The Direct Answer: Sizing a Three Phase Motor Starter

To size a three phase motor starter, multiply the motor's Full Load Amps (FLA) by 1.25 to size the branch circuit conductors and thermal overload, then select an IEC AC-3 rated contactor that meets or exceeds that calculated amperage. For a standard 10 HP, 460V AC squirrel-cage induction motor (14A FLA) driving a centrifugal pump, use a NEMA Size 1 or IEC AC-3 rated 18A contactor (like the Schneider Electric LC1D18) paired with a 12-18A thermal overload relay and a 35A Motor Circuit Protector (MCP).

For 90% of industrial applications—pumps, fans, conveyors, and compressors—the NEMA Design B squirrel-cage induction motor is the correct fit. It demands a starter capable of handling 600% Locked Rotor Amps (LRA) during startup without nuisance tripping, paired with a Short Circuit Protective Device (SCPD) like an MCP or Class J fuses, and a dedicated thermal or electronic overload relay to protect against mechanical overloads.

Motor Load Profiles and Starter Compatibility

Not all three phase motor starters are created equal. The starting method you choose dictates the torque curve applied to the motor shaft and the inrush current drawn from the utility. Below is a comparison of starter types matched to their ideal load profiles.

Starter Type Applied Torque Curve Control Needs Typical Cost (15HP) Best Load Profile
DOL (Direct On Line) Full torque instantly (150-250% FLT) Simple 2-wire or 3-wire control circuit $150 - $250 Low-inertia loads: centrifugal pumps, small fans, machine tools.
Star-Delta (Wye-Delta) Reduced to 33% during start, then steps to 100% Timer relay, 3 contactors, mechanical interlocks $400 - $600 High-inertia, low-starting-torque loads: large blowers, long conveyors.
Soft Starter Linear or S-curve ramp (adjustable 10-100%) SCR firing board, bypass contactor, 24VDC logic $600 - $900 High-inertia loads requiring smooth acceleration: belt conveyors, positive displacement pumps.
VFD (Variable Frequency Drive) Full torque at zero speed, constant flux control PWM inverter, complex parameter programming, shielding $800 - $1,500+ Variable torque loads needing speed control: HVAC fans, process pumps, hoists.
Bench Tip: If your load is a high-inertia flywheel or a heavily loaded conveyor, a DOL starter will cause severe mechanical shock and massive voltage dips. Step up to a Soft Starter or VFD to ramp the torque smoothly and avoid tripping the upstream utility breaker.

Wiring and Terminal Identification for 3-Phase Starters

A standard Direct-On-Line (DOL) three phase motor starter consists of a contactor (the power switch) and an overload relay (the thermal protector). Miswiring the control circuit to the power terminals is a common bench mistake that instantly bricks the coil. Here is the standard IEC terminal nomenclature you will find on modern Eaton and Schneider IEC starters:

  • L1, L2, L3: Line-side power inputs from the disconnect or SCPD.
  • T1, T2, T3: Load-side power outputs feeding the overload relay, then the motor.
  • A1, A2: Contactor coil terminals. A1 is typically the common/neutral or positive, A2 is the switched hot or negative. Never apply 460VAC here if it's a 120VAC or 24VDC coil.
  • NO (13/14) and NC (21/22): Auxiliary contacts used for holding circuits (seal-in) and interlocks. 13/14 is Normally Open; 21/22 is Normally Closed.
  • 95, 96: Overload relay NC trip contacts. These must be wired in series with the contactor coil or PLC stop input to break the circuit on thermal trip.

Sizing Rule of Thumb and Worked Load Example

Do not rely on generic HP-to-kW conversions to size your starter. Motor efficiency and power factor vary wildly between manufacturers. Always size based on the nameplate Full Load Amps (FLA) and the NEC Article 430 guidelines.

The Worked Example: You are wiring a 15 HP, 460V, 3-phase squirrel-cage induction motor driving a high-starting-torque conveyor. The nameplate reads 21A FLA and 126A LRA (Locked Rotor Amps).

  1. Contactor Sizing (AC-3 Category): The IEC AC-3 rating defines a contactor's ability to start and stop squirrel-cage motors. Rule of thumb: size the contactor at 115% to 125% of FLA.
    Calculation: 21A × 1.25 = 26.25A.
    Selection: Choose a 32A AC-3 rated contactor.
  2. Overload Relay Sizing: The thermal element must be set exactly to the nameplate FLA.
    Selection: Choose an overload block with an adjustment range that encompasses 21A (e.g., a 20-25A or 22-32A range). Dial it precisely to 21A.
  3. Short Circuit Protective Device (SCPD): Per NEC 430.52, an inverse-time breaker for an AC motor can be sized up to 250% of FLA.
    Calculation: 21A × 2.5 = 52.5A.
    Selection: Use a 50A 3-pole breaker, or better yet, a Motor Circuit Protector (MCP) with adjustable magnetic trip settings dialed to roughly 10x FLA (210A) to ignore the 126A LRA inrush while catching dead shorts.

Failure Signatures: Diagnosing Hum, Overheat, and Stall

When a three phase motor starter fails to perform, the motor will exhibit distinct physical and auditory signatures. Use these to diagnose the root cause before replacing parts.

Symptom Root Cause Diagnostic Measurement & Fix
Loud Hum / Vibration (Won't start) Single-phasing (lost one power leg) or severe voltage unbalance. Measure phase-to-phase voltage at T1-T2, T2-T3, T1-T3. If one reads 0V or significantly lower, check upstream fuses. Replace blown fuse and investigate the short.
Overheat / Nuisance Tripping Undersized starter, high ambient temperature, or mechanical binding. Measure running current with a clamp meter. If current is < FLA but overload trips, the ambient temp exceeds the relay's compensation range (usually 40°C). Move the panel or upgrade to an electronic overload.
Stall / Breaker Trips Instantly Load torque exceeds motor breakdown torque, or wrong starter type. Decouple the motor from the load and run it unloaded. If it runs fine, the load is jammed or too heavy for a DOL start. Switch to a Soft Starter or VFD to ramp torque.
Safety Warning: Single-phasing is the number one killer of three-phase induction motors. If a contactor pole welds shut or a fuse blows on one leg, the motor will attempt to draw 173% of its normal current on the remaining two legs to maintain output. Always use a starter with built-in phase-loss protection or an electronic overload relay that detects phase unbalance.

The Decision Tree: Picking Your Exact Starter Part Number

Stop guessing at the supply house counter. Use this decision path to lock in your exact bill of materials for a standard DOL three phase motor starter.

Condition Action / Selection
Is the load variable torque (pump/fan) AND requires speed adjustment? Stop. Buy a VFD (e.g., Yaskawa A1000). This guide is for fixed-speed starters.
Is the load high-inertia requiring reduced voltage starting? Stop. Buy a Soft Starter (e.g., ABB PSR series).
Is the load fixed-speed, low-to-medium inertia (conveyor, compressor)? Proceed to DOL sizing.
What is the motor nameplate FLA? Multiply by 1.25. Select the next standard IEC AC-3 contactor frame size above this number.
What is the control circuit voltage? Select coil voltage. (120VAC for standard US panels, 24VDC for PLC-driven panels).

The Concrete Pick for a 15HP (21A FLA) 460V Conveyor Motor:
Do not default to generic, unbranded import contactors for critical industrial loads; the arc chutes and silver-alloy contacts on premium brands last 10x longer under AC-3 switching. For this 21A application, your default, buy-it-today pick is the Eaton XTCE Series:

  • Contactor: Eaton XTCE032B01 (32A AC-3 rated, 110-120VAC 50/60Hz coil, 1NO auxiliary built-in).
  • Overload Relay: Eaton XTOB025BC1 (Class 10 trip, adjustable 17-25A range, dial to 21A).
  • SCPD: Eaton HMCP050 (50A Motor Circuit Protector, adjustable magnetic trip).

Wire the HMCP to the XTCE line side, run the load side through the XTOB, and dial the overload precisely to 21A. You now have a code-compliant, mechanically robust motor starting circuit that will survive a million switching cycles.