This runs NEC 220.83, the Existing Dwelling Unit method — the code path for adding a load to a service that is already in the ground. Nearly every calculator on the first page of Google runs 220.82 instead, which is the new-construction method and gives a different, and for a retrofit a wrong, answer.
Have a licensed electrician verify this before anyone touches a panel. This is a planning tool. Your jurisdiction adopts a particular edition of the NEC and may amend it, your inspector may read a demand factor differently, and only someone standing in front of your equipment can confirm the nameplate ratings this depends on. Nothing here is a permit, an inspection, or a substitute for either.
Local amendments can change this answer. States adopt different editions of the NEC — 2017, 2020 and 2023 are all in force somewhere — and cities amend on top. A calculation that is correct under the code as printed can still be refused by an inspector working to a local rule. Ask your electrician which edition your jurisdiction is on before you rely on any of this.
| Option | Load added | Calculated | Against service | Fits |
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Hand this to your electrician or take it to the permit counter. Every line names the code section it comes from, so it can be checked rather than trusted.
The National Electrical Code contains two optional load calculations for dwellings, and they are not interchangeable.
220.82 — Dwelling Unit is written for a house being designed. It bundles the general lighting load, the small-appliance and laundry circuits and the nameplate ratings of all appliances into one figure, takes the first 10 kVA at 100% and the remainder at 40%, and then adds heating or air conditioning separately at one of six percentages. It assumes you are choosing the service size.
220.83 — Existing Dwelling Unit is written for exactly the question you are asking: can the service already in the ground carry one more thing? Its own text says so — it is permitted to be used to determine whether an existing service or feeder has sufficient capacity for additional loads. It takes the first 8 kVA at 100% and the remainder at 40%, and it splits into two paths depending on whether air conditioning or space heating is part of what you are adding.
Run 220.82 on a retrofit and you will usually get a larger number than the code requires, because the new-construction method is built to size a service conservatively rather than to test an existing one. That difference is frequently the difference between “you need a $4,500 upgrade” and “you do not.” The two methods side by side, with the arithmetic.
Both paths start by summing the same connected load:
Then the demand factors, and this is where the two paths separate:
220.83(A) — no air conditioning or space heating being added. First 8 kVA of the total at 100%, remainder at 40%. Existing air conditioning and heating sit inside that total like any other appliance.
220.83(B) — air conditioning or space heating being added or changed. The air conditioning or heating load is taken at 100% and pulled out of the pot; then the first 8 kVA of everything else at 100% and the remainder at 40%. Because air conditioning and heating never run together, 220.60 lets you drop the smaller of the two — this tool does.
Divide the result by the service voltage, normally 240, and compare against the main breaker rating.
Worked example — the figures this page opens with. An 1,800 sq ft house from the 2000s on a 100 A service, with a gas furnace, and a 48 A charger proposed.
General lighting 1,800 × 3 = 5,400 VA. Small appliance circuits 2 × 1,500 = 3,000 VA. Laundry 1,500 VA. Appliances at nameplate: range 12,000, dryer 5,000, water heater 4,500, air conditioner 3,600, dishwasher 1,200, disposal 900, microwave 1,500 = 28,700 VA. Existing connected load: 38,600 VA.
Existing calculated load: 8,000 at 100% plus (38,600 − 8,000) × 40% = 12,240, giving 20,240 VA — 84.3 A on a 100 A service, so 15.7 A of headroom.
Add the 48 A charger at 11,520 VA and the connected total is 50,120 VA. Calculated: 8,000 + (50,120 − 8,000) × 40% = 16,848, giving 24,848 VA = 103.5 A. 3.5 A over.
A 32 A charger is 7,680 VA, which lands at 23,312 VA — 97.1 A, and it fits. A 40 A charger misses by a third of an amp. With a load-management device the EV load comes out of the calculation entirely and the panel sits at 84.3 A with the 48 A charger installed.
Not necessarily — and he can see the panel, which this page cannot. But ask him for the calculation. If the answer is “everyone needs 200 amps now,” that is a sales position rather than a code position. Print the worksheet above, hand it over, and ask which line he would change.
NEC 220.57 sets a floor: EV supply equipment is calculated at 7,200 VA or its nameplate, whichever is larger. It exists because a small charger is trivially swapped for a large one later. This is one of the details most online calculators get wrong, and it makes small chargers less useful than they look for squeezing into a tight panel.
NEC 625.42 permits the EV load on the service to be taken as the maximum the automatic load management system will allow. A relay device that drops the charger when the panel approaches capacity therefore contributes nothing to the calculation. Devices run roughly $400–$1,200 installed against $3,000–$6,000 for a service upgrade, and your car charges overnight when nothing else in the house is running.
(B) if a heat pump, air conditioner, mini-split or electric heat is part of the same project. (A) if the only new thing is the charger. Choosing (A) when you are also adding a heat pump will understate the load, which is the failure mode that gets a permit rejected.
No. The 80% figure comes from the continuous-load rule for branch circuits and feeders, not from the service calculation. Compare the 220.83 result directly against the main breaker rating. Being at 97 A on a 100 A service is a pass.
Then run them together, not one at a time — the demand factor applies to the whole pot, and each addition is cheaper in calculated terms than the last. Add each nameplate to the appliance list, set the HVAC question to yes, and read the result. That is the real 2026 question and it is the one this method handles best.
That is the code figure for general lighting and general-use receptacles, and it is deliberately generous — it is not a measurement of what you use. Get the area roughly right; being 200 sq ft out moves the calculated load by 240 VA, which is one amp.
Both, depending on your code cycle. The 2026 NEC reorganises the load-calculation rules into a new Article 120, where this method becomes 120.83. The calculation itself is unchanged. Adoption lags publication by years, so most jurisdictions are still enforcing the 2020 or 2023 code, where it is 220.83 — which is the number used throughout this site and on the printed worksheet.