Motor Starter Wiring — DOL, Reversing, and Star-Delta
The classic across-the-line contactor starter — its power poles, its control rung, and the Article 430 protection chain that keeps the wire and the motor each protected by the right device.
Safety. This guide is educational reference material, not a work instruction. Electrical work is performed de-energized and verified by qualified personnel under your site’s LOTO procedures, following the device manufacturer’s manual and the authority having jurisdiction. A reversing starter wired without its interlock can fault line-to-line on the first simultaneous close — prove the interlock before energizing.
Overview
An across-the-line starter switches a motor directly onto the line through an electromechanical contactor. It is the classic alternative to a variable frequency drive: simpler, cheaper, and more rugged, but with no speed control and full starting inrush. Where a drive shapes current and speed, a starter just makes and breaks the connection. This guide covers three common configurations:
- DOL (direct-on-line) — one contactor connects all three phases straight to the motor. Full inrush at start (typically 6–8× FLA, motor-dependent).
- Reversing — two contactors, one per phase sequence; energizing either selects rotation. Requires interlocking so both can never close together.
- Star-delta (wye-delta) — a reduced-voltage starter (main, star, and delta contactors plus a timer) that starts the windings in star to cut inrush, then transitions to delta for run, for larger motors where DOL inrush is unacceptable.
Three terminal groups recur across all three: power (line/load) switched through the contactor main poles; the control coil circuit (start/stop, seal-in, interlocks, timer, overload contact); and the overload relay in the motor power leads with its auxiliary contact in the coil circuit.
flowchart LR
SUP[Three-phase supply] --> OCPD[Branch-circuit OCPD<br/>fuse or breaker]
OCPD --> K[Contactor main poles]
K --> OL[Overload relay]
OL --> M((Motor))
CPT[Control power transformer] --> CC[Control coil circuit<br/>start/stop · seal-in · interlock]
CC -.->|energizes coil| K
OL -.->|aux contact drops coil| CC
This guide covers wiring a single across-the-line starter feeding one induction motor. Terminal designations, coil voltages, contactor and overload catalog numbers, and torque values are vendor-specific — from the manufacturer’s data, never from a guide, including this one.
Before You Start
Have on hand before pulling wire:
- Motor nameplate data — voltage, FLA, service factor, speed, power (motor fundamentals). The nameplate FLA feeds the overload setting; conventional branch-circuit sizing uses the NEC table full-load current (NEC 430.6 principle).
- Starting-method decision (upstream). DOL is simplest but imposes full inrush; reduced-voltage star-delta cuts starting current for large motors where the supply, a generator, or the driven mechanics cannot tolerate DOL inrush. Driven by motor size, supply stiffness, and load — decided in design, not at the panel.
- Contactor and overload selection. Contactors are rated by utilization category — AC-3 for ordinary squirrel-cage starting/running, AC-4 for the far harsher plugging/inching/reversing duty (breaking locked-rotor current). Size the contactor and overload to the motor FLA and the actual duty from the manufacturer tables; a contactor fine for AC-3 may be undersized for AC-4 reversing service.
- Drawings. The branch-circuit design (OCPD type/size, conductor sizes, disconnect location) is decided upstream; this guide assumes you are implementing it.
Sizing & Protection
This is the core. The governing framework is NEC Article 430, and its central discipline applies to a starter exactly as to any motor circuit: the short-circuit/ground-fault protection and the overload protection are two separate devices doing two separate jobs.
- Branch-circuit short-circuit / ground-fault protection (OCPD). A fuse or
breaker that protects the wiring against short-circuit and ground-fault
currents (NEC Art. 430 Part IV). It is sized separately from — and larger
than — the overload, because it must let starting inrush pass without
tripping.
cst motor-branchcomputes the conventional motor-branch chain; conductor sizing follows the wire sizing walkthrough. - Motor overload (running overcurrent). A thermal or electronic overload relay that protects the motor against sustained running overcurrent (NEC Art. 430 Part III), sized to the nameplate FLA and service factor — not to the breaker and not to the wire.
- The fundamental distinction. The OCPD protects the conductors against a fault; the overload protects the motor against overload. Neither can do the other’s job — a breaker large enough to pass inrush never protects the motor from a locked-rotor or running-overload condition, and an overload sized to the motor will not clear a bolted fault. Crossing these two roles is the single most common motor-circuit error (see Common Mistakes).
- SCCR of the starter combination. The combination of OCPD, contactor, and overload has a short-circuit current rating that must equal or exceed the available fault current at its location, established per the UL 508A methodology and dependent on the specific device pairing — verify it against the panel’s SCCR documentation, do not assume it.
Power Wiring
- DOL — line to the contactor main poles, load side through the overload to the motor. Straightforward three-phase; the overload sits in the motor leads.
- Star-delta — both ends of each winding are brought out (six leads). The main and star contactors energize first, putting the windings in star (~58% of line voltage, reduced inrush); after the timer the star contactor drops out and the delta contactor closes, putting the windings across full line voltage for run. Standard star-delta is open-transition — the motor is briefly disconnected during the changeover, producing a current and torque surge as it re-connects. Correct lead identification and contactor sequencing are essential; miswiring can short windings or reverse a phase.
- Reversing — two contactors feed the motor with two phases swapped between them. If both close at once, the swapped phases create a direct phase-to-phase short across the line. Reversing power wiring is only safe behind the interlock below — this is not optional. Terminal torque values and wire ranges come from the device data; record the values used.
Control / Signal Wiring
The control circuit is the low-power ladder that energizes the contactor coil. It is where the starter’s logic lives.
- Start/stop with seal-in (holding contact) — the fundamental rung. A momentary start pushbutton energizes the coil; a normally-open auxiliary contact of that contactor, wired in parallel with the start button, “seals in” and holds the coil after the button is released; a normally-closed stop button in series drops it out. Without the seal-in, the motor stops the instant you let go of the start button.
- Control voltage from a CPT. The coil circuit is commonly fed from a control power transformer stepping the line down to a control voltage, so operators and pilot devices are not exposed to line voltage — see control power wiring.
- Overload auxiliary contact in the coil circuit. The overload relay’s normally-closed auxiliary contact sits in series in the coil circuit, so an overload trip de-energizes the coil and stops the motor.
- Reversing interlock — electrical and mechanical, both. Electrical: each direction contactor’s normally-closed auxiliary contact is wired in series with the other contactor’s coil, so energizing one direction blocks the other. Mechanical: a physical interlock between the two contactors prevents both armatures closing at once even if the electrical interlock is defeated or a contact welds. Both are used together; neither alone is treated as sufficient for reversing duty. Generally accepted practice — verify for your installation.
- Star-delta timing. An on-delay timer holds the star state for the starting interval, then commands the transition to delta. The star and delta contactors are interlocked against each other (they must never close together); transition timing is application-specific.
Grounding, Shielding & EMC
Device-specifics here; the deep treatment is owned by panel grounding & bonding.
- Motor frame and enclosure bonding. Bond the motor frame and the starter enclosure to PE per NFPA 79 Ch. 8 / IEC 60204-1 — sized on the table basis (largest upstream OCPD), procedure per the table, values not reproduced.
- Coil suppression. A DC-operated contactor coil is inductive; a suppression device across the coil (a diode, RC, or varistor per the coil type) limits the switch-off spike that would otherwise stress the controlling contact or a PLC output; AC coils commonly use an RC snubber. Coil-specific — consult the manufacturer. Generally accepted practice.
Common Mistakes
- Reversing starter without an interlock. If both direction contactors can close together, the two swapped phases short line-to-line — a violent fault on the first simultaneous energization. Fit both electrical (aux-contact) and mechanical interlocks; it shows up as a bang and a cleared upstream OCPD the first time both are commanded.
- Overload sized to the breaker rating instead of the motor FLA. The overload is then set far above the motor’s running current and never protects it; a genuine overload runs the motor to insulation failure while the “protection” never trips.
- OCPD relied on for overload protection, or vice versa. The two devices are not interchangeable. A breaker sized to pass inrush cannot protect against a running overload; an overload cannot clear a bolted fault. Crossed roles leave either the motor or the wiring unprotected.
- No seal-in contact. The motor runs only while the start button is physically held and stops the moment it is released — the classic missing holding contact, mistaken for a bad coil or control fault.
- Star-delta open-transition surge misdiagnosed. The current and torque spike at the star-to-delta changeover is inherent to open-transition starting; it is often chased as a fault. Closed-transition starting or corrected timing addresses it where the surge is unacceptable.
- Contactor undersized for the utilization category / duty. Sizing a reversing or plugging (AC-4) application from AC-3 tables under-rates the contactor; the contacts erode or weld under the harsher breaking duty.
Verification Checks
Before and during first energization (evidence-retaining checklists in templates):
- Overload set to the motor FLA (and service factor), and its trip function verified per the relay’s method
- Overload auxiliary contact proven to drop the coil on trip
- Reversing interlock proven — attempt to command both directions and confirm the interlock prevents both contactors closing
- Star-delta transition proven — timer interval correct, sequence star → open → delta, star/delta interlock confirmed
- Rotation verified on a brief bump before coupling the load; correct by swapping two phases if wrong
- OCPD type and rating match the design, and the starter combination SCCR matches the panel’s SCCR documentation
- Terminal torques per the device data, recorded
Standards References
- NEC (NFPA 70), 2023 — Art. 430 (motors, motor circuits, controllers): Part III (overload protection), Part IV (branch-circuit short-circuit and ground-fault protection), Part IX (disconnecting means)
- NFPA 79:2024 — machine electrical wiring practice, protection, and conductor/grounding chapters (chapter-level)
- UL 508A, 3rd Ed. (2018), revised 2025-06-26 — industrial control panel construction and the SCCR methodology for the starter combination (section-level)
- IEC 60947-4-1 / utilization categories — AC-3, AC-4 contactor duty as a concept
Related Pages
- How to wire a VFD — the drive alternative to across-the-line starting
- Wire sizing walkthrough
- Control power wiring
- Panel grounding & bonding
- Engineering toolkit —
cst motor-branchand the motor-circuit calculators - NEC overview
- UL 508A overview
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