Also in: Disaster Response and Recovery
Existing-Facility Load by Peak Demand (NEC 220.87)
Adding load without a service upgrade, on the metered peak.
Example
You enter
- Recorded peak demand (A, metered over the last year) 120
- New load to add (A, continuous-equivalent) 40
- Existing service rating (A) 200
- Recorded data reflects PV / peak-shaving? false
You get
- 125% basis (of metered peak) 150 A
- Total load (basis + new) 190 A
- Headroom under the service 10
Checked against published worked examples from: EC&M (Mike Holt).
Details, formula, and sources
ScopeNEC 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.
Formula or tablebasis = 1.25 x recorded_peak; total = basis + new_load; headroom = service_rating - total; fits = headroom >= 0 and not pv_or_peakshave.
Source and editionThe 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.
Free accessNEC is free to read at nfpa.org/freeaccess. The 220.87 maximum-demand method and its data conditions are in the published code.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesrecorded_peak_a, new_load_a, service_rating_a, pv_or_peakshave, basis_a, total_a, headroom_a
- Metered basis 125% of the maximum demand the utility metered, not the sum of connected loadsNEC 220.87
- Data span at least a year of data, or a 30-day recording per 220.87 conditionsNEC 220.87
- Void condition does not apply where the recorded demand reflects on-site PV or peak-shaving that hides the true peakNEC 220.87