⚡ Electrical

Standby Generator Sizing Calculator

Size standby diesel generators (DG sets) for buildings. Calculate required kVA based on connected loads, demand factors, and motor starting kVA per IS 14280.

📐 Standard: IS 14280 / NBC 2016 Part 8
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ℹ️ About This Calculator

Correctly sizing a standby generator (DG set) is critical for reliable backup power. Under-sizing causes the generator to overload and shut down; over-sizing wastes capital and fuel. This calculator determines the required generator kVA rating by summing critical loads with demand and power factor corrections, and adds a motor starting kVA component for the largest motor per IS 14280.

IS 14280 covers diesel generator sets for standby power in buildings. NBC 2016 Part 8 specifies which loads must be on backup power (emergency lighting, lifts, fire pumps, critical ICU loads). CPCB (Central Pollution Control Board) emission norms set limits on DG set exhaust. DG sets require a stacking / acoustic enclosure above 5 kVA in residential areas. Monthly test run (minimum 30 minutes under load) is mandatory for maintained reliability.

📐 Generator Sizing Formula

IS 14280 / NBC 2016 Part 8

Step 1: Running kW
  kW_running = Σ(P_load × DF_load × FF_load)
  Where DF = demand factor, FF = future expansion factor

Step 2: Running kVA
  kVA_running = kW_running / PF_weighted
  PF_weighted = Σ(kW_load) / Σ(kW_load / PF_load)

Step 3: Motor Starting Allowance
  kVA_start = Largest motor kW × motor start factor (4–8 × FLA for DOL)
  kVA_step = kVA_start - running kVA of that motor

Step 4: Generator Rating
  kVA_gen = (kVA_running + kVA_step) × 1.15 (15% derating for altitude/temp)
  Select next standard rating above kVA_gen

❓ Frequently Asked Questions

What demand factor should I use for generator sizing? +
Lighting loads: 1.0 (assume all on during a power failure). Computers/UPS: 0.9. Air conditioning: 0.6–0.8 (not all zones at peak simultaneously). Motors (pumps, fans): 0.7–0.8. Lifts: 0.6 (not all lifts running simultaneously). Fire pumps: 1.0 (critical safety load, always size at 100%). Total connected load × appropriate demand factors = running load for generator sizing.
Why does motor starting require a bigger generator? +
When a motor starts (DOL – Direct On Line), it draws 5–8× its full-load current for 2–5 seconds. This sudden surge causes a voltage dip on the generator. If the voltage dip exceeds 15–20%, other connected equipment (especially electronics and other motors) may trip. The generator must be sized so that adding the largest motor start kVA does not cause more than 15% voltage dip – this is the "motor starting" sizing criterion.
What is the difference between prime power and standby power ratings? +
Standby power: maximum power for emergency use when grid fails, not more than 200 hours/year, no continuous overload permitted. Prime power: continuous power generation as main source, 80% average load, 100% rated power available continuously. An generator rated "100 kVA standby / 90 kVA prime" means: 100 kVA only for standby duty (grid-parallel or grid-failure backup), 90 kVA if operating continuously as prime power.
What size fuel tank should I provide with a DG set? +
CPCB regulations and NBC 2016: minimum 8 hours' fuel storage at full load. Typical diesel consumption: 0.28–0.35 litres/kWh (about 0.3 L/kWh at 70% load). For a 100 kVA (80 kW) generator at 70% load for 8 hours: 80×0.7×0.3×8 = 134 litres minimum. CPCB allows maximum 2,000 litres above ground without petroleum licence; larger tanks need petroleum storage licence.
Does the DG set need to be synchronized with the grid? +
For pure standby (grid fails, DG starts, grid returns, DG stops): no synchronisation needed. For parallel operation (DG supplements grid supply or export surplus): synchronisation equipment (auto-synchroniser, synchronising breaker, protection relays) is mandatory. Parallel operation requires prior approval from the DISCOM. For most buildings with backup-only DG sets: transfer switch (ATS – Automatic Transfer Switch) is sufficient, no synchronisation needed.

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

⚡ Standby Generator Sizing Calculator
Reference: IS 14280 / NBC 2016 Part 8