Open a motor control panel on any 480V installation built in the last thirty years. Mounted next to the contactor you will find a small, heavy component with four or six terminals and a laminated steel core. Two terminals on one side connect to line voltage. Two terminals on the other side output 24VAC. This is the control transformer, and it divides the panel into two voltage domains: the power circuit running at line voltage and the control circuit running at a fraction of it.
What a Control Transformer Does
A control transformer is a single-phase, step-down transformer. It taps two phases of the incoming 3-phase supply on its primary winding (H1 and H2) and produces a lower voltage on its secondary winding (X1 and X2). In a 208V system, the primary sees 208V line-to-line and the secondary outputs 24VAC. In a 480V system, the primary sees 480V and the secondary still outputs 24VAC. The turns ratio handles the difference.
The result is two distinct voltage domains inside one panel. The power circuit carries full line voltage through the main contacts, overload heaters, and motor leads. The control circuit carries 24V through the STOP button, START button, seal-in contact, contactor coil, and overload auxiliary contact. The 3-wire control logic is identical to a line-voltage starter. Only the voltage source changes.
120VAC and 24VAC are the two standard control voltages in industrial motor starters. 120VAC is the more common in traditional hardwired installations across North America. 24VAC is standard in HVAC and increasingly used in newer industrial panels where operator safety and component cost favor the lower voltage. The choice affects component ratings and fuse sizing, but the circuit topology stays the same.
Why the Transformer Exists
Three reasons drive the use of a separate control voltage.
Operator safety. At 480V, accidental contact with a control wire is a serious shock hazard. At 24V, the same contact is far less dangerous. Pendant stations on overhead hoists, pushbutton stations on packaging lines, selector switches on pump panels: operators handle these devices daily. For cranes and hoists, NEC 610.53 limits pendant pushbutton station voltage to 150V AC maximum. A 24V control circuit provides substantial margin below that limit.
Component cost and availability. Contactors, relays, pilot lights, timers, and pushbutton stations rated for 24VAC are cheaper and more widely stocked than their 480V equivalents. A 24V contactor coil is a fraction of the cost of a 480V coil, and replacement stock is easier to keep on the shelf.
Fault isolation. A short circuit in the control wiring stays on the 24V side. The control transformer's fuses clear the fault without tripping the main circuit breaker, which would shut down the entire feeder. The power circuit and the control circuit fail independently.
Control transformers are standard practice at 480V. They are common at 208V, though 208V installations sometimes use line-voltage control where the control circuit runs at full line-to-line voltage. The NEC does not require a control transformer at 208V. At 480V, the economics and safety case are strong enough that you will rarely encounter a panel without one.
Why 24VAC and Not 24VDC
Modern PLC panels typically run 24VDC from switching power supplies. Hardwired motor starters still use AC control transformers. The reasons are practical.
AC transformers are passive devices. No electronics, no capacitors, no switching regulators. A laminated steel core and two copper windings tolerate heat, vibration, voltage sags, and power quality issues that would shorten the life of a DC supply.
AC transformers are also cheaper. A 150VA control transformer from Hammond, Acme, or Square D costs less than a comparable 24VDC power supply, and it does not require DIN rail mounting, separate wiring, or a dedicated fuse for its DC output.
The NEC treats transformer circuits differently from DC power supply circuits. NEC 430.72(C) specifies fuse sizing rules for control circuit transformers. These rules account for transformer inrush current and allow fuse sizes that would be impermissible on a standard branch circuit. DC power supplies include internal current limiting and thermal shutdown, so their external overcurrent protection follows standard branch circuit rules rather than the transformer-specific provisions in NEC 430.72(C).
The tradeoff is that AC control circuits require more careful fuse sizing, which is the primary skill the control transformer starter simulator teaches.
How to Size a Control Transformer
Control transformers are rated in volt-amperes (VA), not watts. VA rating represents the maximum continuous load the transformer can supply without overheating.
To size a transformer, add up the VA draw of every device connected to the secondary. Contactors are the dominant load, and they have two current ratings that matter.
Sealed VA is the steady-state power draw when the contactor is energized and holding. A typical NEMA Size 1 contactor coil draws 15-25VA sealed at 24VAC. This is the normal running load.
Inrush VA is the momentary surge when the coil first energizes. During the first 5-20 milliseconds after energization, the coil draws 6-10 times its sealed current as the magnetic field builds and the armature travels from open to closed. A NEMA Size 1 contactor with 20VA sealed draw may pull 130-200VA during inrush. This surge is brief, and it determines the minimum transformer size.
Size the transformer for worst-case simultaneous inrush. If the circuit can energize two contactors at the same time (a reversing starter, for example), the transformer must handle both inrush loads simultaneously plus all sealed loads from other energized devices.
Sizing example. A single motor starter with one NEMA Size 1 contactor (20VA sealed, 150VA inrush), two pilot lights (5VA each), and one timing relay (10VA sealed, 30VA inrush). Worst-case inrush: contactor inrush (150VA) + pilot lights (10VA) + timer sealed (10VA) = 170VA. A 200VA transformer handles this with margin. Common industrial sizes are 50, 75, 100, 150, 200, 250, 350, 500, 750, and 1000VA.
Undersizing causes problems. A transformer driven into saturation during inrush cannot maintain its secondary voltage. The contactor coil sees reduced voltage, the armature may not fully close, and the main contacts chatter. Chattering contacts arc, pit, and eventually weld. Oversizing by one standard size is cheap insurance.
Fuse Protection: NEC 430.72(C)
Control transformers require overcurrent protection on every ungrounded conductor. For a 208V-to-24V transformer with dual protection, that means fuses on both primary legs (H1 and H2) and on the secondary hot leg (X1). Primary-only protection uses two fuses; adding the secondary fuse makes three.
X2 is never fused. X2 is the grounded conductor, bonded to the equipment grounding conductor. NEC 250.20(A) requires grounding for AC systems under 50V when the primary exceeds 150V to ground, and NEC 250.30 specifies how to ground the separately derived system. If an X2 fuse blew, the ground-fault return path would break. Fault current reaching the enclosure would have nowhere to go, and the upstream breaker would have no way to sense the fault and open. The enclosure stays energized with no protection. This is why the secondary fuse goes on X1 only.
NEC 430.72(C) governs fuse sizing for control circuit transformers. For the small transformers in most motor starters (primary current under 2A), two protection schemes are commonly applied. The code also provides additional methods for larger transformers and other configurations.
Primary-only protection. When only primary fuses are installed, fuses may be sized up to 300% of primary full-load amps. This percentage accommodates transformer inrush current. A fuse at 100% of FLA would blow every time a contactor energized.
Dual protection (primary and secondary). Adding a secondary fuse on X1 at 125% of secondary FLA provides tighter overload protection for the control wiring. With this secondary fuse in place, the primary fuse percentage increases to 500% of primary FLA. The primary fuses cover short-circuit protection and inrush; the secondary fuse catches sustained overloads that fall below the primary fuse's trip threshold.
Sizing calculation. For a 200VA transformer on a 208V system with a 24V secondary, using dual protection: secondary FLA = 200VA / 24V = 8.33A. At 125%, that is 8.33 x 1.25 = 10.4A. Per NEC 240.6(A), the next standard fuse size is 15A. On the primary side: primary FLA = 200VA / 208V = 0.96A. At 500%, that is 0.96 x 5.0 = 4.8A. A 6A time-delay fuse covers the primary.
Time-delay fuses are specified for control transformer circuits because contactor coils draw 6-10x sealed current during inrush. A non-time-delay fuse would nuisance-trip on this normal operating surge. Time-delay fuses ride through the momentary inrush while still clearing sustained overcurrents.
One additional constraint: NEC 430.72(B) limits fuse size based on control conductor ampacity. If the control wiring is 18 AWG (maximum 7A overcurrent protection), the secondary fuse cannot exceed 7A regardless of what the 125% calculation produces. The conductor ampacity governs when it is more restrictive than the transformer sizing calculation.
Where Control Transformers Are Standard
480V motor control centers. Every bucket in a 480V MCC contains a control transformer. The motor runs at 480V; the pushbuttons, pilot lights, and PLC I/O operate at 24V or 120V. Allen-Bradley Bulletin 509 and 512, Eaton Freedom, Square D 8536 series, and Siemens Class 14 starters are all available with factory-installed control transformers.
HVAC rooftop units. Rooftop package units on commercial buildings run 208V or 480V 3-phase compressors with 24VAC control circuits. The thermostat and control board operate at 24V, fed by a transformer tapping the unit's incoming power. Technicians service these units on rooftops in weather, making the low control voltage a practical safety benefit.
Pump stations and water treatment. Variable-frequency drives and across-the-line starters in pump applications use 24V control for remote start/stop from SCADA panels, float switches, and level transmitters. The control transformer provides isolation between the drive's power section and the low-voltage control network.
Machine tools and packaging equipment. CNC machines, injection molders, and packaging lines use control transformers to supply 24V or 120V for operator stations, safety interlocks, and indicator lights. Machine builders size the transformer to handle all control loads simultaneously, including inrush from multiple contactors on machines with several axes or stations.
When no transformer is used. Some 208V installations use line-voltage control, where the STOP and START buttons, coil, and OL contact operate at 208V directly. The circuit logic is identical. Factory jumpers from L1 and L2 supply control power from the line side of the starter instead of from a transformer secondary. This approach is simpler and cheaper for single-starter installations where operator exposure to control wiring is minimal.
Reading is one thing — wiring it yourself is another. Open the interactive trainer and build this circuit from scratch.
Wire a control transformer circuit and size the fuses in the trainer →Frequently asked questions
What is a control transformer?
A single-phase step-down transformer that converts line voltage (208V or 480V) to a lower control voltage, most commonly 24VAC. It creates two separate voltage domains inside a motor control panel: the power circuit at line voltage and the control circuit at the transformer secondary voltage. The control logic (stop, start, seal-in, overload protection) is identical to a line-voltage starter.
How do you size a control transformer?
Add up the VA draw of every device on the secondary. Use sealed VA for continuously energized devices and inrush VA for the worst-case simultaneous energization scenario. Contactor coils draw 6-10x their sealed current during inrush. Size the transformer to handle worst-case inrush plus all other sealed loads. Common sizes are 50, 75, 100, 150, 200, 250, 350, 500, 750, and 1000VA.
Why do control transformers use 24VAC instead of 24VDC?
AC transformers are passive devices with no electronics to fail. They tolerate wider voltage ranges, cost less, and survive heat and vibration better than DC power supplies. NEC 430.72(C) provides specific fuse sizing rules for transformer circuits that account for inrush current. DC power supplies have internal current limiting and follow standard branch circuit protection rules. Modern PLC panels use 24VDC, but hardwired motor starters still use AC.
What does NEC 430.72(C) require for control transformer fuses?
NEC 430.72(C) governs overcurrent protection for control circuit transformers. For small transformers (under 2A primary current), primary-only fuses may be sized up to 300% of primary FLA. Adding a secondary fuse at 125% of secondary FLA allows the primary percentage to increase to 500%, because the secondary fuse handles overload duty. Time-delay fuses are specified to ride through contactor inrush without nuisance tripping.
Why is the X2 terminal never fused?
X2 is the grounded conductor, bonded to the equipment grounding conductor per NEC 250.20(A) and 250.30. Fusing X2 would mean a blown fuse breaks the ground-fault return path. The control circuit would remain energized through X1, but fault current from a ground fault would have no path to trip the upstream breaker. The enclosure could become energized with no protection. The secondary fuse is always placed on X1, the ungrounded conductor.
What are the standard control voltages used in motor control?
120VAC and 24VAC are the two standard control voltages in hardwired motor starters. 120VAC is the more common in traditional North American installations. 24VAC is standard in HVAC and increasingly used in newer industrial panels. 208V and 480V line-voltage control (no transformer) is used in simple installations where operator exposure to control wiring is minimal. 24VDC is standard in PLC-based systems but not in hardwired motor starters.