Methods: Service headroom
electrify.service-headroom.check · Version 0.1.0
Method
Existing load = maximum demand ÷ power factor × 125%, in amps at the service voltage (single-phase: kVA × 1,000 ÷ V; three-phase: kVA × 1,000 ÷ (√3 × V)); headroom = service rating − existing load.
Assumptions
- The maximum demand is a 15-minute average, the way demand meters and interval files record it.
- Utility meter data is the service total and assumes balanced phases.
Sources
- NFPA 70, National Electrical Code, 2023, Section 220.87, Determining Existing Loads (cited and paraphrased; read it through NFPA's free access); see docs/derivations/service-headroom.md
- Mike Holt, Code Q&A: Calculating feeder or service loads, EC&M, August 8, 2014, Worked existing-load example, 200 A single-phase service
Worked examples
| Example | Independent source | Tolerance | Last result |
|---|---|---|---|
| 120 kW on a 480 V three-phase, 400 A service | electrify/service-headroom spec, scenario "Three-phase service" | ±0.05, ±0.05, ±0.05, ±0.05, ±0.05 | Not yet run in CI |
| Twelve demand-metered bills on a 208 V three-phase, 600 A service | Own example: 120 kW × 1.25 = 150 kVA; 150,000 ÷ (√3 × 208) = 416.4 A | ±0.05, ±0.05, ±0.05 | Not yet run in CI |
Known differences from reference engines
None recorded.
Cite this tool
Service headroom, version 0.1.0. gridprims. https://gridprims.com/tools/electrify/service-headroom/check/