3. Preliminary Panel Equipment Schedule

The values below are intended as a first-pass engineering selection. Where a standard rating is shown, it is the next common rating above the calculated screening value. Always confirm a coordinated starter combination from the MCCB/contactor/overload/VFD manufacturer.

Motor full-load current—Calculated from kW, voltage, PF and efficiency.

Preliminary MCCB nominal current—Breaking capacity & trip curve require site fault-level/coordination check.

Main / line contactor—IEC AC-3 preliminary current class; verify coordination table.

Delta contactor—Shown for star-delta only.

Star contactor—Shown for star-delta only.

Overload relay—Adjust around actual motor nameplate current.

Control MCB—Preliminary control-circuit protection; verify actual coil/SMPS/control-transformer load and inrush.

Preliminary motor cable—Ampacity/derating/short-circuit withstand still require formal cable sizing.

Running voltage drop—At calculated FLC and selected cable.

Estimated starting line current—Actual locked-rotor/start current depends on motor and starter.

Estimated starting voltage drop—Screening estimate only; source impedance is not included.

VFD output current ratingNot applicableShown only for VFD selection.

VFD harmonic distortion & long-cable checks

Indicative VFD input THDi—Typical band only; measure or model the actual system.

Voltage THDv at PCCCannot be inferred from VFD aloneDepends on source impedance, transformer, other loads and system resonance.

Motor cable dv/dt check—Follow the VFD/motor manufacturer's maximum cable-length and filter guidance.

Why VFDs create harmonics: a conventional 6-pulse diode rectifier draws current in pulses near the peaks of the AC voltage waveform. That non-sinusoidal current contains 5th, 7th, 11th, 13th and higher characteristic harmonics. Harmonic current can increase transformer/cable heating, neutral current in some systems, capacitor stress and voltage distortion at the point of common coupling (PCC).

VFD front end / mitigation	Indicative input-current THDi	What it means
6-pulse, no reactor	~35–45%	Common baseline; exact value varies with source impedance/DC bus.
Line reactor / DC choke	~25–35%	Reduces current peaks and improves input waveform.
Passive harmonic filter	~5–10%	Can substantially reduce characteristic harmonics when correctly applied.
AFE / low-harmonic drive	often <5%	Can achieve low input-current distortion under specified conditions.

These are indicative engineering ranges, not guaranteed values. IEEE 519 limits are evaluated at the PCC and depend on the ratio of short-circuit current to maximum demand load current; a VFD's terminal THDi alone does not prove site compliance.

4. Power connection diagram

The diagram automatically changes with the selected starter. It is a functional-learning schematic, not a fabrication drawing.

Power circuitControl signalProtective earth

Power circuit

5. Control diagram

The control diagram shows the logic concept: stop/emergency chain, overload or drive-fault permissive, start command, seal-in/transition logic and coil/output command. A real panel also requires terminal numbering, wire numbers, interlocks, protection contacts, PLC/relay logic and device-specific wiring.

Control circuit

6. How each starter changes the panel

Starter	Typical power devices	Start current / torque behaviour	OLR / motor protection	Best use
DOL	MCCB + 1 AC-3 contactor + OLR	Highest electrical/mechanical starting stress; current commonly several × FLC.	Thermal/electronic OLR normally selected to include nameplate FLC.	Smaller motors or strong supply where full starting torque is useful.
Star-Delta	MCCB + main + star + delta contactors + timer + OLR	Approx. one-third of delta-DOL starting line current and one-third starting torque for a suitable motor.	Line-mounted OLR ≈ FLC setting; inside-delta OLR ≈ 0.58 × FLC.	Loads that can accelerate with reduced starting torque.
Auto-transformer (ATS)	MCCB + autotransformer + start/run/transition contactors + timer + OLR	Tap-adjustable reduced voltage. Supply start current ≈ k² × DOL current; torque ≈ k² × DOL torque.	OLR/motor protection based on motor rated current and starter scheme.	Larger motors needing reduced line current with more start-torque flexibility.
VFD	Input protection + VFD + isolating/bypass devices as required	Controlled frequency/voltage gives low controlled starting current and adjustable torque/speed.	Drive electronic motor protection is commonly used; external protection may still be required by application/standard.	Variable speed, process control, soft acceleration, energy optimization.

7. What the MCCB selection really requires

The tool shows a nominal-current starting point only. A complete MCCB selection must check:

Continuous current: conductor and motor feeder loading.
Breaking capacity (Icu/Ics): must exceed the prospective short-circuit current at the panel.
Trip curve / magnetic pickup: must ride through legitimate motor starting while clearing faults.
Coordination: MCCB + contactor + overload/VFD tested coordination, including Type 1/Type 2 coordination where applicable.
Discrimination: upstream/downstream devices should be coordinated where selective protection is required.
Ambient/enclosure derating: manufacturer ratings may need correction.

8. Contactor selection – why AC-3 matters

Motor contactors are normally selected using an utilization category such as IEC AC-3, not merely by the contactor's thermal ampere rating. AC-3 duty covers starting a squirrel-cage motor and opening the circuit while the motor is running. Reversing, plugging or inching can require a more severe duty category and different selection.

For star-delta, the familiar 0.58 × FLC and 0.33 × FLC values are useful preliminary relationships, but actual contactor selection should follow the manufacturer's star-delta coordination table because connection arrangement, utilization category and transition duty matter.

9. Overload relay selection

An overload relay protects the motor against sustained overcurrent/thermal overload; it does not replace short-circuit protection. Choose an adjustable range that contains the motor's actual nameplate current. Consider trip class, cold/hot start behaviour, phase-loss sensitivity, motor thermal time constant, starting time and number of starts per hour.

Line-mounted star-delta OLR setting ≈ motor nameplate FLC
Inside-delta OLR setting ≈ 0.58 × motor nameplate FLC

10. Cable sizing and total voltage loss

Voltage loss is calculated across the one-way cable length using a simplified 3-phase impedance relation:

ΔV ≈ √3 × I × (R cosφ + X sinφ) × L

where R and X are conductor resistance/reactance per unit length and L is the one-way route length. The tool adjusts conductor resistance approximately for operating temperature and uses a typical reactance screening value. Final cable size still requires:

installation method and number of loaded conductors,
ambient/soil temperature and grouping derating,
conductor/insulation type,
normal and starting voltage drop,
short-circuit thermal withstand,
protective-device coordination,
earth-fault loop/protection requirements,
VFD output-cable compatibility, EMC and bearing-current considerations.

11. VFD long cable: harmonics are not the only issue

Input harmonics occur on the supply side of the VFD. On the motor side, PWM switching creates fast voltage edges (high dv/dt). With long motor cables, reflected-wave effects can increase peak motor-terminal voltage. Depending on VFD carrier frequency, cable type, motor insulation and length, the manufacturer may require an output reactor, dv/dt filter or sine-wave filter. Therefore the page reports a cable-length warning but does not claim a universal cutoff.

12. Example – 30 kW motor, 415 V

With 30 kW shaft output, 415 V, PF 0.86 and efficiency 92%, estimated FLC is around the mid-50 A range. The exact result appears above. A final panel design would then confirm the motor nameplate current, fault level, selected starter method, start time/load torque, cable installation/derating, MCCB coordination, contactor AC-3 ratings, OLR range/trip class and control philosophy.

Frequently asked questions

Can I use the shown MCCB rating directly for purchase?

No. It is a nominal-current screening value. You still need prospective fault current, Icu/Ics, trip-unit settings/curve, cable protection, starter coordination and manufacturer tables.

What does ATS mean on this page?

ATS means auto-transformer starter, not automatic transfer switch. It is a reduced-voltage motor starter using an autotransformer tap during acceleration.

Why is there no external OLR rating for VFD?

Modern VFDs usually provide electronic motor thermal/overload functions based on programmed motor data. Whether an external overload relay or additional motor protection is required depends on the drive, bypass arrangement, multiple motors, process risk and applicable standard.

Can the calculator determine harmonic compliance?

No. It can explain likely VFD input THDi ranges. Compliance at the PCC requires system impedance/short-circuit data, total nonlinear load and usually a harmonic study or measurement.

Does cable length change MCCB size?

Usually cable length affects voltage drop and fault-loop/short-circuit conditions more directly than nominal motor current. It can still affect protection design because the protective device must protect the selected cable and clear faults within required limits.

DEEP LEARNING MODULE • Electrical Design

Understand Motor Panel Equipment Selection & Design Wizard from first principles

This section is intentionally more detailed than a normal calculator page. The aim is to help a learner move from what to enter, to why the formula works, to what must be checked in a real installation or financial decision.

Motor Panel Equipment Selection & Design Wizard — concept flow

1. Core relationship

Start with motor FLC, then verify starter duty, short-circuit protection, overload protection, cable ampacity, voltage drop and control circuit

For a 30 kW, 415 V motor the calculated FLC is typically around the low-50 A range depending on PF and efficiency. Starter type changes starting current, contactor arrangement, overload placement and voltage-drop assessment.

Before calculating, define every symbol, unit and reference point. A technically correct formula can still produce a wrong answer when line/phase quantities, annual/monthly rates, one-way/round-trip length, or input/output power are mixed.

2. Physical or mathematical meaning

Panel selection is a system problem, not a single-current lookup.

MCCB current rating alone is insufficient: breaking capacity and trip settings depend on prospective fault current and coordination.

Contactor selection depends on utilization category, starter topology and manufacturer coordination data.

Overload relay placement matters in star-delta circuits because line current and winding current differ.

VFD panels require drive input/output current checks, EMC/earthing, thermal management and harmonic assessment at the PCC.

3. Step-by-step reasoning workflow

Step 1: Enter motor kW/voltage/PF/efficiency. Record the value, its unit and where it came from before moving to the next step.
Step 2: Choose DOL / Star-Delta / Auto-transformer / VFD. Record the value, its unit and where it came from before moving to the next step.
Step 3: Enter cable length and conductor. Record the value, its unit and where it came from before moving to the next step.
Step 4: Calculate FLC and starting case. Record the value, its unit and where it came from before moving to the next step.
Step 5: Select MCCB/contactors/OLR/control MCB. Record the value, its unit and where it came from before moving to the next step.
Step 6: Review dynamic power and control diagrams. Record the value, its unit and where it came from before moving to the next step.

4. Worked-example method

Use a worked example in three layers: first write the known values, then substitute them into the equation without rounding, and finally interpret whether the result is plausible. After the calculation, change one input at a time and observe the direction and sensitivity of the result. This develops engineering or financial intuition instead of only producing a number.

Known values

Write the rated or assumed inputs with units. If a value is estimated, mark it as an assumption.

Calculation

Substitute values in consistent units. Keep extra precision during intermediate steps.

Reality check

Compare the result with a nameplate, utility bill, manufacturer table, official product terms or a second independent calculation.

5. Sensitivity: what makes the result move?

Do not stop after one answer. Increase and decrease the important inputs and observe what changes. If the result is highly sensitive to an uncertain assumption, that assumption deserves better measurement or a larger design margin.

Question	Why it matters	Action
Which input is least certain?	Uncertainty in a dominant variable can overwhelm calculator precision.	Measure it or run a conservative scenario.
What happens at minimum/maximum operating condition?	Real systems rarely stay at one nominal point.	Calculate boundary cases.
What protection, tax, fee, loss or derating is outside the simple equation?	Real-world limits are often external to the core formula.	Add a separate verification step.

6. Common mistakes

Selecting MCCB breaking capacity without calculating fault level
Using generic contactor multipliers as a substitute for Type-2 coordination tables
Sizing a long motor cable only by ampacity
Ignoring star-delta transition/interlocking
Estimating THDv from VFD kW alone without source impedance and harmonic spectrum

7. Field / decision checklist

Collect motor nameplate values
Confirm prospective short-circuit current
Confirm starter duty and starts/hour
Measure/estimate cable route and installation conditions
For VFD, check line reactor/DC choke/filter needs and motor cable dv/dt

8. How to read the diagram

Read the diagram from left to right. Each block is a stage in the reasoning chain. In electrical topics this usually follows energy or signal flow from the source to the load and protection. In finance topics it follows cash flow through time and compounding. If a stage is unknown, the final answer should be treated as provisional.

9. Why measured or real values may differ

Calculators deliberately simplify reality. Electrical equipment has tolerances, temperature effects, waveform distortion, voltage variation and dynamic behaviour. Financial outcomes have timing, fees, taxes, changing rates and market variability. A difference does not automatically mean the formula is wrong; first compare assumptions, units and the exact quantity being measured.

10. Deeper questions to ask

Does the tool give final MCCB breaking capacity?

No. Breaking capacity requires the prospective short-circuit current at the panel. The tool can recommend that this value be calculated and checked.

What changes with star-delta?

Three contactors and transition logic are required; line and winding currents differ, so contactor and OLR selection depend on their circuit location.

What changes with VFD?

The drive controls motor starting, so a conventional OLR may be replaced by electronic motor protection inside the drive where appropriate, while input protection, output cable effects and harmonics require separate checks.

Learning and design note: This module is designed for understanding and preliminary planning. Final electrical design must be checked against the actual equipment data, fault level, installation conditions, manufacturer coordination information and applicable standards. Financial examples are mathematical illustrations, not guaranteed returns or personalized financial advice.

Continue learning

Use the related motor-current, cable-size, voltage-drop, power-factor, star-delta and VFD guides to understand every part of the panel selection.

Motor Current Cable Size VFD Guide

Motor Panel Equipment Selector

1. Enter motor & installation data