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Part 14 – How to Size Star-Delta Starter for Motors?

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Sizing a Star-Delta Starter with Contactors for a 3-Phase Motor

A Star-Delta starter is used to reduce the inrush current when starting large three phase motors. The starting current of a three-phase induction motor is typically 6 to 8 times higher than its normal full-load current. If this high inrush current is not properly controlled or reduced to a safe level, it can damage or burn the motor. That’s why a Star-Delta starter is used to safely start and operate a three-phase induction motor.

A Y-Δ (Wye-Delta) starter typically consists of three contactors (Main, Star, and Delta), a thermal overload relay, and optionally, a timer. The contactors used in a Star-Delta starter are generally rated smaller than the single contactor used in a Direct-On-Line (DOL) starter. This is because each contactor in a Star-Delta configuration only handles the phase (winding) current, which is approximately 1/√3 (or about 58%) for Main and Delta and 1/3 (33%) for star contractor of the line current (I_L) or full load current (FLC).

The following step by step tutorial shows how to size contactors and components of star-delta starter for three phase motors.

Table of Contents

Steps for Sizing a Star-Delta Starter

To properly size a star-delta starter, determine the motor’s full-load current (FLC from Table) and full-load amperes (FLA from motor’s nameplate), calculate the required current for the main and delta contactor (approximately 58% of the FLC) and 33% of FLC for the star contactor, and then size the components (contactors, thermal overload relay, and conductors) accordingly as shown in the below solved example.

1 – Identify the Motor Nameplate Data

The typical motor nameplate data printed on it generally indicates the motor class type, power rating in kW or HP, Line voltage, full load amperes (FLA), locked-rotor current (LRC), service factor (SF), power factor, efficiently etc. Keep in mind that full load amperes are used to determine the thermal overload setting, while the full load current (FLC) given in NEC Table 430.250 is used to size the short circuit and ground fault protective devices i.e. breakers and fuses.

2 – Determine Full Load Current (FLC)

If FLC is not given, use the NEC Table 430.250 for standard motor currents based on HP and voltage.

3 – Size the Components for Star-Delta Starter

Component	Sizing Rule
Main Contactor	58% of full-load current
Star Contactor	33% of full-load current
Delta Contactor	58 of full-load current
Overload Relay	Set at 100% of motor FLC
Circuit Breaker (CB) / Fuse	Use NEC Table 430.52
Power Cable	Use NEC Table 310.16, select for FLC

To properly size the star/delta contactors, selecting the correct size of conductors, breakers/fuses, and overload protection for motor starters, consider the following example.

Example – Y-Δ (Wye-Delta) Starter for 3-Phase Motor

What are the appropriate sizes for the Main, Star, and Delta contactors, along with the associated starting components, for a Star-Delta starter used with a 50 HP, 3-phase, 480V squirrel-cage induction motor operating at 95% efficiency and a power factor of 0.9? The FLA is 60A. The overload relay is placed after the winding split into main and delta contactor.

Given Data

Motor Power: 50 HP
Voltage: 480V, 3-phase
Power Factor (PF): 0.9
Efficiency (η): 95%
Full Load Amperes (FLA) = 60A (From Motor nameplate)
Full Load Current (FLC) = 65A (From NEC Table 430.250 for a 50 HP, 480V motor)

Step 1: Sizing the Contactors

Main and Delta Contactors:

As mentioned before, the size of all three contactors in start delta starter is smaller than the one main contactor used in DOL starter as they are used to control the phase current in motor winding only, not the line current or full load current.

The main contactor is used for start, transition and running while the delta contactor is used for running after transition from star configuration into delta.

The main contactor and delta contactor should be sized to handle 58% of the full-load current (FLC) or line current of the motor.

The value of winding current:

I_P = 1/√3 × (I_L or FLC)

I_P = 1/1.732 × (I_L or FLC)

I_P = 58% × 65A

I_P = 37.7A

Star Contactor:

Star contactor is used to carry the starting current only. Hence, it is sized 1/3 of the full load current (FLC) or line current. The star contactor carries approximately 33% of the motor’s full-load current.

I_P = 1/3 × (I_L or FLC)

I_P = 33% × 65A

I_P = 21.45A

In short, the required contactor ratings for a Star-Delta starter for the given motor are:

Main Contactor: 58% × 65A = 37.7A ≈ 40A
Star Contactor: 33% × 65A = 21.45A ≈ 25A
Delta Contactor: 58% × 65A = 37.7A ≈ 40A

Selected Contactors: 40A (Main), 25A (Star), 40A (Delta).

The making and breaking capacity of AC3 contactors for squirrel cage induction motor can be calculated as follow:

Making/Breaking Capacity : FLC × 10
Making/Breaking Capacity : 65A × 10 = 650A

Step 2: Size the Overload Relay

According to the NEC, the overload relay should be rated at 125% of the motor’s full-load ampere (FLA) if the motor has a service factor of 1.15 or greater or a temperature rise of 40°C or less. For general-purpose motors that do not meet these conditions, the overload relay should be rated at 115% of the FLA. Refer to the motor nameplate to obtain the full-load amperage (FLA) at the operating voltage.

Since it is a Star-Delta starter and it draws phase current, which is lower than the line current (approximately 58%) the overload relay should be sized accordingly. Hence, the overload relay is typically rated at 58% of the motor’s full-load current, to match the reduced phase current during the start-up and running conditions in the star and delta connection.

Overload Setting = 60A × 58% = 34.8A

According to IEC standards, the thermal overload relay should be rated at 100% of the motor’s full-load current (FLC). However, in a Star-Delta starter configuration, the overload relay is typically sized at 58% of the FLC, since it is installed in the phase where the current is reduced during startup. The relay setting range usually allows adjustment from 70% to 120% of the full-load phase current for minimum and maximum rating based on motor characteristics and application.

OL Relay Setting = 65A × 58% = 38A
Min. Relay Setting = 65A × 70% = 26A
Max. Relay Setting = 65A × 120% = 45A

Step 3: Size the Circuit Breaker

The size of Overcurrent Protection Devices (OCPDs) for short-circuit and ground-fault protection is determined according to NEC 430.52 and Table 430.52(C)(1). Based on the table: Dual-element time delay and non-time-delay fuses are rated at 175% and 300% of the motor’s full-load current (FLC), Inverse time circuit breakers are rated at 250% of the FLC, and Instantaneous trip circuit breakers are rated at 800% of the FLC.

Dual Element Time-delay Fuse Size= 65A × 175% = 113A
Non-time-delay Fuse Size= 65A × 300% = 195A
Inverse Time Breaker Size = 65A × 250% = 162.5A
Instantaneous circuit Breaker Size = 65A × 800% = 520A

If we go with inverse time circuit breaker, the next available size is 175A.

Step 4: Size the Power Cable

Using NEC Table 310.16, for 65A (for continuous load), the selected cable is THHN Copper wire in the 75°C column: 65A → 4AWG

Final Selection for Star-Delta Starter

Component	Selection
Main Contactor	40A
Star Contactor	25A
Delta Contactor	40A
Overload Relay	34A
Circuit Breaker	175A
Power Cable	4 AWG Cu (THHN)

Series Overview: Motor Circuit Calculations

Resources & Tutorials:

Wire Sizing Guides

Part 14 – How to Size Star-Delta Starter for Motors?