Section 220.87/Determining Existing Loads for Service Upgrades
NEC 220.87 permits using measured demand data to calculate existing loads when evaluating service capacity for additional loads.
When adding new loads to an existing service, Section 220.87 allows the existing load to be determined using the maximum demand data from the utility or from demand metering over a minimum 30-day period. The measured demand must be adjusted to account for the highest demand month by applying the correction factors. The new load is then added to the measured existing demand to determine whether the existing service has adequate capacity. This method typically produces a lower existing load than the standard Article 220 calculated method because it uses actual measured demand rather than theoretical connected load.
Why this section exists
When a building owner adds EV chargers, a kitchen remodel, or new HVAC equipment, the standard load calculation (Article 220) often shows the existing service is overloaded on paper, even though the utility records show the building has never approached its service capacity. Section 220.87 provides a practical alternative that avoids unnecessary and expensive service upgrades by using actual measured demand. This is particularly valuable for older buildings where the original load calculation is unavailable.
What plan reviewers look for
Plan reviewers check for the demand data source (utility records or demand meter logs) and verify the measurement period covers at least 30 consecutive days during the highest demand month. They check that the correction factor is applied if the measurement was not during the peak month. They verify the new load is added to the adjusted measured demand and the total does not exceed the service rating.