Using measured demand under NEC 2023 Section 220.87
Adding an EV charger, heat pump, electric water heater, or another large electrical load does not automatically mean an existing house needs a service upgrade. A 200-ampere service rating describes the equipment’s capacity; it does not tell us the highest load the house has actually placed on it. The National Electrical Code (NEC) provides a measured-demand method for evaluating certain existing services and feeders.
What is measured demand?
Demand is the electrical load averaged over a defined interval. Some utilities make interval data available through a smart-meter or advanced metering infrastructure (AMI) portal. A file with one reading for each hour may show energy in kilowatt-hours (kWh), rather than demand in kilowatts (kW):
Average kW = interval kWh ÷ interval length in hours.
For an hourly interval, 10 kWh means 10 kW averaged over that hour. For a 15-minute interval, 2.5 kWh means 10 kW averaged over 15 minutes. Always confirm the interval length and whether the timestamp marks its beginning or end. A monthly or daily kWh total cannot reveal the highest hourly or 15-minute demand.
What does NEC 2023 Section 220.87 require?
Section 220.87 permits actual maximum demand to establish the existing load of a qualifying feeder or service. Its conditions include:
- Maximum-demand data for a one-year period. If that is unavailable, the section has a specific exception for at least 30 days of continuous recording using 15-minute maximum-demand intervals, with the required consideration of occupied operation and seasonal or periodic loads.
- Capacity for the proposed load. The existing maximum demand at 125%, plus the calculated new load, must not exceed the feeder ampacity or service rating.
- Proper protection. Feeder overcurrent protection and service overload protection must satisfy the cited NEC provisions.
In simplified form:
(1.25 × qualifying existing maximum demand) + calculated new load ≤ applicable feeder ampacity or service rating
The new load must be calculated under the rules that apply to that equipment. For example, an EV charger is generally treated as a continuous load; its circuit and service calculation require review under the applicable NEC provisions. Do not apply a percentage to an AMI reading and assume the result is automatically an approved charger size.
A simple illustration
Suppose a qualified review establishes 80 A as the existing maximum demand on a 200 A service, and the proposed equipment contributes a calculated new load of 40 A:
(1.25 × 80 A) + 40 A = 140 A
The result is below 200 A. The designer must still verify the data, service conductors, equipment, protection, and other applicable requirements. This example illustrates the arithmetic; it is not a determination for a particular house.
Three important limits of smart-meter data
Hourly data is an hourly average. A higher 15-minute load can occur inside that hour. The 30-day recording exception specifies a 15-minute interval; an hourly CSV should not be represented as that recording. Ask the authority having jurisdiction (AHJ) what demand records it accepts for the one-year path.
Grid-import readings can differ from house load when solar is present. Rooftop generation can supply the house behind the utility meter. An import-only AMI file may therefore understate the load on equipment within the home. More importantly, NEC 2023 Section 220.87 excludes the measured-demand method where the feeder or service has a renewable energy system or peak-load shaving. A solar home’s grid-import peak should not be placed into the formula as if it qualified under that section. Use an applicable alternative calculation method, such as NEC 220.83 for a qualifying existing dwelling, and coordinate with the AHJ.
kW is not automatically amperes. Current depends on voltage, power factor, phase arrangement, and what the meter actually measures. An engineer must verify the conversion and the capacity of the specific service or feeder being evaluated.
What to request from the utility
Ask for the service or feeder’s one-year maximum-demand records, including the demand interval and whether values are in kW, kVA, or amperes. If the portal exports hourly kWh, request the full year and confirmation of whether it shows grid imports, exports, or both. Identify solar, batteries, and other load-management equipment before choosing the calculation method.
Planning an upgrade? ZigTech can review the electrical service, utility data, and proposed equipment to determine an appropriate calculation path for your project. Contact us if you have questions or need help planning your upgrade.
Learn more: NEC 2023 Section 220.87: Using Measured Demand for Existing Electrical Services.
