Calculator
Example Data Table
| Motor | HP | Quantity | Voltage | Phase | Power Factor | Efficiency |
|---|---|---|---|---|---|---|
| Pump motor | 5 | 2 | 415 V | 3 | 0.85 | 90% |
| Hoist motor | 10 | 1 | 415 V | 3 | 0.85 | 90% |
| Mixer motor | 7.5 | 1 | 415 V | 3 | 0.85 | 90% |
Formula Used
For single phase current:
I = HP × 746 / (V × Efficiency × Power Factor)
For three phase current:
I = HP × 746 / (√3 × V × Efficiency × Power Factor)
Total running current is calculated by multiplying each motor current by quantity.
Total Running Current = Sum of all motor row currents
The feeder motor load adds extra allowance for the largest motor.
Feeder Current = Total Running Current + 25% of Largest Motor Current
The final ampacity target includes demand, safety, spare capacity, and derating.
Minimum Ampacity = Feeder Current × Demand × Safety × Spare Allowance / Derating
How to Use This Calculator
Enter the supply phase and voltage first.
Add the common power factor and motor efficiency.
Enter demand factor, safety factor, spare capacity, and derating.
Add each motor horsepower and quantity.
Use the FLA override when nameplate current is known.
Press the calculate button to view the load result.
Download the CSV or PDF report after calculation.
Article
About Multiple Motor Load Planning
A construction site often runs several motors at once. Pumps, hoists, mixers, fans, compressors, and saws can share one feeder. Each motor adds running current. The largest motor also needs extra allowance because it can pull more current during heavy work. This calculator helps estimate that combined load before equipment is connected.
Why This Estimate Matters
Motor load planning protects feeders, panels, temporary boards, and generators. A weak allowance can cause nuisance trips. It can also create voltage drop. Too much guessing can raise rental cost or material cost. A structured estimate gives a better starting point for site discussions.
The calculator accepts horsepower, quantity, voltage, phase, efficiency, power factor, derating, demand factor, spare capacity, and starting multiplier. You may enter a known full load amp value. When that value is entered, it overrides the horsepower based current. This helps when nameplate data is already available.
How The Load Is Built
The running current is the sum of all motors. The design current adds twenty five percent of the largest single motor. This mirrors common motor feeder practice. Then demand, safety, spare capacity, and derating are applied. The result is an estimated ampacity target. The tool also estimates apparent power, real power, and peak starting current.
Good Inputs Improve Results
Use nameplate full load amps when possible. Use realistic efficiency and power factor values. For older motors, avoid overly optimistic values. For temporary construction power, include future tools that may connect later. Long cable runs need extra attention because voltage drop increases with distance.
Planning Notes
This calculator is for estimating. Final design should follow local code, equipment labels, site conditions, and a qualified electrical review. Different jurisdictions may use special tables or inspection rules. The result should guide early sizing, comparison, and reporting. It should not replace engineered drawings for critical installations.
Exporting Results
After calculation, export the numbers as a CSV file for spreadsheets. You can also download a simple PDF for field notes, approvals, or contractor records. Keep exported assumptions with the result. Later changes in motor quantity, voltage, cable route, or duty cycle can change the required capacity and the final recommendation. Review each change before ordering equipment and feeder hardware.
FAQs
1. What is a multiple motor load calculation?
It estimates the combined current of several motors. It also adds allowance for the largest motor, safety margin, spare capacity, derating, and starting current.
2. Why is the largest motor treated differently?
The largest motor can create the highest stress on the feeder. Adding an extra allowance helps estimate a safer feeder load for grouped motor service.
3. Should I use horsepower or nameplate FLA?
Use nameplate FLA when available. It reflects the actual motor label. Horsepower estimates are useful for early planning when the exact motor data is missing.
4. What does derating mean here?
Derating reduces usable ampacity because of temperature, cable grouping, conduit fill, installation method, or local code rules. Lower derating increases required ampacity.
5. What is starting current multiplier?
It estimates motor inrush during starting. Many motors draw several times their running current at startup. The calculator applies this multiplier to the largest motor.
6. Can this size a generator?
It can support early generator planning by showing running kVA and starting current. Final generator sizing should consider motor starters, duty cycle, and site loads.
7. Does this replace electrical design?
No. It is an estimating tool. Final construction power design should follow local code, approved drawings, equipment labels, and a qualified electrical review.
8. Why include spare capacity?
Construction sites often change. Spare capacity helps allow for added motors, temporary tools, longer duty cycles, and future equipment connections.