Will my panel handle a heat pump, or do I need a service upgrade?
Usually the compressor is not the problem; the backup strip heat is. For a 1,900 ft² house on a 150 A service with an electric range, dryer and water heater, replacing the gas furnace and 3-ton air conditioner with a 3-ton heat pump and 10 kW of strip heat calculates to 129.7 A under NEC 220.83(B), which fits with 20.3 A to spare. With 15 kW of strip heat the same house reaches 150.5 A and needs an upgrade; with the compressor interlocked so it cannot run with the strips, 10 kW calculates to 114.7 A. The heat pump’s strip heat rating is the number to negotiate with the installer.
Updated September 23, 2026. Numbers on this page are computed by the same engine as the calculators, under NEC 2023.
Worked example: 1,900 ft² house on 150 A, heat pump with 10 kW strip heat
Calculated per NEC 2023Existing dwelling method (220.83)
Inputs
| NEC edition | NEC 2023 |
|---|---|
| Floor area | 1,900 ft² |
| Small-appliance circuits | 2 |
| Laundry circuits | 1 |
| Cooking | Electric range 10 kW |
| Clothes dryer | 5,000 VA |
| Appliances fastened in place | Electric water heater 4,500 VA; Dishwasher 1,200 VA; Garbage disposal 900 VA |
| Heating | Gas or none (no electric heating load) |
| Air conditioning | 3,600 VA |
| Existing service | 150 A, 120/240 V |
| Being removed | Gas furnace and 3,600 VA air conditioner |
| Being added | Heat pump, 3,600 VA compressor with 10,000 VA supplemental strip heat, not interlocked |
Result
| Calculated load before the change | 18,960 VA79.0 A, with the air conditioner |
|---|---|
| Calculated load with the heat pump | 31,120 VA129.7 A at 240 V |
| Verdict on 150 A | Fits20.3 A to spare |
| Line item | NEC | Connected | Demand | Basis |
|---|---|---|---|---|
| General lighting and receptacles (existing) | 220.41 | 5,700 VA | (in subtotal) | 1,900 ft² × 3 VA/ft² |
| Small-appliance and laundry circuits | 220.52 | 4,500 VA | (in subtotal) | 3 circuits × 1500 VA |
| Existing appliances, ranges, dryers and other loads | 220.83 | 21,600 VA | (in subtotal) | At nameplate |
| Existing EV supply equipment | 220.83 | — | (in subtotal) | None |
| Air-conditioning or central / fewer-than-four-unit space heating at 100% | 220.83(B) | 17,200 VA | 13,600 VA | Larger of cooling 3,600 VA and heating 13,600 VA (220.60); added heat pump 3,600 VA + supplemental 10,000 VA, cooling 3,600 VA |
| Added appliances and other loads | 220.83(B) | — | (in subtotal) | None |
| Added EV supply equipment | 220.83 | — | (in subtotal) | None |
| All other loads after demand factors | 220.83(B) | 31,800 VA | 17,520 VA | First 8,000 VA at 100%; remaining 23,800 VA at 40% = 9,520 VA |
Because heating is being added, 220.83(B) takes the heat pump and its strip heat at 100% (13,600 VA) and only the other loads through the 8 kVA / 40% tiers. The house goes from 79.0 A to 129.7 A, still inside the 150 A service.
- The heat pump compressor doubles as the cooling load, so the old air conditioner drops out of the calculation rather than stacking with it.
- Under 220.83(B) the added air conditioning and central or fewer-than-four-unit heating are compared under 220.60 and the larger is taken at 100%.
Same house, three strip-heat sizes and an interlock
Calculated per NEC 2023Existing dwelling method (220.83)
Inputs
| Compressor | 3,600 VA (3-ton) in every case |
|---|---|
| Existing loads | 18,960 VA (79.0 A) calculated before the change |
Result
| 5 kW strip heat | 108.8 A · fits26,120 VA calculated |
|---|---|
| 10 kW strip heat | 129.7 A · fits31,120 VA calculated |
| 15 kW strip heat | 150.5 A · over36,120 VA calculated |
| 10 kW strip heat, compressor interlocked | 114.7 A · fitsStrips cannot run with the compressor (220.82(C)(3) treatment in the added-heating line) |
Up to 10 kW of strip heat fits this 150 A service; 15 kW takes it to 150.5 A. Each 5 kW of strips adds 20.8 A because added heating counts at 100%.
Why heat pumps trip the calculation and air conditioners did not
A 3-ton heat pump compressor draws about what the 3-ton air conditioner it replaces drew, so on its own it is nearly a wash. What changes is the auxiliary heat: most air-source heat pumps ship with 5 to 20 kW of electric resistance strips for defrost and cold snaps, and the NEC counts those at nameplate. Under the existing-dwelling method for an added heating load, 220.83(B), that nameplate goes in at 100%, not through the 40% tier.
On this house the compressor plus 10 kW of strips is 13,600 VA of heating at 100%. That single line is larger than the entire existing appliance load after demand factors, which is why the calculation moves 50.7 A while the compressor alone would have moved it almost nothing.
Ways to make it fit without a new service
If the calculation lands over the service rating, the installer has options short of a service upgrade:
- Specify less strip heat. In milder climates 5 kW is common; a cold-climate heat pump sized to the house can often run without strips at all.
- Interlock the strips with the compressor so the two cannot run together. The calculation then takes the larger of the two rather than their sum; on this house that is the difference between the 10 kW rows above.
- Use a load-management or energy-management system (NEC 750) that sheds the strips or another large load when the service is near its limit.
What the electrician will actually run
Exactly this method, on your nameplates. The Pro load calculator’s existing-dwelling mode takes the house as it is, the loads being removed and the loads being added, and prints the 220.83 line items for the permit. Under NEC 2026 the same check is 120.83 with a single set of tiers, and the calculator runs that edition too.
Frequently asked questions
- Does the old air conditioner still count?
- Not if it is removed. The heat pump compressor becomes the cooling load, and 220.60 compares heating and cooling and carries the larger. Keeping a separate air conditioner in addition to the heat pump would add its nameplate back in.
- What is "interlocked" strip heat?
- A control that prevents the resistance strips from energizing while the compressor runs (except during defrost on some units). When it is present the NEC allows the heating load to be the larger of compressor or strips rather than their sum. Ask the installer whether the air handler supports it.
- Would a 100 A service take this heat pump?
- Not with 10 kW of strips: the house calculates to 129.7 A. Even the existing loads alone are 79.0 A on a 100 A basis, so a 100 A house making this change almost always needs an upgrade or a strip-free cold-climate unit.
- Does a heat pump water heater count the same way?
- No. A heat pump water heater is an appliance fastened in place, not space heating, so it goes into the 8 kVA / 40% tiers with the other appliances. See the water-heater answer for the numbers.
Related questions
- 100 A vs 200 A service: which one does my house need?The same 1,600 ft² house calculated three ways under NEC 2023: gas appliances, all-electric, and all-electric with an EV charger. The optional and standard methods both shown, with the minimum service each requires.
- Electric water heater vs gas: what does it do to my panel?The same 1,700 ft² house with a gas water heater, a 4,500 W electric tank and a 120 V heat pump water heater, calculated under the NEC 2023 optional and standard methods, with the amps each option adds.
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