Calculator Inputs
Example Data Table
| Case | HP | Voltage | Phase | Length | Material | Planning Use |
|---|---|---|---|---|---|---|
| Pump feeder | 10 | 460 | 3 | 100 ft | Copper | General motor branch planning |
| Fan feeder | 25 | 480 | 3 | 220 ft | Aluminum | Long run voltage drop check |
| Shop motor | 5 | 240 | 1 | 75 ft | Copper | Small equipment feeder estimate |
Formula Used
Three phase current: I = HP × 746 / (√3 × V × Efficiency × PF)
Single phase current: I = HP × 746 / (V × Efficiency × PF)
Feeder ampacity: Largest motor amps × ampacity multiplier + other motor amps
Corrected ampacity: Table ampacity × ambient correction × conductor adjustment
Three phase voltage drop: VD = √3 × I × R × Length / 1000
Single phase voltage drop: VD = 2 × I × R × Length / 1000
Breaker guide: Largest motor amps × breaker multiplier + other motor amps
Conduit fill: Total conductor area / conduit area × 100
How to Use This Calculator
- Enter motor horsepower, voltage, phase, efficiency, and power factor.
- Enter nameplate current when the motor data plate is available.
- Add largest motor current and other motor current for group feeders.
- Set feeder ampacity and breaker planning multipliers.
- Enter one way length and your maximum voltage drop target.
- Select material, conduit size, correction factors, and raceway conductors.
- Press the calculate button and review the result above the form.
- Download the CSV or PDF report for project notes.
Motor Feeder Planning Guide
A motor feeder is more than one wire size. It must carry starting duty, running load, and practical installation limits. This calculator gives a structured estimate before detailed engineering review. It is useful for shops, panels, pumps, fans, compressors, and machine circuits.
What The Tool Checks
The first check is motor current. You may use calculated current from horsepower, voltage, efficiency, and power factor. You may also enter nameplate current when that value is known. Nameplate current usually gives a better project estimate.
The second check is feeder ampacity. A common method sizes conductors at one hundred twenty five percent of the largest motor current plus other motor loads. This supports heating margin during normal operation. The multiplier field lets you model local rules or project standards.
The third check is voltage drop. Long feeders can start motors poorly. They can also waste energy and create nuisance trips. The calculator estimates drop from conductor resistance, length, phase type, and parallel sets. It then compares the result with your selected limit.
Conduit And Breaker Review
Conduit fill is also estimated. The tool uses selected conduit area, conductor area, grounding conductors, spare conductors, and parallel sets. This helps reveal crowded raceways early. It does not replace the final raceway schedule.
Breaker guidance is shown separately. Motor short circuit protection often differs from normal load protection. The breaker multiplier helps create a planning value, then the tool rounds it to a standard size. Always coordinate protection with equipment ratings and local code.
Good Design Practice
Use conservative entries when the final field path is unknown. Measure one way distance carefully. Check termination temperature ratings. Confirm copper or aluminum choices with the project specification. Verify ambient corrections and conductor adjustment factors. For multiple motors, enter the largest motor current and the total of other motor loads. Keep a record of assumptions.
This calculator is for planning, estimating, and learning. It helps compare options quickly. A licensed professional should approve final feeder sizes, overcurrent devices, and installation details. Document each change after field checks. Small changes in route length, conduit type, or terminal rating can affect the answer. Recalculate safely before ordering cable, lugs, disconnects, raceway, and final material release today.
FAQs
1. What does this motor feeder calculator size?
It estimates feeder conductor size, adjusted ampacity, voltage drop, conduit fill, and breaker planning value. It is for early design, estimating, and comparison. Final installation choices should be checked by a qualified electrical professional.
2. Should I use horsepower or nameplate current?
Use nameplate current when it is available. It reflects the actual motor data better than a calculated value. Horsepower calculation is useful when the motor has not been selected yet.
3. Why is the feeder multiplier set to 125 percent?
Many motor feeder methods include extra ampacity for the largest motor. The default supports common planning practice. You can change the multiplier to match your project rule or local requirement.
4. Does this replace an electrical code review?
No. It is a planning calculator. It does not replace local code review, engineered drawings, equipment labeling, field verification, or authority requirements.
5. How is voltage drop handled?
The tool uses one way length, running current, conductor resistance, phase type, and parallel sets. It compares the result with your selected voltage drop percentage.
6. Why is conduit fill included?
Conduit fill helps check whether selected conductors can fit within a chosen raceway. The estimate includes phase conductors, grounding conductors, spare conductors, and parallel sets.
7. Can I size multiple motor feeders?
Yes. Enter the largest motor current and the total current of the other motors. The calculator applies the feeder multiplier to the largest motor only.
8. Why did the conductor become larger than expected?
Long distance, voltage drop limits, derating, ambient correction, and parallel set choices can increase conductor size. Review each input before changing the design.