Existing-Facility Load by Peak Demand (NEC 220.87)

Adding load without a service upgrade, on the metered peak.

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Details, formula, and sources

NEC 220.87 lets an EXISTING building use the maximum demand the utility actually metered (a year of data, or a 30-day recording) instead of summing connected loads, which overstates what a service carries. basis = 125% of the recorded peak; total = basis + new load; headroom = service rating - total. A 200 A service with a 120 A recorded peak adding a 40 A EVSE is 1.25 x 120 + 40 = 190 A -> 10 A of headroom, fits with no upgrade; a busier 145 A peak is 221 A, over the 200 A service. VOID where the recorded data reflects on-site PV or peak-shaving that hides the true peak. A design aid; the AHJ and utility data govern.

basis = 1.25 x recorded_peak; total = basis + new_load; headroom = service_rating - total; fits = headroom >= 0 and not pv_or_peakshave.

The existing-load determination of NEC 2023 220.87 -- 125% of the maximum demand the utility metered over a year (or a 30-day recording) plus the new load -- by name. A computational aid; the AHJ-adopted NEC edition and the utility data govern.

NEC is free to read at nfpa.org/freeaccess. The 220.87 maximum-demand method and its data conditions are in the published code.

Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.

Field names used by the API: recorded_peak_a, new_load_a, service_rating_a, pv_or_peakshave, basis_a, total_a, headroom_a

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