Electrical Load Calculator — Building Load Estimation Tool | MEP Master Guide

⚡ Electrical Load Calculator

Building Load Estimation — IEC Standard | MEP Master Guide

⚙ Project Settings

Project Name

Supply Voltage

System Type

Overall PF Target: 0.85 – 0.95

Spare Capacity % IEC: 20% minimum

Connected Load

0.00

Design Load

0.00

kW (after demand factors)

Total kVA

0.00

kVA (incl. spare)

Current / Phase

0.0

Amps

Power Factor

0.85

Overall PF

💡 Lighting

0.00

kW design load

🔌 Small Power

0.00

kW design load

❄️ HVAC

0.00

kW design load

⚙️ Mechanical

0.00

kW design load

🔧 General

0.00

kW design load

💡 Lighting Load

Demand factor for lighting: 1.0 (100%) per IEC — all lighting circuits assumed fully loaded. Use diversity factor for large installations.

Quick add:

Description

Unit

Qty / Area Number of fittings

Watts each / m² Per fitting or per m²

Demand Factor Usually 1.0

📋 Complete Load Schedule

#	Category	Description	Qty	Unit Load (W)	Connected (kW)	Demand Factor	Design Load (kW)
No items added yet. Add items from the category tabs above.

🧮 Calculations Summary

Add items to see calculations.

⚙️ Section 1 — Project Settings

Field	What to Enter	Where to Get It	Typical Value
Project Name	Name of the building or area you are calculating the load for	Project brief, drawing title block, or your own reference	e.g. "Office Block — Level 3"
Supply Voltage	The voltage of the main electrical supply to the building	Utility supply letter, incoming switchboard single line diagram. Middle East/UK/Europe = 400V	400V (most countries)
System Type	Whether the supply is 3-phase or single phase. Almost all commercial buildings use 3-phase	Utility supply letter or incoming switchboard drawing	3-Phase for all commercial
Overall Power Factor	Ratio of real power (kW) to apparent power (kVA). Affects kVA and current calculations	Utility requirement (usually 0.90 minimum) or CIBSE Guide K. Use 0.85 if unknown	0.85 – 0.92 typical
Spare Capacity %	Additional capacity for future expansion. IEC 60364 requires minimum 20% spare on all new installations	Client brief or project specification. Hospitals and data centres: 25–30%	20% standard, 25% critical

💡 Section 2 — Lighting Load

Field	What to Enter	Where to Get It	Typical Value
W per fitting	Use when you know exact number of fittings and wattage of each. Best for detailed design stage	Lighting layout drawing (count fittings) and luminaire datasheet (get wattage)	LED downlight: 18–24W
W per m²	Use at early design stage when only floor area is known. A power density figure is used instead of counting fittings	Floor area from architectural drawing. Power density from CIBSE Guide A or SLL Lighting Guide	Office LED: 10–15 W/m²
Demand Factor	Proportion of lighting assumed on simultaneously. Always 1.0 for lighting — all lights could be on at the same time	IEC 60364-3 or CIBSE Guide K. Emergency lighting: 1.0. General lighting: 1.0	1.0 for all lighting

🔌 Section 3 — Small Power / Outlets

Field	What to Enter	Where to Get It	Typical Value
Power per outlet	The rated wattage of each outlet or piece of connected equipment	Equipment datasheet, client equipment schedule, or standard assumption per outlet type	Office outlet: 150–200W
Quantity	Total number of outlets or equipment items of this type in the area	Power layout drawing or outlet schedule — count from electrical drawing	Count from drawing
Demand Factor	Proportion of outlets assumed in use simultaneously. Not all outlets will be used at the same time — this accounts for diversity	CIBSE Guide K Table 4.1, BS 7671 Appendix 4. Office general outlets: 0.4–0.5. Dedicated equipment: 0.7–0.9	Office outlets: 0.4–0.5

❄️ Section 4 — HVAC Equipment Load

Field	What to Enter	Where to Get It	Typical Value
Power (kW shaft)	The mechanical shaft power of the motor — NOT cooling capacity. Enter motor kW, not tonnage or kW of cooling	HVAC equipment datasheet — look for "Motor kW" or "Input Power kW". Mechanical engineer equipment schedule or vendor technical submittal	AHU motor: 5.5–55kW
Motor Efficiency	Efficiency of the electric motor. Tool calculates: Electrical kW = Shaft kW ÷ Motor Efficiency	Motor datasheet or IEC 60034-30 efficiency class tables. IE3 motors required in most countries. If unknown use 0.88	IE3: 0.88–0.93
Demand Factor	Proportion of HVAC running simultaneously at full load. Chillers rarely run at 100% — this accounts for part-load operation	CIBSE Guide K or project M&E specification. Chillers: 0.8–0.9. AHUs: 0.75–0.85. FCUs: 0.7	Chiller: 0.85 \| AHU: 0.8

❄️ Important: For chillers enter the compressor motor kW (electrical input), not cooling capacity. Example: a 500kW cooling chiller typically has a 150–200kW compressor input. Check the chiller datasheet for "Compressor Input kW".

⚙️ Section 5 — Mechanical Equipment (Motors & Pumps)

Field	What to Enter	Where to Get It	Typical Value
Power (kW/HP)	Shaft power rating of the motor or pump — the mechanical output power. Can be entered in kW, W or HP	Motor nameplate data, pump datasheet, mechanical equipment schedule or pump curve. Look for "Shaft Power kW" or "Motor Rating kW"	CHW pump: 5.5–22kW
Motor Efficiency	Efficiency of the electric motor. Formula: Electrical kW = Shaft kW ÷ Motor Efficiency	Motor nameplate, manufacturer datasheet or IEC 60034-30 standard efficiency tables. IE3 motors mandatory in most countries from 2023	IE3 motor: 0.88–0.95
Demand Factor	Proportion of motors running simultaneously at full load. Do NOT add standby pumps — they do not contribute to load	CIBSE Guide K Table 4.2. CHW pumps: 0.8. Fire pumps: 1.0 (must always be 100%). Standby: 0 — do not add	Duty pump: 0.8 \| Fire: 1.0

⚙️ Standby equipment: Do NOT add standby pumps or standby fans to the load calculation. Only add duty equipment. Standby equipment does not contribute to electrical load as it only runs when the duty unit fails.

📊 Understanding the Results

Result	What it Means	How it is Used
Connected Load (kW)	Total rated power of ALL equipment assuming everything runs at 100% simultaneously — worst case maximum	Sizing individual circuit protective devices (MCBs) and cable sizes at equipment level
Design Load (kW)	Connected load multiplied by demand factors — the realistic expected maximum load at peak conditions	Sizing main LV switchboard, incoming supply cables and transformer
Total kVA	Design load with spare capacity added, divided by power factor. This is what the transformer and utility supply must provide	Transformer sizing (must be ≥ total kVA), utility capacity request, generator sizing
Current per Phase (A)	Current flowing in each phase. Formula: I = kVA × 1000 ÷ (√3 × V) for 3-phase	Sizing main incoming cable, main incomer switch and busbar rating
MCB/MCCB Rating	Recommended standard IEC breaker size = Current × 1.25 rounded up to next standard size	Starting point for main incomer protection. Verify with fault level and discrimination study

📋 Recommended Step-by-Step Workflow

Step 1 — Set Project Details

Enter project name, supply voltage (400V for Middle East/UK), select 3-phase, set power factor (0.85 default) and spare capacity (20% minimum per IEC).

Step 2 — Add Lighting Load

Use W/m² at early design stage or W per fitting at detailed design. Get floor areas from architectural plans. Use LED power densities from CIBSE Guide A.

Step 3 — Add Small Power

Count outlets from electrical drawing. Use client equipment schedule for specific loads. Apply demand factor 0.4–0.5 for general office outlets per CIBSE Guide K.

Step 4 — Add HVAC Loads

Get motor kW from mechanical engineer's equipment schedule or HVAC datasheets. Enter shaft kW and motor efficiency. Do NOT add standby equipment.

Step 5 — Add Mechanical & General

Add pumps, lifts, compressors from mechanical schedule. Add UPS, EV chargers under General. Fire pumps always use demand factor 1.0.

Step 6 — Review Results

Check Full Schedule tab for complete load list. Use kVA for transformer sizing. Use current (Amps) for main cable and incomer sizing. Check PF recommendation.

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.