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Emergency & Standby Power

Generator sizing, load pickup sequencing, UPS & fuel autonomy

NEC 700NEC 701NEC 702IEC 60364AS/NZS 3000BS 7671

Standard

System Parameters

Load Schedule

Generator Options

Configure loads and click Calculate

How Emergency Power Works

The emergency power calculator sizes generator and UPS systems to supply essential loads during mains failure, ensuring compliance with code-required backup power provisions.

The calculator classifies loads per NEC Article 700 (emergency — life safety), Article 701 (legally required standby), and Article 702 (optional standby). The total emergency demand is calculated with appropriate diversity, then a generator kVA rating is selected accounting for motor starting kVA, step loading sequence, and altitude/temperature derating.

IEC 60364-5-56 defines the requirements for safety services supply. ISO 8528 establishes generator set ratings (standby, prime, continuous). AS/NZS 2293 covers emergency lighting requirements. Results include the generator/UPS sizing, fuel consumption estimate, automatic transfer switch rating, load shedding priority schedule, and compliance summary for each load classification.

Frequently Asked Questions

How do I size a standby generator for a building?
Generator sizing requires summing all loads that must be supplied during an outage, including motor starting requirements. Per ISO 8528-1, the generator must handle the maximum continuous load (PRP or COP rating) plus the largest motor starting transient without exceeding 15-20% voltage dip. Calculate: running load (kW), apply diversity, add the largest motor starting kVA (typically 6x motor FLC x voltage / 1000), and convert total to kVA at the system power factor. Select a generator rated above this value from standard ratings (e.g., 100, 150, 200, 250, 300, 500 kVA per ISO 8528-5).
What is the difference between NEC Article 700, 701, and 702?
NEC Article 700 covers emergency systems required by law for life safety (exit lighting, fire alarms, smoke evacuation) with mandatory 10-second transfer time and 2-hour fuel supply. Article 701 covers legally required standby systems (ventilation, communications, industrial processes) required by code but not for immediate life safety, with 60-second maximum transfer time. Article 702 covers optional standby systems (owner-elected backup for convenience) with no mandated transfer time. Each has different wiring requirements — Article 700 demands completely independent wiring per 700.10, while 701 and 702 permit shared raceways.
What transfer switch type should I use for emergency power?
Transfer switches are classified as automatic (ATS) or manual, and open-transition or closed-transition. NEC 700.5 requires automatic transfer for emergency systems. Open-transition (break-before-make) ATS is the standard, creating a brief interruption during transfer. Closed-transition (make-before-break) ATS provides momentary paralleling (typically less than 100ms per NEMA standards) for seamless transfer but requires utility approval and paralleling protection. IEC 60947-6-1 classifies transfer switching equipment. For critical loads like hospitals, a bypass-isolation type ATS per NFPA 110 allows maintenance without de-energising loads.
How do I calculate UPS battery autonomy?
Battery autonomy in minutes is determined by the load power (kW), battery capacity (Ah), system voltage, and end-of-discharge voltage. The basic formula is: autonomy = (battery_Ah x battery_V x efficiency) / (load_kW x 1000). Per IEEE 485 (lead-acid) or IEEE 1188 (VRLA), battery sizing accounts for ageing factor (1.25), temperature correction (per IEEE 485 Table 1), and design margin (10-15%). For a 100kW load requiring 15 minutes autonomy with a 480V DC bus and 0.85 system efficiency: minimum battery = (100,000 x 0.25 hours) / (480 x 0.85) = 61 Ah, then multiply by ageing and temperature factors.
What are the fuel storage requirements for emergency generators?
NEC 700.12(B)(2) requires on-site fuel sufficient for 2 hours of full-load operation for emergency systems. NFPA 110 Level 1 (hospitals, high-rise) typically requires 24-96 hours of fuel depending on the occupancy classification and local authority requirements. Fuel consumption can be estimated at approximately 0.27 litres per kWh for diesel generators at 75% load. For a 500kVA (400kW) generator at 75% load: consumption is approximately 81 litres/hour, so 24-hour storage requires approximately 1,944 litres. Tank installation must comply with local fire codes, EPA regulations, and IEC 62271 for equipment rooms.

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Key Terms

max demandcable sizingdiversity factor

Related FAQ

standards FAQ

Standards Reference

  • NEC Articles 700-702 — Emergency systems
  • IEC 60364-5-56 — Safety services
  • AS/NZS 2293 — Emergency lighting