Calculator
Example Data Table
| Motor | Voltage | Phase | Length | Material | Suggested check |
|---|---|---|---|---|---|
| 5 hp | 230 V | Single phase | 75 ft | Copper | Check ampacity and drop |
| 10 hp | 480 V | Three phase | 100 ft | Copper | Common industrial run |
| 25 hp | 480 V | Three phase | 250 ft | Aluminum | Long feeder review |
Formula Used
Three phase full load current: I = W ÷ (√3 × V × efficiency × power factor)
Single phase full load current: I = W ÷ (V × efficiency × power factor)
DC full load current: I = W ÷ (V × efficiency)
Design current: Design current = full load current × service factor × conductor multiplier
Required base ampacity: Required ampacity = design current ÷ combined derating factor ÷ parallel runs
Single phase or DC voltage drop: Vd = 2 × K × I × L
Three phase voltage drop: Vd = √3 × K × I × L
K is conductor resistance per foot. L is one way length in feet.
How to Use This Calculator
Enter the motor power from the nameplate. Select the correct power unit. Choose the motor phase and voltage. Add efficiency and power factor if known. Enter conductor material, insulation rating, ambient temperature, conductor count, length, voltage drop limit, and parallel runs. Press calculate. The result appears above the form.
About Electric Motor Wire Sizing
Electric motors need conductors that can carry running current without excess heat. They also need enough copper or aluminum area to control voltage drop. A motor can start poorly when the supply voltage falls too far. It may pull more current, run hotter, and lose torque. This calculator brings those checks into one workflow. It estimates full load current from power, phase, voltage, efficiency, and power factor. Then it applies a conductor multiplier, ambient correction, raceway adjustment, and optional parallel runs. The selected size is the smallest listed conductor that meets ampacity and voltage drop limits.
Why Derating Is Important
A wire table gives base ampacity under standard conditions. Real installations are often different. Hot spaces reduce cooling. Many current carrying conductors in one raceway also reduce cooling. Derating accounts for those limits. The tool raises the required base ampacity when correction factors are below one. This helps avoid choosing a wire that looks acceptable on a table but fails after installation details are considered.
Voltage Drop Planning
Voltage drop is not only a comfort number. It affects motor torque, starting, and efficiency. Long runs usually need larger conductors than ampacity alone would require. The calculator checks running drop and estimates starting drop with a multiplier. This gives a better view of feeder behavior during inrush. For sensitive machines, a stricter limit may be useful.
Input Tips
Use the motor nameplate when possible. Rated voltage, power, efficiency, and power factor are better than guesses. Use the real one way distance from source to motor. Pick the insulation temperature allowed by terminals and local rules. For small conductors, terminal ratings can control the final answer. Aluminum conductors need larger sizes because their resistance is higher.
Final Use
Treat this calculator as a planning aid. It does not replace local electrical codes, manufacturer data, or licensed design review. Motor branch circuits may also need overload protection, short circuit protection, disconnects, grounding, and enclosure checks. Use the result to compare options, estimate material, and discuss the installation with a qualified electrician or engineer.
Keep Records
Save each result with job notes, because future maintenance becomes easier when wire size choices, voltage assumptions, and derating factors are documented clearly.
FAQs
What does this calculator estimate?
It estimates a practical motor conductor size using current, derating, voltage drop, material, temperature rating, and run length.
Does this replace electrical code?
No. It is a planning tool. Always follow local code, nameplate data, equipment instructions, and qualified electrical design review.
Why is 125 percent often used?
Motor conductors are commonly sized above running current. This gives thermal margin for continuous operation and starting conditions.
Should I use copper or aluminum?
Copper has lower resistance and usually needs a smaller size. Aluminum can be economical but often needs a larger conductor.
What length should I enter?
Enter the one way distance from the power source to the motor. The calculator applies the return path where needed.
What is a good voltage drop limit?
Three percent is a common design target for branch circuits. Long motor feeders may need closer review.
Why does ambient temperature matter?
Higher ambient temperature reduces conductor cooling. The calculator applies correction factors to raise the required ampacity.
Does it check starting voltage drop?
Yes. It estimates starting drop with the starting current multiplier. Use motor data when exact starting current is known.