When new electrical equipment is added to an existing building, the designer must determine whether the existing service or feeder can carry the additional load. NEC 2023 Section 220.87, “Determining Existing Loads,” provides a way to use actual maximum demand for that evaluation when its conditions are met.
The method applies to existing installations. It can be useful for homes and other buildings, but a utility bill or a single smart-meter reading does not, by itself, complete a code-compliant load calculation.
What is maximum demand?
Maximum demand is the highest measured load during the qualifying period, averaged over the demand interval used for the measurement. It is different from the equipment’s service rating and from the total energy used in a month.
For example, a 200-ampere service is rated to supply up to its applicable capacity. It does not mean the building has actually reached 200 amperes. A monthly bill showing 1,000 kilowatt-hours (kWh) tells us how much energy was used, but not the highest demand reached during a short interval.
The Section 220.87 calculation
For an eligible existing service or feeder, the basic capacity check is:
125% of the qualifying existing maximum demand + calculated new load ≤ service rating or feeder ampacity
The calculated new load must follow the NEC rules applicable to the proposed equipment. The service or feeder must also have the required overload or overcurrent protection. Passing the arithmetic alone does not establish that the installation complies with every applicable electrical requirement.
Example: If a qualifying measurement establishes an existing maximum demand of 60 A, and a proposed addition contributes 30 A of calculated new load, the result is:
(60 A × 1.25) + 30 A = 105 A
That result can be compared with the rating or ampacity of the specific service or feeder being evaluated. The example is only an illustration; it does not size an actual installation.
What data is required?
Section 220.87 normally requires maximum-demand data for a one-year period. Where a year of that data is unavailable, its exception permits a minimum 30-day continuous recording under specific conditions. That recording uses the highest average demand over a 15-minute interval, is connected to the highest loaded phase based on initial loading, and must account for occupied operation and seasonal or periodic loads that may not appear during the recording period.
| Data supplied | What it can tell you |
|---|---|
| Monthly or daily kWh totals | Energy consumption; these totals do not establish maximum demand. |
| Hourly kWh intervals | Average kW for each hour, because the interval is one hour; these do not reveal a higher 15-minute demand within an hour. |
| Qualifying one-year maximum-demand records | A potential basis for the one-year path, subject to verification of what was measured and acceptance by the authority having jurisdiction. |
| Qualifying 30-day recording | A potential basis for the exception when all of its 15-minute recording and other conditions are met. |
If a utility provides interval energy rather than demand, divide the kWh in each interval by its length in hours to calculate average kW for that interval. For a 15-minute interval, multiply kWh by 4. The engineer must still confirm the meter’s location, the system voltage and phase, the units needed for the calculation, and whether the records represent the load on the service or feeder under review.
Important limitation for solar and load shaving
NEC 2023 does not permit the Section 220.87 measured-demand method where the feeder or service has a renewable energy system, such as solar PV or wind, or employs peak-load shaving. Generation can reduce what the utility meter sees without reducing the building’s underlying electrical load. Use another applicable load-calculation method and coordinate the approach with the authority having jurisdiction.
Before deciding on a service upgrade
Identify the service or feeder being evaluated, obtain suitable demand records, calculate the proposed new load, and verify the equipment and protection. For an existing dwelling, NEC 220.83 may provide an alternative calculation path when its conditions are met. Check which NEC edition and local amendments the project jurisdiction has adopted.
Have questions about an existing electrical service or a planned upgrade? Contact ZigTech for help reviewing your project and selecting an appropriate calculation method.
