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BLD-ELEC05-08 · Lesson 3 · 108 clock hours · lab

Panelboard Wiring, Motor Control Circuit, and PLC Integration

Objective. Students will wire a residential/light-commercial panelboard from a completed rough-in, then build and commission a motor control circuit—incorporating a magnetic starter, overload relay, and a PLC input/output sequence—that starts, stops, and protects a three-phase motor load according to the design ladder diagram.

Panelboard Wiring, Motor Control Circuit, and PLC Integration

Overview

This lab brings together three interconnected skill sets: landing branch-circuit conductors in a residential/light-commercial panelboard, building and testing a three-wire motor control circuit with a magnetic starter, and integrating a programmable logic controller (PLC) to automate that circuit. Completing all three tasks—accurately and safely—demonstrates job-readiness at the NCCER Level 3–4 standard and prepares you for journeyman licensing work.

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Part 1: Panelboard Wiring

Safety first—LOTO is non-negotiable. Before touching any conductor inside the panelboard, follow the full lockout/tagout (LOTO) procedure: isolate the upstream source, apply a lockout hasp and personal padlock, attach a danger tag, and verify absence of voltage with your multimeter using the test–de-energize–test sequence. Log every step on the project LOTO log. Your instructor must sign the log before any work begins and again before energization.

With the panel de-energized:

1. Land branch-circuit conductors. Strip insulation to the manufacturer's strip length—no nicks in the copper. Insert each conductor into its assigned terminal (hot to breaker, neutral to the neutral bar, equipment grounding conductor to the ground bar).

2. Torque terminations. Panelboard terminals have a manufacturer-specified torque value, typically printed on the terminal block or in the installation sheet. Use a calibrated torque screwdriver and apply one steady motion until the driver clicks or the indicator mark aligns. Mark each terminal with a torque-verification mark (a short paint or marker line that bridges the screw head to the terminal body) so the inspector can confirm it at a glance.

3. Install breakers per the panel schedule. The approved panel schedule assigns each circuit a breaker ampere rating and pole count. Installing the wrong size breaker—even temporarily—is a code violation and a fire hazard. Double-pole breakers must be handle-tied where required.

4. Grounding electrode system (GES) connection. Verify that the grounding electrode conductor lands on the main bonding jumper terminal and that the neutral-to-ground bond exists only at the service entrance, never at a downstream distribution panel.

5. Complete the directory label. A legible, accurate panel directory is required by the NEC. Use the label provided; abbreviations must be clear enough that a firefighter or EMT could identify circuits under stress.

Your instructor inspects torque marks, the LOTO log, grounding, and panel schedule accuracy before authorizing energization.

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Part 2: Motor Control Circuit—Magnetic Starter and Overload Relay

A three-wire control circuit uses momentary start and stop pushbuttons. When you press START, current flows through the starter coil; the main contacts close, and an auxiliary (holding) contact wired in parallel with the START button seals in the circuit. Releasing START does not drop the motor—the auxiliary contact holds the coil energized. Pressing STOP breaks the control circuit, the coil de-energizes, and all contacts open.

Wiring sequence:

1. Connect L1, L2, L3 incoming power to the magnetic starter line terminals.

2. Run T1, T2, T3 from the starter load terminals through the overload relay heater elements to the motor terminals.

3. Wire the control circuit: one side of 120 V control power → STOP (NC, normally closed) → START (NO, normally open) → auxiliary contact (NO, in parallel with START) → starter coil → neutral.

4. Set the overload relay to the motor's nameplate full-load ampere (FLA) rating. Bimetallic or electronic overload relays have an adjustment dial or setting; match it to the FLA. An overload set too high will fail to protect the motor windings from heat damage.

Testing: Run five start/stop cycles. Use a clamp meter around one motor lead to confirm current is within the expected range. To verify the overload trip function, your instructor will demonstrate the manual trip button—do not manually overheat the heaters. Record voltage, current, and trip test results on the motor-control test record form.

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Part 3: PLC Integration

A programmable logic controller (PLC) replaces hardwired relay logic with software-based ladder logic while keeping the physical field devices (pushbuttons, starters, sensors) connected through its I/O terminals.

Mapping I/O:

Ladder logic structure: Your program contains at least one rung: a normally open (NO) examine-if-closed (XIC) instruction representing the start input, a latch or seal-in rung using an output bit, and an output coil instruction energizing the starter. The STOP function can be wired in hardware (maintaining the hardwired stop as a safety layer) or added as an examine-if-open (XIO) instruction in the ladder.

Upload and commission: Connect the PLC to the laptop via the programming cable, download the program, switch to RUN mode, and verify all indicator lights match the expected I/O states before operating the motor. Demonstrate three documented start/stop cycles. On your PLC printout, annotate each rung in plain language: what condition is being examined, what output is being controlled, and why.

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Arc-Flash and PPE

Even at 120/240 V, arc-flash energy can cause severe burns. Before the instructor authorizes energization, confirm you are wearing safety glasses, an arc-flash face shield rated for the incident energy level of this task, insulated gloves appropriate for the voltage, and a hard hat. No jewelry, no loose clothing near energized terminals.

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Connecting This to Your Credentials

Key terms10

Lockout/Tagout (LOTO)A federally required procedure (OSHA standard 29 CFR 1910.147) for isolating hazardous energy sources before work begins; involves applying a physical lock and a danger tag so the energy source cannot be re-energized while workers are exposed.
Torque terminationThe act of tightening an electrical terminal to the manufacturer's specified torque value, measured in inch-pounds or foot-pounds, to ensure a reliable, low-resistance connection that will not loosen under vibration or thermal cycling.
Magnetic starterAn electrically operated switch (contactor) combined with an overload relay, used to start, stop, and protect a motor; the coil pulls in the main power contacts when energized and opens them when de-energized.
Three-wire control circuitA motor control wiring scheme using momentary start and stop pushbuttons along with an auxiliary holding contact on the starter, so the circuit is maintained (sealed in) after the start button is released and drops out only when the stop button is pressed or power is lost.
Overload relayA protective device wired in series with the motor load conductors that trips and breaks the control circuit when motor current exceeds the set full-load ampere value for longer than a preset time, preventing motor winding damage from sustained overheating.
Full-Load Ampere (FLA) ratingThe maximum continuous current a motor draws at its rated load, voltage, and frequency; found on the motor nameplate and used to set overload relays and size branch-circuit conductors and overcurrent devices.
Programmable Logic Controller (PLC)An industrial digital computer with discrete input and output terminals that executes a user-written control program (commonly ladder logic) to automate machinery and processes in place of hardwired relay circuits.
Grounding Electrode System (GES)The network of electrodes (ground rods, metal water pipe, concrete-encased electrodes, etc.) and the conductors connecting them to the service neutral, providing a reference to earth potential and a path for fault current.
Panel scheduleAn approved document listing every circuit in a panelboard by circuit number, breaker ampere rating, number of poles, and load description; required by the NEC and used to verify correct breaker installation and to complete the panel directory label.
Auxiliary (holding) contactA small supplementary contact on a magnetic starter or contactor that closes when the main coil energizes; wired in parallel with the momentary START button to keep the coil circuit complete after the button is released.

Worked examples4

1. The motor nameplate reads: 1 HP, 208 V, 3-phase, FLA = 4.6 A. What value should you set on the overload relay, and within what adjustment range would you expect a typical adjustable-dial overload to be set?
  1. Read the nameplate FLA directly: 4.6 A.
  2. The overload relay must be set to protect the motor at its nameplate FLA. Set the relay dial to 4.6 A.
  3. Most adjustable overload relays have a dial range (for example, 3–5 A or 4–6 A for a relay in this size class). Confirm the 4.6 A value falls within the relay's adjustment range before installation.
  4. Do not set the overload higher than the nameplate FLA to 'avoid nuisance trips'; doing so removes motor protection. If nuisance trips occur, investigate the mechanical or electrical cause rather than raising the overload setting.
Answer: Set the overload relay to 4.6 A, matching the motor nameplate FLA exactly.
2. You are landing a 20 A, 120 V branch-circuit conductor on a breaker terminal. The manufacturer's specification for that terminal is 35 in-lb of torque. Your calibrated torque screwdriver is set to 35 in-lb. After tightening, you need to apply a torque-verification mark. Describe the process step by step.
  1. Confirm the torque screwdriver reads 35 in-lb on its setting window or scale.
  2. Insert the stripped conductor end (stripped to the marked strip-length) fully into the terminal.
  3. Apply steady, controlled clockwise torque with the screwdriver until you feel or hear the click (or the indicator aligns), signaling that 35 in-lb has been reached. Stop immediately—do not continue turning.
  4. Using a paint pen or permanent marker, draw a short line that bridges from the screw head across to the terminal body. This torque-verification mark will show an inspector the screw has not rotated since it was set.
  5. Record the terminal location and torque value on the inspection checklist or LOTO/project log as required.
Answer: The terminal is torqued to 35 in-lb and marked with a verification line bridging the screw head to the terminal body.
3. During the five-cycle motor test, your clamp meter reads 4.2 A on one motor lead. The nameplate FLA is 4.6 A. The overload relay is set at 4.6 A. Is the motor operating within acceptable limits? What would a current reading significantly above 4.6 A suggest?
  1. Compare the measured current (4.2 A) to the nameplate FLA (4.6 A). Because 4.2 A < 4.6 A, the motor is drawing less than its rated full-load current, which is acceptable—the motor is running under full load.
  2. Calculate the percentage of FLA: (4.2 ÷ 4.6) × 100 = 91.3%. The motor is at about 91% of FLA, a normal operating condition.
  3. If the clamp meter read, for example, 5.5 A (significantly above 4.6 A), the motor would be overloaded. Sustained overcurrent generates excess heat in the windings; the overload relay should trip to interrupt the control circuit before damage occurs.
  4. A reading above FLA also signals you to investigate the mechanical load, voltage supply balance, or motor condition before resuming operation.
Answer: 4.2 A is within acceptable limits (91% of FLA). A reading significantly above 4.6 A would indicate an overloaded or faulted motor and should trigger an overload-relay trip.
4. In the PLC ladder logic for this motor control application, you have a single rung with one XIC (examine-if-closed) instruction mapped to input I:0/0 (start pushbutton) and one OTE (output energize) instruction mapped to output O:0/0 (starter coil). A second rung latches the output using a bit from O:0/0. The STOP pushbutton is hardwired in series with the starter coil circuit. If the start pushbutton is momentarily pressed and released, trace the logic state through one complete seal-in cycle.
  1. START pressed: Input I:0/0 goes TRUE. The XIC instruction on rung 1 passes power; the OTE instruction energizes output O:0/0. The starter coil energizes, closing the main contacts (motor starts) and closing the auxiliary hardware contact.
  2. Seal-in rung activates: A second rung has an XIC instruction examining the O:0/0 output bit. Because O:0/0 is now TRUE, that rung also passes power and keeps O:0/0 energized independently of the start pushbutton state.
  3. START released: Input I:0/0 goes FALSE. Rung 1's XIC opens, but rung 2 (the seal-in rung) continues to energize O:0/0. The motor stays running.
  4. STOP pressed (hardwired NC contact opens): The physical control circuit to the starter coil is broken regardless of the PLC output state. The coil de-energizes, main contacts open, motor stops. The auxiliary hardware contact opens, and the O:0/0 output bit resets on the next PLC scan because the physical output feedback changes; the seal-in rung loses its condition. Releasing STOP will NOT restart the motor—a new START press is required.
Answer: The motor starts on a momentary START press, seals in via the latch rung, and stops only when the hardwired STOP contact is opened, requiring a new START command to run again.
In class: With the main breaker locked out and verified de-energized (full LOTO procedure documented on the project LOTO log), students land all branch-circuit conductors and the grounding electrode system on the panelboard, torque terminations to manufacturer specs using a calibrated torque screwdriver, install breakers per the approved panel schedule, and complete the panel directory label; the instructor verifies the LOTO log, torque marks, and panel schedule accuracy before authorizing energization. · On the motor-control training board, students wire a magnetic starter with a three-wire control circuit (momentary start/stop pushbuttons, auxiliary contact for hold-in), set the overload relay to the motor nameplate full-load ampere rating, and test the circuit through five start/stop cycles while monitoring voltage, current with a clamp meter, and overload trip function; students record all readings on the motor-control test form. · Students connect the PLC (trainer unit) to the motor control circuit—mapping one digital input (start pushbutton) and one digital output (starter coil) in the PLC program—upload the ladder logic, and demonstrate that the PLC sequence controls the motor through at least three documented cycles; students annotate the PLC printout with rung-by-rung explanations and attach it to the project documentation package.
Materials: Residential panelboard (training unit, 120/240 V single-phase, 100 A or 200 A), THWN conductors (various gauges), torque screwdriver (calibrated), LOTO kit (lockout hasp, tags, padlocks), clamp meter, multimeter, three-phase motor-control training board, magnetic starter, overload relay, momentary pushbutton station, PLC trainer unit with ladder-logic software, motor nameplate data sheet, motor-control test record form, NEC codebook, NCCER Electrical Levels 3–4 modules, PPE (safety glasses, arc-flash face shield rated for the task, insulated gloves, hard hat)
Check: Two-part competency check: (1) Panelboard—instructor verifies torque marks on all terminals, correct LOTO documentation, proper grounding, and matching panel schedule before energizing; (2) Motor/PLC—student must demonstrate correct start/stop operation, successful overload-relay trip test, and accurate PLC documentation. Both parts scored on a 10-point rubric each; student must score ≥8/10 on each part to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

OSHA 10 – Control of Hazardous Energy / Electrical Safety
1. During the panelboard lab, a student is preparing to land conductors. Which action must be completed and documented BEFORE any conductors are touched inside the panel?
NCCER Electrical L3–4 – Panelboards and Distribution Equipment
2. Why is a torque-verification mark applied to a terminal screw after torquing to specification?
NCCER Electrical L3–4 – Motor Controls
3. In a three-wire motor control circuit, what is the purpose of the auxiliary contact wired in parallel with the momentary START button?
NCCER Electrical L3–4 – Motor Controls
4. A motor nameplate states FLA = 6.8 A. To which value should the overload relay be set?
State Journeyman License Exam – Grounding and Bonding / NEC
5. Where should the neutral-to-ground bond (main bonding jumper) be installed in a building's electrical system?
NCCER Electrical L3–4 – Motor Controls
6. A student's clamp meter reads 7.9 A on a motor lead whose nameplate FLA is 6.8 A. The overload relay is correctly set at 6.8 A. What is the most likely outcome if this condition persists?
NCCER Electrical L4 – Programmable Logic Controllers
7. In PLC ladder logic, an XIC (examine-if-closed) instruction mapped to a digital input will pass power (go TRUE) when:
OSHA 10 – Electrical Hazards and PPE
8. Which personal protective equipment item is specifically required because of arc-flash exposure risk when working near an energized or recently energized panelboard?
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