NEC 220.87 Explained: Load Calculations From Measured Demand
A plain-language guide to NEC 220.87, the code section that lets an existing service load be based on what the meter actually recorded. What it requires, how the 125% rule works, and when it beats a traditional load calculation.
NEC 220.87 is the section of the National Electrical Code that lets a load calculation for an existing service or feeder be based on actual measured maximum demand, instead of nameplate ratings and generalized demand factors. The 2026 edition moves it to Article 120 as 120.87 and changes how the new load is counted. See "Under the 2026 edition" below.
In short: it is based on how a building actually performs, not on a modeled, worst-case assumption. For a contractor adding a heat pump, EV charger or water heater to an older home, it is often the difference between "the panel is fine" and "you need a service upgrade first."
What 220.87 requires
The section is short. Paraphrased, the existing load may be taken from measured demand when all of the following are true:
- Maximum demand data is available for a one-year period. Twelve months captures both the heating and the cooling season, which is why utility smart-meter (AMI) interval data is such a good fit.
- The measured maximum demand at 125 percent, plus the new load, does not exceed the rating of the service (or the ampacity of the feeder).
- The feeder has overcurrent protection in accordance with 240.4, and the service has overload protection in accordance with 230.90.
There is an exception for buildings with no year of data: demand can be recorded continuously for a minimum of 30 days with a recording ammeter or power meter connected to the highest-loaded phase of the service, starting from the loading at the beginning of the recording. The recording has to be taken while the building is occupied and has to include, by measurement or calculation, the larger of the heating or cooling load and other seasonal loads. The exception is not allowed where the service has a renewable energy system (solar PV or wind) or uses any form of peak load shaving. That limit applies to the 30-day exception only, not to the one-year route.
The one-year requirement does not set an interval length. The code's definition of maximum demand, "the highest average kilowatts reached and maintained for a 15-minute interval," sits in the 30-day exception. So 15-minute data is the natural match, and if you use hourly data, say so on the report: hourly averages can understate short peaks.
220.87 is explicitly permitted by the NEC. It is not a workaround. It asks for good data and clear documentation, and the Authority Having Jurisdiction (AHJ) always retains full authority to decide whether the method was applied appropriately.
The code text itself is free to read on NFPA's free-access site. Always work from the edition your jurisdiction has adopted.
The arithmetic, with an example
The whole calculation is three lines:
| Step | Example |
|---|---|
| Measured 15-minute peak over 12 months | 9.7 kW, which is about 40 A at 240 V |
| Multiply by 125% | 50 A |
| Add the new load | 50 A + 32 A heat pump = 82 A |
| Compare with the service rating | 82 A is less than 100 A, so the service is adequate |
The 125 percent factor is the code's built-in safety margin. It is part of the method, not something you add on top.
Under the 2026 edition
The 2026 NEC renumbers the section as 120.87 and rewrites condition (2): the measured maximum demand at 125 percent, plus the new load minus any removed load, determined in accordance with Article 120 Parts I, III and IV, must not exceed the service rating or feeder ampacity.
Two practical differences from 2023:
- Load being removed can be subtracted. Replacing a central air conditioner with a heat pump, for example, the old AC comes off the total.
- The new load is determined under the code's calculation rules (Parts I, III and IV), where the 2023 text just says "the new load."
Conditions (1) and (3) and the 30-day exception are unchanged.
Two ways to calculate load
Traditional calculations size a service using standardized tables built around an average home from decades ago. 220.87 instead asks what this specific home actually draws, using the utility meter already on the wall.
| Traditional load calculation | NEC 220.87 | |
|---|---|---|
| What it's based on | Nameplate ratings and standardized demand factors | Actual measured demand from the utility meter |
| What it reflects | A modeled, worst-case estimate | How the building actually performs |
| Where the data comes from | Manufacturer specs and code tables | 12 months of utility interval (AMI / smart meter) data |
| Typical result | Often overstates real-world demand | Reflects the true measured peak, almost always lower |
For a closer comparison with the optional method for existing dwellings, see NEC 220.83 vs. 220.87.
The research behind it
Two Lawrence Berkeley National Laboratory (LBNL) studies form the evidence base:
- Less & Walker, Applied Energy 410:127422
- Murphy et al., Journal of Building Engineering 120:115576
Across 11,940 homes, the mean measured 15-minute peak demand was just 9.7 kW. A standard 100-amp, 240-volt service delivers 24 kW. On average, 220.87 produces roughly 40% lower calculated loads than the 220.83 method for existing dwellings, which is often the deciding factor in whether a service upgrade is actually required.
When 220.87 is a good fit
- An existing building undergoing renovation or electrification: a heat pump, EV charger, heat pump water heater, induction range.
- A service upgrade is being suggested because of a calculated overload, not an observed one.
- Utility AMI / smart-meter interval data is available for the property. (How to get it.)
It is not a fit for new construction, for a home with no usage history under its current occupants, or where a big load was added so recently that the last twelve months do not reflect it.
Recognized by regulators
"An acceptable means of calculating an existing service load." — Dennis Blair, Vermont Chief State Electrical Inspector
Acceptance still varies by jurisdiction, mostly because the method is unfamiliar rather than because it is disputed. What inspectors want to see in a 220.87 report covers how to make the submission easy to approve.
How TrueLoad helps
TrueLoad turns a utility's raw interval data into an AHJ-ready NEC 220.87 report, so a contractor doesn't have to build the calculation by hand.
- Request the utility's AMI data with a ready-made authorization request.
- Upload the file. TrueLoad finds the true measured peak automatically.
- Add the new equipment being installed. TrueLoad checks it against the service capacity.
- Print a permit-ready report showing PASS or FAIL in minutes, not weeks.
No interval data for the home yet? The same project runs the traditional existing-dwelling calculation instead, so the job does not wait. Measured demand vs. the traditional calculation.
TrueLoad is a service adequacy evaluation aid. This guide is a plain-language summary, not the code text and not engineering advice. Final responsibility for code compliance remains with the preparer of record and the Authority Having Jurisdiction.
Put it into practice
TrueLoad runs the measured-demand calculation from the utility's interval data, or the traditional calculation when there is none, and produces the report. Your first project is free.