Service Capacity for a Heat Pump and EV Charger Addition
A 2,400 sq ft single-family dwelling has a 200 A, 120/240 V single-phase service with 4/0 Al service-entrance conductors and a 200 A panel with six open spaces. The owner adds a 48 A (11.52 kW) hardwired EVSE in the garage and replaces a gas furnace with a heat pump: outdoor unit nameplate 4,800 VA, plus 15 kW of electric auxiliary heat. The thermostat is field-configured to lock out the auxiliary heat whenever the compressor runs. Existing loads are a 12 kW range, 5.5 kW dryer, 4.5 kW water heater, 1.2 kVA dishwasher, 900 VA disposer, and 1,000 VA built-in microwave, along with standard lighting and small-appliance circuits. The electrician's calculation applies Table 220.55 to reduce the range to 8 kW and lands at 148 A, so he tells the owner no service upgrade is needed.
1. Redo the calculation using the optional method for a dwelling unit and state the service demand in amps. Was the electrician's method correct?
Read the worked answer
Method was wrong even though the answer happens to be close; here is the correct approach.
Step 1 – 220.82(B) general load (nameplate values, no Table 220.55 reduction):
| Item | VA |
|---|---|
| Lighting, 2,400 sq ft x 3 VA | 7,200 |
| Small appliance, 2 x 1,500 | 3,000 |
| Laundry | 1,500 |
| Range (nameplate) | 12,000 |
| Dryer | 5,500 |
| Water heater | 4,500 |
| Dishwasher | 1,200 |
| Disposer | 900 |
| Microwave | 1,000 |
| EVSE, 48 A x 240 V | 11,520 |
| Total | 48,320 |
Demand: first 10,000 at 100% + 40% of the remainder = 10,000 + 0.40(38,320) = 25,328 VA.
Step 2 – 220.82(C) heating/cooling. The heat pump is not added in (B); it is handled here only. Cooling = 4,800 VA. Heating under (C)(2) = compressor + 65% of supplemental = 4,800 + 0.65(15,000) = 14,550 VA. The larger of heating vs. cooling governs: 14,550 VA.
Step 3 – Total: 25,328 + 14,550 = 39,878 VA / 240 V = 166 A.
The 200 A service is adequate. Note the judgment point: because the thermostat locks out the aux heat, (C)(2) would allow dropping the compressor and using 0.65(15,000) = 9,750 VA, giving 146 A - but that depends on documented, verifiable lockout control, so the conservative 166 A is the number to carry unless the AHJ accepts the manufacturer's sequence-of-operation literature.
2. The inspector says the EVSE must be treated as a continuous load at 125% in the service calculation. Recompute and state whether the service still passes.
Read the worked answer
It still passes, with margin.
Treating the EVSE as continuous (625.42) puts 48 A x 1.25 = 60 A, or 60 A x 240 V = 14,400 VA in place of 11,520 VA, an increase of 2,880 VA.
- Revised (B) total: 48,320 + 2,880 = 51,200 VA
- Demand: 10,000 + 0.40(41,200) = 26,480 VA
- Add the governing heating load of 14,550 VA = 41,030 VA
- 41,030 / 240 = 171 A against a 200 A service (85%).
Why this is a judgment call worth flagging: the 40% factor in 220.82(B) is applied to the appliance total, so whether the 125% continuous multiplier is applied before the demand factor (conservative, 171 A) or the EVSE is taken at nameplate inside the (B) total (166 A) changes the number by about 5 A. Both pass here. The important point is that the conclusion is stable - this service does not need upgrading for either the heat pump or the EV charger - so the argument should be over documentation, not over the outcome.
Also verify the service conductors themselves: 4/0 Al on a 200 A single-phase dwelling service is permitted by Table 310.12, so no conductor change is triggered by the added load.
3. Size the branch circuit for the 48 A EVSE. What wiring methods work and which ones fail?
Read the worked answer
Load side: 48 A continuous x 125% = 60 A minimum circuit ampacity; a 60 A overcurrent device is the standard selection.
Conductors must have 60 A ampacity after any adjustment.
| Option | Ampacity | Verdict |
|---|---|---|
| 6 AWG Cu NM-B (60 C column, 334.80) | 55 A | Fails |
| 4 AWG Cu NM-B | 70 A | Works |
| 6 AWG Cu THHN/THWN-2 in conduit, 75 C terminations | 65 A | Works |
| 6 AWG Cu MC cable, 75 C | 65 A | Works |
The trap is the common habit of running 6 AWG NM-B for a 60 A circuit. Because 334.80 limits NM cable to the 60 C column, 6 AWG NM-B is only good for 55 A and cannot serve a 60 A EVSE circuit. Either upsize to 4 AWG NM-B, switch to a 75 C wiring method, or commission the EVSE at 40 A output (50 A circuit), which 6 AWG NM-B does support.
Other items:
- A disconnecting means is required only where the EVSE is rated more than 60 A or more than 150 V to ground (625.43); this 48 A/240 V unit does not require a separate disconnect beyond the breaker.
- A hardwired EVSE does not require a receptacle GFCI device; verify the unit's integral CCID requirement in its listing instructions (110.3(B)).
- The EVSE must be listed for the location; cord-and-plug installations have separate receptacle and GFCI rules.