Electrical Calculator
Formula Used
Base motor current = Single motor FLC × Number of motors
Total load current = Base motor current + Auxiliary load
Minimum disconnect rating = Total load current × Sizing factor × (1 + Design margin ÷ 100)
Recommended size = Next common disconnect ampere rating above the calculated minimum.
SCCR check = Device SCCR should be greater than or equal to available fault current.
How To Use This Calculator
- Select table FLC or manual FLC.
- Choose the motor phase, voltage, and horsepower.
- Enter the number of motors served by one disconnect.
- Add auxiliary amps when they pass through the same device.
- Keep 115% for common motor disconnect planning.
- Add a design margin when future load changes are expected.
- Enter a selected disconnect rating for a pass or fail check.
- Press calculate, then export the result as CSV or PDF.
Example Data Table
| Phase | Voltage | HP | FLC | Motors | Factor | Minimum Rating | Common Size |
|---|---|---|---|---|---|---|---|
| 3 | 460 V | 10 HP | 14 A | 1 | 115% | 16.10 A | 20 A |
| 3 | 230 V | 25 HP | 68 A | 1 | 115% | 78.20 A | 100 A |
| 1 | 230 V | 5 HP | 28 A | 2 | 115% | 64.40 A | 100 A |
What This Calculator Does
A motor disconnect must be selected with care. It is not just a convenient switch. It must handle the running current of the motor. It must also suit the voltage, phase, enclosure, and available fault level. This calculator helps you size the ampere rating for planning. It uses motor full load current, motor count, auxiliary load, and a chosen sizing factor. The default factor is 115 percent, which is common for motor disconnect planning.
Why Full Load Current Matters
Motor nameplates can show full load amps. Code tables may also be used in many design cases. This tool lets you choose a table based value or a manual current value. Manual entry is useful when the table does not match your motor. It is also useful for special duty equipment, torque motors, or imported machines. The calculated current is multiplied by the motor quantity. Auxiliary control load can be added as well.
Interpreting The Output
The result shows the base current, minimum disconnect rating, and the next common ampere size. It also checks a selected disconnect size. If you enter a short circuit value, the tool compares it with the disconnect rating. This is only a planning check. The final device must be listed for the circuit, controller, horsepower, voltage, and installation location.
Good Design Practice
Always check local rules before buying equipment. Disconnects for motors often need horsepower ratings. Fused and non fused devices may have different limits. An enclosure may need dust, rain, washdown, or hazardous location protection. Lockable handles may be required for maintenance safety. A calculator cannot inspect the site. It also cannot judge coordination or arc flash duties. Use this page for estimates, study, and early selection. Then confirm the final answer with approved drawings, manufacturer data, and a qualified electrical professional.
Inputs That Improve Accuracy
Better inputs give better results. Select the correct phase first. Match the voltage to the motor circuit. Enter the real number of motors served by one disconnect. Add heater, brake, or control loads when they pass through the same switch. Use a margin when future motor changes are likely. Keep notes with every export for easier review during procurement meetings and field coordination reviews too.
FAQs
What does this calculator size?
It estimates the minimum ampere rating for a motor disconnect. It also suggests the next common rating and checks selected device values.
What is the default sizing factor?
The default factor is 115 percent. You can change it to 125 percent or 150 percent for planning comparisons.
Should I use nameplate FLA or table FLC?
Use the value required by your design rule. The calculator supports both table lookup and manual current entry for flexibility.
Does this size the fuse?
No. This page sizes the disconnect ampere rating. Fuse and breaker sizing need separate motor branch circuit calculations.
What is SCCR?
SCCR means short-circuit current rating. The device rating should meet or exceed the available fault current at the installation point.
Can this handle multiple motors?
Yes. Enter the number of motors served by one disconnect. The calculator multiplies motor FLC and adds auxiliary load.
Why does the common size jump higher?
Disconnect switches are sold in standard ampere ratings. The calculator rounds up to the next common value in its list.
Is this final engineering approval?
No. It is a planning aid. Always verify with local code, manufacturer data, drawings, and a qualified electrical professional.