How to do an NEC service load calculation, start to finish
A load calculation answers one question: how big does the service (or feeder) have to be for this building? The inspector will ask for it with the permit application, the utility will ask for it before they set a transformer, and the owner will ask why you're quoting a 400 A service when the neighbor has 200 A. The free worksheet above does the math for you. This guide explains what goes on each line, which NEC sections apply, and the mistakes that get load calcs kicked back at plan review.
One thing up front: use the NEC edition your AHJ adopted. The 2023 NEC reorganized Article 220 — the dwelling 3 VA per square foot moved from Table 220.12 to 220.41, the non-dwelling unit load table became Table 220.42(A), the lighting demand factors moved to Table 220.45, and non-dwelling receptacle demand moved from 220.44 to 220.47. The method didn't really change, but the section numbers did, and plan reviewers notice. Every line in the worksheet shows both the 2023 and 2020 numbers.
Standard method for a dwelling (Article 220, Part III)
The standard method applies a separate rule to each kind of load, then adds them up. Here's the order the worksheet follows.
1. General lighting, small-appliance and laundry circuits
Measure the floor area from the outside dimensions. Leave out open porches and unfinished spaces that can't be adapted for future use. Note that the 2023 NEC no longer excludes garages in 220.5(C) — if you're on 2023, the garage counts. Multiply by 3 VA per square foot (220.41 in 2023, Table 220.12 in 2020). Add 1,500 VA for each small-appliance branch circuit — at least two — and 1,500 VA for the laundry circuit (220.52). Bathroom circuits are not added.
Then apply the lighting demand factors: for a dwelling, the first 3,000 VA at 100% and the next block at a reduced percentage, with a further step for very large loads. The worksheet pre-fills the first two steps with their table reference and leaves the top step for you to enter — look it up in our NEC general lighting load guide or your code book.
2. Fastened-in-place appliances (220.53)
List every appliance that's fastened in place: water heater, dishwasher, disposer, built-in microwave, trash compactor, well pump. Don't include the range, dryer, space heating, or air conditioning — they have their own rules. When there are four or more, 220.53 lets you take 75% of the total. The worksheet counts the appliances and applies the factor automatically at the threshold you set.
3. Dryer (220.54)
Use 5,000 VA or the nameplate rating, whichever is larger. The worksheet handles the "larger of" automatically. For five or more dryers (multifamily), enter the demand percentage from Table 220.54.
4. Cooking appliances (220.55)
This is the line people get wrong most. Table 220.55 depends on how many appliances there are, their kW rating, which column you're allowed to use, and the notes under the table (note 4 covers a cooktop and up to two wall ovens in the same room). The worksheet deliberately makes you enter the demand VA instead of the nameplate — see how to read Table 220.55 for worked examples.
5. Heating or air conditioning, not both (220.60)
Heating and cooling don't run at the same time, so 220.60 lets you count only the larger of the two. Enter fixed electric heat at 100% (220.51). If you have a heat pump with strip heat, the compressor and strips can run together in heat mode — put both on the heating line. The sheet takes the larger number.
6. Largest motor
Add 25% of the largest motor in the calc (220.50 and 430.24). On most houses that's the A/C compressor. Use the nameplate — or the full-load current from the NEC motor tables for general-purpose motors (see our motor full-load current chart) — times voltage.
7. EV chargers and everything else
The 2023 NEC added 220.57: EVSE load is 7,200 W or the nameplate rating, whichever is larger. A 48 A wall charger on a 60 A circuit is 11,520 VA — that alone can push a 150 A house to 200 A. Add hot tubs, well pumps, shop equipment and anything else that isn't already counted.
Optional method (220.82, 220.83 and 220.84)
For a single dwelling served by 120/240 V or 208Y/120 V with service conductors of 100 A or more, 220.82 offers a simpler calc that usually comes out smaller for all-electric homes:
- List all "general loads" at nameplate — lighting at 3 VA/ft², the small-appliance and laundry circuits, and every range, oven, cooktop, dryer, water heater and fastened-in-place appliance, plus motors not otherwise counted.
- Take the first 10 kVA at 100% and the rest at 40%.
- Add the largest of the heating/air-conditioning options in 220.82(C), each with its own percentage.
Existing dwellings use 220.83 — the same idea with a different first block depending on whether heat or A/C is being added. The worksheet's Optional tab covers both: change the first-block VA and remainder percentage to switch.
For three or more dwelling units, 220.84 applies a single demand factor (from Table 220.84, based on the number of units) to the combined connected load, then you add the house loads calculated by the standard method. See our 220.84 multifamily guide for when a building qualifies. And for single-family services, the 310.12 rule lets you size service conductors at 83% of the service rating — see dwelling service conductor sizing.
Commercial service load calculation
Non-dwelling calcs follow the same structure but with different rules. The worksheet's Commercial tab walks through it:
- General lighting — building area times the unit load for the occupancy from Table 220.42(A) (2023) or 220.12 (2020). If the actual connected lighting on the fixture schedule is higher, it governs. Most commercial occupancies take no lighting demand reduction.
- Continuous loads at 125% — anything that runs three hours or more (lighting in a store or office, parking-lot lights, water heaters under 422.13, EV chargers) gets 125% for feeder and service conductor sizing (215.2 and 230.42). Mark them "Y" and the sheet applies it.
- Receptacles — 180 VA per yoke (220.14(I)), then the first 10 kVA at 100% and the remainder at 50% (220.47 in 2023, 220.44 in 2020).
- Signs, show windows and multioutlet assemblies — 220.14(F), (G) and (H).
- Kitchen equipment — 220.56 demand factors for three or more units.
- HVAC — larger of heating or cooling (220.60), plus 25% of the largest motor.
- Growth allowance — not an NEC requirement, but most engineers carry 20–25% spare. It's a separate line so it's visible.
For existing buildings, 220.87 lets you use the maximum demand from a year of utility data (or a 30-day recording) instead of calculating — often the only way to prove an existing service can take a new load.
Converting VA to amps
| System | Formula | 100,000 VA equals |
|---|---|---|
| 120/240 V single-phase | VA ÷ 240 | 417 A |
| 120/208 V single-phase (network) | VA ÷ 208 | 481 A |
| 208Y/120 V three-phase | VA ÷ (208 × 1.732) | 278 A |
| 480Y/277 V three-phase | VA ÷ (480 × 1.732) | 120 A |
Three-phase amps assume the load is balanced. Apartments fed from a 208Y/120 V system get 120/208 V single-phase, three-wire services — divide by 208, not 240, and watch the neutral.
Mistakes that get load calcs rejected
- Wrong code edition. Citing 220.12 on a 2023 job (or 220.41 on a 2020 job) is the easiest plan-review comment there is.
- Range at nameplate in the standard method. Use the Table 220.55 demand, not 12 kW.
- Counting heat and A/C together. They're noncoincident — larger one only.
- Forgetting the 25% largest motor.
- Missing the EV charger. The owner is going to buy one. Ask.
- Skipping 125% on continuous commercial loads.
- Treating the calc as the service size. The calc is the minimum; pick the rating with room to grow.
Where the load calc goes next
Once the service is sized, the same loads land on your panel schedules — use the free panel schedule template to lay out circuits and check phase balance, and the voltage drop calculator for long feeders. For conductors, see the wire ampacity chart and standard breaker sizes.
Run the whole electrical job, not just the calc
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This worksheet is a calculation aid, not engineering advice. The electrical designer of record and the authority having jurisdiction have the final say. Verify every factor against the NEC edition adopted by your AHJ and any local amendments.
