Description
Product Introduction
The Cutler-Hammer C340FG is a industrial control transformer designed to step down higher distribution voltages to manageable levels for control circuits. Manufactured under the Eaton Cutler-Hammer lineage, this unit provides electrical isolation and consistent voltage regulation across motor starters, contactors, relays, and PLC power supplies in harsh industrial enclosures.
Unlike generic commercial power supplies, the C340FG features heavy-duty copper windings and high-grade silicon steel laminations engineered to handle high inrush currents without dipping secondary voltage. Its compact foot-mount footprint allows quick retrofits into existing motor control centers (MCCs) and control cabinets where standard DIN-rail units fall short.
Key Technical Specifications
| Parameter | Value |
| Manufacturer | Cutler-Hammer / Eaton |
| Part Number | C340FG (Variant: C340FGO) |
| Product Type | Industrial Control Transformer |
| Power Rating | 0.275 kVA (275 VA) |
| Primary Voltage | 240 x 480 VAC |
| Secondary Voltage | 120 VAC |
| Frequency | 50 / 60 Hz |
| Insulation Class | 105°C (Class A) |
| Mounting Type | Foot / Base Plate Mounting |
| Winding Material | Copper |
| Termination | Screw Terminals with Pressure Plates |
| Operating Temp | -20°C to +40°C Ambient |
Application Scenarios & The “Trench” Experience
It’s 2:15 AM on a Sunday. The main conveyor line at a automotive stamping plant trips on a control power fault. The culprit? An ancient control transformer inside a legacy MCC bucket insulation-shorted after 20 years of continuous vibration and thermal cycling. The secondary voltage plummeted from 120VAC down to 78VAC, dropping out every pilot relay on the line. Replacing it with an exact-fit C340FG gets the panel control bus back to a crisp 120VAC, bypassing the nightmare of re-drilling mounting plates or re-engineering panel wiring while downtime costs tally thousands per hour.
Specific Application Scenarios:
- Automotive & Assembly: Motor Control Center (MCC) bucket control power isolation for starter coils and overload relays.
- Water/Waste Water Treatment: Stepping down panel supply voltages to power 120VAC solenoid valves and field instrumentation transmitters.
- Material Handling: Panel-mounted control power source for crane hoists, variable frequency drive (VFR) contactors, and safety interlocks.
- HVAC & Industrial Chillers: Supplying clean AC voltage to local control boards and interlocking circuits in high-vibration compressor skids.
C340FG
C340FG
Transparency SOP: Quality Assurance & Testing
Every Cutler-Hammer C340FG unit pulled from our surplus inventory passes through a bench-testing sequence before shipping:
- Inbound Inspection: Visual verification of core laminations, potting compound integrity, terminal blocks, and OEM labeling to catch physical damage or severe oxidation.
- Winding Resistance Checks: DC resistance measurements across primary and secondary taps to verify balanced winding geometry and check for internal shorted turns.
- Insulation Resistance (Megger Test): 1000V DC insulation test between primary-to-secondary windings and primary-to-ground frame. Minimum threshold: 100 MΩ.
- No-Load & Full-Load Voltage Verification: Energized on our test bench at rated primary voltage (240V/480V) to verify precise 120V secondary output under open-circuit and simulated rated kVA load.
- Thermal & Noise Evaluation: Monitored for excessive core hum, vibration, or localized hot spots during a 30-minute burn-in cycle.
- Final Packaging: Vacuum-sealed with desiccant and secured in custom high-density foam packaging to prevent winding or terminal damage during transit.
The Veteran’s Tech Trap Guide
⚠️ 1. Check Primary Jumper Configuration First
Do not drop this transformer in without confirming how the primary windings are strapped. If your field feed is 480VAC, the primary windings must be wired in series. If it’s 240VAC, they must be wired in parallel. Hooking up 480V to a 240V parallel connection will instantly smoke the primary coil and blow your line fuses.
❗ 2. Watch Out for Inrush VA Ratings
Standard contactor coils draw up to 5 to 10 times their sealed VA rating when energizing. If your control loop pulls more than 1.5 kVA inrush, this 275VA transformer will experience a momentary voltage dip, causing adjacent sensitive relays to chatter or drop out. Calculate your total panel inrush, not just sealed VA.
⚠️ 3. Grounding the Secondary Neutral
Always follow NEC 250.20(B) for separately derived systems. Ensure one leg of the 120V secondary (X2) is bonded to the enclosure ground terminal unless you are intentionally running an ungrounded control circuit with ground fault monitoring. Leaving the secondary floating often leads to weird phantom voltage induction on long field wiring runs.
Frequently Asked Questions
Q: Can I use this Cutler-Hammer C340FG on a 208V or 230V line?
A: You can connect 230V to the 240V primary taps without issue, giving you roughly 115V on the secondary. However, connecting 208V directly to the 240V primary tap will yield a proportional secondary drop (~104V AC). This low voltage can cause 120V contactor coils to hum, overheat, or fail to pull in reliably.
Q: Is this unit hot-swappable?
A: No. Absolutely not. Never attempt to replace or rewire a control transformer on a live panel. Isolate the main incoming breaker, execute Lockout/Tagout (LOTO), and verify zero potential across all primary and secondary terminals with a calibrated multimeter before touching a screwdriver to the unit.
Q: Why buy “New Surplus” or “Refurbished” when Cutler-Hammer/Eaton has newer lines?
A: Legacy machine panels and MCC buckets are built to specific mounting bolt patterns and spatial envelopes. A modern replacement transformer often has completely different physical mounting dimensions or terminal locations, requiring extra panel drilling, tap changes, or wire extensions. Sourcing a tested original C340FG provides a 1:1 drop-in replacement that slashes installation downtime to minutes.
Q: What kind of warranty comes with this module?
A: We back all our New Surplus and refurbished Cutler-Hammer inventory with a full 12-month functional warranty. If the transformer fails under normal operating conditions within a year of purchase, we replace it or issue a refund immediately.
Q: Does this transformer include secondary fuse clips mounted directly on the frame?
A: The standard C340FG relies on external panel-mounted fuse blocks or a supplementary terminal-mounted fuse kit. Check the product photos or physical unit specs to confirm if a secondary fuse block accessory is pre-installed on the top frame before wiring.
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