring method is NM-B cable?
Read the worked answer
The EVSE is a continuous load, so the branch-circuit conductors and overcurrent device must be at least 125% of 48 A: 48 × 1.25 = 60 A. Use a 60 A overcurrent device and a circuit rated 60 A.
With NM-B, the 60°C ampacity column applies. 6 AWG copper NM-B is rated 55 A at 60°C, which is below 60 A, so 6 AWG is not acceptable. 4 AWG copper NM-B is rated 70 A at 60°C, so 4 AWG copper is the minimum size. If the wiring method were THHN in conduit with 75°C terminals, 6 AWG copper at 75°C would be rated 65 A and would be acceptable; the answer changes with the wiring method.