VFD Sizing Calculator & Circuit Breaker Selection Guide | SimplyBuy
Free Industrial Selection Tools

Industrial Electrical
Selection Guides — VFD, Contactor,
Soft Starter & More

Specify Correctly. Order Confidently.

Eight free selection guides for industrial buyers, resellers and panel builders — VFD selection, circuit breaker guide, motor starter sizing, contactor sizing, soft starter comparison, automation selection, PLC guide and cable cross section table. Use the guides to specify correctly, then request a confirmed quote from SimplyBuy within 12 hours.

8
Selection Guides
4
Major Brands
12h
Quote Response
50+
Countries DDP
Input motor kW + load type → recommended VFD rating and brand references across Schneider, Siemens, ABB and Danfoss.
Selection Guide
MCB vs MCCB vs ACB — step-by-step guide with brand comparison tables for Schneider and Siemens.
Selection Guide
Select contactor, overload relay and MPCB by motor kW — Schneider, Siemens and ABB comparison table.
Selection Guide
I/O-based PLC selection guide + relay type comparison — when to use each, and which brand to specify.
Selection Guide
Select the right AC contactor by motor kW and duty cycle — Schneider TeSys, Siemens SIRIUS and ABB AF series.
Selection Guide
DOL vs Soft Starter vs VFD — when to choose each, and how to size a soft starter for pump and fan applications.
Selection Guide
PLC vs DCS vs SCADA — how to select the right automation architecture for your application and scale.
Selection Guide
Select cable cross section (mm²) by current rating, material and installation method — IEC 60364 reference table.
Selection Table
Selection Guide

VFD Selection Guide — How to Select the Right Variable Frequency Drive

Selecting the correct VFD starts with matching the drive to your motor power (kW), load type and supply voltage. Oversizing wastes budget; undersizing causes thermal stress and premature failure. Follow this 3-step VFD selection guide to determine the right drive, then confirm with SimplyBuy.

Step 1 — Identify Your Load Type
Variable Torque
Pumps · Fans · Blowers
Load torque increases with speed squared. Current demand is lower at partial speed — significant energy savings possible. Select VFD at 100–110% of motor FLC. Most common application.
VFD rated current ≥ motor FLC
Apply 10% derating above 40°C or 1,000m altitude
Constant Torque
Conveyors · Mixers · Compressors
Full torque required at all speeds. VFD must sustain rated current continuously. Select VFD at 110–125% of motor FLC. Ensure VFD thermal rating matches duty cycle.
VFD rated current ≥ 110% motor FLC
Check 150% overload capability for 60s
High Inertia
Centrifuges · Large Fans · Crushers
High inrush during acceleration phase. Extended ramp time required. Select VFD at 125–150% of motor FLC. Consider extended acceleration time or braking resistor.
VFD rated current ≥ 125% motor FLC
Check braking requirement — resistor or regen unit
Step 2 — Motor kW to Full Load Current Reference (380–415V, 3-phase)
Motor (kW)FLC @ 400V (A)VT Min VFD (A)CT Min VFD (A)
0.75 kW2.1 A2.1 A2.5 A
1.5 kW3.8 A3.8 A4.2 A
2.2 kW5.5 A5.5 A6.1 A
4.0 kW9.5 A9.5 A10.5 A
5.5 kW12.5 A12.5 A13.8 A
7.5 kW16.5 A16.5 A18.2 A
11 kW23 A23 A25.3 A
15 kW30 A30 A33 A
22 kW44 A44 A48 A
30 kW58 A58 A64 A
37 kW72 A72 A79 A
45 kW86 A86 A95 A
55 kW104 A104 A115 A
75 kW140 A140 A154 A
Step 3 — Brand Comparison
BrandRecommended SeriesBest ApplicationKey Feature
Schneider ElectricAltivar ATV320Pump, fan, conveyorEcoStruxure ready, built-in EMC filter
SiemensSINAMICS G120General industrialTIA Portal integration, PROFINET
ABBACS880Demanding processDirect torque control, wide voltage range
DanfossFC302Pump & fan specialistBuilt-in cascade control, energy optimiser
⚠️ Always verify final VFD selection with the manufacturer's selection software or SimplyBuy's engineering team before ordering — especially for high-inertia, regenerative or multi-motor applications.

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Selection Guide

Circuit Breaker Selection Guide — MCB, MCCB or ACB?

Choosing the wrong circuit breaker type is one of the most common specification errors in LV panel design. This circuit breaker selection guide covers the three main types — MCB, MCCB and ACB — and how to match the right protection device to your rated current and application.

Step 1 — Select Circuit Breaker Type by Rated Current
MCB
Up to 125A
Miniature Circuit Breaker — for residential, light commercial and branch circuit protection. Fixed trip characteristics, DIN rail mounting. Quick and cost-effective for standard final distribution.
Residential distribution boards
Light commercial panels
Branch circuit protection
MCCB
16A – 1,600A
Moulded Case Circuit Breaker — adjustable trip settings, higher breaking capacity. For motor protection, industrial feeders and sub-distribution. Accepts motor operators, auxiliary contacts and electronic trip units.
Industrial feeder protection
Motor protection (>15kW)
Sub-distribution panels
ACB
630A – 6,300A
Air Circuit Breaker — for main distribution and generator protection in large industrial facilities and data centres. Fully withdrawable, energy metering and Modbus/PROFIBUS communication built in.
Main LV distribution
Generator protection
Data centres & substations
Step 2 — Match Protection Requirement to Product Type
Protection RequirementRecommended ProductNotes
Overload + short circuit onlyStandard MCB or MCCBMCB for ≤125A; MCCB for higher ratings
Earth leakage protectionRCCB / RCBO / MCCB + ELR30mA for personnel; 300mA for equipment
Motor protection (high inrush)MPCB (Manual Motor Protector)Replaces MCCB + overload relay in one device
Adjustable trip settingsMCCB with electronic trip unitSchneider ComPact NSX / Siemens 3VL with ETU
Remote operation / motorisedMCCB or ACB + motor operatorAdd rotary motor operator accessory to MCCB/ACB
Metering + communicationACB with Modbus / PROFIBUS trip unitSchneider MasterPact MTZ / Siemens 3WL
Step 3 — Brand Comparison
Schneider Electric
SeriesTypeRangeKey Feature
Acti 9 iC60MCB1A–63ADIN rail, residential & commercial
EasyPact CVSMCCB16A–630AEconomic, IEC 60947-2
ComPact NSXMCCB16A–630AMicrologic trip unit, PROFIBUS
MasterPact MTZACB800A–6,300AFully digital, IEC/UL dual listed
Siemens SENTRON
SeriesTypeRangeKey Feature
SENTRON 5SLMCB0.5A–63AIEC 60898-1, DIN rail
SENTRON 3VLMCCB16A–1,600AETU trip unit, withdrawable
SENTRON 3WLACB630A–6,300AModbus / PROFIBUS, draw-out
💡 For motor protection applications, an MPCB (Manual Motor Protector) + contactor combination may replace both the MCCB and overload relay — simplifying wiring and reducing panel space. Ask SimplyBuy to recommend the right combination for your motor rating.

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Selection Guide

Motor Starter Sizing Guide — Contactor, Overload Relay & MPCB Selection

A motor starter consists of three components: a contactor, an overload relay and a short-circuit protection device (MPCB or fuse). Use this motor starter sizing guide to select the correct rating for each — then match to the right brand.

Step 1 — Select Starter Type
DOL
Up to ~15 kW
Direct On Line — simplest, lowest cost. Inrush 6–8× FLC. Best for small motors, simple loads.
Star-Delta
15–200 kW
Reduces inrush to 2–3× FLC. Requires 3 contactors. Standard for conveyors and pumps above 15 kW.
Soft Starter
Any Size
Controls inrush to 1.5–4× FLC. Smooth ramp-up, soft stop. No speed control during run.
VFD
Any Size
Full speed control + energy saving. Inrush ≤1× FLC. Higher cost. Best for pumps and fans.
Step 2 — Component Sizing by Motor kW (DOL, 380V, AC-3 duty)
Motor (kW)FLC (A)Schneider ContactorSchneider O/L RelaySiemens ContactorABB Contactor
0.75 kW2.1 ALC1D09LRD053RT2015AF09
2.2 kW5.5 ALC1D09LRD073RT2015AF09
5.5 kW12.5 ALC1D12LRD123RT2016AF12
7.5 kW16.5 ALC1D18LRD163RT2016AF16
11 kW23 ALC1D25LRD213RT2026AF26
15 kW30 ALC1D32LRD213RT2026AF26
22 kW44 ALC1D40ALRD223RT2027AF38
30 kW58 ALC1D50ALRD3323RT2035AF52
37 kW72 ALC1D65ALRD3403RT2036AF65
45 kW86 ALC1D80LRD3503RT2037AF80
💡Coil voltage must match your control circuit voltage — typically 24V DC, 110V AC or 220–240V AC. Specify coil voltage when ordering. Most models have multiple coil voltage options available.

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Selection Guide

PLC & Relay Selection Guide — How to Select the Right Control Device

Selecting a PLC or relay involves matching I/O count, logic complexity and communication requirements to your application. This guide covers the most commonly specified PLCs and relay types supplied by SimplyBuy.

🖥️
PLC Selection Guide
How to select a PLC for your application
Step 1 — Define Your I/O Requirements
I/O CountPLC TierTypical Use
Up to 14Nano / Micro PLCSimple machines, standalone equipment
16 – 128Compact PLCSmall to mid-size machines, packaging
128 – 1,024Modular PLCProcess control, large panel systems
1,000+Rack-basedPlant-wide automation, DCS integration
Step 2 — Siemens PLC Selection Guide
SeriesI/O RangeBest For
S7-1200Up to 284Standalone OEM machines, PROFINET
S7-1500Up to 32,768Complex process, motion control, safety
S7-300Up to 32,768Retrofit and maintenance projects
PLC vs Relay Control — Quick Decision
Use PLC when
Logic is complex or needs reprogramming
Fault logging / diagnostics required
HMI or SCADA integration needed
I/O count exceeds 8 points
Use Relay when
Logic is simple and fixed
Budget is primary driver
I/O count is 4 or fewer
No reprogramming ever needed
🔌
Relay Selection Guide
General purpose, timing, monitoring & safety relays
4 Main Relay Types — When to Use Each
⚡ General Purpose Relay
Switching control signals, PLC output interface. Standard NO/NC contacts, DIN rail socket mount.
Schneider RSL · Siemens 3TH · Omron MY
⏱️ Timing Relay
Star-delta timing, motor restart delay, process sequence. On-delay / off-delay / interval modes.
Schneider RE8 · Siemens 3RP · Omron H3CR
📊 Monitoring Relay
Phase failure detection, voltage / current monitoring. Critical process and motor protection.
Schneider RM35 · Siemens 3UG · ABB CM
🛡️ Safety Relay
E-stop circuits, light curtain interface, door interlock. SIL2/SIL3 rated, self-monitoring.
Schneider XPSAC · Siemens 3SK · Pilz PNOZ
Relay Selection — Key Specs to Check
Relay TypeKey SpecsTiming Range
General PurposeCoil voltage, contact A rating, NO/NCN/A
TimingTiming mode, voltage, output contact0.1s – 300h
MonitoringFunction, trip threshold, output contactAdjustable
SafetySIL rating, PLe level, self-monitoringN/A
Selection Guide

Contactor Sizing Guide — Select the Right AC Contactor

AC contactors are sized by their rated operational current at AC-3 duty — the standard duty for squirrel-cage induction motors. Follow this contactor sizing guide to select the correct frame size, then match coil voltage and brand.

Step 1 — Identify Duty Class
Duty ClassApplicationContactor Selection Rule
AC-3Standard squirrel-cage motor — normal start/stopSize to motor FLC. Most common selection.
AC-4Plugging, inching, reversing, jogging dutyStep up one contactor frame size vs AC-3. Heavier duty contacts required.
AC-2Slip-ring motor starting and stoppingConsult manufacturer — current profile differs from AC-3.
AC-1Non-inductive or slightly inductive loads — heaters, resistorsContactor can be significantly smaller than AC-3 equivalent.
Step 2 — Contactor Sizing Table (AC-3, 380–415V)
Motor (kW)FLC (A)Schneider TeSysSiemens SIRIUSABB AFChint NC
1.5 kW3.8 ALC1D093RT2015AF09NC1-0910
5.5 kW12.5 ALC1D123RT2016AF12NC1-1210
11 kW23 ALC1D253RT2026AF26NC1-2510
15 kW30 ALC1D323RT2026AF26NC1-3210
22 kW44 ALC1D40A3RT2027AF38NC1-4010
37 kW72 ALC1D65A3RT2036AF65NC2-6510
55 kW104 ALC1D953RT2038AF96NC2-9510

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Selection Guide

Soft Starter Sizing Guide — When to Use a Soft Starter vs VFD

A soft starter controls the voltage ramp during motor startup and stopping — reducing mechanical shock and inrush current without providing speed control during normal operation. Use this guide to determine whether a soft starter is the right choice for your application.

Step 1 — Soft Starter vs VFD vs DOL: Decision Guide
RequirementDOLSoft StarterVFD
Reduce inrush current
Smooth mechanical start/stop
Speed control during operation
Energy saving on pumps/fans
Lower cost vs VFD✓✓
Built-in motor protection
Step 2 — Soft Starter Brand Comparison by Motor kW (380–415V, 3-phase)
Motor (kW)FLC (A)Schneider AltistartSiemens 3RWABB PSR/PST
5.5 kW12.5 AATS01N212QN3RW4023PSR9-600-70
11 kW23 AATS01N225LU3RW4026PSR25-600-70
22 kW44 AATS48C21Q3RW4036PST37-600-70
37 kW72 AATS48C32Q3RW4045PST72-600-70
55 kW104 AATS48C48Q3RW4055PST105-600-70
90 kW169 AATS48C66Q3RW4074PSTB170-600-70

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Selection Guide

Industrial Automation Selection Guide — PLC vs DCS vs SCADA

Selecting the right industrial automation architecture depends on application scale, process complexity and communication requirements. This guide covers the three main tiers and how to choose between them.

PLC
Programmable Logic Controller
Best for machine-level control and discrete manufacturing. Fast scan times (1–10ms). IEC 61131-3 programming. Typical I/O: 8 to 32,768 points.
Machine tools, conveyors, packaging lines
Motor control panels, LV automation
Siemens S7-1200/1500, Schneider M221/M241
DCS
Distributed Control System
Best for large-scale continuous process control — oil & gas, chemical, water treatment. Distributed I/O architecture. High redundancy and reliability built-in.
Oil & gas refineries, chemical plants
Water and wastewater treatment
Schneider M580 PAC, Yokogawa CENTUM
SCADA
Supervisory Control & Data Acquisition
Supervisory layer over PLCs and RTUs — data acquisition, visualisation and alarm management across geographically distributed assets.
Power grids, pipelines, remote monitoring
Building management systems
Ignition, Wonderware, WinCC, iFIX
PLC Selection by I/O Count — Siemens & Schneider Quick Reference
I/O CountTierSiemensSchneiderTypical Use
Up to 14Nano/MicroLOGO! 8Zelio LogicSimple machines, standalone equipment
16–128CompactS7-1200M221 / M241OEM machines, packaging, motor panels
128–1,024ModularS7-1500M580 ePACProcess control, large panel systems
1,000+Rack / PACS7-1500HM580 Hot StandbyPlant-wide, redundant, DCS-class

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Selection Table

Cable Cross Section Selection Guide — Select by Current, Material & Installation Method

Cable cross-section (mm²) must be matched to the continuous current rating, conductor material and installation method. Under-sized cables cause overheating; over-sized cables add unnecessary cost. This table is based on IEC 60364-5-52 — the international standard for wiring installation.

Method B1 / B2
Cable in conduit on wall, or clipped directly to surface. Most common installation method in panels and buildings.
Method E / F
Cable on open tray, ladder or free in air. Better heat dissipation — allows higher current rating for same cross-section.
Method D
Cable buried directly in ground or in underground duct. Lower rating than tray — soil thermal resistance limits heat dissipation.
Copper Conductor — Current Capacity by Installation Method (A) · IEC 60364-5-52 · 30°C ambient
Cross SectionMethod B1/B2
In conduit / clipped
Method E/F
On tray / in air
Method D
Underground
Typical Use
1.5 mm²16 A21 A22 ALighting circuits, control wiring
2.5 mm²21 A28 A29 ASmall socket outlets, small motors ≤1.5kW
4 mm²27 A36 A38 AMotors 2.2–4kW, sub-distribution
6 mm²34 A46 A47 AMotors 5.5–7.5kW, large sockets
10 mm²46 A63 A63 AMotors 11kW, feeder circuits
16 mm²61 A85 A79 AMotors 15–18.5kW, main feeders
25 mm²80 A112 A101 AMotors 22–30kW, incoming feeders
35 mm²99 A138 A122 AMotors 37kW, sub-main cables
50 mm²119 A168 A144 AMotors 45–55kW, main distribution
70 mm²151 A213 A178 AMotors 75kW, large feeders
95 mm²182 A258 A211 AMotors 90–110kW, main cables
120 mm²210 A299 A240 AMotors 132kW+, main incomer
Aluminium Conductors
Aluminium has approximately 80% of the current capacity of copper for the same cross-section. To convert: multiply copper current values by 0.8, or select aluminium cable one size larger (e.g. use 35mm² Al where 25mm² Cu is specified). Minimum practical size for aluminium is 16mm² due to connection reliability.
Derating Factors
Ambient temp 40°C× 0.91
Ambient temp 50°C× 0.82
3 cables grouped on tray× 0.82
6 cables grouped on tray× 0.73

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Product Catalogs

Product Selection Guide Downloads — PDF Catalogs by Category

Official product catalogs for VFD, circuit breaker and motor control selection. Download for immediate reference — no registration required.

⚡ VFD — Variable Frequency Drive Selection Guide PDF
Schneider Electric
Altivar ATV320 VFD — Selection & Type Code Reference
0.18–22kW · 200–500V · Frame sizes · IP ratings · Communication options · Complete type code table for VFD sizing and ordering.
Download VFD Selection Guide PDF
Inovance
Inovance VFD — Product Selection Catalog
Inovance AC drive series selection guide — power range, voltage options, application notes for pumps, fans and general industrial use.
Download VFD Catalog PDF
🔒 Circuit Breaker — Selection Guide PDF
Schneider Electric
Compact NSX MCCB — Accessories & Selection Guide PDF
NSX 100–630 accessories: motor operator, MX/MN trip coils, Vigi earth leakage module, rotary handle. Complete part numbers for circuit breaker selection.
Download Circuit Breaker Selection PDF
Square D by Schneider Electric
PowerPact MCCB — UL 489 Selection Guide PDF
PowerPact B/H/J/L/P/R frame series · UL 489 certified · North American standard MCCB · Trip unit configurations & accessory part numbers.
Download MCCB Selection PDF
⚙️ Motor Control — Contactor & Starter Selection Guide PDF
Schneider Electric
TeSys Deca LC1-D Series — Contactor Selection Guide PDF
9A–150A AC contactors · Coil voltage options · Auxiliary contact accessories · Overload relay matching · Motor starter wiring diagrams.
Download Contactor Selection PDF
Schneider Electric
TeSys Motor Starters & Protectors — Selection Guide PDF
DOL and reversing motor starters · Manual motor protectors · MPCB selection tables · Motor branch circuit wiring diagrams and part numbers.
Download Motor Starter PDF
Need a Siemens or ABB datasheet not listed? Email us — we'll send it within 24 hours.
FAQ

Frequently Asked Questions — Industrial Product Selection

Q
How do I size a VFD for a pump?
For a variable torque pump application, select a VFD rated at 100–110% of the motor's full load current. Size to the VFD's output current rating, not just motor kW. A 15kW pump at 380V draws approximately 30A — select a VFD rated for at least 30A output. Apply a 10% margin for ambient above 40°C or altitude above 1,000m. SimplyBuy recommends Schneider Altivar ATV320 or Siemens SINAMICS G120 for standard pump duty.
Q
What is the difference between MCB and MCCB?
An MCB (Miniature Circuit Breaker) has a fixed trip current, rated up to 125A, used for final distribution circuits. An MCCB (Moulded Case Circuit Breaker) has adjustable trip settings and covers 16A to 1,600A — used for motor protection, feeder protection and sub-distribution. For motor protection above 15kW, an MCCB or MPCB is almost always the correct choice over an MCB.
Q
What size contactor do I need for a 15kW motor?
A 15kW motor at 380V 3-phase draws approximately 30A full load current. For AC-3 duty, you need a contactor rated for at least 30A AC-3 — for example Schneider LC1D32, Siemens 3RT2026 or ABB AF26. Always match the overload relay: Schneider LRD21 for LC1D32, Siemens 3RU2126 for 3RT2026.
Q
VFD vs soft starter — which should I choose?
Choose a VFD when you need speed control during operation, energy saving on variable torque loads, or precise process control. Choose a soft starter when you only need to control inrush current at start/stop and the motor always runs at full speed during normal operation. A soft starter costs 30–50% less than a VFD for the same motor rating but provides no speed control.
Q
How do I select a PLC for my application?
Start with three questions: (1) How many I/O points do you need? (2) Do you need motion control, PROFIBUS/PROFINET, or safety functions? (3) Which software ecosystem does your team use? For Siemens TIA Portal users, the S7-1200 covers most standalone machine applications up to 284 I/O. For larger systems or process control, the S7-1500 is the correct choice.
Q
When should I use a monitoring relay instead of a PLC?
Use a monitoring relay when the protection function is single and fixed — for example, phase failure detection on a 3-phase motor, or voltage monitoring on a generator output. A monitoring relay costs a fraction of a PLC and requires no programming. Use a PLC when multiple monitoring functions need to be combined with control logic, fault logging, or HMI integration.

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