Every service upgrade starts with the same question from the homeowner: will the panel I have carry what I want to add? A new range, a heat pump or an EV charger in the garage all raise that question. The answer is a load calculation, and there is a lot of guesswork around it. People add up every nameplate in the house and conclude they need 400 amps, or look at the age of the panel and say 200 without running the numbers at all.
This guide walks through the optional method for a single dwelling in Article 220 of the National Electrical Code (NFPA 70), step by step, with a worked example. It also covers where the calculation fits in a quote, because for an electrician the load calc is usually the first billable piece of a bigger job.
One thing up front. A load calculation that sizes a real service has to be run and signed by a licensed electrician, using the actual nameplate ratings and the code edition and local amendments your jurisdiction has adopted. What follows explains the method so you can scope a job and talk it through with a customer. It is not a substitute for the permit calculation.
What an electrical load calculation tells you
A load calculation estimates the demand a house will place on its service, in volt-amps (VA), and converts it to amps at 240 volts. That figure is then rounded up to the next standard service size, such as 100, 150 or 200 amps.
The key word is demand. A house never runs every appliance at full power at the same moment, and the code accounts for that with demand factors. That is why a straight total of nameplates overstates what the service needs, often by a wide margin.
The NEC gives two ways to do it for a dwelling. The standard method applies separate demand factors to lighting, ranges and dryers from their own tables. The optional method groups the general loads and applies a single demand factor, which is simpler and usually produces a smaller result. The IAEI Magazine walkthrough of residential service calculations works the same house both ways and gets 198 amps from the standard method and 142 from the optional one. The optional method applies to a single dwelling served by one 120/240 volt, 3-wire service or feeder with an ampacity of 100 amps or more.
How to calculate electrical load, step by step
Step 1: Add up the general loads
Under the optional method, general loads cover almost everything except heating and air conditioning:
- Lighting and general receptacles: 3 VA per square foot of floor area, measured from the outside dimensions and excluding open porches, garages and unfinished spaces not adaptable for future use.
- Small-appliance circuits: 1,500 VA for each 20 amp kitchen and dining countertop circuit. The code requires at least two.
- Laundry circuit: 1,500 VA.
- Fixed appliances at nameplate rating: range, wall oven and cooktop, dryer, water heater, dishwasher, disposal, built-in microwave, and anything else permanently connected, such as a well pump, pool pump or hot tub.
Use the nameplate figure, not a typical one. For resistive loads like a water heater or range the VA matches the wattage. For motor loads the two can differ, so take the VA rating where the nameplate gives one.
Step 2: Apply the demand factor
Take the first 10,000 VA of the general load at 100 percent and everything above that at 40 percent. That single step is where the optional method gets its realistic result, because it reflects the fact that the range, the dryer and the water heater are rarely all at full draw together.
Step 3: Add the largest heating or cooling load, not both
A house is not heated and cooled at the same time, so the method takes only the largest of the applicable options:
- Air conditioning and cooling equipment at 100 percent.
- A heat pump compressor at 100 percent, or the compressor at 100 percent plus 65 percent of any supplemental electric heat that can run alongside it.
- Electric space heating at 65 percent, or at 40 percent where there are four or more separately controlled heating units.
Adding heating and cooling together is an easy way to land a service size higher than the house needs.
Step 4: Account for an EV charger
An EV charger draws its full rating for hours at a time, so the code treats it as a continuous load. IAEI's overview of EV charging requirements notes that the feeders and branch circuits supplying charging equipment are sized for continuous duty at not less than 125 percent of the equipment's maximum load rating. A 40 amp charger at 240 volts is 9,600 VA, which becomes 12,000 VA at 125 percent.
How the charger is folded into a service calculation is where readings differ, and where a load management system can change the answer entirely by capping what the charger draws. Our free calculator adds the charger at 125 percent outside the demand factor, which is the more conservative reading and gives the larger service size. Check which approach your inspector expects.
Step 5: Convert to amps and round up
Add the demand-adjusted general load, the heating or cooling figure and any EV load, divide by 240 volts, and round up to the next standard service size. If the result is 99 amps, a 100 amp service passes on paper, though it leaves almost no headroom for what the homeowner adds next year.
A worked example
Here is a 2,000 square foot house with an electric range, an electric dryer and a central air conditioner. The appliance ratings are illustrative, round numbers, so substitute the real nameplates on any actual job.
| Load | Calculation | VA |
|---|---|---|
| Lighting and receptacles | 2,000 sq ft x 3 VA | 6,000 |
| Small-appliance circuits | 2 x 1,500 VA | 3,000 |
| Laundry circuit | 1 x 1,500 VA | 1,500 |
| Range | Nameplate | 12,000 |
| Dryer | Nameplate | 5,000 |
| Water heater | Nameplate | 4,500 |
| Dishwasher, disposal, microwave | 1,200 + 900 + 1,500 | 3,600 |
| General load | 35,700 |
Apply the demand factor: 10,000 VA at 100 percent, plus 25,700 VA at 40 percent (10,280 VA), gives 20,280 VA.
Add the air conditioner at 3,500 VA and the total is 23,780 VA. Divided by 240 volts, that is 99.1 amps, so the house fits a 100 amp service with no margin at all.
Now the homeowner wants a 40 amp EV charger. At 125 percent it adds 12,000 VA, taking the total to 35,780 VA, or 149.1 amps. The calculation now points to a 150 amp service, and there is a case for 200 if a heat pump is on the cards. That conversation, with the numbers written down, is what turns a charger install into a service upgrade quote.
Swap the air conditioner for a 4,000 VA heat pump with 10,000 VA of strip heat instead, and the heating figure becomes 4,000 plus 65 percent of 10,000, or 10,500 VA. Without the charger that gives 30,780 VA, about 128 amps, which is past 100 before anything else is added.
Run the numbers, then quote and bill the job, no signup
Mistakes that throw the number off
- Totalling nameplates with no demand factor. It feels safe, but it sizes the service for a moment that never happens and prices the customer out of a job they would have said yes to.
- Counting heating and cooling together. Take the largest applicable option only, with the heat pump plus supplemental heat case as the one combination the method allows.
- Using typical wattages instead of nameplates. A tankless water heater or a large induction range can be several times the figure people assume.
- Measuring the wrong floor area. Garages, open porches and unfinished spaces not adaptable for future use are excluded from the 3 VA per square foot figure.
- Forgetting the loads that are coming. Ask about planned additions such as a charger, a hot tub or a switch from gas to electric heat. A service sized exactly to today's load will need upgrading again.
Put the load calculation in the quote
For an electrician, the load calc is not paperwork, it is the reason the customer trusts the price. A quote that says a 200 amp upgrade is needed is easy to push back on. A quote that shows the calculated load at 149 amps with the new charger, and explains why 200 leaves room for the heat pump they mentioned, is much harder to argue with.
Include the method used (optional or standard), the main loads and the calculated amps as a line in the estimate or as an attachment. If the calculation took a site visit and measurements, decide in advance whether you bill that time separately or fold it into the job, and say so on the quote. Our guides to pricing electrical work and estimating electrical jobs cover labour rates, materials and minimums.
Send the estimate, get it approved, invoice it in one click
Billbooks lets you send a professional estimate with the load calculation attached, collect the customer's approval online and turn it into an invoice without retyping a line, with deposits and online payments built in. Free for 30 days, no credit card needed.
Frequently asked questions
How do you calculate the electrical load of a house?
Add the general loads (3 VA per square foot, 1,500 VA for each small-appliance circuit and the laundry circuit, and the nameplate rating of every fixed appliance), apply the demand factor of 100 percent on the first 10,000 VA and 40 percent on the rest, add the largest heating or cooling load, then divide by 240 volts to get amps. That is the NEC optional method for a single dwelling.
What size electrical service does a 2,000 square foot house need?
Floor area alone does not decide it. A 2,000 square foot house with typical electric appliances and central air can calculate out at around 100 amps, while the same house with an EV charger or electric heat can need 150 amps or more. Run the calculation with the real appliance ratings.
Is 100 amp service enough for a house?
It can be for a smaller home with gas heating and cooking and few large electric loads. Electric heat, a heat pump with strip heat, an electric range and an EV charger are the loads that most often push a house past 100 amps, so run the load calculation before adding any of them.
Why do heating and air conditioning not add together in a load calculation?
Because a house is not heated and cooled at the same time, so the optional method takes only the largest applicable heating or cooling load. The exception is a heat pump with supplemental electric heat that can run alongside it, where the compressor counts in full and the supplemental heat at 65 percent.
How does an EV charger affect the load calculation?
EV charging is treated as a continuous load, so it is counted at 125 percent of its rating. A 40 amp charger at 240 volts is 9,600 VA, which becomes 12,000 VA. A load management system that caps the charger can reduce that figure, so check what the equipment and your inspector allow.
Can I do my own load calculation for a permit?
You can run the numbers to understand what a house needs, but a calculation used to size and permit a service has to reflect the actual nameplate ratings and the local code amendments, and in most places it has to be prepared by a licensed electrician.
