Service Upgrade With Heat Pump and EV Charger
An 1,800 ft² single-family dwelling has a 100 A overhead service and a 24-space panel. The owner is adding a 3-ton heat pump with nameplate 240 V, compressor RLA 24 A, outdoor fan 1/6 hp, MCA 28 A, maximum overcurrent device 40 A; a 48 A Level 2 EV charger; an 8 kW electric range; a 5 kW dryer; and a 4.5 kW water heater. Existing general lighting and receptacles are unknown, but the panel schedule shows 12 existing 15/20 A circuits. The utility will not increase the overhead drop until the calculated load is documented. The contractor proposes a 200 A service and says the existing 100 A panel can be reused as a subpanel. The service disconnect is currently at the meter, 35 ft from the panel, and the water pipe is metal with 8 ft in contact with earth.
1. Before concluding that a 200 A service is adequate, what load-calculation steps and demand factors must be documented? Identify the data you still need.
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Use the standard method unless the optional method is clearly allowed and documented. Steps: compute general lighting at 3 VA/ft² for 1,800 ft², add small-appliance and laundry branch circuits at 1,500 VA each, apply the 220.42 demand factors, add the range at 220.55, dryer at 220.54, water heater at nameplate, and the larger of heating or cooling at 100%. The heat pump MCA 28 A at 240 V is about 6.72 kVA; the 48 A EV charger is a continuous load at 240 V = 11.52 kVA, and its branch circuit must be sized at 125% (60 A). The range and dryer nameplates must be used, not assumed. You also need the existing connected load and any 220.87 demand data if using the existing service. A 200 A service may be adequate, but do not conclude until the calculation, conductor ampacity, meter/main, and panel bus rating are checked.
2. If the existing 100 A panel becomes a subpanel, what grounding, bonding, and service-disconnect requirements apply?
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If the existing 100 A panel becomes a subpanel, it must be fed with a 4-wire feeder: two ungrounded, one neutral, and one equipment grounding conductor. In the subpanel, the neutral bar must be isolated from the enclosure and from the equipment ground; remove the main bonding jumper and land the EGC on a separate ground bar. The grounding electrode conductor and main bonding jumper stay at the service. The metal water pipe is a grounding electrode only if it has at least 10 ft in contact with earth; the GEC must connect within 5 ft of the pipe's entry and be supplemented by another electrode, such as a ground rod. Bond around the water meter and any insulating joints. The service disconnect must be readily accessible and located at a readily accessible point nearest the point of entry, or as permitted by the adopted code and utility; 35 ft may be acceptable only if the authority agrees.
3. Can the 48 A EV charger be set lower or use load management to avoid a service upgrade? Explain the conditions.
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Yes, if the EVSE is listed and adjustable, setting it to 40 A continuous reduces the service load to 9.6 kVA and the branch circuit to 50 A. Use a 50 A circuit with 6 AWG copper or as the terminals allow, and set the EVSE maximum in hardware/firmware, not just a label. If the calculated load is still too high, a listed energy-management system can limit the charger based on the service load, but it must be installed per its listing and the adopted code. The load calculation must use the actual limited input, and the setting must be documented and not easily changed by the homeowner. Do not simply install a 48 A charger on a 50 A circuit.