Electrical Load Estimation
Connected load, maximum demand, diversity and design kVA per NBC 2016 and IS 732.
Free · no sign-in requiredPurpose
Establish the maximum demand and design kVA for a building or plant so that the incoming supply, transformer, main panel and standby generator can be sized at concept stage.
Inputs required
Connected load by category, the demand factor applicable to each, an overall diversity factor, system voltage, phase configuration, power factor and a spare-capacity allowance for future growth.
Standards followed
National Building Code of India 2016 Part 8 Section 2, IS 732 for electrical wiring installations, and IS 8623 for switchgear assemblies. Load categories and demand factors follow established consulting practice for the occupancy.
Inputs
Results
- Total connected load
- 590.0 kW
- Sum of category demands
- 407.5 kW
- After diversity + spare
- 426.0 kW
- Reactive demand
- 245.4 kVAr
- Design apparent power
- 491.7 kVA
- Resultant power factor
- 0.866
- Load factor (demand/connected)
- 72.2 %
- Full-load current at 415 V
- 684 A
- Lighting
- 40.0 kW × DF 0.90 = 36.0 kW
- Small power
- 60.0 kW × DF 0.50 = 30.0 kW
- HVAC
- 250.0 kW × DF 0.85 = 212.5 kW
- Motors
- 90.0 kW × DF 0.70 = 63.0 kW
- Lifts
- 45.0 kW × DF 0.60 = 27.0 kW
- Fire
- 75.0 kW × DF 0.20 = 15.0 kW
- Misc
- 30.0 kW × DF 0.80 = 24.0 kW
- Connected load = Σ (kW × qty)
- Category demand = kW × qty × demand factor
- Maximum demand = Σ demand / diversity × (1 + spare)
- kVAr = Σ P·tan(cos⁻¹ PF); kVA = √(kW² + kVAr²)
- I = kVA × 1000 / (√3 × V)
- Category power factors: lighting 0.95, small power 0.90, HVAC/motors/lifts 0.85, misc 0.90.
- Standby loads (fire pumps, sprinkler) are non-coincident with normal demand and carry a low demand factor; DG sizing must still cover them.
- Three-phase four-wire 415 V / 50 Hz system assumed for current calculation.
- Demand factors must be confirmed against the actual tenancy schedule and utility tariff.
Engineering reference & methodology
Connected load is not demand
Connected load is the arithmetic sum of every rated load in the installation. It never occurs, because not all equipment runs at once or at full rating. Maximum demand is the connected load reduced by a demand factor per category and then by an overall diversity factor across categories. Sizing a supply on connected load produces a transformer and cable that are oversized, expensive and running at poor efficiency and poor power factor at part load.
Demand and diversity factors
The demand factor accounts for equipment within a category not operating simultaneously or at full load — lighting typically approaches unity, small power and socket circuits are much lower, and motor loads depend on process duty. The diversity factor then accounts for different categories peaking at different times of day: cooling peaks in the afternoon, lighting in the evening. Both factors must be justified against the occupancy and the operating pattern, and stated as assumptions in the design basis.
From kW to design kVA
Apparent power is real power divided by the power factor, so design kVA equals maximum demand in kW divided by the design power factor. Line current for a three-phase supply is the apparent power divided by the product of the square root of three and the line voltage; for single phase it is apparent power divided by voltage. Equipment ratings, cable ampacity and protection settings are all governed by this current, not by the kW figure.
Power factor and its consequences
A poor power factor raises the current for the same useful power, which increases cable and transformer losses, increases voltage drop, consumes switchgear capacity and attracts a utility penalty in most tariffs. Sizing at an optimistic power factor is a common error: the installed plant then cannot deliver the design load, and correction equipment has to be retrofitted into a switchroom with no space allocated for it.
Spare capacity
A spare allowance is added after diversity so that future fit-out, added process equipment or electric vehicle charging can be accommodated without replacing the incoming infrastructure. This is a commercial decision as much as a technical one, because transformer and incoming cable capacity is expensive and disruptive to increase later, whereas panel and busbar capacity is comparatively easy to extend.
Verification before the result is used
Cross-check the estimate against a measured or benchmarked load density in watts per square metre for the same building type; a result far from that benchmark usually indicates a category with a wrong demand factor. Confirm that the resulting current is consistent with the intended transformer rating and its permitted loading, that the standby generator has been sized on the essential portion of the demand rather than the whole, and that the figure has been reconciled with the utility's declared supply capacity before the connection application is made.
Results are preliminary design guidance. Verify every value against the edition of each standard adopted by your project or authority having jurisdiction, and against manufacturer performance data, before using it in design or procurement.
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