To wire a standard 120/240V reversible single-phase AC motor (like a 1.5HP Leeson or Baldor compressor motor), you must reconfigure the internal run and start winding jumpers on the terminal board and route the start winding leads through the center poles of a drum switch. Reading wiring diagrams for motors becomes straightforward when you stop looking at the whole page and start tracing individual nodes from the power source to the load. This guide walks through a 240V reversible setup using a standard 6-pole drum switch, detailing exact terminal mappings, the physical trace, and how to verify the circuit before throwing the breaker.

Decoding the Schematic: Symbols and Terminal Mapping

Motor schematics rely on standardized NEMA (National Electrical Manufacturers Association) symbols. On your diagram, squares with numbers (T1, T2, etc.) represent physical terminal studs on the motor's connection plate. Zig-zag lines represent the copper windings. A circle with an 'S' or a centrifugal switch symbol indicates the internal mechanical switch that disconnects the start winding once the rotor reaches roughly 75% of synchronous speed.

Unlike three-phase motors where swapping any two leads reverses rotation, single-phase motors require you to reverse the polarity of the start winding relative to the run winding. The run winding remains fixed. Below is the physical terminal mapping for a standard NEMA MG-1 compliant 6-lead single-phase motor.

Terminal ID Internal Winding Function / Role Typical Internal Lead Tag
T1 Run Winding 1 (Start) Main magnetic field generation 1 (Often Black)
T2 Run Winding 1 (Finish) Main magnetic field generation 2 (Often White)
T3 Run Winding 2 (Start) Main magnetic field generation 3 (Often Red)
T4 Run Winding 2 (Finish) Main magnetic field generation 4 (Often Blue)
T5 Start Winding / Switch Phase-shifted starting torque 5 (Often Yellow)
T8 Start Winding / Switch Phase-shifted starting torque 8 (Often Orange)
Bench Note: Internal wire colors vary wildly between manufacturers (Baldor, Leeson, WEG). Never trust the insulation color blindly. Always read the printed number tags on the wire ferrules or use a multimeter to identify the windings if the tags are missing.

Before tracing to the switch, you must set the motor's internal jumpers for your supply voltage. Assuming a 240V supply (which halves the amperage draw and allows for smaller feeder wire), the run windings must be wired in series.

Voltage Setup Run Winding Jumper Configuration Line Connections (Run) Line Connections (Start)
120V (Parallel) Link T1 to T3; Link T2 to T4 L1 to T1/T3 group; L2 to T2/T4 group Start winding across L1/L2
240V (Series) Link T2 to T3 (Series Jumper) L1 to T1; L2 to T4 Start winding across L1/L2

Node-by-Node Trace: Source to Load and Ground Path

With the motor internally jumpered for 240V (T2 linked to T3), we trace the external circuit through a standard 6-terminal Forward/Reverse drum switch (e.g., Dayton 2X442 or Furnas R10). This switch has Line terminals (L1, L2) and Load terminals (T1, T2, T3, T4).

1. The Source and Disconnect
Power originates at a 2-pole 15A breaker in your main panel. We run 14/2 NM-B cable (or four 14 AWG THHN wires in EMT conduit) to the drum switch. The Black wire is L1 (Line 1), the Red/White wire is L2 (Line 2), and the Bare/Green wire is the Equipment Grounding Conductor (EGC).

2. The Drum Switch Routing (Forward)

  • L1 (Black) enters the switch at terminal L1.
  • L2 (Red) enters the switch at terminal L2.
  • In the 'Forward' position, the switch internally bridges L1 to switch-terminal T1, and L2 to switch-terminal T2. It also bridges L1 to T3, and L2 to T4.

3. Switch to Motor Terminals (The Reversal Logic)

  • Run Winding Path (Fixed): Switch T1 wires to Motor T1. Switch T2 wires to Motor T4. Because Motor T2 and T3 are jumpered, the current flows through both run windings in series. This path never changes polarity.
  • Start Winding Path (Switched): Switch T3 wires to Motor T5. Switch T4 wires to Motor T8.
When you flip the drum switch to 'Reverse', the internal contacts cross the start winding lines. Switch T3 now connects to Motor T8, and Switch T4 connects to Motor T5. By swapping T5 and T8, you reverse the magnetic polarity of the start winding relative to the run winding, forcing the rotor to spin in the opposite direction.

4. The Ground Path (EGC)
The ground path is entirely independent of the switching logic. The bare copper EGC runs continuously from the panel's ground bar, through the cable or conduit, bypasses the drum switch completely, and terminates directly under the green grounding screw on the motor's exterior cast-iron frame. Never route the ground through a switch, and never bond the neutral to the motor frame. According to NFPA 70 (NEC) Article 250, the equipment ground must provide a permanent, low-impedance fault path back to the source.

Verifying the Connections with a Multimeter

Before energizing a newly wired motor, you must verify the circuit. A melted terminal lug or a tripped main breaker is the penalty for skipping this step. Set your digital multimeter (DMM) to the Ohms (Ω) setting. For insulation checks, a dedicated Megohmmeter (Megger) is ideal, but a standard DMM on its highest resistance scale (usually 20MΩ) will catch dead shorts.

Safety Warning: Ensure the 2-pole breaker is OFF and locked out. Verify the circuit is dead by testing L1 to L2, L1 to Ground, and L2 to Ground with your DMM set to AC Volts. Readings must be 0V.
  1. Verify Run Winding Continuity: Place probes on Motor T1 and Motor T4. With the T2-T3 series jumper in place, you are reading through both run windings. Expect a reading between 2Ω and 8Ω (depending on motor HP). If it reads OL (Open Line), a jumper is loose or an internal thermal overload has tripped.
  2. Verify Start Winding Continuity: Place probes on Motor T5 and Motor T8. Expect a slightly higher resistance, typically 5Ω to 15Ω, because start windings use thinner gauge wire. If this reads OL, the internal centrifugal switch may be stuck open, or the start capacitor (if externally wired) is disconnected.
  3. Verify Switch Logic (The Reversal Check): Disconnect power. Place probes on the switch's load-side terminals (T3 and T4). Have a helper toggle the drum switch between Forward and Reverse. You should see continuity (near 0Ω) in both positions, confirming the switch poles are making contact.
  4. Check for Ground Faults: Set your DMM to the highest Ohms scale (20MΩ). Place one probe on Motor T1 (and later T5) and the other probe on the bare metal motor frame (scrape away paint to ensure bare metal contact). The reading must be OL (Infinite). Any reading below 1 Megohm indicates degraded winding insulation or a pinched wire touching the chassis. As noted in Fluke's motor testing guidelines, marginal insulation resistance will cause nuisance GFCI/AFCI tripping or eventual frame energization.
  5. Verify Ground Path Continuity: Place one probe on the motor's green ground screw and the other on the panel's ground bar. Expect a reading of less than 1Ω. This confirms your EGC is solid and will clear a fault instantaneously.

Once these five checks pass, you can confidently close the connection boxes, remove the lockout, and apply power. If the motor hums but fails to rotate, immediately cut power; this indicates the start winding circuit is open (often a failed centrifugal switch or a disconnected T5/T8 wire), and leaving it energized will burn out the run winding in under a minute. For deeper standard specifications on motor terminal markings and enclosure types, refer to the NEMA Motors and Generators standards library.