Ask a homeowner what their electrical service is and you get one of three answers: "100 amps, I think," "200 — the guy said it was fine," or a blank stare. The number matters more than most people realize. It decides whether you can add a heat pump, an EV charger, a hot tub, or a workshop — or whether the utility has to pull new wires from the street. And the number that decides it is not a hunch. It is a calculation, and in the United States that calculation lives in Article 220 of the National Electrical Code.
This guide walks through the whole idea in plain English: what a service load calculation actually measures, how the Standard Method works step by step, how the Optional Method differs, and a complete worked example for a 2,400 sq ft home so you can follow the arithmetic with a calculator in hand. It ends with the honest rules of thumb for 100, 200, and 320-amp services — and the mistakes that skew the math in both directions.
Service size is decided by demand, not by breaker count. Article 220 adds up the loads the code assumes you'll actually run at once — 3 VA per square foot of living area, small-appliance and laundry circuits, nameplate values for fixed appliances — then trims the total with demand factors, because nothing runs everything simultaneously. A 2,400 sq ft home with gas heat and typical electric appliances calculates to roughly 100–137 amps; the same house all-electric with an EV charger wants 200. 100 amps is usually enough for a gas home without a charger. 200 amps is the safe default for anything electric.
What a Service Load Calculation Actually Measures
The core misunderstanding to clear up first: your panel's job is not to carry the sum of its breakers. A panel with 40 breakers marked 20 amps each is not a "800-amp panel" in any sense. Breakers are fuses with opinions — they trip when a circuit misbehaves. What actually flows through your service is the coincident demand: the appliances that happen to run at the same time, on the worst plausible evening.
Article 220 is the NEC's method for estimating that worst plausible evening without installing monitoring equipment for a year. It converts your house into a list of volt-amps (VA — for residential math, effectively watts), applies reduction factors where the code has studied real diversity, and hands you a single number. Divide that number by 240 volts and you get the calculated load in amps, which the service — the conductors, meter, and main breaker — must be able to carry.
Two legal paths get you there. The Standard Method (NEC 220.41 through 220.60 in the 2023 code) builds the estimate piece by piece. The Optional Method (220.82) is a compressed version available for dwellings served by a single 120/240-volt service of 100 amps or more. Electricians and plans examiners accept both; they just land on different numbers, as the worked example below will show.
The Standard Method, Step by Step
Step 1 — General lighting and small circuits
Start with floor area. The code assigns 3 VA per square foot of dwelling space, measured from the outside dimensions — walls included, open porches and garages typically excluded. For a 2,400 sq ft home that's 7,200 VA. Then add the circuits the code insists every kitchen and laundry has:
- Two 20-amp small-appliance circuits for kitchen and dining receptacles — 1,500 VA each (NEC 220.52(A)).
- One 20-amp laundry circuit — 1,500 VA (NEC 220.52(B)).
That gives a subtotal of 11,700 VA for the 2,400 sq ft example — lighting, convenience outlets, and the plug-in life of the house, before a single appliance is counted.
Step 2 — The lighting demand factor
Here is where the code admits nobody runs every light and outlet at once. NEC 220.42 applies demand factors to the total from Step 1: the first 3,000 VA at 100%, everything above that at 35%. Our 11,700 VA subtotal becomes 3,000 + (35% × 8,700) = 6,045 VA. Almost half the subtotal vanishes — that's not a loophole, that's eighty years of measured behavior in real homes.
Step 3 — Fixed appliances, at 75% when there are four or more
Nameplate everything that's fastened in place: water heater, dishwasher, disposal, exhaust fans, water softener. Dryers, ranges, and HVAC are handled separately, and so is anything on a receptacle. If the list of fastened-in-place appliances reaches four or more, NEC 220.53 lets you count them at 75% — the garbage disposal and the dishwasher genuinely never run together. Three or fewer: 100%.
Step 4 — Range and dryer
The electric range gets the code's demand table, not its nameplate. A single range up to 12 kW counts as 8,000 VA (NEC 220.55, Column C) — a 12 kW nameplate reduced to 8 because nobody boils every burner on high for an hour. The clothes dryer counts at nameplate, with a floor of 5,000 VA (NEC 220.54). Gas appliances count as zero here, which is exactly why gas homes calculate so much smaller.
Step 5 — Heating and cooling: the larger one only
Furnace and air conditioner never peak together, so NEC 220.60 says count the larger of the heating or cooling load, not both. For a gas-furnace house that's the cooling side: the AC compressor and air-handler nameplates. One more motor rule rides on top (NEC 430.24): add 25% of the largest motor — usually the compressor — because motors draw extra current while starting.
A Worked Example: 2,400 Square Feet
Here is the promised arithmetic, for a concrete but typical home: 2,400 sq ft, gas furnace, central air conditioning (5,280 VA compressor, 660 VA air-handler blower), 12 kW electric range, 5 kW electric dryer, 4,500 VA electric water heater, 1,200 VA dishwasher, 900 VA disposal. Three fastened-in-place appliances means no 75% factor yet.
| Load | Calculation | Volt-amps |
|---|---|---|
| General lighting | 2,400 sq ft × 3 VA | 7,200 |
| Small-appliance circuits | 2 × 1,500 VA | 3,000 |
| Laundry circuit | 1 × 1,500 VA | 1,500 |
| Subtotal | 11,700 | |
| Demand factor (220.42) | 3,000 + (35% × 8,700) | 6,045 |
| Range (220.55) | 12 kW → Column C demand | 8,000 |
| Dryer (220.54) | 5,000 VA minimum | 5,000 |
| Water heater | Nameplate | 4,500 |
| Dishwasher | Nameplate | 1,200 |
| Disposal | Nameplate | 900 |
| AC + blower (220.60) | Non-coincident — cooling is larger | 5,940 |
| Largest motor (430.24) | 25% × 5,280 | 1,320 |
| Calculated load | 32,905 VA |
32,905 VA ÷ 240 V = 137 amps. By the Standard Method this house needs a 150-amp service to clear the number with any margin — and most electricians would call 200 amps on the spot, because nobody upgrades a service twice.
The Optional Method: Same House, Different Answer
NEC 220.82 compresses the same judgment into fewer lines. Add up all the general loads — lighting, small circuits, laundry, and the nameplate values of every appliance including the range and dryer — then apply one demand factor: 100% of the first 10 kVA, 40% of the remainder. Heating and cooling are added at full nameplate (largest of heat or cool), with no motor add-on.
For our example: general loads total 35,300 VA (the range now counts its full 12,000). The demand factor turns that into 10,000 + (40% × 25,300) = 20,120 VA. Add cooling at 5,940 VA and the total is 26,060 VA ÷ 240 V = 109 amps.
Same house, same code: 137 amps by the Standard Method, 109 by the Optional Method. Both are defensible on a permit application. The Optional Method rewards the statistical reality that big loads almost never coincide, which is why it typically lands 15–25% lower. When an electrician tells you your house "calculates fine" for an upgrade, ask which method — the answer changes the panel size.
100 Amps, 200 Amps, or More: The Honest Rules of Thumb
The calculation is the law; the rules of thumb below are just how the arithmetic tends to come out. Use them to sanity-check plans, not to sign contracts.
| Service | Typically works for |
|---|---|
| 100 A | Gas heat, gas water heater, gas range or modest electric range, no EV charger, under ~2,500 sq ft. The classic 1950s–1980s house that has been fine for decades usually stays fine. |
| 200 A | The default for new construction and any upgrade since the 2000s: electric range and dryer, heat pump, and headroom for one 48-amp EV charger. |
| 320–400 A | All-electric homes over ~3,500 sq ft with two EVs on fast charging, workshops with welders or compressors, or homes with planned additions and subpanels. |
The item that moves homes between rows fastest is the EV charger. A Level 2 charger is a continuous load: the code counts it at 125% of its rating, so a 48-amp charger demands 60 amps of the calculation all by itself. On a house already calculating to 80 amps, that tips past 100 — which is why utilities and inspectors now ask about charging plans before approving panel upgrades.
The Mistakes That Skew the Math
- Adding up breaker handles. The most common homeowner calculation is also the least meaningful. Forty breakers × 20 amps tells you nothing about demand — Article 220 exists precisely because this number is always wrong.
- Counting heat and cooling together. The furnace and the compressor never peak simultaneously; the code takes the larger (220.60). Adding both inflates the result by 5–10 kW.
- Skipping the demand factors. Conservative in the other direction: counting every light and appliance at 100% sizes the service one or two steps bigger than the code requires. Oversizing costs money now and buys nothing later — the conductors and meter are priced by the amp.
- Forgetting the charger is continuous. A "48-amp" charger contributes 60 amps to the calculation. Planning around the faceplate rating is how homes end up with a panel that trips at the worst moment.
- Confusing panel rating with service size. A 200-amp panel fed through a 100-amp main breaker and 100-amp conductors is a 100-amp service with spare slots. The upgrade happens at the conductors, meter, and main — not the box.
Frequently Asked Questions
Can I size my electrical service by adding up all the breaker handles?
No. Breaker handles are trip ratings, not consumption — a 20-amp breaker might feed a phone charger or a space heater. Article 220 exists precisely because connected capacity is not demand. Adding up 40 breakers will overstate the load, usually by a factor of two or three.
Is a 100-amp service enough for a house?
Often, yes — for homes with gas heat, a gas water heater, and no electric vehicle charging. A 100-amp service at 240 volts is 24,000 VA of capacity, and a modest gas-appliance home typically calculates between 60 and 100 amps. All-electric homes, heat pumps, and EV charging almost always want 200 amps.
Do I need 200 amps for an EV charger?
Not always, but a Level 2 charger is usually the straw that breaks a 100-amp service. An 11.5 kW charger counts at 125% — about 60 amps of calculation. On a home already calculating to 80 amps, that tips the total past 100. A formal load calculation, or an energy management system that sheds other loads while charging, is the honest way to know.
What's the difference between the Standard Method and the Optional Method?
Both are legal Article 220 calculations. The Standard Method applies demand factors piece by piece — 35% beyond the first 3,000 VA of lighting and small circuits, demand tables for ranges and dryers. The Optional Method pools all general loads, takes 100% of the first 10 kVA and 40% of the rest, then adds heating or cooling at nameplate. For the same house it typically lands 15–25% lower.
What's the difference between service size and panel size?
The service is limited by the weakest link — service conductors, meter base, or main breaker. The panel rating is just the box. A 200-amp panel fed by 100-amp conductors and a 100-amp main is a 100-amp service with headroom for circuits, not power. Upgrading means replacing the conductors from the street, the meter, and the main — which is why it's a utility coordination job, not a panel swap.
This guide explains the code's arithmetic for planning purposes. Service upgrades and load calculations for permits are the domain of a licensed electrician and your local authority having jurisdiction — the adopted code edition and local amendments always win.
Where to Read Next
- Water heater sizing: tank vs tankless — the other half of the home's biggest circuits, and why an electric tankless wants 150 amps of its own.
- The Home Maintenance category — the seasonal rhythm that keeps measurement problems from becoming emergencies.
- Solar panel system sizing calculator — the other big electrical question on a modern roof, worked the same way: numbers first, opinions second.
The bottom line: your service size is a demand calculation, not a breaker tally. Three VA per square foot, the circuits the code requires, nameplates for what's fastened in place, demand factors for human behavior, and the larger of heat or cool. Run the arithmetic before you plan the heat pump or the charger — and let a licensed electrician run it again before the permit.