How to Read a 2 Speed Electric Motor Wiring Diagram (NEMA 6-Lead)

When wiring a 2 speed electric motor, the most common industrial and HVAC configuration is the NEMA standard 2-speed, 2-winding, 6-lead motor (often found in exhaust fans, compressors, and pump stations). The direct answer for terminal mapping is simple: T1, T2, and T3 connect to your high-speed contactor, while T4, T5, and T6 connect to your low-speed contactor. Power is applied to one set or the other, never both. For a default, bulletproof starter setup on motors up to 10HP at 480V, use two Schneider Electric TeSys D LC1D09 contactors paired with a LA9D09978 mechanical interlock block and a LRD series thermal overload.

The Terminal Map and Diagram Symbols

Before tracing the circuit, you must identify the physical terminals on the motor peckerhead (connection box) and understand the standard schematic symbols used in 2 speed electric motor wiring diagrams.

Diagram Symbol Key:

• Disconnect/Fuses: A box with a switch symbol and zig-zag lines (fuses). Represents the main isolating point.
• Contactors (KM1, KM2): Drawn as a coil (circle or rectangle) with associated Normally Open (NO) and Normally Closed (NC) auxiliary contacts. These switch the 3-phase power.
• Overloads (OL): A box with a bimetallic strip symbol. Placed in series with the motor leads to trip the circuit on thermal overcurrent.
• Interlock (Mechanical/Electrical): Dotted lines connecting two contactor coils or specific NC auxiliary contact blocks (e.g., Schneider LADN04) wired in series with the opposing coil.

Node-by-Node Trace: Source to Load

Assuming a 480V 3-phase supply using copper THHN wire in EMT conduit, here is the exact node-by-node path from the service to the motor windings.

1. Main Disconnect: L1, L2, and L3 enter the fused disconnect switch. The EGC (green/bare) passes straight through the conduit bonding path.
2. Branch Fuses: Power passes through dual-element time-delay fuses (sized per NEC 430.52, typically 175% of motor FLA) to protect against short circuits.
3. Contactor Line Side: L1, L2, L3 branch to the top (line) terminals of both Contactor 1 (High) and Contactor 2 (Low).
4. Contactor Load Side to Overloads: The bottom (load) terminals of KM1 route to Thermal Overload 1. The bottom terminals of KM2 route to Thermal Overload 2. (Note: Some setups use a single shared overload if the FLA of both speeds is identical, but dual overloads are best practice for differing ampacities).
5. Overload to Motor Terminals: Overload 1 outputs wire directly to motor terminals T1, T2, T3. Overload 2 outputs wire directly to motor terminals T4, T5, T6.
6. The Ground Path: The EGC never passes through a fuse, contactor, or overload. It runs continuously from the panel ground bar, through the conduit or as a dedicated green THHN wire, terminating directly on the motor casing's green grounding screw. This ensures equipotential bonding regardless of which speed is active.

The Mandatory Interlock: Preventing Catastrophic Failure

The most critical aspect of a 2 speed electric motor wiring diagram is the interlocking logic. If KM1 and KM2 close simultaneously, you are effectively tying two separate, inductively coupled 3-phase windings together out of phase. This creates a massive phase-to-phase short circuit that will weld the contactor contacts shut, explode the arc chutes, and destroy the motor windings.

You must implement both electrical and mechanical interlocks:

• Electrical Interlock: Wire a Normally Closed (NC) auxiliary contact from KM1 into the coil circuit of KM2, and vice versa. If KM1 is energized, its NC contact opens, physically breaking the control circuit to KM2's coil.
• Mechanical Interlock: Install a physical blocking device (like the LA9D09978) between the two contactors. This uses a physical lever to prevent the armature of KM2 from pulling in if KM1 is already pulled in, providing fail-safe protection even if the electrical auxiliary contacts weld shut.

How to Verify Your Connections with a Multimeter

Never assume the factory terminal labels are correct or that the windings are intact. Before terminating the motor leads, use a digital multimeter (DMM) like the Fluke 117 to verify the internal winding topology.

Step 1: Winding Continuity (Ohms)
Set your meter to the lowest Ohms range. Measure across T1 to T2, T2 to T3, and T1 to T3. You should read a low, balanced resistance (e.g., 1.5Ω to 5.0Ω for a 5HP 480V motor). Repeat for T4-T5-T6. The low-speed winding (T4-T6) will typically read slightly higher resistance due to having more turns of finer wire to create more poles.

Step 2: Winding Isolation (Over-Limit)
Measure between the two separate windings: T1 to T4, T2 to T5, T3 to T6. Your meter must read OL (Over Limit) or infinite resistance. If you read continuity between T1 and T4, the internal insulation has failed, and the motor must be rewound or replaced. Do not apply power.

Step 3: Ground Fault Check
Measure from T1 to the motor casing ground lug, and T4 to the ground lug. Both must read OL. For a true dielectric test, use a Megohmmeter (Megger) at 1000V DC; acceptable insulation resistance should be >2 Megohms per NEMA MG-1 standards.

Decision Tree: Selecting the Right Motor Starters

Stop guessing contactor sizes. Use this decision matrix to select the exact hardware for your 2-speed motor based on the nameplate Full Load Amps (FLA). This table terminates in a specific, orderable part configuration.

Final Execution Rule: Always size your branch circuit wire and fuses based on the highest FLA listed on the nameplate (usually the high-speed winding), applying the 125% continuous load multiplier required by NEC Article 430.22. If the high speed draws 14A, your wire must be sized for 17.5A (use 12 AWG THHN), and your time-delay fuses should be set to the nearest standard size above the 175% starting surge allowance.