Load Calculation and Rated Current: Demand and Diversity Factors
Load calculation: Confusing connected load with actual demand either inflates the switchboard or leaves it undersized. The route runs through the demand factor.
ContentsLoad calculation is the first step that decides how many amperes of busbar, which incoming device and which cable size a switchboard will be built with. A wrong figure is expensive in both directions: an oversized board wastes capital, an undersized one runs hot and trips early.
Connected load is not demand
Connected load is the sum of the nameplate ratings of every consumer fed from the board. In real operation those loads never run simultaneously at full output. Demand is the power actually drawn from the supply within a given interval. Two coefficients bridge the gap: the utilisation (demand) factor and the simultaneity factor.
Demand factor and simultaneity factor
The utilisation factor states how much of its rating a machine actually draws; pumps and fans typically operate below nameplate. The simultaneity factor expresses the probability that several circuits on the same board are loaded at the same moment. Multiplying both and applying them to the connected load yields the design load. These coefficients are not guesses: they should come from the process description, the shift pattern and, where available, metered data.
How the rated current is established
IEC 61439-1 uses the rated diversity factor (RDF) to describe assemblies: the ratio of the current that outgoing circuits can carry simultaneously to the sum of their rated currents. The rated current of the assembly is chosen from the continuous capability of the main busbar and incoming device at the actual ambient temperature, and the temperature-rise verification is an inseparable part of that step. Spare capacity is added on top of the design load, but excessive spare inflates busbar and enclosure cost quickly.
Typical diversity factors by circuit count
| Number of outgoing circuits | Typical RDF | Comment |
|---|---|---|
| 2 – 3 | 0.9 | Few circuits, high simultaneity |
| 4 – 5 | 0.8 | Mixed load profile |
| 6 – 9 | 0.7 | Common range for industrial distribution boards |
| 10 and above | 0.6 | Sub-boards with many ways |
| Measured profile | As measured | Preferred where energy meter data exists |
Once the rated current is set, temperature correction follows: as internal air temperature rises, the carrying capability of devices and busbars falls. See our articles on derating, internal heat load and, for the cable side, cable sizing and voltage drop.
Four mistakes seen most often on site
- Treating connected load as the rated current and building an unnecessarily large board
- Sizing breakers and cables for motor loads without considering starting current
- Leaving spare capacity only on the main busbar and none in the outgoing ways
- Assuming 35 °C ambient while summer conditions inside the room are far higher
Key takeaways: load calculation
- Connected load is not demand — Connected load is the sum of the nameplate ratings of every consumer fed from the board.
- Demand factor and simultaneity factor — The utilisation factor states how much of its rating a machine actually draws; pumps and fans typically operate below nameplate.
- How the rated current is established — IEC 61439-1 uses the rated diversity factor (RDF) to describe assemblies: the ratio of the current that outgoing circuits can carry simultaneously to the sum of their rated currents.
- Typical diversity factors by circuit count — Once the rated current is set, temperature correction follows: as internal air temperature rises, the carrying capability of devices and busbars falls.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Where do I get a realistic demand factor?
The most reliable source is metered data from the plant itself. For new installations, the operating scenario supplied by process engineering plus data from comparable plants is used, and the accepted coefficient with its justification is recorded in the design file.
How much spare capacity is reasonable?
A common practice is 20 to 30 per cent above the design load. Reserving part of it in busbar capability and part as empty modules and terminal space makes later additions far easier.
Is the assembly rated current the same as the incoming breaker rating?
No. The incoming device is one component; the rated current of the assembly is valid only when busbars, connections and enclosure behaviour have been verified together. Fitting a 630 A breaker does not make the board a 630 A assembly.
Related articles
- Switchboard Heat Load Calculation and Choosing a Cooling Solution
- Busbar Current Carrying Capacity and Derating Calculation
- Cable Sizing and Voltage Drop Calculation
- EV Charging Station Distribution Board: Protection and Load Management
- All technical articles
Related Devpan solution: electrical panel design services.
Devpan verifies design load and diversity factor against the customer load schedule, then closes the busbar and enclosure selection with a temperature-rise calculation. Send us your load list and we will settle the rated current together.
