LiftMaster K21-10340 115V Transformer Kit, 24VAC 20VA for Mechanical Commercial Door Operators
The LiftMaster K21-10340 is a replacement transformer kit for select LiftMaster mechanical commercial door operators. It steps 115V line power down to 24VAC control voltage at a 20VA rating, supplying the low-voltage side of the operator controls. A bad transformer can leave the operator control circuit dead, cause unstable accessory power, or create repeated low-voltage faults depending on the operator design.
This transformer is part-number and rating sensitive. It should not be substituted with a different transformer just because the voltage looks similar. The 20VA output, wire connections, mounting arrangement, and original diagram callout all need to match. It is commonly found in parts references as K21-10340, 21-10340, 2110340, 215115LD, and G215115LD.
Compatible With / Common Applications
Commonly used where the LiftMaster mechanical commercial door operator parts diagram calls for K21-10340 or its related numbers:
- Select LiftMaster mechanical commercial door operators
- Commercial door operator control circuits requiring 115V input and 24VAC 20VA output
- Applications listing 21-10340, 2110340, 215115LD, or G215115LD
- Mechanical operator families using a 20VA low-voltage transformer by diagram
- Commercial sectional, rolling, or industrial door operators where the original transformer label matches
Features and Specifications
| Feature / Specification | Details |
|---|
| Manufacturer | LiftMaster |
| Product Type | Transformer kit |
| Part Number | K21-10340 |
| Input Voltage | 115V AC |
| Output Voltage | 24VAC |
| Power Rating | 20VA |
| Application | Mechanical commercial door operator control power |
| Compatible Operator Type | Select LiftMaster mechanical commercial operators |
| Wire Reference | Blue/yellow secondary wire references appear in some listings |
| Notes | Not the same rating as K21-5115 / 21-5115 40VA transformer |
Also Known As / Related Part Numbers
- K21-10340
- 21-10340
- 2110340
- 215115LD
- G215115LD
- 20VA-215115LD
- LiftMaster 115V commercial transformer
- 24VAC 20VA transformer
Technician Note
If the old transformer failed because of a low-voltage short, simply installing a new transformer may cause another failure. Check control wiring and accessories before energizing the operator.
Before Ordering Checklist
- Confirm the old transformer or parts diagram calls for K21-10340
- Match 115V primary and 24VAC 20VA secondary rating
- Do not substitute a 40VA transformer unless the manufacturer diagram supports it
- Inspect low-voltage wiring, relay wiring, and accessory wiring for shorts
- Verify proper fuse protection and control voltage after installation
Professional Advisory
This part is connected to line-voltage circuits. Disconnect power and use a qualified commercial door technician for safe diagnosis and replacement.
Product Questions and Answers
What is the output of K21-10340?
K21-10340 is commonly listed as a 115V transformer kit with 24VAC, 20VA output.
Is K21-10340 the same as 2110340?
Yes. 2110340 is one of the common related references for the K21-10340 transformer kit.
Can I replace K21-10340 with K21-5115?
Not by assumption. K21-10340 is a 20VA transformer path, while K21-5115 is commonly listed as a 40VA transformer path.
What type of operators use this transformer?
It is used on select LiftMaster mechanical commercial door operators where the parts diagram calls for it.
What should be checked if the operator has no control power?
Check incoming voltage, fuses, transformer output, accessory wiring, and downstream shorts before assuming the transformer is the only failed part.
