Liftmaster K21-35057 Transformer 460V 24VAC 21-35057 Commercial Door Operators
The LiftMaster K21-35057 is a 460V to 24VAC replacement transformer used in select LiftMaster commercial door operator control circuits. In these systems, the transformer steps the incoming line voltage down to the low-voltage control power required by the operator controls, relays, and accessories.
Transformer selection is voltage-specific. This part is intended for operators that call for a 460V primary transformer and should not be substituted into an operator with a different incoming voltage. The operator nameplate, wiring diagram, and parts diagram should all be checked before ordering.
A failed transformer can be the result of age, heat, shorted low-voltage wiring, an accessory short, water damage, or incorrect supply voltage. If the underlying problem is not corrected, a new transformer may fail again.
Compatible With
For use with select LiftMaster commercial door operators that specify K21-35057 or 21-35057 in the parts diagram and require a 460V primary to 24VAC secondary transformer. Compatibility must be verified by operator model, voltage, phase, wiring diagram, and original transformer number.
Features & Specifications
| Manufacturer | LiftMaster |
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| Product Type | Commercial door operator transformer |
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| Part Number | K21-35057 |
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| Also Known As | 21-35057 |
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| Primary Voltage | 460V |
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| Secondary Voltage | 24VAC |
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| Application | Select LiftMaster commercial door operators |
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| Notes | Match by voltage, nameplate, wiring diagram, and parts diagram |
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Also Known As / Related Part Numbers
Technician Note
Do not choose a transformer by physical size alone. Incoming voltage, secondary voltage, VA rating, wiring leads, and the operator diagram must match.
Before Ordering
- Confirm the operator nameplate voltage is 460V.
- Match K21-35057 or 21-35057 in the parts diagram.
- Inspect low-voltage wiring and accessories for shorts.
- Disconnect and lock out power before service.
- Use a qualified technician for high-voltage commercial operator repairs.
Important Ordering Warnings
- Not for 120V, 208V, 230V, or 575V applications unless the manual specifically confirms this part.
- A failed transformer may indicate a shorted control circuit or accessory.
Professional Advisory
Transformer replacement involves high voltage. Do not attempt live testing or wiring changes unless properly trained and authorized.
Product Questions & Answers
What voltage is the K21-35057 transformer?
K21-35057 is commonly listed as a 460V primary to 24VAC secondary transformer for select LiftMaster commercial door operators.
21-35057 is commonly listed as an also known as or related number for K21-35057.
Can this transformer be used on 120V or 230V operators?
No. This listing is for 460V primary applications. The transformer must match the operator voltage and parts diagram.
What happens if the wrong transformer is installed?
The wrong transformer can damage the operator controls, create a safety hazard, or fail immediately.
Who should replace this transformer?
A qualified commercial door technician or electrician should handle transformer replacement because high voltage is involved.
