Enter Motor Feeder Data
Example Data Table
| Item | Example value | Reason |
|---|---|---|
| Supply | 480 V, three phase | Common construction motor feeder service. |
| Largest motor | 50 hp | Main pump or compressor. |
| Other motors | Two motors at 10 hp each | Fans, conveyors, or smaller pumps. |
| Power factor and efficiency | 0.88 and 92% | Used for early current estimates. |
| Spare margin | 15% | Allows modest future site growth. |
Formula Used
The calculator estimates motor current when a direct full load current is not entered.
Three phase FLC = kW × 1000 ÷ (√3 × V × PF × Efficiency)
Single phase FLC = kW × 1000 ÷ (V × PF × Efficiency)
Feeder ampacity = 125% × largest motor FLC + other motor FLC total + 125% × continuous load + noncontinuous load
Corrected ampacity = feeder ampacity ÷ correction factor
Final planning ampacity = corrected ampacity × (1 + spare capacity)
Planning OCP estimate = 250% × largest motor FLC + other motor FLC total + auxiliary loads
How to Use This Calculator
Choose the supply phase and enter the line voltage. Add the largest motor rating. Enter its full load current if you know it. Add the number and size of additional motors. Include continuous and noncontinuous auxiliary loads. Adjust demand, correction, spare, starting, length, and voltage drop settings. Press the calculate button. Review the result above the form. Compare the result with conductor tables, protection rules, and project specifications.
Motor Feeder Planning for Construction Work
Motor feeders carry current to one motor group or one large motor. On construction sites, they often serve pumps, hoists, fans, compressors, crushers, and temporary plant equipment. A feeder should be sized with more care than a simple nameplate sum. Motors draw higher current during starting. They also run for long periods under changing load. A weak feeder can create heat, nuisance trips, poor voltage, and short equipment life. Clear records also help crews inspect temporary power before each shift starts.
Why The Largest Motor Matters
Feeder sizing gives extra weight to the largest motor. This is because the largest unit creates the biggest running load and the highest starting stress. A common design method uses one hundred twenty five percent of the largest full load current. Then it adds the other motor currents. Continuous auxiliary loads may also need one hundred twenty five percent. The calculator follows that planning logic. It also allows correction factors and spare capacity.
Important Input Choices
The most reliable input is full load current from an approved motor table or project schedule. Nameplate current can differ from code values. Horsepower based estimates help during early planning, but final feeders should be checked against local rules. Power factor and efficiency affect estimated current when a direct current value is not entered. Demand factor is useful when additional motors do not run together. Spare capacity helps future expansion.
Voltage Drop And Practical Checks
Voltage drop is not the same as ampacity. A conductor may carry the load safely, yet still cause low voltage at the motor. Long temporary feeders need special review. This tool estimates a maximum circuit resistance for the selected voltage drop target. Designers can compare that value with conductor data during cable selection. Starting voltage drop should be checked separately for heavy motors, direct on line starters, and remote equipment.
Use Results With Field Judgment
The result is a design aid, not a permit approval. Always confirm conductor ampacity, insulation rating, ambient temperature, raceway fill, grounding, short circuit rating, starter settings, and overload protection. Local electrical codes and manufacturer data control final work. For critical systems, review the load schedule with a qualified electrical professional before ordering cable, switchgear, or protection devices. Keep records for site inspections.
FAQs
What is a motor feeder load?
It is the calculated current that a feeder must carry for one motor group or a large motor circuit. It includes the largest motor, other motors, auxiliary loads, correction factors, and often a planning spare margin.
Why is the largest motor multiplied by 125%?
Motor feeder rules often require extra ampacity for the largest motor. The added twenty five percent helps handle heating and long running conditions. Always confirm the exact rule used by your local code.
Should I use horsepower or full load current?
Use full load current from approved motor tables or project schedules when possible. Horsepower estimates are useful during early planning. Final conductor sizing should not rely only on rough horsepower conversion.
Does this calculate motor overload protection?
No. It gives feeder ampacity and a planning overcurrent value. Motor overload protection depends on motor nameplate data, service factor, starter type, and local electrical code requirements.
What is the demand factor for?
Demand factor reduces the additional motor load when all extra motors do not operate at the same time. Use one hundred percent when simultaneous operation is expected or when the schedule is uncertain.
How does correction factor affect ampacity?
A lower correction factor increases the required ampacity. It represents ambient temperature, bundling, raceway conditions, or other derating issues. Use a project approved value for final work.
What starting factor should I enter?
Direct starting may be near six hundred percent of full load current. Soft starters and drives can be lower. Use manufacturer data when available, especially for pumps, compressors, hoists, and crushers.
Is voltage drop included?
Yes, the tool estimates a maximum resistance target for the entered voltage drop. It does not select an exact wire size. Compare the resistance result with conductor data and installation conditions.
Can I use this for three phase motors?
Yes. Select three phase and enter the line voltage. The calculator uses the square root of three in the current estimate when a direct full load current is not entered.
Can this replace an electrical design review?
No. It supports early planning and checking. A qualified person should confirm conductors, grounding, short circuit ratings, protective devices, voltage drop, installation method, and local code compliance.
Why add spare capacity?
Spare capacity helps when the site adds future tools, pumps, fans, or temporary loads. It should not hide poor design. Use a realistic margin that matches the project scope and equipment plan.