Enter AHU equipment data
Use nameplate values where possible. Fan ratings are treated as mechanical motor output. Other listed loads are treated as electrical input.
Example Data Table
This example uses a 400 V three-phase AHU with a supply fan, return fan, electric heater, and control load.
| Input | Example value | Purpose |
|---|---|---|
| Supply fan motor output | 7.50 kW | Primary air movement |
| Return fan motor output | 3.00 kW | Return air movement |
| Electric heater | 12.00 kW | Air heating duty |
| Fan motor efficiency | 92% | Converts output to input power |
| VFD loss allowance | 3% | Allows for drive losses |
| Future capacity allowance | 20% | Supports later expansion |
Formula Used
The calculator uses simplified electrical planning formulas. It separates fan motor conversion from directly stated electrical loads.
Fan input kW = (Fan motor output kW ÷ Motor efficiency) × (1 + VFD loss)
Connected kW = Supply fan + Return fan + Compressor + Heater + Humidifier + ControlsDesign kW = Connected kW × Demand factor × (1 + Spare capacity)
kVA = Design kW ÷ Power factorThree-phase A = Design kW × 1000 ÷ (√3 × Voltage × Power factor)Single-phase A = Design kW × 1000 ÷ (Voltage × Power factor)
Minimum ampacity = Line current × Continuous-load factorMonthly kWh = Diversity-adjusted operating kW × Daily hours × Monthly days
How to Use This Calculator
- Select the phase arrangement and enter the actual feeder voltage.
- Enter fan motor shaft ratings and their expected efficiency.
- Add direct electrical loads for heaters, humidifiers, compressors, and controls.
- Set demand, spare capacity, and continuous-load values from the project basis.
- Enter operating hours to estimate monthly energy consumption.
- Review the results above the form, then export or print them.
AHU Electrical Load Planning
An air handling unit combines several electrical loads. Fan motors usually dominate the base load. Electric heaters can quickly become the largest connected load. Humidifiers, compressors, controls, drain pumps, and accessories also matter. A useful calculation lists every item before selecting a feeder.
Start with the equipment schedule. Use actual nameplate values when they are available. A fan motor rating may show shaft output instead of electrical input. The calculator converts that rating with motor efficiency. It also adds a variable frequency drive allowance. This gives a better planning value for supply and return fans.
Electric heaters require close attention. Their rating is normally direct electrical input. A heater bank can raise current sharply during cold-weather operation. Humidifiers can behave similarly. Do not assume these loads operate at the same time unless the control sequence confirms it. Use a documented demand factor when diversity is valid.
Power factor affects feeder current. The same real power draws more current when power factor is lower. Three-phase current uses line voltage and the square-root-of-three relationship. Single-phase current uses voltage directly. Use the supply voltage that reaches the equipment, not a nominal value copied from another project.
The calculated minimum circuit ampacity includes your continuous-load factor. This is not the final conductor selection. Conductor size also depends on insulation rating, ambient temperature, installation method, grouping, voltage drop, and local rules. Breaker selection must consider motor starting, equipment protection limits, and manufacturer instructions.
Energy results help with budget planning. They use diversity-adjusted operating power, daily hours, and monthly operating days. Actual energy will vary with airflow, static pressure, weather, load control, and fan speed. A VFD can reduce fan energy substantially at part load. It does not eliminate the need for a proper full-load electrical check.
Keep the calculation with the design record. Update it after approved equipment submittals arrive. Compare the final nameplate MCA and MOCP values with the preliminary feeder. Review coordination with upstream protection. Confirm isolation, service access, earthing, control power, and emergency operating requirements before construction begins.
Good records also simplify commissioning. Mark each circuit source, disconnect, voltage, phase, load, and protection rating. Coordinate AHU controls with the electrical sequence. A locked rotor or inrush condition can influence protection. Review manufacturer starting guidance before finalizing distribution equipment for final feeder planning.
Frequently Asked Questions
1. What loads should an AHU calculation include?
Include supply and return fans, electric heaters, humidifiers, compressors, controls, pumps, dampers, lights, and manufacturer-listed accessories. Use dedicated feeder loads where equipment requires separate circuits.
2. Is fan motor kW always electrical input?
Not always. Some schedules show mechanical shaft output. Divide output by motor efficiency to estimate electrical input. Use the nameplate input or full-load current when it is available.
3. Why include a VFD loss allowance?
A variable frequency drive has conversion and control losses. Adding a modest allowance improves preliminary input-power estimates. Final design should use the selected drive manufacturer data.
4. What demand factor should I use?
Use 100% when listed loads can operate together. Reduce it only when a documented operating sequence prevents simultaneous operation. Do not use diversity merely to make a feeder smaller.
5. Does the calculator select the final breaker?
No. It shows a preliminary next standard size from calculated circuit ampacity. Final overcurrent protection must follow equipment instructions, motor requirements, coordination, and local electrical rules.
6. Why is power factor needed?
Power factor converts real power to apparent power and affects current. Lower power factor means higher current for the same kW. Use equipment data or a reasonable project estimate.
7. Can I use this for a packaged rooftop unit?
Yes, for preliminary planning. Enter electrical input values for compressor and heater sections. Verify the packaged unit nameplate MCA and MOCP before choosing conductors or protection.
8. What is the purpose of spare capacity?
Spare capacity reserves electrical headroom for future changes, options, or small upgrades. It is a project decision. It does not replace verification of actual installed equipment.
9. Does monthly energy equal utility billing energy?
No. It is an estimate based on entered hours and diversity. Billing energy changes with weather, fan control, occupancy, static pressure, heating demand, and maintenance condition.
10. Should electric heat use a motor efficiency value?
No. Electric resistance heater ratings are normally electrical input. Enter the heater kW directly. Do not divide heater load by fan motor efficiency.
11. What should be checked after the calculation?
Check nameplates, MCA, MOCP, short-circuit rating, voltage drop, conductor derating, disconnect rating, upstream coordination, isolation requirements, and the final approved control sequence.