NEC 220.82 vs 220.83, worked out

The two sections, in plain terms

220.82 — Optional Calculation for a Dwelling Unit. Written for a house being built or a service being designed from scratch. It bundles the general lighting load, the small-appliance and laundry circuits and the nameplate ratings of every appliance into a single general-load figure, takes the first 10 kVA of that at 100% and everything above it at 40%, then adds heating or air conditioning separately at whichever of several listed percentages applies to the equipment. The question it answers is how big a service does this house need?

220.83 — Optional Calculation for an Additional Load in an Existing Dwelling Unit. The section title gives the game away, and so does its opening line: it is permitted to be used to determine whether an existing service or feeder has sufficient capacity for additional loads. It sums the same categories, takes the first 8 kVA at 100% and the remainder at 40%, and splits into two paths — (A) where no air conditioning or space heating is being added, and (B) where it is. The question it answers is can what is already here take one more thing?

Two different questions. You are asking the second one.

Why the wrong one is used so often

Partly because 220.82 is the better-known section — it is what an apprentice learns, it is what appears in most textbook examples, and it is the one used thousands of times a year on new construction. Partly because it is easier to code: one demand factor, no branching on whether HVAC is part of the job.

And partly, if you are being uncharitable, because 220.82 usually returns a larger number, and a larger number sells a service upgrade. That is not necessarily anyone acting in bad faith. But if you are looking at a free calculator published by a company that installs panels, it is worth knowing which section it ran.

The same house, both ways

An 1,800 sq ft house built in the 2000s, gas heat, on a 100 A service. Electric range 12,000 VA, dryer 5,000, water heater 4,500, central air conditioner 3,600, dishwasher 1,200, disposal 900, built-in microwave 1,500. The owner wants a 48 A Level 2 charger, which NEC 220.57 calculates at 11,520 VA.

Fourteen amps apart on the same house, from the same inputs, in the same code book. One says you are marginally over and a smaller charger solves it. The other says you are seventeen percent over a 100 A service, which reads as “you need a service upgrade.” The difference is roughly $4,500.

And 220.83 is not a loophole. It is the section the code provides for precisely this situation, and its more generous treatment reflects a real fact: an existing house has a demand history, its occupants have habits, and the code writers were willing to be less conservative about a service that has been carrying a household for twenty years than about a service being sized for a house nobody has lived in yet.

Which path within 220.83

Use (A) when the only thing you are adding is the charger, or an induction range, or a heat pump water heater — anything that is not air conditioning or space heating. First 8 kVA of the whole total at 100%, the rest at 40%. Existing air conditioning sits inside that pot like any other appliance.

Use (B) when air conditioning or electric space heating is being added or changed as part of the same work — a heat pump going in, a mini-split, resistance heat. The heating or cooling load comes out of the pot and is taken at 100%, then the first 8 kVA of everything else at 100% and the remainder at 40%. This is the path people get wrong when electrifying, and getting it wrong understates the load, which is the version that gets a permit refused.

Because air conditioning and heating do not run at the same time, 220.60 lets you count only the larger of the two. That is worth remembering in an all-electric house where the heat strips are 10,000 VA and the compressor is 3,600.

Three details most calculators get wrong

• The EV charger minimum. NEC 220.57 requires electric vehicle supply equipment to be calculated at 7,200 VA or its nameplate, whichever is larger. A 16 A charger has a 3,840 VA nameplate and still counts as 7,200. It exists because a small charger is trivially swapped for a large one later, and it means dropping to a very small charger buys you much less headroom than you would expect.
• The 80% rule does not apply here. The continuous-load rule that sizes a circuit at 125% of its load applies to the branch circuit feeding the charger — a 48 A charger needs a 60 A circuit. It does not apply to the service calculation. Compare the 220.83 result directly against the main breaker rating; 97 A on a 100 A service is a pass, not a near miss.
• Load management removes the load, not part of it. NEC 625.42 permits the EV load on the service to be taken as the maximum the automatic load management system allows. A relay device that disconnects the charger when the panel approaches capacity therefore contributes nothing at all to the calculation. Devices run roughly $400–$1,200 installed. This is the single most common way a marginal panel is rescued and it is missing from most calculators entirely.

The method that beats both: 220.87

If your utility can give you twelve months of interval data — and many now can, from a smart meter, in a web portal — then 220.87 lets you use the maximum demand actually recorded in place of a calculated existing load. Add the new load to that measured peak and compare.

It is almost always kinder than 220.83, because 220.83 sums nameplates and no household ever runs the range, the dryer, the water heater and the air conditioner simultaneously at full rating. Where a calculation lands marginally over, 220.87 is the first thing to try, and the data costs nothing. Where twelve months of data does not exist, a recording ammeter left on the service for thirty days is the sanctioned alternative.

What to do with this

Run the calculation, print the worksheet, and hand it to your electrician. Not the conclusion — the worksheet. It names the code section behind every line, which turns "do I really need an upgrade?" from a difference of opinion into a specific question about a specific number. If your electrician disagrees, ask which line he would change and why. That is a conversation both of you can win.

And if the answer is that you genuinely do need the upgrade, you will know it for a reason you can see.

A note on the section number

The 2026 edition of the NEC reorganises the load-calculation rules into a new Article 120, and this method is renumbered 120.83 there. The arithmetic is unchanged — the same 8 kVA at 100%, the same 40% on the remainder.

Which number your inspector uses depends entirely on which edition your jurisdiction has adopted, and adoption runs years behind publication. Most of the United States is still enforcing the 2020 or 2023 code as things stand, so 220.83 remains the reference you will see on a permit and the number this site uses throughout. If your inspector cites 120.83, you are on the newer code and looking at the same rule under a new address.