How to Wire a Magnetic Motor Starter

A magnetic motor starter, also called a full-voltage or across-the-line starter, is the standard way to switch power to a three-phase motor while protecting it from overload, and it's built from two components working together: a contactor that does the actual switching, and an overload relay that monitors current and interrupts the circuit if the motor draws more than it should. Getting the wiring right means understanding the difference between the power circuit and the separate, lower-voltage control circuit that operates it, along with correctly sized circuit breakers upstream and a disconnect switch for safe servicing. This guide covers how a magnetic starter works, the difference between 3-wire and 2-wire control, and the correct wiring order.

Contactor vs. Overload Relay: Two Different Jobs

The contactor is the heavy-duty electromechanical switch that carries the motor's actual current. When its coil is energized, internal contacts close and connect the three power legs to the motor; when the coil de-energizes, the contacts open and cut power. The contactor does the switching, nothing more; it has no built-in awareness of whether the motor is drawing a safe or unsafe amount of current.

The overload relay is wired in series with the contactor and continuously monitors motor current. If the current exceeds the relay's set trip point for long enough, it opens a normally closed contact in the control circuit, de-energizing the contactor coil and dropping power to the motor. The relay is sized to the motor's full-load amperage (FLA) from the nameplate, not from the general conductor-sizing tables used elsewhere in the circuit, and its heater elements or electronic trip setting need to be matched to that specific motor.

Power Circuit vs. Control Circuit

A magnetic starter installation actually involves two separate circuits operating at different voltages, and keeping this distinction clear is the key to understanding the wiring:

Where the motor's supply voltage is too high for the contactor coil to use directly, a control transformer steps that voltage down to the coil's rated control voltage, keeping the pushbuttons, switches, and other control devices operating at a safer, lower voltage than the motor circuit itself.

3-Wire Control vs. 2-Wire Control

3-wire control uses momentary pushbuttons, a normally open Start button and a normally closed Stop button, along with an auxiliary "seal-in" contact on the contactor itself that holds the coil energized after the Start button is released. This is the standard configuration in industrial control because it provides inherent undervoltage protection: if power is lost, the contactor drops out and does not automatically restart when power returns, since the seal-in contact also opens when the coil de-energizes. Restarting requires someone to press Start again, which prevents equipment from unexpectedly restarting unattended after an outage.

2-wire control uses a maintained switch, such as a selector switch, float switch, or pressure switch, that keeps the control circuit closed as long as its condition is met. This configuration will automatically restart the motor once power returns after an outage, since the maintained switch is still in its "call for run" position. 2-wire control is generally limited to simple on/off applications where automatic restart after a power interruption is acceptable or even desired, such as some pump and compressor applications.

Wiring a 3-Wire Start-Stop Circuit: Step by Step

• De-energize and lock out the circuit at the disconnect before any wiring begins.
• Wire the power circuit first. Connect the three incoming power legs (L1, L2, L3) to the line side of the contactor. Connect the contactor's load side (T1, T2, T3) to the line side of the overload relay, and the overload relay's load side to the motor.
• Confirm the overload relay is sized and set to the motor's actual full-load amperage from its nameplate before proceeding.
• Wire the control circuit's Stop button first. Run control voltage from one side of the control transformer (or control source) to one terminal of the normally closed Stop button.
• Connect the Stop button's other terminal to one terminal of the normally open Start button.
• Connect the Start button's other terminal to the contactor coil terminal, and also to one side of the contactor's auxiliary (seal-in) contact.
• Wire the other side of the seal-in contact back to the same point as the Start button's output, so the seal-in contact is wired in parallel with the Start button.
• Route the overload relay's normally closed contact in series with the contactor coil circuit, so an overload trip breaks the coil circuit and drops the contactor out regardless of the pushbuttons' state.
• Connect the other side of the contactor coil to the return side of the control voltage source, completing the circuit.
• Double-check every connection 
• Restore power and test. Confirm Start energizes the motor and holds it running after the button is released, and confirm Stop, and an intentional overload trip test where practical, both de-energize it correctly.

Why the Seal-In Contact Matters

The seal-in (or "holding") contact is what makes 3-wire control work with a momentary pushbutton at all. Without it, releasing the Start button would immediately open that side of the circuit and drop the contactor out, since nothing else would be holding the coil energized. The seal-in contact, wired in parallel with the Start button and mechanically linked to the contactor itself, closes the instant the contactor energizes and keeps the coil circuit complete after the Start button is released, right up until the Stop button or the overload relay breaks the circuit.

Common Wiring Mistakes

• Sizing the overload relay from the wrong number: the overload relay must be set from the motor's nameplate full-load amperage, not the FLC (full-load current) table value used to size conductors and short-circuit protection; mixing these two up is one of the most common motor-circuit errors.
• Forgetting the seal-in contact: a 3-wire circuit without it will only run while the Start button is physically held down, since nothing holds the coil circuit closed once it's released.
• Wiring 2-wire control where 3-wire is needed: using a maintained switch on an application where unattended automatic restart after a power loss would be unsafe is a real hazard, not just a wiring preference.
• Mismatched control voltage: connecting a 24V-rated contactor coil directly to a 120V control source (or vice versa) without the correct transformer will damage the coil or fail to energize it.

The Bottom Line

Wiring a magnetic motor starter comes down to correctly connecting two related but distinct circuits: the power circuit, running full motor current through the contactor and overload relay, and the control circuit, running a much smaller signal current that energizes the contactor coil through pushbuttons or switches. A 3-wire start-stop circuit, using momentary buttons and a seal-in contact for undervoltage protection, is the standard, safer configuration for most industrial applications, while 2-wire control with a maintained switch suits specific cases where automatic restart is acceptable. Sizing the overload relay from the motor's actual nameplate FLA, not the conductor-sizing table, and confirming the seal-in contact is correctly wired are the two details most likely to trip up an otherwise correct installation.