DOL Motor Starter, Contactor & Overload Sizer
Precision sizing engine for three-phase Direct-On-Line (DOL) motor starters,AC-3 / AC-3e contactor selection, Class 10/20 thermal overload dial settings,BS 88-2 Type 2 coordination back-up fusing, BS 7671 feeder cable voltage drop, and BS EN 61439-2 Form 4 MCC panel segregation.
Starter Engineering Telemetry
Standard DOL Wiring & Form 4 Compartment Reference
BS EN 60617 Standard Engineering Symbols • BS EN 61439-2 Physical Segregation Boundaries
Show Full Mathematical Derivation (AM2 Working Standard)BS 7671:2018+A4:2026
Engineering Design Guidance: BS EN IEC 60947-4-1:2019 & BS EN 61439-2
1. AC-3 vs AC-3e Contactor Utilisation
Standard AC-3 covers typical squirrel-cage motors (starting and switching off during running). However, under UK Eco-Design statutory mandates (SI 2021/1095), motors from 0.75 kW to 1000 kW must meet IE3 (Premium) or IE4 (Super Premium) efficiency. These motors feature higher copper slot fill and lower rotor resistance, resulting in locked-rotor starting inrush currents reaching 7.5× to 8.5× FLC (compared to 6× for legacy IE1 motors). Always verify contactor compliance with AC-3e (introduced in BS EN IEC 60947-4-1:2019) to prevent premature contact erosion and dynamic contact welding during cold starts.
2. BS EN IEC 60947-4-1:2019 Type 2 Coordination
Type 2 Coordination requires that under a short-circuit fault, the starter and contactor cause no hazard to operators or surrounding plant and remain fully functional without replacement of components (light contact tacking is permissible if easily freed with a screwdriver without significant deformation). To achieve Type 2 coordination:
- BS 88-2 gM Fuses: Dual-rated motor fuses (e.g.
32M50) provide a continuous 32A thermal rating with a 50A melting time-current curve to absorb starting inrush without nuisance aging. - Partial-range aM Fuses: Provide high-speed short-circuit protection only; the thermal overload relay F1 handles all overload currents.
- Motor Protection Circuit Breakers (MPCB): Combined units offering instantaneous magnetic tripping (12 to 14× FLC) and temperature-compensated thermal protection.
3. BS EN 61439-2 Forms of Internal Separation
In industrial Motor Control Centres (MCCs), internal barriers mitigate risk to personnel during inspection or maintenance. The primary forms of separation for starter cubicles are:
| Form | Busbars | Functional Starters | External Terminals | Typical UK Sector |
|---|---|---|---|---|
| Form 2b | Separated from starters | Not separated from each other | Separated from busbars (common chamber) | Light commercial & HVAC |
| Form 3b | Separated from starters | Separated from each other | Separated from busbars (common chamber) | BMS plantrooms & institutional |
| Form 4b Type 6 | Separated by metallic barriers | Separated in individual cubicles | Separated in dedicated individual terminal boxes | Standard UK industrial MCC / Factories |
| Form 4b Type 7 | Fully segregated busbar chamber | Withdrawable or fixed cubicles | Individual terminal boxes with dedicated glanding plates | WIMES 3.01 (Water) & HTM 06-01 (Health) |
4. Feeder Cable & Voltage Drop Sizing (BS 7671)
Motor feeders must satisfy Regulation 433.1 (Ib ≤ In ≤ Iz) under continuous running, and limit transient starting voltage drop to < 8%. A starting voltage drop exceeding 10% can cause contactor coil chattering, thyristor misfiring, or extended run-up times triggering premature thermal overload trips.