Contactor Size Calculator

1. Select Standard

1. Use the toggle at the top to choose between IEC and NEC standards.
2. The required input fields will automatically adjust based on your selection.

2. Enter System Details

1. Input your motor's power, system voltage, phase, and efficiency.
2. For IEC, select the Utilization Category (e.g., AC-3).
3. For NEC, provide the motor nameplate current and service factor.

3. Calculate & Review

1. Apply any necessary derating factors for temperature, altitude, or grouping.
2. Click "Calculate" to view the results.
3. The tool will display the required ratings and further considerations based on your chosen standard.

1. Define Application & Duty

First, identify the load type to select the correct Utilization Category. This is critical for ensuring the contactor's endurance.

• AC-1: For resistive loads like heaters.
• AC-3: For standard squirrel-cage motors (most common).
• AC-4: For severe duty like jogging or plugging.
• AC-3e: For high-efficiency IE3/IE4 motors.

2. Establish System Ratings

Confirm your system's electrical characteristics to match the contactor's ratings.

• Ue: Rated Voltage (must be ≥ system voltage).
• Ui: Insulation Voltage (must be ≥ Ue).
• Uimp: Impulse Voltage (e.g., 6-8 kV).

IEC ratings assume <40°C ambient and <2000m altitude. Apply derating if conditions exceed this.

3. Size for Current (Ie)

Calculate the motor's Full Load Current (FLC), then apply all derating factors to find the required operational current (Ie).

Required Current Formula:

Select a contactor where its standard Ie ≥ Required Ie.

4. Ensure Short-Circuit Coordination

This is a critical safety step. Select a coordination type based on your needs:

• Type 1: Contactor may be damaged after a fault.
• Type 2: No damage allowed, ensuring quick return to service.

Always use manufacturer-tested tables to select a contactor, overload, and SCPD (fuse/breaker) combination that is proven to work together.

5. Pair Overload Protection

A contactor does not protect against overloads. It must be paired with an overload relay.

• Setting: The relay's current range must include the motor's nameplate FLC.
• Trip Class: Use Class 10 for normal starts, or Class 20/30 for heavy loads with long acceleration times.

6. Select Control Coil

The coil is the electromagnet that activates the contactor.

• Voltage: Match the coil voltage (e.g., 24V DC, 230V AC) to your control circuit supply.
• Burden: Size the control transformer to handle the high inrush (pickup) VA, not just the sealed VA.

Consider low-consumption electronic coils if driving directly from a PLC.

7. Specify Accessories

Select any required add-ons for your application:

• Auxiliary Contacts: Add NO/NC contacts for status signals or control logic (e.g., seal-in circuit).
• Interlocks: Mechanical interlocks are mandatory for reversing starters.
• Terminals: Ensure terminals can fit your wire size and apply correct tightening torque.

8. Final Verification

Perform a final check before installation:

• Markings: Ensure the contactor is marked with IEC 60947-4-1 and all ratings (Ue, Ie, Ui, Uimp).
• Environment: Confirm the IP rating is suitable for the panel location.
• Endurance: For high-cycle duties, verify the electrical life meets your requirements.

Worked Example: Sizing an IEC Contactor

Scenario: Select a contactor for an 11 kW, 400V, 50 Hz IE3 motor with a nameplate Full Load Amp (FLA) of 20 A. The panel has an available fault current of 25 kA and the control circuit is 230V AC.

1. Application & Duty: Standard IE3 motor, so the duty is AC-3. For better performance, we can also look for an AC-3e rating.
2. Current Sizing (Ie): The motor FLA is 20 A. No derating is mentioned, so we need a contactor where its AC-3 rating at 400V is Ie ≥ 20 A. We would select the next standard size up for a safety margin.
3. Short-Circuit Coordination: We need high reliability, so we select Type 2 coordination. We consult a manufacturer's coordination table for a 25 kA fault level. A typical tested combination might be:
   • Contactor: Frame size rated for ~25A AC-3.
   • Overload Relay: A model with a setting range that includes 20 A (e.g., 18-25 A), with Trip Class 10.
   • SCPD: A 50 A aM type fuse or a specific MCCB model listed in the table.
4. Control Circuit: We select a contactor with a 230V AC coil. We check its data sheet for inrush VA to size the control transformer correctly.
5. Accessories: We will add a 1 NO + 1 NC auxiliary block for the seal-in circuit and a status signal to the PLC.
6. Final Verification: The chosen contactor is marked with IEC 60947-4-1, its terminals accept our wire size, and its electrical endurance is suitable for the expected switching frequency.

Controller HP Rating

Contactor must have an HP rating ≥ the motor's HP at the operating voltage (NEC 430.83).

Conductors

Branch-circuit conductors must be sized to at least 125% of the motor's FLC from NEC tables.

Branch OCPD

Size the fuse or breaker according to NEC Table 430.52 (e.g., 250% for inverse-time breakers).

Overload Protection

Set the overload relay based on the motor's nameplate current (not table FLC), typically at 115% or 125%.

SCCR

The entire assembly's Short-Circuit Current Rating (SCCR) must be ≥ the available fault current at the panel.

Disconnecting Means

The disconnect switch must be rated for at least 115% of the motor's FLC.