Also in: Disaster Response and Recovery
Critical Load Priority Shedding
When the available source is smaller than the connected load.
Example
You enter
- Available source capacity (kW) 128
- Tier 1, emergency loads, Art. 700 (kW) 40
- Tier 2, legally required standby, Art. 701 (kW) 55
- Tier 3, optional standby, Art. 702 (kW) 70
- Tier 4, further user tier (kW) 0
- Usable stored energy if energy-limited (kWh, 0 if none) 400
You get
- Served 95
- Shed 70.0 kW: optional standby (Art. 702)
- Served (%) 57.5758
- Source utilization 74% (33.0 kW headroom)
- Runtime by shed level 10
- Runtime tiers12 hours 4.21053
- Runtime all hours 2.42424
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
Scopea derated generator, one unit where two were planned, a battery -- the loads must be served in priority order, and the NEC names the order: emergency systems (Article 700), then legally required standby (Article 701), then optional standby (Article 702), with the selective load pickup and load shedding provision of 700.4(B). The calculator takes each tier's connected kW and the source actually available, adds whole tiers in order while they fit, and sheds the first tier that does not and everything below it, because shedding schemes switch whole tiers on transfer switches or shunt-tripped breakers. It reports the served and shed kW, the served share of the total, the source utilization and headroom, and -- for a battery or a fixed fuel supply in kWh -- the runtime at each shed level. A shelter and clinic with 40 kW emergency, 55 kW legally required, and 70 kW optional load on a generator derated to 128 kW serves 95 of 165 kW (58%) at 74% utilization, shedding the optional tier. On a 400 kWh battery everything runs 2.42 hours, the top two tiers 4.21, and the emergency tier alone 10.0 -- the number that decides whether the optional loads are worth carrying. Which loads belong to which article is the AHJ's call, and the 700.4(B) numbering should be checked against the adopted edition. It does not design shedding controls or selective coordination, or derate a battery (the Off-Grid Battery Bank Sizing calculator). The NEC as adopted and the AHJ govern.
Formula or tabletiers in priority order (1 emergency, 2 legally required standby, 3 optional standby, 4 user tier); a tier is served while the cumulative kW <= source kW, and the first tier that does not fit and all below it are shed; served fraction = served / total; utilization = served / source; runtime at each shed level = usable kWh / cumulative served kW.
Source and editionNFPA 70 Articles 700 (emergency systems), 701 (legally required standby systems), and 702 (optional standby systems), and the selective load pickup and load shedding provision of 700.4(B), cited by number with the edition and section numbering to be verified against the adopted code; the AHJ named as governing.
Free accessCumulative addition and a division on the user's own tier loads; the NEC articles are cited by number and not reproduced.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namessource_kw, tier1_kw, tier2_kw, tier3_kw, tier4_kw, usable_kwh, served_kw, shed_kw, served_pct, utilization_pct, runtime_tier1_hours, runtime_tiers12_hours, runtime_all_hours
- NEC priority order Article 700 loads first, 701 next, 702 lastNFPA 70 Articles 700, 701, 702
- Whole tiers shed a tier is connected or not, as load-shedding schemes are wiredNFPA 70 700.4(B), edition to be verified
- Usable energy is net battery energy entered after depth of discharge and temperature deratingproject method