Advanced Motor Calculator
Example Data Table
| Example |
FLC |
Nameplate amps |
Device percent |
Branch conductor |
Raw protection |
| Three phase pump |
14 A |
13.5 A |
250% |
17.50 A |
35.00 A |
| Single phase fan |
8 A |
7.6 A |
250% |
10.00 A |
20.00 A |
| Feeder group |
22 A largest |
21 A |
175% |
27.50 A |
38.50 A |
Formula Used
Branch conductor ampacity = Motor FLC × conductor percentage.
Adjusted branch ampacity = branch ampacity ÷ adjustment factor.
Short-circuit and ground-fault protection = Motor FLC × selected device percentage.
Overload setting = nameplate current × overload percentage.
Feeder ampacity = 125% of largest motor FLC + other motor FLC + non-motor load.
Disconnect rating = Motor FLC × 115%.
Single phase voltage drop = 2 × K × I × D ÷ CM.
Three phase voltage drop = 1.732 × K × I × D ÷ CM.
K is 12.9 for copper and 21.2 for aluminum. I is current. D is one-way feet. CM is circular mil area.
How To Use This Calculator
Enter the motor horsepower and phase first. Add the system voltage. Then enter the motor full-load current from the correct motor current table.
Enter the nameplate current separately. This value is used for overload calculations. Choose the protective device type. Use a custom percentage only when your design requires it.
For feeder sizing, enter the largest motor FLC. Add the total FLC of other motors. Add any non-motor load in amps.
For voltage drop, enter the one-way run length and conductor circular mil area. Review the result. Then export the report as CSV or PDF.
About This Motor Calculation Tool
Motor circuit sizing has several steps. A motor load is not handled like a simple heater or lamp load. Starting current can be high. Running current can stay steady for hours. This calculator gives one clear worksheet for common study and field planning.
The tool is built around NEC 2014 style motor methods. It keeps the motor full-load current separate from nameplate current. That matters. Branch-circuit conductors and short-circuit protection normally use the table full-load current. Motor overload devices normally use nameplate current. Keeping both values visible reduces mistakes.
What The Calculator Checks
You can estimate branch conductor ampacity, feeder ampacity, overload setting, disconnect rating, and short-circuit protection. You can also check a simple voltage drop estimate. The form includes service factor, temperature rise, adjustment factor, conductor material, conductor circular mil area, and route length.
The protection section lets you pick common device percentages. You may also enter a custom percentage. This is useful for class work, design checking, and comparing device choices. A next standard ampere option is included. It should be used only when allowed by the applicable rule and authority.
Why FLC Values Matter
Motor nameplates do not always match code table current. Two motors with the same horsepower may have different nameplate amps. Efficiency, design, and manufacturer choices can change that value. Code motor tables create a common base for conductor and protection sizing. This calculator asks for that FLC directly.
How Results Should Be Used
The output is a guide, not a permit-ready design. Always verify the exact NEC edition adopted in your area. Check terminal temperature ratings. Check conductor insulation. Check equipment markings. Check manufacturer overload tables. Local amendments may also change the final answer.
The CSV download is useful for records. The PDF download gives a simple job note. Both exports repeat the input values and final calculated values. That makes review easier.
For best results, start with a known FLC. Then enter the real nameplate amps. Add other motor loads when feeder sizing is needed. Finally, review voltage drop if the run is long. A careful worksheet saves time. It also helps explain each selected circuit value.
Use licensed judgment before applying any result to installed work.
FAQs
What is motor FLC?
Motor FLC means full-load current. In code calculations, it often comes from motor current tables, not from the motor nameplate.
Is this an official Mike Holt tool?
No. This is an independent educational calculator. It is not affiliated with Mike Holt, NFPA, or any code publisher.
Why does the calculator ask for nameplate amps?
Nameplate amps are commonly used for motor overload calculations. They may differ from table full-load current values.
Why is the conductor calculated at 125 percent?
Many motor branch-circuit conductor calculations use 125 percent of motor full-load current for continuous motor duty planning.
What does next standard ampere rating mean?
It rounds the raw protective device result up to a standard ampere size. Use it only when the applicable rule allows that step.
Can I use this for final electrical design?
Use it for study and checking only. Final design must follow the adopted code, equipment markings, and local authority requirements.
Why is voltage drop included?
Long motor runs can lose voltage. The voltage drop estimate helps you review conductor size and route length effects.
What if my motor has special markings?
Follow the equipment marking and manufacturer instructions. Marked limits can override a larger calculated value.