14.2 Motor Control Circuits

Control Circuits on the Prov Motors Block

After you can size motor power conductors and place the disconnect/controller, Prov still expects you to protect the control circuit correctly. Article 430 Part VI (430.71–430.74) is the home base. These questions look “small” (14 AWG, control transformers, start-stop stations) but they are easy points if you open the right table — and easy misses if you apply 240.4 branch-circuit habits blindly.

Arkansas Journeyman candidates should tab 430.71, 430.72 (especially Table 430.72(B)), 430.73, and 430.74, plus a mental cross-link to Article 725 only when the stem clearly leaves motor-control Article 430 and enters remote-control/signaling Class rules.

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What Is a Motor Control Circuit?

A motor control circuit carries the electrical signals that direct the controller — coil circuits for contactors/starters, stop-start stations, interlocking contacts, overload relay control contacts, permissive contacts from safety devices, and similar.

It is not:

• the motor branch-circuit power conductors sized under 430.22,
• the equipment grounding conductor sized under 250.122,
• or a Class 2 thermostat cable run that the stem never ties to an Article 430 motor controller.

If the stem says “control circuit of a magnetic motor starter,” stay in Part VI until an exception or Informational Note pushes you elsewhere.

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Two Source Patterns (Always Identify First)

1) Control circuit tapped from the motor branch circuit

Control power is taken from the line side (or as permitted) of the starter / motor circuit. Protection of those control conductors is tightly limited because a fault on small control wire must clear via the motor branch-circuit device or a supplementary control OCPD. This is where Table 430.72(B) earns its keep.

2) Separate control power source

A dedicated control circuit (often from a control transformer or a separate panelboard circuit) supplies coils and stations. Protection then follows the control-circuit OCPD rules in 430.72 for that arrangement, and transformer primary/secondary protection may also invoke Article 450 / control-transformer footnotes in 430.72.

Exam first question: Is control power tapped from the motor circuit or separately derived / separately protected? Wrong source → wrong column in the table.

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Overcurrent Protection — 430.72

430.72 requires control-circuit conductors to be protected against overcurrent. The details depend on conductor size, whether the conductors are tapped from the motor branch circuit, and whether protection is provided by the branch-circuit device, a supplementary protective device, or both.

Table 430.72(B) — the Prov lookup

Table 430.72(B) lists maximum ratings of overcurrent devices for control-circuit conductors. Themes you must verify in the published table (do not invent ampacities under time pressure):

• Smaller control conductors (for example, 18 AWG and 16 AWG where permitted) have low maximum protective-device ratings.
• 14 AWG and 12 AWG control conductors have higher allowable protective-device ceilings than 18/16 AWG, still often far below ordinary power-circuit breaker sizes.
• Columns distinguish conditions such as protection by the motor branch-circuit protective device versus protection by a supplementary control-circuit overcurrent device.
• Notes address copper vs aluminum, flexible cords, and special conditions — read the notes when the stem mentions them.

Workflow:

1. Identify control conductor size and material.
2. Identify how the control circuit is supplied (motor branch tap vs separate).
3. Open Table 430.72(B) and select the correct column.
4. Pick the maximum OCPD rating allowed — or recognize that a supplementary fuse/breaker in the control circuit is required because the motor branch device is too large.

Classic trap: A 30 A or 40 A motor branch-circuit breaker protecting 14 AWG control conductors tapped ahead of the starter without a control fuse. Table 430.72(B) usually will not allow that arrangement — the fix is a properly rated control-circuit protective device, not upsizing the control wire only in your head.

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Mechanical Protection & Routing — 430.73 Themes

430.73 and related installation practice expect control conductors to be protected from physical damage where exposed. On industrial equipment, control wiring often leaves the starter enclosure to field devices. Exam cues:

• Use a wiring method suitable for the location (raceway, cable tray, machine wireways, etc.).
• Where control conductors enter the same raceway as power conductors, watch insulation voltage ratings and fill — Chapter 3 wiring-method rules still apply.
• Keep control wiring arranged so a fault does not defeat the stop function (orderly wiring, proper terminal landing). Stems about a grounded control circuit causing a motor to start unexpectedly are really about failure modes and disconnection rules in 430.74, not ampacity.

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Control Circuit Transformers

Many 480 V motor starters use a control transformer (often 480–120 V or 480–24 V) so pushbuttons and coils operate at a safer control voltage.

Exam checkpoints:

• Protect the transformer primary (and secondary when required) per 430.72 provisions that address control-circuit transformers, coordinated with Article 450 overcurrent philosophy.
• Do not size the control-transformer primary OCPD as if it were a motor branch-circuit device under 430.52.
• If the stem gives transformer VA and voltages, compute primary/secondary currents (I = VA / V) before applying percentage or table limits.
• Grounding/bonding of control-transformer secondaries follows the separately derived / control-circuit grounding rules indicated by the installation — do not assume every 120 V control secondary is ungrounded or grounded without reading the stem.

Practical example (method, not a substitute for the table): A 100 VA, 480–120 V control transformer draws about 0.21 A on the 480 V primary and about 0.83 A on the 120 V secondary. Primary protection will look “small” compared with the motor fuses — that is expected. Your job is to match 430.72 / 450 limits, not to match the motor FLC.

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Disconnecting Control Circuits — 430.74 (Lockout Favorite)

430.74 requires that motor control circuits be arranged so that they are disconnected from all sources of supply when the disconnecting means is in the open position, or otherwise arranged to prevent energization as the section specifies. Intent for the exam:

• Opening the motor/controller disconnect should not leave an alternate control source capable of closing the contactor unexpectedly.
• Where a separate control power source exists, additional disconnecting means or interlocking may be required so lockout is effective.
• Control transformers tapped on the line side of the disconnect are a classic hazard: if control power remains live with the disconnect open, the contactor could still pick up if a start circuit faults or is operated.

Prov pattern: “Motor disconnect open, but starter coil can still be energized from a separate 120 V control circuit” → noncompliant with the 430.74 disconnection concept unless additional compliant disconnection/interlock is provided.

Also coordinate with 430.102 lockable disconnect practice: lockout must isolate power and effective control energy.

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Start-Stop Stations, Interlocks, and Undervoltage

While deep machine-tool standards (NFPA 79) are outside the core Prov booklet, NEC motor-control questions still assume basic control logic literacy:

• Stop buttons are normally closed in the coil circuit; Start buttons are normally open and typically sealed in by an auxiliary contact.
• Undervoltage release / three-wire control prevents automatic restart after power failure when arranged that way — two-wire control (maintained contact) can restart when voltage returns.
• If a stem asks which arrangement prevents automatic restart after an outage, look for three-wire stop-start with maintaining contact, not a maintained-on hand switch alone.

Use that logic only to choose among Code-compliant wiring descriptions; the enforceable Part VI lookup remains 430.72 / 430.74.

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Related Articles (When the Stem Leaves 430)

Do not drag 725 Class 2 power limitations into a 430.72 motor-control conductor protection item unless the question explicitly frames a Class circuit.

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Timed Open-Book Tips for §14.2

• First classify: tapped motor-circuit control vs separate control source.
• Open Table 430.72(B) for conductor/OCPD limits — do not “remember” 15 A for every 14 AWG control wire.
• For control transformers, calculate currents, then apply 430.72 / 450, not 430.52.
• For unexpected-start / lockout stems, go to 430.74 and ask whether all control sources are disconnected or safely disabled.
• Keep power-circuit percentages (125%, 115%, 250%) out of control-circuit answers unless the cited section actually uses them.