Current-limit ownership
A breaker frame, trip setting, cable ampacity, VFD current limit, and motor rated current are not interchangeable. Identify which boundary the entered amperes represents.
Electrical Engineering
Motor Capacity Calculator
Find usable continuous motor shaft capacity from the lower of derated nameplate output and available three-phase current capacity.
MOTOR CONTINUOUS CAPACITY
Identify whether the motor thermal rating or the available electrical current is the bottleneck
This calculator evaluates two independent steady-state ceilings: a nameplate shaft-output branch reduced by entered ambient and altitude allowances, and a line-current branch converted through power factor and efficiency. The lower branch defines usable output at the reviewed point.
Usable shaft capacity (kW)—
Limiting branch—
Thermal/nameplate capacity (kW)—
Current-limited capacity (kW)—
Usable torque at speed (N·m)—
Estimated rated-output current (A)—
MOTOR CONTINUOUS CAPACITY
Motor dual-constraint capacity ledger
The smaller branch is a screening bottleneck, not permission to load the motor. Verify rated current, service factor, VFD output, conductor and protection limits, cooling, duty, and driven-equipment torque.
| Capacity branch | Input A | Allowance/input B | Factor | Capacity | Meaning |
|---|
CURRENT CALCULATION PROCESS
Formula, current substitution, intermediate values, and reconciliation
P_usable = min[P_rated(1−d_ambient−d_altitude), √3VI_limitPFη/1000]
One branch derates mechanical nameplate output. The other converts available three-phase current into electrical input and then shaft output. Both use the same operating voltage, power-factor, efficiency, and speed basis.
| Symbol | Engineering meaning | Unit | Default |
|---|---|---|---|
| P_rated | Rated continuous shaft output | kW | 75 |
| V_LL | Available line voltage | V | 400 |
| I_limit | Available line-current ceiling | A | 120 |
| PF | Power factor at reviewed load | ratio | 0.87 |
| eta | Efficiency at reviewed load | ratio | 0.93 |
| d_ambient | Ambient derating allowance | % | 5 |
| d_altitude | Altitude derating allowance | % | 3 |
Intermediate values remain unrounded until display formatting.
HOW TO USE THIS MODEL
Evaluate both continuous capacity branches
- Enter the mechanical nameplate output and actual installation allowances for ambient and altitude.
- Use a continuous line-current limit from the motor, drive, conductor, or protection study as applicable.
- Enter operating-point power factor and efficiency rather than assuming unity.
- Compare the two shaft-output branches and note which equipment boundary is represented by the current limit.
- Check starting, overload, VFD, cooling, and process torque separately before selecting or uprating equipment.
MOTOR CONTINUOUS CAPACITY FUNDAMENTALS
Why motor capacity has multiple boundaries
- Mechanical rating
- Nameplate kW is rated shaft output under stated electrical and environmental conditions.
- Thermal derating
- Ambient and altitude can reduce cooling and allowable continuous output.
- Electrical limit
- Available current may be constrained by the motor, VFD, feeder, breaker, generator, or transformer.
- Conversion chain
- Electrical current capacity becomes shaft power only after power factor and efficiency.
- Minimum rule
- A series system cannot deliver more than its most restrictive continuous component.
- Torque capacity
- At a selected speed, shaft kW converts to torque; variable-speed applications need a full capability curve.
MODEL AND FORMULA
Select the lower of thermal and electrical shaft limits
P_usable = min[P_rated(1−d_ambient−d_altitude), √3VI_limitPFη/1000]
One branch derates mechanical nameplate output. The other converts available three-phase current into electrical input and then shaft output. Both use the same operating voltage, power-factor, efficiency, and speed basis.
DEEPER ENGINEERING ANALYSIS
Capacity decisions that need system context
Low-speed cooling
A self-cooled motor may lose continuous torque at low speed even when current is available. Use motor/VFD derating curves.
Starting can size the system
A motor with adequate running capacity may exceed voltage-drop, generator, VFD, or protection limits while accelerating.
WORKED DECISION CASES
Two capacity bottleneck examples
VFD current-constrained retrofit
A replacement process needs more shaft power, but the existing drive current limit produces a lower capacity than the derated motor nameplate. The drive is the first upgrade candidate.
High-altitude installation
A motor nameplate appears adequate at sea level. Entered altitude and ambient allowances show whether thermal capacity becomes lower than the available electrical branch.
TECHNICAL LANGUAGE
Motor capacity terms
- Continuous rating
- Output sustainable under specified conditions without exceeding temperature limits.
- Current limit
- Maximum permitted continuous or controlled current at a defined boundary.
- Thermal derating
- Reduction in allowable output due to cooling or temperature conditions.
- Shaft capacity
- Mechanical output available at the motor shaft.
- Bottleneck
- Lowest applicable capacity among series constraints.
- Base speed
- Speed below which a suitable drive may provide approximately constant torque before field weakening.
EVIDENCE AND DATA LINEAGE
Capacity decision records
Retain motor and VFD nameplates, continuous current-limit source, conductor and protection data, voltage and speed, power-factor and efficiency source, ambient, altitude, enclosure, cooling method, duty, torque curve, process requirement, and the identified limiting branch.
LIMITS AND EXCLUSIONS
Single-point steady-state boundaries
- The model evaluates balanced three-phase steady operation at one voltage and speed.
- It excludes service-factor operation, starting, acceleration, overload duration, VFD waveform/current control, low-speed cooling, field weakening, harmonics, and protection coordination.
- Entered ambient and altitude allowances are additive planning inputs and must come from applicable manufacturer or engineering guidance.
RELIABLE SOURCES
References for this page's method and boundaries
FREQUENTLY ASKED QUESTIONS
Motor capacity questions
Why is the lower branch selected?
The motor system cannot sustainably exceed either its thermal/nameplate output or the shaft power possible through the available current boundary.
Can I use breaker size as current limit?
Not automatically. Breaker rating or pickup may not equal motor continuous current capacity, conductor ampacity, or VFD output capability.
Does service factor increase capacity here?
No. This page intentionally treats continuous derated nameplate output as the thermal branch. Apply service factor only under its stated conditions and a separate policy.
What happens below base speed?
Cooling and drive capability can change. Use manufacturer constant-torque/constant-power curves and VFD limits rather than this single-point screen.
Why enter both power factor and efficiency?
Power factor converts apparent electrical input to real input; efficiency converts real input to mechanical output. They describe different losses.
Can this select a motor for starting a load?
No. Starting requires torque-speed curves, inertia, acceleration time, voltage drop, current, duty, and protection analysis.
RELATED CALCULATORS
Continue the electrical engineering review
Use the next model to test a separate operating boundary without hiding it inside this result.
IMPORTANT ENGINEERING NOTE
Capacity is a system property, not only a motor number
Qualified engineering review should confirm every electrical, thermal, mechanical, control, environmental, starting, duty and protection boundary. Do not uprate a motor or feeder from the calculated lower branch alone.