A soft start wiring diagram always splits into two distinct circuits: the high-current power path (Line to Load) and the low-voltage control logic. If you are wiring a standard 3-phase workshop compressor or conveyor using an industry workhorse like the Schneider Electric Altivar ATS22, the direct answer is this: you will wire 3-phase mains to the L1/L2/L3 terminals, route the T1/T2/T3 output through a thermal overload relay to the motor, and feed 120VAC control power to the CL1/CL2 logic terminals.
But reading the schematic and physically landing the wires in the lugs are two different jobs. Miswiring the control logic can fry the internal logic board, and undersizing the power conductors will cause thermal faults under load. Below, we will walk through the physical terminals, trace the exact current path, and show you how to prove your connections with a multimeter before you ever throw the disconnect.
Decoding the Soft Start Wiring Diagram Symbols
Before you strip a single wire, you need to understand what the symbols on your soft start wiring diagram actually represent in physical space. Manufacturers use standard IEC and NEMA symbols, but a few specific to solid-state starters trip up beginners.
- Anti-Parallel Thyristors (SCRs): You will see two diode symbols pointing in opposite directions, stacked in series for each phase. This represents the Silicon Controlled Rectifiers inside the starter that chop the voltage waveform to limit inrush current. They sit directly between the Line (L) and Load (T) terminals.
- Thermal Overload Relay (OL): Represented by a square with a bimetallic strip symbol and a 95/96 Normally Closed (NC) auxiliary contact. On the diagram, the power flows through the OL, but the 95/96 contact is wired back into the soft starter's logic loop to drop the Run command if the motor overheats.
- Logic Inputs (LI): Shown as a generic input block with a COM (Common) reference. These are not power terminals; they are high-impedance opto-isolators that just need a voltage signal (usually 24VDC or 120VAC) to tell the microprocessor to begin the ramp-up sequence.
Terminal Mapping and Physical Device Layout
When you pop the cover off a Schneider ATS22 (or a similar Siemens SIRIUS 3RW40), the physical layout separates the heavy copper power lugs from the small screw-terminal control blocks. Here is your exact pin mapping for a standard 3-wire control setup.
| Terminal Label | Function | Wire Size / Type (Typical) | Connection Notes |
|---|---|---|---|
| L1, L2, L3 | Line Power In | Sized per NEC 430.22 (e.g., 8 AWG THHN for 40A) | Incoming 3-phase mains from the disconnect/fuses. |
| T1, T2, T3 | Load Power Out | Same as Line (e.g., 8 AWG THHN) | Outgoing power to the thermal overload relay. |
| PE | Protective Earth | Same as Line or per NEC 250.122 | Chassis ground. Must be bonded to the panel ground bar. |
| CL1, CL2 | Control Power | 18 AWG or 16 AWG stranded | 120VAC or 230VAC feed for the internal logic board and cooling fan. |
| LI1 | Logic Input 1 (Run) | 18 AWG stranded | Receives voltage from the Start pushbutton to initiate the ramp. |
| COM | Logic Common | 18 AWG stranded | Return path for the logic inputs. Tied to the control transformer neutral/0V. |
Node-by-Node Trace: Source to Load
Let's trace the current path from the utility feed all the way to the motor windings, explicitly calling out the control polarity and the critical ground path.
The Power Path (High Current)
- Source: 3-phase power leaves the main panel breaker and enters the fused disconnect switch.
- Line Terminals: From the load side of the disconnect, three hot conductors (typically Black, Red, Blue for 208/480V) land on L1, L2, and L3.
- The SCRs: Inside the soft starter, the current passes through the anti-parallel thyristors. During startup, the SCRs fire late in the AC cycle, reducing the RMS voltage to the motor.
- Load Terminals: The chopped voltage exits via T1, T2, and T3.
- Overload Protection: T1/T2/T3 wire directly into the line side of the external Thermal Overload Relay (OL).
- Motor: The load side of the OL connects directly to the motor's T1, T2, and T3 terminal box leads.
The Control Path and Polarity
The control circuit powers the microprocessor and the internal bypass contactor (if equipped).
- AC Control (CL1/CL2): If your diagram specifies 120VAC control power, you are feeding this from a step-down transformer. Because it is alternating current, polarity does not matter—Hot goes to CL1, Neutral goes to CL2.
- DC Control (If using 24VDC logic): If you are using an external 24VDC PLC supply for the logic inputs, polarity is strict. +24VDC must go to the logic input (LI1 via the pushbutton), and the 0VDC (Common) must land on the COM terminal. Reversing this will prevent the opto-isolators from forward-biasing, and the starter will ignore your start command.
The Ground Path (Crucial)
Do not rely on the mounting DIN rail or backplate for grounding. The incoming Equipment Grounding Conductor (EGC) from the main panel lands on the enclosure's ground bar. From there, run a copper bonding jumper to the PE (Protective Earth) terminal on the soft starter chassis. A second jumper must run from the PE terminal out to the motor frame. Never route the equipment ground through the soft starter's power terminals or SCRs.
Meter Verification: Proving Your Connections
Before you remove your LOTO locks and energize the circuit, grab your multimeter. Here is how to verify the physical wiring matches your soft start wiring diagram.
- Verify Dead Circuit: Set your meter to AC Voltage (V~). Measure L1 to L2, L2 to L3, and L1 to Ground at the line side of the disconnect. All readings must be 0.00V.
- SCR Integrity Test (Diode Mode): Set your meter to Diode Test mode. Place the red probe on L1 and the black probe on T1. A healthy SCR will read 'OL' (Open Line) or a very high resistance. Swap the probes (black on L1, red on T1); it should also read 'OL'. Repeat for L2/T2 and L3/T3. If you read 0.00V or a dead short in either direction, the internal thyristor is blown and the unit must be replaced before applying power.
- Control Circuit Continuity: Set the meter to Continuity (the beep setting). With the start pushbutton unpressed, measure across LI1 and COM. It should read 'OL'. Have a helper press and hold the Start button; the meter should beep, confirming the logic path is intact.
- Ground Bond Verification: Set the meter to Resistance (Ohms Ω). Measure from the soft starter's PE terminal to the motor frame ground lug. You want to see less than 0.5 Ω. If it's higher, your bonding jumper is loose or corroded.
Frequently Asked Questions
Can I adapt a 3-phase soft start wiring diagram for a single-phase motor?
No. Standard industrial soft starters like the ATS22 or Siemens 3RW series rely on three sets of SCRs to monitor and choke all three phases. If you only wire single-phase power (L1 and L2) and leave L3 open, the internal microprocessor will detect a phase loss and throw a fault code (e.g., 'PHF' on Schneider drives). If you need to soft-start a single-phase compressor, you must buy a soft starter specifically designed for single-phase applications, which uses a different internal topology and zero-crossing detection scheme.
Why does my soft start wiring diagram include a bypass contactor?
SCRs are not perfect conductors; even when fully turned on, they drop about 1 to 1.5 volts per phase. On a 50A motor, that voltage drop translates to roughly 150 watts of heat generated continuously inside the starter enclosure. A bypass contactor is wired in parallel with the SCRs. Once the soft starter finishes the ramp-up and the motor hits full speed, the starter energizes the bypass contactor. Current takes the path of least resistance through the contactor's mechanical contacts, completely bypassing the SCRs and eliminating the heat. If your diagram includes one, do not skip it, or you will drastically shorten the life of the starter.
What size wire do I need for the control circuit on a soft start wiring diagram?
While the power circuit wire size is dictated by the motor's Full Load Amps (FLA) and NEC Article 430, the control circuit draws very little current (typically under 0.5A for logic inputs). However, for mechanical reliability and to prevent voltage drop over long runs to a remote pushbutton station, industry standard practice is to use 18 AWG or 16 AWG stranded copper wire for all control and logic wiring. Always use ferrules on the ends of stranded wire before landing them in the soft starter's small screw terminals to prevent stray strands from causing a short across the logic board.
