⚡ Electrical

Generator Critical Load Calculator

Identify and calculate critical electrical loads that must be backed up by the emergency generator per NBC 2016 and IS 14280 requirements.

📐 Standard: NBC 2016 Part 8 / IS 14280
✅ Free to use
📄 PDF export
📱 Mobile friendly
⚡
Generator Critical Load Calculator Calculator
Reference: NBC 2016 Part 8 / IS 14280
⚡ Electrical
Identify and calculate critical electrical loads that must be backed up by the emergency generator per NBC 2016 and IS 14280 requirements.
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ℹ️ About This Calculator

The critical load calculator helps identify which building loads must be connected to the emergency generator (essential bus) and calculates the total generator kVA required for those loads. NBC 2016 Part 8 mandates that certain safety-critical systems remain powered during a grid failure. This tool ensures code compliance and adequate backup capacity for all life-safety and critical business loads.

NBC 2016 Part 8 (Electrical and Allied Installations) specifies that essential services must be supplied from the emergency generator within 15 seconds of grid failure. The emergency generator bus must be separated from the normal supply bus and connected via an Automatic Transfer Switch (ATS) or Manual Transfer Switch (MTS). Generator room: minimum 4-hour fire rating, ventilated, with fuel storage for minimum 8 hours' full-load operation.

📐 Critical Load Schedule

NBC 2016 Part 8 / IS 14280

NBC 2016 Mandatory Essential Loads:
  1. Fire detection and alarm systems
  2. Fire fighting pumps (main + jockey)
  3. Emergency lighting (escape routes, exit signs)
  4. Lifts (at least one per bank for evacuation)
  5. Medical equipment (hospitals/ICU)
  6. Public address system

Optional Critical Loads:
  7. CCTV and access control
  8. UPS systems (IT/servers)
  9. Selected air handling units

Generator kVA = Σ(critical loads) / PF + motor start allowance
Typical essential load = 15–25% of total building load

🧮 Worked Example

A 10-storey commercial building has a total connected load of 1200 kW. Essential loads are listed: fire pumps 45 kW, emergency lighting 20 kW, one lift per bank 30 kW, PA and fire alarm 5 kW, and selected UPS/IT loads 60 kW, totalling 160 kW (about 13% of the building load, within the typical 15–25% range). Applying an average PF of 0.8 and a 20% motor-starting allowance for the fire pump, generator kVA = (160/0.8) × 1.2 = 240 kVA. The next standard size, a 250 kVA generator, is selected and connected to the essential bus through an ATS with a 15-second transfer time.

❓ Frequently Asked Questions

What is the difference between essential, critical, and normal loads? +
Normal loads: connected to main supply only, shed during power failure (example: most general lighting, regular power outlets). Essential loads: must transfer to generator within 15 seconds (emergency lighting, fire systems, lifts). Critical loads (hospitals/data centres): must transfer to UPS instantly (within 10 ms) then generator takes over from UPS. In a hospital, ICU and OT are critical; corridors and admin are essential; canteen is normal.
Which lifts must be on generator power per NBC? +
NBC 2016: in buildings above 15 m, at least one lift per bank must be powered by the emergency generator for evacuation of mobility-impaired persons. In hospitals: all patient lifts (including stretcher lifts) must be on emergency power. Fire service lifts (mandatory for buildings above 30 m) must be exclusively on the generator supply, not shared with normal loads. Fire service lifts must operate within 60 seconds of grid failure.
How do I size the generator for high motor-starting loads? +
Critical systems with large motors (fire pumps, main HVAC): use automatic soft starters or star-delta starters to reduce starting current to 2–3× instead of 5–7×. If DOL starters are used for fire pumps (common for reliability): the generator must be sized so the largest motor start does not cause more than 15% voltage drop. Add: motor starting kVA = motor FLA × start factor × motor voltage, and check against generator transient capability.
Can the generator supply both essential and non-essential loads? +
It can, but it's not recommended. If the generator supplies all loads (not just essential), it must be sized for the full building load – a much larger, more expensive generator. Better practice: selective switching via the ATS. The generator supplies only essential loads (15–25% of building). Non-essential loads remain shed until normal supply is restored. Load shedding relays can progressively restore additional loads as the generator stabilises.
What automatic transfer switch (ATS) do I need? +
ATS specifications: transfer time ≤ 15 seconds (per NBC for emergency loads); voltage sensing on all three phases; frequency monitoring; manual bypass for maintenance; retransfer delay (adjustable, typically 30 minutes) to avoid repeated transfer/retransfer during unstable grid. For hospitals and data centres: make-before-break ATS for load-critical systems. Standard ATS: MCCB-based, 3P+N, with mechanically interlocked mechanism.

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⚠️ Disclaimer: For preliminary engineering design only. Verify all results with a licensed engineer before use. Full disclaimer →