Using actual demand data to size added load
An existing service's load can be based on its actual peak demand from 12 months of utility data, or in some cases a 30-day recording. The peak times 125%, plus the new load, must not exceed the feeder ampacity or service rating.
What the code requires
Instead of calculating an existing building's load on paper, 220.87 lets you use what the service actually drew. Three conditions must all be met: maximum-demand data covers a 1-year period; the maximum demand times 125%, plus the new load, does not exceed the feeder ampacity or the service rating; and the feeder and service have the protection the code requires under 240.4 and 230.90.
Where the 30-day option is used, the recording is taken continuously with a recording ammeter or power meter on the highest-loaded phase, and demand is the highest average kW held over a 15-minute interval. The building must be occupied, and the recording must include the larger of the heating or cooling load, plus any seasonal or periodic loads, by measurement or calculation.
On the job
- On a home with solar, a 30-day logger study does not qualify. Use 12 months of utility demand data or a load calculation instead.
- A 30-day recording should not be run while the building is unoccupied or during an unusual period, such as a holiday.
- The measured peak must be multiplied by 125% before the new load is added. Leaving that step out overstates how much capacity is left.
- Utilities may require the owner's written permission before releasing demand data.
Exceptions that matter
- If 12 months of data is not available, a recording of at least 30 days may be used instead, but not on a service with solar PV, wind generation or any form of peak-load shaving.
What changed in 2023
In 2020, a service with solar PV, wind or peak-load shaving could not use this method at all; in 2023 that limit applies only to the 30-day recording option, so a home with solar can now use 12 months of utility data.
Related sections
- NEC 220.83: Adding new loads to an existing home's service
- NEC 220.70
- NEC 230.42: Sizing service-entrance conductors
- NEC 230.79: Minimum service size, including 100 A for homes
- NEC 230.90
- NEC 240.4
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