NEC 220.87: Do You Have Room for an EV Charger

"Hundred-amp service — you'll want to budget for an upgrade before you hang an EV charger on it." The upgrade goes onto the quote as a line item with four figures next to it, and at no point has anyone looked at a single day of what the house actually used. That is the ordinary order of events, and NEC 220.87 is the section that can make the line unnecessary.

It is not laziness. Without measurements, the only defensible way to size an existing service is to assume the house draws roughly everything it was built to draw. That assumption is baked into how load calculations normally work, and it is deliberately unkind.

Almost no house draws that. What 220.87 offers instead of an argument is a document: twelve months of recorded maximum demand, or a 30-day continuous recording if the twelve months do not exist. Either way the conversation stops being about what the panel looks like and starts being about a number somebody can check.

An estimated load and a measured load are two different documents

Article 220 of the NEC is where service and feeder loads get calculated, and most of it works forward from the building. Part III is the standard method: square footage, appliance nameplates, demand factors applied load by load. Part IV holds the optional methods, including 220.82 for a dwelling unit and 220.83 for working out whether an existing dwelling's service can carry something new. They are quicker, and they still start from what the house contains rather than what it uses.

For a 1960s house with a gas furnace, an electric range nobody runs at full tilt, and a dryer used twice a week, the gap between those two things is enormous. The calculation says the service is nearly full. The meter says it spends most of the year at a fraction of capacity.

220.87 is the door that lets the meter answer. Titled Determining Existing Loads, it permits a feeder or service calculation on an existing installation to be based on actual maximum demand — the real high-water mark — rather than an estimate assembled from nameplates.

One thing to be clear about before any of the arithmetic. You are not doing this calculation. The one that matters is signed by the licensed electrician doing the work and reviewed by your authority having jurisdiction, and every decision behind it — conductors, breaker, panel space, terminations — belongs to them. I am not an electrician or an inspector; what this site is and is not sets that boundary out. What you can do is fetch the data and run the numbers privately, so that when a quote says "service upgrade required" you know whether that is a measurement or a default.

What 220.87 actually asks for

Three conditions, all of which have to hold. The wording below is from the 2023 edition as hosted on UpCodes, read 17 August 2026; NFPA's own free online access to NFPA 70 is the text to check it against. Adoption is what makes any of it enforceable, though: a permit is judged under whichever edition your jurisdiction adopted, local amendments included, and that may not be 2023.

1. Maximum demand data is available for a one-year period. Twelve months, so the calculation catches your January and your August rather than a mild April.
2. That maximum demand at 125 percent, plus the new load, does not exceed the ampacity of the feeder or the rating of the service. The 25 percent is the cushion — headroom against a peak the twelve months happened not to contain.
3. Overcurrent protection is in order — the feeder per 240.4, the service per 230.90.

If the year of data does not exist, there is a fallback, and it is more demanding than people expect. The calculated load may instead rest on the maximum demand — described as the highest average kilowatts reached and maintained for a 15-minute interval — continuously recorded over a minimum 30-day period with a recording ammeter or power meter on the highest loaded phase. The recording has to be taken while the building is occupied, and it has to include, by measurement or calculation, the larger of the heating or cooling load plus anything else that comes and goes with the seasons.

Two clauses in that fallback do most of the damage in practice. A 30-day study run in October measures neither your air conditioning nor your heat, so the seasonal load has to be added on top rather than hoped for. And the exception does not apply at all where the feeder or service has a renewable energy system or employs any form of peak load shaving. Solar homes are excluded from that route by the text.

Getting the number out of your utility

Start by not looking at your bill. Residential billing is usually energy-only — kilowatt-hours across a month — and 220.87 runs on demand in kilowatts. Those are different measurements, and the second one is rarely printed anywhere on the statement.

Route one: download your own interval data. With a smart meter the file usually sits somewhere in your online account, under the Green Button standard. What the standard does not promise is resolution. The Department of Energy's own description says the data "can be provided in 15-minute, hourly, daily, or monthly intervals depending on what a utility decides to make available" (energy.gov/data/green-button, read 17 August 2026). If yours is monthly, stop — one number covering 730 hours is an average, and 220.87 wants a peak. There is no way to recover the second from the first.

What that looks like on a real account varies more than the standard suggests. Con Edison advertises "up to one year of your past energy-use data" as a CSV or XML file (coned.com, read 17 August 2026). PG&E routes residential customers through its Energy Data Hub (read 17 August 2026). Whatever the label on the button, you are hunting for the same two columns: a timestamp and a usage figure.

Then the catch that decides whether any of this is usable. If your file is hourly, each row is energy over an hour, and dividing it by one gives you an average kilowatt figure for that hour. A fifteen-minute spike gets averaged against forty-five quiet minutes and comes out looking smaller than it was. The 220.87 exception explicitly names a 15-minute interval; the one-year path does not name an interval at all. So hourly data may be accepted, or it may not, and the person to ask is your electrician — who will ask the inspector.

Route two: ask the utility directly. Email or call and use their words: twelve months of recorded maximum demand, in kW, for my service. Some utilities will produce it from the meter's demand register even on an energy-only rate. Some will not. Ask in writing so you have the answer to hand to the electrician either way.

Route three: pay for the 30-day study. This is the electrician's meter on your service for a month, and it costs money and a month of calendar. Worth it when the alternative is a service upgrade quote, and it comes with all the seasonal fine print above.

Two names are worth saying out loud on any of those calls. The download is Green Button Download My Data, which the Green Button Alliance describes as the component of NAESB's REQ.21 Energy Services Provider Interface covering customer downloading of usage data, in an industry-standard XML format meant to look the same across every provider's website (greenbuttonalliance.org, read 18 August 2026). Its sibling, Connect My Data, is authorised third-party access rather than a file — the one to ask about if you would rather the electrician pulled the twelve months himself than took a spreadsheet from you.

The arithmetic, with invented numbers

Every figure below is made up to show the shape of the calculation. Do not use them as benchmarks.

Say the service is 100 amps at 240 volts. Say the highest demand across twelve months of data comes out at 8.4 kW, on a July evening.

Now the new load. The 2023 edition added 220.57, which puts the EVSE load at 7,200 volt-amperes or the equipment's nameplate rating, whichever is larger — a floor, so that a service sized around a small charger today is not sized out of a bigger one tomorrow. A 40-amp unit at 240 V is 9,600 VA, comfortably above the floor, so 40 amps is what goes in.

Then a second 125 percent appears, and this is the part worth being careful about. Article 625 treats vehicle charging as a continuous load, and a continuous load has its branch-circuit overcurrent protection set 25 percent above the equipment's maximum draw — which is why a 40-amp unit lands on a 50-amp breaker. But condition 2 above reads maximum demand at 125 percent plus the new load. It multiplies the existing demand. It does not, on its face, say to multiply the new one as well.

So two sums are defensible, and the gap between them is not academic:

Three of those four fit under a 100-amp service. One does not — which means the 48-amp unit is or is not a service upgrade depending on which sum the plan examiner runs. That makes it a question to ask rather than a number to assume, and it is short enough to ask in one breath: for a 220.87 calculation, do you want the new EVSE load at the 220.57 figure or at 125 percent of it?

Ask it before you shop, because for a great many cars the step from 40 to 48 amps buys no extra charging speed at all — the onboard charger in the vehicle sets the ceiling, not the wall box. It would be a poor trade to inherit that argument in exchange for nothing.

Change one input and the two readings stop disagreeing. Same house, but the twelve-month peak reads 12.8 kW because of a heat pump: 12,800 VA ÷ 240 V is 53.3 A, times 1.25 is 66.7 A, and adding either 40 or 50 on top clears 100 amps either way. Nothing to arbitrate there.

When your figure and the electrician's disagree, the cause is rarely the arithmetic. Three inputs move it. The interval, because an hourly average is always a smaller number than the fifteen-minute peak buried inside it. The window, because 220.87 asks for a full year and a partial download will miss whichever season produced the high-water mark. And net metering, because a solar house's imported kilowatt-hours are not the load its service carried. So the useful question is not "what did you get" but "which row did you use" — the timestamp of the peak, and the interval length it was averaged over. Two people reading one file at two resolutions can disagree indefinitely without either of them being wrong.

When the number doesn't fit

Three directions, and this page only names them — each is a subject of its own.

An energy management system can cap the combined draw so the service never sees the full charger load; in the 2023 NEC that allowance sits in 625.42, the rating section, which routes you on to Article 750, and whether your AHJ accepts a given product is its own conversation. A smaller circuit is the cheapest fix and often costs you nothing real overnight, since a 24-amp charger still recovers a normal commute in a few hours. And a service upgrade is the honest answer sometimes, at which point the utility's own charges enter the quote alongside the electrician's. Each one gets its own page under install and inspection.

Why the edition matters more here than usual

220.57 is the newest thing on this page, and new is a problem. It arrived with the 2023 edition — UpCodes indexes it under NFPA 70 2023 and under no earlier printing, while 220.87 itself goes back through 2020, 2017 and 2014. Cite 220.57 to someone working from a 2020 book and there is nothing at that number to look at.

Whose book that is depends on a map. NFPA's enforcement pages report that as of 3 August 2026 the 2026 NEC was in effect in six states, the 2023 in twenty, the 2020 in fifteen, the 2017 in three and the 2008 in two (nfpa.org, read 17 August 2026). Those counts only cover states carrying a statewide regulation; elsewhere the edition is whatever your city adopted, which is a different question with a different answer three towns over. Working out which one is yours is a short procedure, and the charger-sizing piece on this site walks through where to look it up.

It is also rarely called the NEC where it counts. San Diego's Information Bulletin 187, revised May 2026, sends applicants to the California Electrical Code and accepts a Circuit Card — form DS-1779 — in place of electrical plans for a charger in a residential private garage, with load calculations required on the card itself (sandiego.gov, read 17 August 2026). That California Electrical Code is Part 3 of the 2025 Title 24 code, effective 1 January 2026 (California Building Standards Commission) — the 2023 NEC underneath, state amendments on top. Same section numbers, different cover, plus local text that can move the answer.

While the permit office has you on the line, spend the second question well: do they accept utility interval data for a 220.87 calculation, and at what interval. It costs nothing to ask, and it decides whether the file you downloaded is evidence or a hobby.

In California, part of that may already be published. Government Code 65850.7(g)(1)(A) required every city and county to adopt an ordinance creating an expedited, streamlined permitting process for electric vehicle charging stations — those over 200,000 residents by 30 September 2016, the rest by 30 September 2017 — and, in doing so, to adopt a checklist of every requirement an application must satisfy. Subdivision (g)(3) adds that each checklist and the required permitting documentation "shall be published on a publicly accessible internet website," alongside electronic submittal and electronic signatures, and points local agencies at the Governor's Office of Business and Economic Development's Electric Vehicle Charging Station Permitting Guidebook (statute text read at leginfo.legislature.ca.gov, 18 August 2026). Look for your city's checklist before you look for its phone number. Outside California the call is still the whole procedure.

From the download to an amp figure

Open your utility account, find the usage or data section, and take twelve months. Sort the usage column descending and read the top row. If that row is kilowatt-hours over an interval, divide by the interval length in hours — 0.25 for 15-minute data, 1 for hourly — and what you are holding is maximum demand in kilowatts.

Then the units, which is where this goes wrong quietly. Multiply the kilowatts by 1,000 to get volt-amperes, then by 1.25, then divide by your service voltage. On the 8.4 kW example above: 8,400 × 1.25 ÷ 240 = 43.75 amps. Subtract that from your service rating and the remainder is the room you are shopping inside.

Then send your electrician one line: "I have twelve months of interval data at [hourly / 15-minute] resolution. Will you quote using a 220.87 calculation, and tell me the maximum demand figure you use?"

If the answer is that they would rather not, that is information too. Between a quote built on a measurement and a quote built on an assumption, only one of them can tell you an upgrade is genuinely necessary — and the contact page is open if your permit office told you something that contradicts any of this.