Service headroom
Preview Version 0.1.0How much room is left on an existing electric service, from the demand the meter already measured.
Headroom
210.1A
About 210.1 A (174.7 kVA) of this 400 A service is left by the measured-demand method in 2023 NEC 220.87.
NFPA 70 · Mike HoltCross-checked · The spec's 480 V scenario (126.3 kVA, 157.9 kVA, 189.9 A, 210.1 A, 174.7 kVA) and a twelve-bill example pass. Two published worked examples (EC&M, 2014; Electrical Contractor, 2011) are reproduced to the whole amps they print: 123 A, 173 A, and 27 A spare; 929 A, 1,180 A, and 20 A spare. What this means
A starting point for your electrician, not a load calculation for a permit: the inspector decides. · Example values · Review pending Report a problem
How we got this
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.
Steps
- Data period: the highest 15-minute demand from a recording of at least 30 days: 120.0 kW.
- Demand in kVA: 120.0 kW ÷ power factor 0.95 = 126.3 kVA.
- Times 125%: 126.3 × 1.25 = 157.9 kVA.
- In amps: 157.9 kVA × 1,000 ÷ (√3 × 480 V) = 189.9 A.
- Headroom: 400 A − 189.9 A = 210.1 A, or 174.7 kVA.
- At power factor 0.90 instead, the headroom is 199.5 A.
- Utility meter data is the total for the service and assumes the phases are balanced. The 30-day option is based on the highest-loaded phase, so per-phase recordings, when you have them, replace the meter total.
- Method: 2023 NEC 220.87, paraphrased: the existing load is the maximum demand from a year of data, or from 30 days or more of 15-minute recording, taken at 125%. The authority having jurisdiction decides which edition applies and whether this method may be used.
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
Known limits
- Not a code compliance decision; conductor sizing and full load calculations are out of scope.
- Sites with solar, wind, a battery, or peak shaving cannot use the 30-day option.