12.3 Motor Control Circuits and VFDs
Key Takeaways
- NEC 430.122(A) sizes VFD INPUT conductors at 125% of the VFD rated input current; 430.122(B) sizes VFD OUTPUT conductors at 125% of the motor full-load current per 430.6 — unless the VFD is marked 'Suitable for Output Motor Conductor Protection,' then use the larger of 125% of motor FLC or the marked minimum.
- NEC 430.74 requires the motor controller disconnect to also disconnect the control circuit, or a separate listed disconnect must be provided.
- NEC 430.72 protects control-circuit conductors tapped from the motor branch circuit per Table 430.72, using the motor branch-circuit overcurrent device and the tap conductor size.
- A reversing starter uses two contactors with both mechanical and electrical interlocks so both cannot close simultaneously.
- NEC 430.126 requires overtemperature protection for VFD-driven motors because the shaft fan cools less at reduced speed (430.126(B) requires individual protection per motor in multi-motor applications), and 430.120 requires VFDs grounded per manufacturer instructions as output conductors can induce bearing currents and stress motor insulation.
Why This Matters for the TX Journeyman Exam
Motor control circuits and variable-frequency drives (VFDs) appear in both exam parts. The Knowledge part tests Article 430 Part VII rules and starter operating principles; the Calculations part tests 430.122 VFD conductor sizing — a two-part calculation that trips up candidates who apply the input-current rule to the output conductors, or who reach for the wrong current value. Chapter 11 covers motor disconnects, controllers, and starter names; this section goes deeper into control-circuit protection, starter operating principles, and VFD rules.
Article 430 Part VII — Control Circuits
Scope (430.71)
Part VII covers motor control circuits — the circuits that carry the command and holding signals to magnetic starters, contactors, and solid-state controllers. A control circuit can be tapped from the motor branch circuit (the most common arrangement) or supplied from a separate source.
Overcurrent Protection (430.72)
| Source of control circuit | Overcurrent protection rule |
|---|---|
| Tapped from the motor branch circuit | Protected by the motor branch-circuit overcurrent device per Table 430.72 — the tap conductor size vs. the branch OCPD rating determines the maximum tap length and required conductor ampacity |
| Separate source | Protected per Article 240 (general overcurrent rules) based on the control-circuit conductor ampacity |
Table 430.72 gives the maximum overcurrent protection for control-circuit conductors based on conductor size and the rating of the upstream motor branch-circuit protective device. For example, a 14 AWG control-circuit conductor tapped from a circuit protected at 60 A is permitted only within the tap-length limits of Table 430.72(C).
Grounding (430.73)
Control-circuit conductors shall be grounded where the system is grounded. A control transformer secondary operating at ≤ 50 V is permitted to be ungrounded, but a grounded secondary is required where the secondary is not independently grounded and the primary is ≤ 300 V to ground.
Disconnecting Means (430.74)
The disconnecting means for the motor controller shall also disconnect the control circuit, or a separate listed disconnect for the control circuit shall be provided. This rule prevents a live control circuit from re-energizing the starter coil after the controller disconnect is opened. In practice, the standard 3-pole motor disconnect ahead of the starter interrupts the line side of both the power circuit and the control transformer primary, satisfying 430.74 in one device.
Motor Control Device Operating Principles
| Device | Operating principle |
|---|---|
| Across-the-line (full-voltage) magnetic starter | A contactor with overload relays applies full line voltage to the motor terminals on command; the holding circuit seals-in via an auxiliary contact after the start button is released |
| Reversing starter | Two contactors (one forward, one reverse) with both mechanical and electrical interlocks so that both cannot be closed simultaneously; reversing is achieved by swapping two line leads (T1 and T3) to the motor |
| Two-speed starter | Two separate contactors (one for low speed, one for high speed) with interlocks; designed for two-speed, two-winding or consequent-pole (Dahlander) motors |
| Autotransformer starter | Reduced-voltage starting via an autotransformer with taps (e.g., 50%, 65%, 80%); reduced starting current and torque, then full voltage is applied after a timed transition |
| Primary-resistance starter | A resistor is inserted in series with the motor during start, reducing applied voltage; the resistor is shorted out after a time delay |
| Part-winding starter | The motor's two parallel windings are energized sequentially — first one winding (reduced current), then both; only for part-winding motors |
| Wye-delta (Y-Δ) starter | The motor starts with windings connected in wye (reduced voltage per phase = 1/√3 of line), then transitions to delta for full-voltage running; requires a motor with all six leads brought out |
| Solid-state soft starter | SCRs control the applied voltage by phase-angle or voltage-ramp control during start, then bypass; smoothest mechanical start, lowest inrush among reduced-voltage methods |
Reversing starter interlocks are a favorite exam point. The mechanical interlock physically prevents both contactors from closing at once; the electrical interlock uses normally-closed auxiliary contacts so the forward coil cannot energize while the reverse contactor is closed, and vice versa. Both are required.
Variable Frequency Drives (VFDs)
A VFD varies motor speed by rectifying the AC input to DC and synthesizing a variable-frequency, variable-voltage AC output via pulse-width modulation (PWM). The VFD, not the across-the-line starter, controls motor torque and speed. The NEC rules below are exam-critical.
NEC 430.122 — VFD Input and Output Conductors
430.122 splits the conductor-sizing rule into input (line side, VFD supply) and output (load side, VFD-to-motor):
| Conductors | Rule (2023 NEC) |
|---|---|
| 430.122(A) — Input (branch/feeder to VFD) | Ampacity ≥ 125% of the VFD rated input current (the nameplate input current, which includes VFD losses and harmonics and is typically higher than the motor FLC) |
| 430.122(B) — Output (VFD to motor) | Ampacity ≥ 125% of the motor full-load current determined per 430.6 (Table 430.250 for 3-phase AC motors) |
| 430.122(B) Exception | If the VFD is marked 'Suitable for Output Motor Conductor Protection,' the output conductor ampacity shall be not less than the larger of (i) 125% of the motor FLC per 430.6, or (ii) the ampacity of the minimum conductor size marked on the VFD |
Input conductor ampacity ≥ VFD rated input current × 1.25
Output conductor ampacity ≥ motor FLC (430.6) × 1.25
The common error is applying the 430.122(A) input-current rule to the output conductors. The output rule (430.122(B)) is based on the motor FLC, not the VFD input current — unless the VFD carries the 'Suitable for Output Motor Conductor Protection' marking, in which case the VFD-marked minimum may govern.
NEC 430.126 — Motor Overtemperature Protection with VFDs
430.126 addresses a hazard unique to adjustable-speed drives: a motor can overheat at reduced speed because the shaft-mounted cooling fan delivers less airflow at low RPM, even when the motor current is below rated full-load current. Standard overload relays sized to 430.32 protect against overcurrent, not against the reduced-cooling condition, so 430.126 adds overtemperature protection.
430.126(A) — Protection methods. One or more of the following:
- Thermal protectors integral to the motor
- Overload relays with thermal memory that accounts for the reduced-cooling effect
- Embedded temperature sensors (thermistors/RTDs) in the motor windings — recommended for externally cooled (TEFC/ODP) motors that lose fan airflow at low speed
- Other approved overtemperature-protection methods
430.126(B) — Multiple motors. Where one VFD drives multiple motors, each motor shall have individual overtemperature protection. A single drive-level device does not protect individual motors against a localized cooling failure.
Do not confuse 430.126 (overtemperature) with 430.32 (overload) — the VFD's electronic overload function protects against overcurrent; 430.126 protects against the reduced-cooling overheating that overcurrent relays cannot detect.
NEC 430.120 — Grounding of VFDs
VFDs shall be grounded per the manufacturer's instructions and Article 250. The VFD frame, the motor frame, and the equipment grounding conductor shall be bonded. VFD manufacturers specify shielded motor output cables and a high-frequency bonding path to limit common-mode current.
VFD Effects on Motor Insulation and Bearings
PWM output from a VFD produces voltage spikes (typically 2–3× the DC bus voltage) at the motor terminals due to reflected wave on long motor leads; NEMA MG 1 Part 31 defines inverter-duty motors with insulation rated for these spikes. Bearing currents from common-mode voltage can cause electrical discharge machining (EDM) of bearing races — mitigation includes shaft grounding rings, insulated bearings, and shielded symmetric VFD cables.
Worked Example: VFD Input and Output Conductor Sizing
A VFD feeds a 5 hp, 460 V 3-phase motor. The VFD nameplate rated input current is 27 A. The VFD is not marked 'Suitable for Output Motor Conductor Protection.' Size both the input and output conductors (copper, 75 °C terminations).
Part A — Input (Branch/Feeder to VFD) Conductors, 430.122(A)
Input ampacity ≥ VFD rated input current × 1.25 = 27 A × 1.25 = 33.75 A
From Table 310.16, 75 °C Cu column: 10 AWG = 35 A (≥ 33.75 A). The 12 AWG row is 25 A (too small). Minimum input conductor: 10 AWG Cu. Installers often upsize to 8 AWG (50 A) for voltage drop on long feeder runs.
Part B — Output (VFD-to-Motor) Conductors, 430.122(B)
Because the VFD is not marked 'Suitable for Output Motor Conductor Protection,' the output conductors use the motor FLC basis.
Motor FLC (Table 430.250, 5 hp, 460 V 3-phase) = 7.6 A
Output ampacity ≥ motor FLC × 1.25 = 7.6 A × 1.25 = 9.5 A
From Table 310.16, 75 °C Cu column: 14 AWG = 20 A (≥ 9.5 A). However, 240.4(D) limits the 14 AWG overcurrent protection to 15 A, and 9.5 A ≤ 15 A, so 14 AWG is permitted. Minimum output conductor: 14 AWG Cu. In practice, installers upsize to 12 AWG or 10 AWG to address VFD output voltage drop and reflected-wave voltage stress on long motor leads.
Part B Exception — VFD Marked 'Suitable for Output Motor Conductor Protection'
If the VFD were so marked, the output conductor ampacity would be the larger of:
- 125% of motor FLC = 9.5 A (→ 14 AWG), or
- The ampacity of the minimum conductor size marked on the VFD (e.g., if the VFD nameplate says 'Minimum output conductor 12 AWG,' use 12 AWG = 20 A).
Step C — Verify Equipment Grounding Conductor (250.122)
The EGC is sized from Table 250.122 based on the rating of the overcurrent protective device feeding the VFD, not the VFD input current. If the VFD branch circuit is protected at 40 A, Table 250.122 requires 10 AWG Cu EGC.
Common Exam Traps
- 430.122(A) is input; 430.122(B) is output. The input rule uses VFD rated input current; the output rule uses motor FLC per 430.6 (unless the VFD is marked 'Suitable for Output Motor Conductor Protection'). Applying the input-current rule to the output conductors is the single most common VFD error.
- 125%, not 115%. 430.122 uses 125% (matching the 430.22 motor-conductor rule); 440.15 uses 115% for HVAC disconnects. Don't cross them.
- 430.126 is overtemperature, not overload. Reduced-speed fan cooling is the hazard; embedded temperature sensors are the recommended method for externally cooled motors.
- Reversing starter requires both interlocks. Mechanical alone is not enough; electrical alone is not enough.
- 430.74 control-circuit disconnect is satisfied by the controller disconnect only if it opens the control circuit too; otherwise a separate listed disconnect is required.
Per NEC 430.122(B), VFD OUTPUT (VFD-to-motor) conductors are sized at 125% of which value, unless the VFD is marked 'Suitable for Output Motor Conductor Protection'?
A reversing magnetic starter uses two contactors. What two interlocks are required so both contactors cannot close simultaneously?
Per NEC 430.74, the motor controller disconnecting means must also accomplish which of the following?