Motor FLA Calculator

Calculator overview

Inputs and outputs

This summary comes from the calculator's published input and output contract.

Inputs

Efficiency: About this input

The motor efficiency as a fraction between 0 and 1, for example 0.9 for 90 percent. It converts shaft output to the electrical input that sets the current.

Unit fraction Default 0.895 Range 0 to 1
Phase: About this input

Single-phase or three-phase supply. The full-load current formula differs by a factor of the square root of three between them.

Default Three phase Allowed Single phase, Three phase
Power Factor: About this input

The motor power factor as a fraction between 0 and 1. A lower value raises the current drawn for the same real power.

Unit fraction Default 0.86 Range 0 to 1
Power Unit: About this input

Whether the rated power is entered in horsepower or kilowatts. Horsepower is read as shaft output; kilowatts as electrical input.

Default Horsepower Allowed Horsepower, Kilowatts
Rated Power: About this input

The motor nameplate power, in the unit chosen above. In horsepower this is the mechanical shaft output the motor is rated to deliver.

Unit hp Default 10 Range At least 0
Service Factor: About this input

The nameplate service factor, for example 1.15, which sets how far above rated load the motor may run continuously and caps the overload setting.

Unit factor Default 1.15 Range At least 1
Starter Type: About this input

How the motor is started, such as direct-on-line or star-delta. It sets the multiple of full-load current used to estimate inrush.

Default Direct on line Allowed Direct on line, Star-delta, Autotransformer, 65% tap, Soft starter, Variable frequency drive
Voltage: About this input

The line-to-line supply voltage the motor runs on, in volts.

Unit V Default 460 Range At least 0

Outputs

Apparent Power: About this output

The apparent power drawn, in kilovolt-amperes: real power divided by power factor.

Unit kVA
Estimated Starting Current: About this output

The estimated inrush at start, in amperes: the derived full-load current times a fixed illustrative multiple for the selected starter. Actual inrush depends on the motor code letter, the drive settings and the manufacturer's data, none of which this model reads. An estimate for coordination studies, not a measured value.

Unit A
Full Load Current: About this output

The current the motor draws at rated load, in amperes, derived from power, voltage, power factor and efficiency rather than read from a code table, so expect about 10 percent difference from a nameplate or code value.

Unit A
Input Power: About this output

The real electrical power the motor draws, in watts: shaft output divided by efficiency.

Unit W
Losses: About this output

The power lost as heat, in watts: input power minus output power.

Unit W
Maximum Overload Setting: About this output

An illustrative overload-relay setting, in amperes: 125 percent of the derived full-load current when the service factor is 1.15 or more, otherwise 115 percent. Codes base overload protection on the motor NAMEPLATE current and also allow 125 percent for a marked temperature rise of 40 C or less, a condition this model does not carry. Use the nameplate and the applicable code for the actual setting.

Unit A
Minimum Conductor Ampacity: About this output

An illustrative branch-circuit conductor ampacity, in amperes: 125 percent of the full-load current this model derives from your efficiency and power factor. Electrical codes size conductors from the published table full-load current for the horsepower and voltage, which is typically about 10 percent higher than a nameplate-equation figure, so treat this as a screening estimate and use the code table figure for any code-governed sizing.

Unit A
Minimum Disconnect Rating: About this output

An illustrative disconnect ampere rating: 115 percent of the derived full-load current. Codes require the disconnect to be rated from the table full-load current, not a derived figure, so confirm against the code table for the horsepower and voltage.

Unit A
Model Status: About this output

The overall check on your entries, shown above the results. It reads OK when the inputs are usable, NOT VALID with a reason when an entry makes the model meaningless, or CHECK with a reason when a result is valid but worth a second look. Read it before you trust the numbers below.

No unit declared
Output Power: About this output

The mechanical shaft power the motor delivers, in watts.

Unit W
Reactive Power: About this output

The reactive power drawn, in kilovolt-amperes reactive, the part that loads the supply without doing work.

Unit kvar

What it is

The Motor FLA Calculator estimates a motor's full-load current from its rating, voltage, efficiency and power factor, and derives the branch-circuit figures that follow: conductor ampacity, disconnect rating, overload setting and starting inrush. It handles single- and three-phase motors and accepts the rating in either horsepower or kilowatts.

Currents are in amperes, power in watts or kilovolt-amperes, and voltages in volts.

The current here is calculated, not looked up, and that matters for code work. Electrical codes size branch-circuit conductors and disconnects from a published table full-load current for the horsepower and voltage, not from a nameplate-equation figure. The table value is typically about ten percent higher, so every code-sizing number on this page comes out lower than the code-required one. They are screening estimates. Use the code table for anything a jurisdiction will inspect.

Methodology

Purpose and model boundary

This model estimates a motor’s running current from rated output power, supply, efficiency, and power factor. It then applies workbook multipliers to show indicative conductor ampacity, disconnect rating, overload setting, and starting current.

The calculation is an equation-based screening model. It does not use electrical-code full-load-current tables or the motor’s nameplate current, so its protection and conductor figures are not code selections.

Inputs and units

Power unit selects horsepower or kilowatts. In both branches the entered rating is treated by the workbook as motor output power; horsepower is converted at 746 W/hp and kilowatts at 1000 W/kW.
Phase selects single-phase or three-phase supply.
Rated power is the motor output rating in the selected unit and must be positive.
Voltage is the line supply voltage in volts and must be positive.
Efficiency is a fraction greater than zero and no greater than one.
Power factor is a fraction greater than zero and no greater than one.
Starter type selects a fixed illustrative starting-current multiple from the workbook table.
Service factor is the nameplate factor used only to select the overload multiplier; the contract accepts values of at least one.

Governing relationships

Let P_out be rated output power in watts, eta efficiency, V voltage, PF power factor, and m the phase multiplier:

P_out = rated hp x 746, or rated kW x 1000

P_in = P_out / eta

m = 1 for single phase and m = sqrt(3) for three phase

I_FLA = P_out / (m x V x eta x PF)

S = m x V x I_FLA / 1000

Q = S x sqrt(max(0, 1 - PF^2))

Losses = P_in - P_out

The workbook then applies:

Indicative conductor ampacity = 1.25 x I_FLA

Indicative disconnect rating = 1.15 x I_FLA

Indicative overload setting = 1.25 x I_FLA when service factor is at least 1.15, otherwise 1.15 x I_FLA

Estimated starting current = I_FLA x starter multiple

The starter multiple comes from the workbook lookup for direct-on-line, star-delta, autotransformer, soft-starter, or variable-frequency-drive starting.

Calculation sequence

Convert the selected power unit to watts.
Divide output power by efficiency to obtain electrical real input power.
Select the single- or three-phase multiplier and calculate full-load current.
Derive apparent power, reactive power, and losses.
Apply the conductor, disconnect, and overload multipliers to the derived current.
Look up the selected starter’s fixed multiple and estimate starting current.

Outputs and interpretation

Full-load current is the main result. It is derived from the entered operating values and will generally differ from a code-table or nameplate current. Input power, output power, and losses show the energy balance. Apparent power and reactive power show the supply burden associated with power factor.

The conductor, disconnect, overload, and starting-current outputs are labeled as estimates because their multipliers are applied to the model-derived current. For an actual installation, code rules may require table current, nameplate current, temperature-rise markings, or device-specific data instead.

Validation and status logic

The workbook evaluates status in this order; the first matching row is returned.

Condition	Returned status
Rated power is zero or negative	`NOT VALID: rated power must be greater than zero`
Voltage is zero or negative	`NOT VALID: voltage must be greater than zero`
Efficiency is zero or negative, or greater than one	`NOT VALID: efficiency must be between 0 and 1`
Power factor is zero or negative, or greater than one	`NOT VALID: power factor must be between 0 and 1`
Efficiency is below 0.70 after the preceding validity checks	`CHECK: efficiency below 0.70 is unusual for an industrial motor`
None of the preceding conditions applies	`OK`

Assumptions and limitations

The model assumes balanced sinusoidal operation at one rated load point. Efficiency and power factor are user-supplied constants rather than curves that vary with load. Three-phase voltage is treated consistently with the workbook’s line-voltage formula. The reactive-power result uses a scalar power factor and therefore does not represent waveform distortion.

No code current tables, conductor ampacity tables, ambient or bundling adjustment, termination temperature limits, voltage-drop checks, short-circuit/ground-fault protection, or multi-motor feeder rules are implemented. The overload calculation does not read nameplate current or temperature-rise marking. Starting-current multiples are illustrative and do not use locked-rotor code letter, actual drive configuration, acceleration time, torque, or manufacturer curves.

Restrictions and non-computing states

Rated power and voltage must be positive. Efficiency and power factor must be in (0, 1]. Dropdown inputs must match the delivered lists. Invalid states can still produce intermediate spreadsheet numbers, but the NOT VALID status takes precedence and those figures must not be used.

Errors and warnings

The LogicCommons proxy may reject a value that violates the contract type, dropdown, or bound before the workbook runs. The workbook returns NOT VALID when its own domain checks fail and CHECK for the unusual-low-efficiency condition. A transport or SpreadsheetWeb service failure is not a calculated status and does not mean that an electrical condition passed or failed.

References

The workbook derives its relations rather than reproducing any table, chart or figure from a code or standard. The full-load current comes from the power equation and the sizing percentages are applied to it directly.

Wikipedia. Power factor, for why current exceeds what real power alone implies. https://en.wikipedia.org/wiki/Power_factor
Wikipedia. Electric motor, for efficiency and loss mechanisms. https://en.wikipedia.org/wiki/Electric_motor
Schneider Electric. Electrical calculation formulas, a manufacturer reference for the motor relations. https://download.schneider-electric.com/files?p_Doc_Ref=39000-323-01&p_EnDocType=Instruction+sheet&p_File_Name=39000-323-01.pdf

No code tables are reproduced here and none are implemented. NFPA 70 and the equivalent code adopted where you work carry the table full-load currents that govern conductor and disconnect sizing, the ampacity tables, the derating rules and the protective-device sizing rules. Every one of those is outside this tool. Starter multiples are illustrative. No trademark or organisation name appearing here implies endorsement by its owner.

Additional source notes migrated from Methodology

The workbook uses standard single- and three-phase power relationships and an internal set of illustrative protection and starting multipliers. It does not reproduce electrical-code tables. See the page’s References section for the power-factor, motor, and manufacturer formula sources, then use the governing code tables and motor nameplate for design decisions.

Frequently asked questions

Why is my calculated current lower than the code table value?

Because they are different quantities. This tool derives current from the specific efficiency and power factor you entered; code tables publish conservative composite values across motor designs for each horsepower and voltage. For a 10 hp, 460 V, three-phase motor the table is about 14 A against the 12.16 A calculated here, roughly 15 percent. Codes require the table figure for conductor and disconnect sizing precisely so that the answer does not depend on a manufacturer's efficiency claim.

Can I size my branch circuit from this page?

No, not for anything a jurisdiction will inspect. Every sizing output here applies the conventional percentage to a **derived** current, and the code applies it to the **table** current. In the shipped example that is 15.21 A against a code-required 17.5 A, which can be a different wire size. Use the page to sanity-check an order of magnitude, then size from the code table.

What is the difference between efficiency and power factor?

Efficiency is about real power: the fraction of what the motor draws that becomes shaft work rather than heat. Power factor is about current: how much current is needed to deliver that real power, given that a motor's magnetising current lags the voltage. A motor can be highly efficient and still draw large current at a poor power factor, and current, not efficiency, is what sizes the conductor.

Why is the overload setting based on a different rule?

Because codes base motor overload protection on the motor's **nameplate** current rather than either the table value or a derived one, and they permit a higher percentage for motors with a service factor of 1.15 or more or a marked temperature rise of 40°C or less. This tool applies the service-factor branch to its derived current and cannot see the temperature-rise marking. Read the nameplate and apply the code.

How accurate is the starting current estimate?

Only as an order of magnitude. It multiplies full-load current by a fixed illustrative factor for the starter type, six times for direct-on-line. Real inrush depends on the motor's code letter, its actual locked-rotor current, and for drives on the programmed ramp, none of which the tool reads. Use it to see roughly what a coordination study will face, not as an input to one.

Does this cover the fuse or breaker size?

No. Motor short-circuit and ground-fault protection is a separate code rule with its own percentages, its own table, and different limits by device type. Those percentages are much larger than the ones here because the device must let the inrush through without tripping. Nothing on this page addresses it.

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This page is provided by LogicCommons for informational purposes only. Results are analysis outputs computed from the inputs you supply and are not engineering advice, a design, or a substitute for review by a licensed professional under the codes adopted where the work is built. Verify all inputs and results independently.

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