Electrical Load Calculations for Dwellings and Commercial Buildings
Work a dwelling service by the standard and optional methods, check an existing service before adding a load, run a basic commercial load, and catch the errors that change a service size.
Table of contents
A load calculation decides how big a service or feeder must be, and it is usually the first thing a plans examiner reads. Nearly every customer now wants to add something to an old panel, such as a car charger or a heat pump. The real question is often whether a service can take one more large load, and that needs a method you can defend. Choosing the method and getting the inputs right matter more than the arithmetic. This works through full calculations with real numbers, and it assumes you can already find a load in the code and do basic power math.
What You Will Be Able to Do
- Work a dwelling load by the standard method.
- Work the same house by the optional method, and say when that is allowed.
- Decide whether an existing service can take a new load.
- Run a basic commercial load.
- Size dwelling service conductors using the 83 percent rule.
- Review someone else's calculation.
Section numbers follow the 2023 National Electrical Code (NEC), where load calculations are in Article 220. The 2026 edition moved them to Article 120 and changed some values, as shown later. Your state or city may be on another edition, with amendments. Use the adopted one, and treat these examples as practice. A real calculation is the responsibility of the licensed person who submits it.
Why Demand Factors Exist
Loads are stated in volt-amperes (VA), which is volts times amps. Adding up every nameplate in a building gives a number far higher than the building ever draws, since everything does not run at once. A demand factor is the percentage the code lets you count. Each one reflects how a type of load behaves, so they cannot be moved from one load to another.
The Standard Method for a Dwelling
Take the loads in this order.
| Step | Load | How it counts |
|---|---|---|
| 1 | General lighting | 3 VA per square foot, from outside dimensions |
| 2 | Small appliance circuits | 1,500 VA each, at least two |
| 3 | Laundry circuit | 1,500 VA |
| 4 | Demand on steps 1 to 3 | First 3,000 VA at 100 percent, the next part up to 120,000 VA at 35 percent |
| 5 | Range | Table 220.55, 8 kW for one range up to 12 kW |
| 6 | Dryer | 5,000 VA or nameplate, whichever is larger |
| 7 | Fixed appliances | 75 percent if there are four or more |
| 8 | Heat or cooling | The larger of the two, not both |
| 9 | Largest motor | Add 25 percent of it |
| 10 | Electric vehicle equipment | 7,200 VA or nameplate, whichever is larger |
A few details decide whether the result holds up:
- Floor area. Read the rule for what it leaves out, such as open porches.
- Ranges over 12 kW. For one range, add 5 percent to the 8 kW for each kW above 12. A 14 kW range counts as 8 x 1.10 = 8.8 kW.
- Fixed appliances. The 75 percent applies only with four or more, and it leaves out ranges, dryers, and heating and cooling equipment.
- Heat or cooling. Use the larger load, because they do not run together.
Worked Example: A New 2,400 Square Foot House
The house has a 12 kW range, a 5 kW dryer, a 5,000 VA air conditioner, gas heat, and a 9,600 VA car charger. Its four fixed appliances are a 4,500 VA water heater, a 1,500 VA dishwasher, a 900 VA disposal and a 1,200 VA microwave, for 8,100 VA.
| Load | Math | VA |
|---|---|---|
| General lighting | 2,400 x 3 | 7,200 |
| Small appliance | 2 x 1,500 | 3,000 |
| Laundry | 1,500 | 1,500 |
| Subtotal | 11,700 | |
| After demand | 3,000 + (8,700 x 0.35) | 6,045 |
| Range | Table value | 8,000 |
| Dryer | Nameplate | 5,000 |
| Fixed appliances | 8,100 x 0.75 | 6,075 |
| Air conditioner | Nameplate | 5,000 |
| Largest motor | 5,000 x 0.25 | 1,250 |
| Car charger | Nameplate | 9,600 |
| Total | 40,970 |
40,970 / 240 = 170.7 amps, so a 200-amp service passes. The neutral is worked out separately under 220.61, because it carries only the unbalanced load, and ranges and dryers count at a reduced percentage.
The Optional Method
NEC 220.82 is a simpler method for a single dwelling supplied at 120/240 or 208Y/120 volts, with service or feeder conductors of 100 amps or more. It lumps most loads together.
- Add the general loads at nameplate: lighting, small appliance and laundry, plus range, dryer, fixed appliances and other equipment.
- Count the first 10,000 VA at 100 percent and the rest at 40 percent.
- Add the heating and cooling load, using the largest of the options listed in 220.82(C). Air conditioning counts at 100 percent, and central electric heat usually at 65 percent.
Worked Example: The Same House
| Load | VA |
|---|---|
| Lighting, small appliance and laundry | 11,700 |
| Range at nameplate | 12,000 |
| Dryer | 5,000 |
| Fixed appliances | 8,100 |
| Car charger | 9,600 |
| General load | 46,400 |
| First 10,000 at 100 percent | 10,000 |
| Remaining 36,400 at 40 percent | 14,560 |
| Air conditioning at 100 percent | 5,000 |
| Total | 29,560 |
29,560 / 240 = 123.2 amps. The same house came to 170.7 amps by the standard method. Neither result is wrong. The optional method is allowed, and it gives a much lower number. A customer who adds a load later will want the larger service, so a low number is a minimum to design from, not a target.
What the 2026 Edition Changes
If your job is under the 2026 NEC, some inputs change, and so do the numbers.
| Item | 2023 NEC | 2026 NEC |
|---|---|---|
| Article | 220 | 120 |
| Dwelling general lighting | 3 VA per square foot | 2 VA per square foot for load, with 3 still used to count circuits |
| Optional method, first step | First 10 kVA at 100 percent | First 8 kVA at 100 percent |
| Car charger in optional method | In the general load | At nameplate, at 100 percent |
For the standard example, lighting drops to 4,800 VA. The subtotal is 9,300 VA, the demand total is 5,205 VA, and the house comes to 40,130 VA, or 167.2 amps. The service is the same size, but a submitted calculation using the wrong edition can still be rejected.
Adding a Load to an Existing Service
Two methods answer "can this panel take one more load?"
Using Measured Data
NEC 220.87 lets you start from what the building really draws. Take the highest demand from a year of utility data, multiply by 125 percent, and add the new load. If a full year of data is not available, the code allows a recording of at least 30 days, with conditions. The building must be in use. The result must also account for the larger of the heating or cooling load, by measurement or calculation, and for other seasonal loads.
Worked Example: A Car Charger on a 100-Amp Service
A house has a 100-amp service. Utility data shows a peak of 10.8 kW in the last year, and the owner wants a 9,600 VA charger.
- Existing load: 10,800 x 1.25 = 13,500 VA.
- Add the charger: 13,500 + 9,600 = 23,100 VA.
- Current: 23,100 / 240 = 96.3 amps.
That passes on the numbers, with little margin. Now suppose the data was a 30-day recording taken in April, with the air conditioner off. That reading misses the largest seasonal load, so the calculation cannot stand as it is. Add the cooling load before you trust it.
Using a Calculation
NEC 220.83 covers additions to an existing dwelling by calculation. It counts the first 8 kVA at 100 percent and the rest at 40 percent, with the heating and cooling treated according to its own conditions. Use it when no reliable demand data exists.
Recent editions also let an energy management system limit the load a service sees. Those rules are new and local approvals differ, so ask before you design around one.
Sizing Dwelling Service Conductors
NEC 310.12 allows conductors that carry the entire load of a dwelling to be sized at 83 percent of the service rating.
For a 200-amp service: 200 x 0.83 = 166 amps. That is 2/0 copper, good for 175 amps at 75 degrees C, or 4/0 aluminum, good for 180 amps. This works only where no correction or adjustment factors apply.
This rule covers only conductors that carry the whole dwelling load. A feeder to a garage subpanel carries part of the load, so it does not qualify. That feeder is sized from the normal ampacity table.
A Commercial Load
Commercial work has different factors, and more of the load is continuous, meaning it runs for 3 hours or more.
- General lighting. Table 220.42(A) in the 2023 NEC sets VA per square foot by occupancy, such as 1.3 for offices and 1.9 for retail. The 125 percent for continuous loads is already built in, so do not add it again. Editions before 2020 used higher values for offices.
- Receptacles. Count 180 VA each. Under 220.47, the first 10 kVA counts at 100 percent and the rest at 50 percent.
- Motors. Add 25 percent of the largest motor.
Worked Example: A 10,000 Square Foot Office
The office has 120 general-purpose receptacles and three rooftop units of 10,000 VA each. The largest motor in any unit is 7,500 VA. The service is 208Y/120 volts, three-phase.
| Load | Math | VA |
|---|---|---|
| Lighting | 10,000 x 1.3 | 13,000 |
| Receptacles | 120 x 180 = 21,600 | |
| After demand | 10,000 + (11,600 x 0.50) | 15,800 |
| Rooftop units | 3 x 10,000 | 30,000 |
| Largest motor | 7,500 x 0.25 | 1,875 |
| Total | 60,675 |
Current: 60,675 / (208 x 1.732) = 168.4 amps. The code minimum is about 170 amps, and a real design usually adds spare capacity. The code sets a floor for safety, and it does not guarantee room to grow.
Reviewing a Calculation
When someone hands you a calculation, check these before the arithmetic:
- Which edition does it use, and is that the adopted one?
- Is the floor area right?
- Did the 75 percent go on four or more appliances only?
- Is heat or cooling counted once, using the larger?
- Is the largest motor's 25 percent included?
- Does the car charger use nameplate, if that is larger?
- If this is an existing service, does the data cover the heaviest season?
- Did anyone apply 125 percent twice on commercial lighting?
Common Mistakes
| Mistake | What it costs |
|---|---|
| Counting both heat and cooling | An oversized service |
| Applying 75 percent to three appliances | A calculation that is too low |
| Leaving out the 25 percent for the largest motor | A rejected submittal |
| Using a spring recording for a summer peak | An overloaded service in July |
| Using 83 percent on a subpanel feeder | Undersized conductors |
| Adding 125 percent again to commercial lighting | An oversized service |
| Copying a calculation from another edition | A rejected or wrong result |
Check Your Understanding
1. A coworker's calculation for a tiny cabin lists one kitchen circuit at 1,500 VA, because the kitchen is so small. What do you change?
The standard method counts at least two of those circuits, whatever the kitchen size. Add the second 1,500 VA before the demand step.
2. A heat pump adds 12 kW of electric heat, and the cooling side is 4 kW. Which goes in the standard calculation?
The larger load, which is the heat at 12 kW. Do not add both.
3. A cabin on a 60-amp service needs a new calculation, and the electrician wants the lower result the shortcut method gives. Is that allowed?
No. NEC 220.82 needs service or feeder conductors of 100 amps or more, so this cabin uses the standard method unless the service is upsized.
4. A salesperson tells a homeowner that a 100-amp panel can never handle new equipment without an upgrade. How would you test that claim?
Run the numbers. With a year of demand data, use 125 percent of the peak plus the new load. Without it, use a qualifying recording or the calculation for existing dwellings, then compare the result to 100 amps.
5. A retail shop of 5,000 square feet is calculated at 1.9 VA per square foot. What is the lighting load, and should you add 125 percent?
9,500 VA. Do not add 125 percent, because the table value already includes it.
Key Takeaways
- Demand factors belong to specific loads, so apply each one only where the code says.
- The standard and optional methods can give very different results for the same house.
- Check which edition applies, because the 2026 NEC changed the article and several values.
- Measured data needs a full picture, including the heaviest season.
- The 83 percent rule covers only conductors that carry the whole dwelling load.
- Code results are minimums, so plan for what the customer will add.