13.1 Motor-Starter Wiring Diagrams & Control Symbols
Key Takeaways
- RA 7920 Section 19 explicitly names motor-starter wiring diagrams with protection as PRC RME exam content.
- Ladder (schematic) diagrams show control logic; wiring (connection) diagrams show physical terminal-to-terminal connections — read the ladder diagram first to understand how a circuit works.
- A DOL starter's seal-in contact keeps the contactor coil energized after the start button is released; without it the motor only runs while the button is held down.
- Star-delta and reversing starters both require their transition or direction contactors to be electrically and mechanically interlocked so they can never close together and short the motor windings or line phases.
- The most common RME diagram-reading errors are confusing NO and NC symbols, misjudging overload reset behavior, and confusing auxiliary control contacts with main power contacts.
13.1 Motor-Starter Wiring Diagrams & Control Symbols
Republic Act No. 7920 Section 19 names "motor-starter wiring diagrams with protection" as one of the specific items the Board of Electrical Engineering may draw exam questions from, so this is not background reading — it is directly examinable material. A Registered Master Electrician (RME) is routinely handed a starter diagram for equipment they did not design and must be able to read it correctly on the first pass: to install it, to troubleshoot a nuisance trip, or to explain to a client why a motor will not run.
Two Ways to Draw the Same Circuit
Every motor starter is documented with two different kinds of drawings, and confusing them is a common exam trap.
A ladder (schematic) diagram strips away physical layout and shows only electrical logic. Two vertical "rails" represent the two sides of the control power supply, and horizontal "rungs" run between them, each rung showing one control function — start/stop logic, an indicator lamp, an interlock. Ladder diagrams are read top to bottom in the sequence a technician would think through the control logic, and left to right along each rung. They are the diagram an RME should reach for first when the question is "how does this thing work?" or "why didn't this contactor pull in?"
A wiring (connection) diagram shows the actual physical terminals, terminal numbers, and point-to-point wire runs exactly as they exist inside the enclosure — device by device, arranged by physical position rather than by logical sequence. Wiring diagrams are what an electrician follows when actually landing conductors on terminal blocks, or when tracing a specific wire with a meter during troubleshooting. They are harder to read for "what does this circuit do," because functionally related contacts may be scattered across the drawing wherever they happen to sit inside the enclosure.
Standard Control-Circuit Symbols
| Symbol | How it is drawn | What it does |
|---|---|---|
| Normally-open (NO) contact | Two short parallel lines with an open gap between them | Open in the de-energized/unactuated state; closes when its coil energizes or its button is pressed |
| Normally-closed (NC) contact | Two parallel lines with a diagonal slash across the gap | Closed (conducting) in the de-energized/unactuated state; opens when its coil energizes or its button is pressed |
| Coil | Circle or small rectangle labeled M, CR, K, or similar | The electromagnet winding of a contactor or relay; pulling it in operates its associated contacts |
| Pushbutton — START | NO contact, usually shown boxed and labeled "Start" | Momentary; closes only while physically held down |
| Pushbutton — STOP | NC contact, usually shown boxed and labeled "Stop" | Momentary; opens only while physically held down |
| Pilot light | Circle containing a letter (R = running, G = stopped/ready, A = alarm/tripped) | Signals equipment status to the operator |
| Overload relay contact | NC contact labeled OL | Opens on excessive motor current, breaking the control circuit |
| Fuse | Zig-zag line, or a rectangle crossed by a diagonal line | Overcurrent protection for a conductor |
| Disconnect / isolator | An angled blade drawn across the line | Manually opens all ungrounded conductors for isolation, lockout, or maintenance |
An RME does not need to memorize every manufacturer's house style, but must recognize this core set on sight, because the exam will present a diagram cold and ask what a specific symbol does or what happens if a specific contact fails to close.
Anatomy of a Direct-On-Line (DOL) Starter
The direct-on-line (DOL, also called "across-the-line") starter is the simplest motor-starting method and the base diagram every more complex starter builds from.
Power circuit. The three-phase supply passes through a disconnect, then through branch-circuit fuses or a breaker sized to the motor, into the contactor's main (power) contacts, through the overload relay's current-sensing heaters, and out to the motor. These lines are drawn heavy because they carry the full motor running current.
Control circuit. A much smaller current — often stepped down through a control transformer — operates the contactor coil. Reading the ladder rung left to right: the NC stop button comes first, then the NO start button, then the coil, with the overload relay's NC auxiliary contact in series somewhere on the same rung. A second NO contact — an auxiliary contact on the contactor itself — is wired in parallel with the start button. This is the seal-in (holding) contact: the instant the coil energizes, this contact closes and keeps the coil energized after the operator releases the start button. Without it, the motor would run only while a finger held the start button down — a "jog" circuit, not a maintained-run circuit.
Because the stop button is normally closed and wired in series with everything downstream of it, pressing stop breaks the entire rung — coil, seal-in contact, and all — regardless of what else on the rung is doing. This is why the stop button sits first (closest to the supply) on most ladder diagrams: it guarantees stop always wins over start.
The overload relay's NC auxiliary contact sits in series in the control circuit, separate from the heater elements that live in the power circuit. When motor current exceeds the overload's setting for long enough to trip, this NC contact opens, de-energizing the coil and dropping the contactor out — removing power from the motor. Some overload relays reset automatically once they cool down; most industrial overloads used on larger motors require a manual reset button to be pressed before the circuit can be re-energized, and an RME must be able to identify which behavior a given diagram or nameplate specifies before troubleshooting a "won't restart" complaint.
Star-Delta (Wye-Delta) Starter Diagrams
Star-delta starting reduces inrush current on larger motors by starting the motor with its windings connected in star (wye) and then transitioning to delta once it is up to speed. The diagram shows three contactors — a main (line) contactor, a star contactor, and a delta contactor — plus a timer. On start, the main and star contactors close together, connecting the motor windings in star for reduced starting current and torque. After a timed interval, the timer opens the star contactor and closes the delta contactor, reconnecting the same windings for full running torque at line voltage.
The star and delta contactors must never be closed at the same time. If both were closed together, the delta contactor would connect the winding ends directly across two phases already joined by the star contactor, producing a phase-to-phase short circuit through the motor windings. For this reason, star-delta diagrams always show the star and delta contactor coils interlocked — each contactor's NC auxiliary contact wired in series with the other contactor's coil, so energizing one physically prevents the other from energizing, backed up by the timer's sequencing so the two can never overlap even for an instant.
Reversing Starters and Interlocking
A reversing starter uses two contactors sharing one motor and one set of overloads: a forward contactor and a reverse contactor, wired so the reverse contactor swaps two of the three phase conductors to reverse rotation direction. As with star-delta, the forward and reverse contactors must never close simultaneously — doing so would connect two phases directly together through the contactors, a bolted short circuit. Reversing starters are protected with two layers of interlocking: an electrical interlock, where each contactor's NC auxiliary contact is wired in series with the other's coil circuit, and a mechanical interlock, a physical linkage or lockout bar between the two contactors that prevents both from being pulled in at once even if the electrical interlock somehow failed — for example, if an auxiliary contact welded shut and stopped opening as designed. Many reversing pushbutton stations add a third layer — double-circuit forward/reverse buttons wired so pressing one also mechanically opens the other's control path.
Common Wiring-Diagram Reading Traps for the RME Exam
- NO vs. NC confusion. The open-gap symbol (NO) and the slashed-gap symbol (NC) look similar at a glance; misreading one for the other flips the entire logic of a rung and is the single most common diagram-reading error on licensure exams.
- Assuming all overloads reset the same way. Some trip contacts reset automatically once the motor cools; many industrial overload relays require a manual reset. A diagram or nameplate that specifies "manual reset" changes the correct troubleshooting sequence and the correct answer to "why won't it restart."
- Confusing auxiliary contacts with main power contacts. Main contacts are drawn heavy and carry full motor current; auxiliary contacts (seal-in, interlock) are drawn light and carry only small control currents. Treating an auxiliary contact as if it could switch the motor load — or vice versa — is a scope-of-practice error as well as a diagram-reading error.
- Missing the seal-in contact entirely. A diagram without a seal-in contact around the start button describes a motor that runs only while the button is physically held — a legitimate but different control scheme that candidates sometimes mistake for a wiring defect, or a genuine defect that candidates mistake for an intentional jog circuit. The exam expects the reader to tell the two apart from the diagram alone.
In a direct-on-line (DOL) motor starter's control circuit, what is the purpose of the auxiliary contact wired in parallel with the start pushbutton?
On a DOL starter ladder diagram, the overload relay's protective contact is wired as a normally-closed (NC) contact placed in series in the control circuit. What happens when the overload relay trips?
In a star-delta (wye-delta) starter, why must the star contactor and delta contactor be electrically and mechanically interlocked?
A reversing motor starter's forward and reverse contactors are protected by both an electrical interlock (crossed NC auxiliary contacts) and a mechanical interlock (a physical linkage bar). What specific fault does the mechanical interlock guard against that the electrical interlock alone might not catch?