What is Electrical Load Calculation?
Electrical load calculation is the process of determining the total electrical power demand of a building or facility. It involves identifying every piece of electrical equipment, determining its power rating, applying demand factors to account for diversity, and calculating the resulting current that the supply system must carry.
An accurate electrical load calculation is required for:
- Sizing the utility supply — requesting the correct supply capacity from the electricity authority
- Transformer sizing — selecting the correct kVA rating for the HV/LV transformer
- Main LV switchboard sizing — busbar ratings, incomer switch and protection devices
- Standby generator sizing — ensuring the generator can carry all essential loads
- Main incoming cable sizing — selecting the correct cable cross-section area
- UPS system sizing — for critical and essential power loads
- Power factor correction — sizing capacitor banks to meet utility PF requirements
Connected Load vs Design Load — Key Difference
The two most important figures in any electrical load calculation are the connected load and the design load. These are different and must not be confused.
Connected Load
The connected load is the sum of the rated power of all electrical equipment in the building, assuming every single item is running at 100% of its rated power simultaneously. This is the theoretical maximum — in practice it never occurs because not all equipment runs at the same time or at full load.
Connected Load (kW) = Sum of all equipment rated power in kW
Design Load
The design load (also called the maximum demand) is the connected load multiplied by demand factors that account for the reality that not all equipment operates simultaneously or at full rated power. The design load is what the electrical system must actually be designed to carry.
Design Load (kW) = Connected Load (kW) x Demand Factor
| Parameter | Connected Load | Design Load |
| Definition | Sum of all rated equipment power | Realistic maximum demand |
| Demand factor applied | No — assumes 100% everything | Yes — accounts for diversity |
| Used for | Individual circuit sizing | Main switchboard, transformer, generator |
| Typical ratio | 100% | 60 to 80% of connected load |
What is Demand Factor in Electrical Load Calculation?
The demand factor is the ratio of the maximum demand of a system to its total connected load. It accounts for the fact that not all electrical loads operate simultaneously and not all equipment runs at its full rated power at the same time.
Demand Factor = Maximum Demand / Total Connected Load
A demand factor of 1.0 means 100% of the connected load is assumed to be on simultaneously — used for lighting and critical equipment. A demand factor of 0.5 means only 50% of the connected load is assumed active at peak — used for general power outlets in office buildings.
| Load Category | Demand Factor | Reference Standard | Notes |
| Lighting — general | 1.0 | IEC 60364-3 | All lights could be on simultaneously |
| Lighting — emergency | 1.0 | BS 5266 | Must always be available |
| Office power outlets | 0.4 – 0.5 | CIBSE Guide K | Not all outlets used at once |
| Dedicated workstations | 0.6 – 0.7 | CIBSE Guide K | Higher usage than general outlets |
| Server room / IT | 0.9 | CIBSE Guide K | Near continuous full load |
| Chillers | 0.8 – 0.9 | CIBSE Guide K | Part load most of the time |
| AHUs and fans | 0.75 – 0.85 | CIBSE Guide K | Variable speed drives common |
| FCUs and split ACs | 0.7 | CIBSE Guide K | Not all spaces at peak load |
| Duty pumps | 0.75 – 0.85 | CIBSE Guide K | Variable flow systems |
| Fire pumps | 1.0 | NFPA 20 / BS 9990 | Must always be available at full load |
| Lifts and escalators | 0.5 | CIBSE Guide K | Not all running simultaneously |
| Catering equipment | 0.7 – 0.8 | CIBSE Guide K | Diversity in kitchen equipment |
How to Calculate kVA from kW
Once the design load in kW is established, it must be converted to kVA (kilovolt-amperes) to size the transformer, generator and main supply. The relationship between kW and kVA is determined by the power factor.
kVA = kW / Power Factor
The power factor is the ratio of real power (kW) to apparent power (kVA). A power factor of 1.0 means all the apparent power is doing useful work. In practice, electrical loads in buildings have power factors below 1.0 due to inductive loads such as motors and fluorescent lighting.
| Building Type | Typical Overall Power Factor | Notes |
| Modern office building (LED lighting, VFDs) | 0.90 – 0.95 | After PF correction |
| Commercial building without PF correction | 0.75 – 0.85 | Before capacitor bank |
| Industrial facility | 0.70 – 0.85 | Large motor loads |
| Hospital | 0.85 – 0.90 | Mixed loads |
| Data centre | 0.90 – 0.95 | UPS systems typically have good PF |
| Retail / Shopping mall | 0.80 – 0.90 | Mixed lighting and power |
How to Calculate Current per Phase
The current per phase (in Amps) is calculated from the total kVA and the supply voltage. This figure is used to size the main incoming cables and the main incomer protective device.
For 3-phase supply:
Current (A) = kVA x 1000 / (Square Root of 3 x Voltage)
Current (A) = kVA x 1000 / (1.732 x 400) — for 400V 3-phase supply
For single phase supply:
Current (A) = kVA x 1000 / Voltage
Current (A) = kVA x 1000 / 230 — for 230V single phase supply
| Total kVA | Current at 400V 3-Phase (A) | Cable Size (approx) | MCCB Rating (A) |
| 50 kVA | 72 A | 25 mm² copper | 100 A |
| 100 kVA | 144 A | 70 mm² copper | 200 A |
| 200 kVA | 289 A | 150 mm² copper | 400 A |
| 315 kVA | 455 A | 240 mm² copper | 630 A |
| 500 kVA | 722 A | 2 x 240 mm² copper | 800 A |
| 630 kVA | 909 A | 2 x 300 mm² copper | 1000 A |
| 1000 kVA | 1443 A | 3 x 300 mm² copper | 1600 A |
Motor Efficiency and Electrical Load Calculation
When calculating the electrical load of motors — including HVAC fans, pumps, compressors and other mechanical equipment — it is important to use the electrical input power, not the shaft output power. The relationship between them is the motor efficiency.
Electrical Input Power (kW) = Shaft Power (kW) / Motor Efficiency
For example, a pump with a shaft power of 7.5 kW driven by an IE3 motor with 90% efficiency will draw 7.5 / 0.90 = 8.33 kW of electrical power.
| IEC Efficiency Class | Efficiency Range | Application | Notes |
| IE1 — Standard | 0.80 – 0.88 | Being phased out | Not permitted for new installations in EU/UK |
| IE2 — High Efficiency | 0.84 – 0.91 | Limited applications | Being replaced by IE3 |
| IE3 — Premium Efficiency | 0.88 – 0.95 | Standard for new installations | Mandatory in EU, UK, Middle East from 2023 |
| IE4 — Super Premium | 0.90 – 0.96 | High efficiency applications | VFD driven motors |
Power Factor Correction
Most utilities require buildings to maintain a minimum power factor — typically 0.90 or above. Buildings that fail to meet this requirement may face financial penalties on their electricity tariff. Power factor correction is achieved by installing capacitor banks that supply reactive power locally, reducing the reactive power drawn from the utility.
Required kVAR for PF correction = kW x (tan(arccos(existing PF)) – tan(arccos(target PF)))
| Existing PF | Target PF 0.95 | kVAR per 100 kW load | Notes |
| 0.70 | 0.95 | 72 kVAR | Large correction needed |
| 0.75 | 0.95 | 62 kVAR | Significant correction |
| 0.80 | 0.95 | 42 kVAR | Common in older buildings |
| 0.85 | 0.95 | 29 kVAR | Moderate correction |
| 0.90 | 0.95 | 16 kVAR | Minor correction |
| 0.95 | 0.95 | 0 kVAR | No correction needed |
IEC 60364 Requirements for Electrical Load Calculation
IEC 60364 (Electrical Installations of Buildings) is the primary international standard governing electrical load calculation and system design. Key requirements include:
- All electrical installations must be designed with a minimum 20% spare capacity above the design load per IEC 60364-3
- Demand factors must be applied based on the type of installation and actual usage patterns
- Motor circuits must be sized for the full load current with an additional 25% for starting current per IEC 60364-4-43
- Power factor must be maintained above the utility requirement — typically 0.90 minimum
- Standby equipment must NOT be included in load calculations — only duty equipment
- Fire protection systems must use a demand factor of 1.0 and must be supplied from a dedicated circuit
- Hospital essential systems (Category 1 and 2) must have dedicated circuits with UPS and generator backup
Spare Capacity Requirements
IEC 60364 and most project specifications require a minimum spare capacity to be added to the design load before sizing the main electrical infrastructure. This spare capacity allows for future load growth and unforeseen additions without requiring a complete infrastructure upgrade.
| Building Type | Minimum Spare Capacity | Reference |
| Standard commercial building | 20% | IEC 60364-3 |
| Hospital — general areas | 25% | HTM 06-01 |
| Hospital — critical care | 30% | HTM 06-01 |
| Data centre | 25 – 40% | TIA-942 / Uptime Institute |
| Industrial facility | 20 – 25% | IEC 60364-3 |
| Mixed use development | 20% | IEC 60364-3 |
Frequently Asked Questions
What is the difference between connected load and maximum demand?
The connected load is the sum of all rated equipment power assuming everything operates at 100% simultaneously — the theoretical maximum. The maximum demand (design load) is the connected load multiplied by demand factors that account for diversity — the realistic peak load the system must carry. The maximum demand is always lower than the connected load for buildings with mixed load types.
How do I calculate kVA from kW for a building?
Divide the design load in kW by the overall power factor to get kVA. For example, a building with a design load of 500 kW and a power factor of 0.85 requires 500 / 0.85 = 588 kVA of transformer capacity. Always add the required spare capacity (minimum 20% per IEC 60364) before dividing by the power factor.
What demand factor should I use for office power outlets?
Per CIBSE Guide K, general office power outlets should use a demand factor of 0.4 to 0.5 (40 to 50%). This means only 40 to 50% of the outlet capacity is assumed to be in use simultaneously at peak load. Dedicated equipment outlets such as server racks, photocopiers and AV equipment should use higher demand factors of 0.7 to 0.9.
Should standby pumps and fans be included in the electrical load calculation?
No — standby equipment must NOT be included in the electrical load calculation. Standby equipment only operates when the duty equipment fails and does not contribute to the normal running load. Including standby equipment would double-count the load and result in oversized and unnecessarily expensive electrical infrastructure.
What is a typical power factor for a commercial building?
A modern commercial building with LED lighting and variable frequency drives on HVAC equipment typically achieves an overall power factor of 0.88 to 0.95 after power factor correction. Without correction, the natural power factor of a commercial building is typically 0.75 to 0.85 due to inductive motor loads and older lighting equipment. Most utilities require a minimum power factor of 0.90.
How much spare capacity should I add to the electrical load?
IEC 60364-3 requires a minimum of 20% spare capacity above the design load for all new electrical installations. For critical facilities such as hospitals and data centres, 25 to 30% spare capacity is typically specified to allow for future expansion without requiring transformer or switchboard replacement.