Existing dwelling load calculation: can the service take the new load?

What the existing-dwelling method is for

The standard dwelling calculation is built for new construction and tends to produce a big number. For a dwelling that is already built, the Code gives a shorter optional method that applies to an existing 120/240 V or 208Y/120 V, 3-wire service or feeder. You use it to answer one question: with the new load added, is the calculated load still at or under the service rating? It is the right tool for the job when someone asks "can I add a load to my existing service" before the permit, not after the breaker trips.

It does not size the new circuit, and it does not replace a utility load study or what your AHJ requires. It only checks the service. For the conductors, use the wire size calculator.

NEC 2023 220.83 became NEC 2026 120.83

The same calculation has a new number and a different structure. In the NEC 2023 it is 220.83; in the NEC 2026 it is 120.83. Check which edition your jurisdiction has adopted before you use either set of numbers.

Two differences matter in practice. First, the 2023 edition uses 3 VA per square foot for general lighting and the 2026 edition uses 2, so the same house starts from a smaller number in 2026. Second, 2023 has two methods: Table 220.83(A) when you are not adding air conditioning or electric space heating, and Table 220.83(B) when you are. In 2026 one table covers every case, and it gives a new EV charger 80 percent, new electric resistance heat 80 percent and other new loads 50 percent.

The steps, as the calculator runs them

1. General lighting and receptacles: floor area times 2 VA per ft² (2026) or 3 VA per ft² (2023).
2. Small-appliance and laundry circuits: 1,500 VA for each small-appliance circuit and each laundry circuit. A dwelling needs at least two small-appliance circuits (210.11(C)(1)).
3. Nameplate ratings of the other fixed appliances: range, dryer, water heater, dishwasher and the like.
4. Heating and air conditioning: count the larger of the two, never both. When the two are equal, the calculator counts heating.
5. Add the new loads (EV charger, new heat pump, new range) in VA: amps times volts.
6. Apply the table: the first 8 kVA counts in full, then each part above that counts at its percentage. As the calculator reads it, the 8 kVA is taken from the existing load first (the Code table does not say in what order); each line is rounded each line to whole VA.
7. Convert to amps and compare with the service rating.

Load amps = total demand VA / volts
Spare amps = service rating - load amps
Fits when load amps <= service rating

In 2023 method A the whole pool, new loads included, is 100% of the first 8 kVA and 40% of the rest. In method B the larger of air conditioning or heating is taken on its own line at its table row (100% for air-conditioning equipment, 100% for central electric space heating), and everything else goes through that same 8 kVA and 40% split. The calculator always takes heating at 100 percent in method B because it does not ask how many separately controlled heating units there are. That errs on the safe side.

Example 1: a 48 A EV charger on a 100 A service

The house is 2,000 ft² with two small-appliance circuits and one laundry circuit, a 12,000 VA range, 5,000 VA dryer, 4,500 VA water heater, 1,200 VA dishwasher and 3,500 VA of air conditioning, on a 100 A, 240 V service. The charger is 48 A at 240 V, which is 11,520 VA. Lighting alone is 2,000 ft² x 2 = 4,000 VA in 2026. These sizes are example inputs, not Code values.

2026 before the charger: 77.83 A, within the 100 A rating
2026 with the charger: 27,896 VA / 240 V = 116.23 A, over the 100 A rating
2023 before the charger: 81.17 A, within the 100 A rating (Table 220.83(A))
2023 with the charger: 24,088 VA / 240 V = 100.37 A, over the 100 A rating (Table 220.83(A))

The two editions do not give the same answer for the same house. Before the charger, the house is 77.83 A under 2026 and 81.17 A under 2023, a difference that comes from the lighting figure (2 versus 3 VA per ft²) and from how the two editions treat the existing load. With the charger it is 116.23 A under 2026 and 100.37 A under 2023: 2026 counts a new charger at 80 percent, while the 2023 table has no charger row and counts it with all other loads at 40 percent above the first 8 kVA. Which edition comes out higher depends on the house.

Example 2: a 200 A service with electric heat

Now a 2,800 ft² house on a 200 A service with 15,000 VA of existing electric resistance heat, 4,000 VA of air conditioning, the same range, dryer, water heater and dishwasher, and a 40 A charger (9,600 VA). Only the larger of heating and cooling counts, so the air conditioning is listed but left out of the total.

2026 before the charger: 99.67 A, within the 200 A rating
2026 with the charger: 31,600 VA / 240 V = 131.67 A, within the 200 A rating
2023 before the charger: 104.33 A, within the 200 A rating (Table 220.83(A))
2023 with the charger: 28,880 VA / 240 V = 120.33 A, within the 200 A rating (Table 220.83(A))

Spare capacity with the charger is 68.33 A in 2026 and 79.67 A in 2023. Enter existing electric heat in the heating field, not as a nameplate appliance. As an appliance it would still be counted in the 40 percent remainder, but the larger-of-heating-or-cooling rule would be skipped and the air conditioning would be counted as well.

When the answer is "over the rating"

If the load comes out above the service rating, the new load does not fit as entered. The usual answers are a service upgrade or load management; an installer and the AHJ decide whether a power control or energy management route is allowed. In the first house the result is over the 100 A rating, so the calculator's next common size is 125 A. The calculator's size ladder is a list of common residential ratings, not an NEC requirement, and your utility has a say.

If you do upgrade, the service conductors are the next question. The 200 amp service wire size and 100 amp service wire size pages show the conductors for a dwelling service, and the wire size calculator covers other cases.

This is an estimate for planning. It does not replace the edition your AHJ enforces, local amendments, or the utility load study.