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Example for calculating amperage draw:
Below is an example of text for a label that could be placed on accessory devices:
Output Rating…5 AMPS 28VAC or DC Max
Power…24VAC @ 30ma
The “Power” rating is required. The transformer’s secondary side is 24VAC and the transformer is rated for 20VA. The
example accessory above uses 30 milli-amps, which is .03 Amps. Multiply the volts being used (24V) and the amps
(.03A) to get the VOLT Amps (VA) used by the accessory (24Vx0.03A=0.72VA). That leaves (20-0.72=) 19.28VA left
for other accessories (photo-eyes, loop detectors, etc). Once the total VA exceeds 20VA, the operator may experience
failures. For operator setups requiring multiple accessories, calculate total VA draw and upgrade the transformer if
necessary. (40VA transformers or greater are available).
The output rating is how much electricity the accessory is able to have pass through it. This number is useful when
determining if an accessory will properly function long term with the commercial operator. The output rating has
nothing to do with the required amount of electricity to make the accessory function.
LiftMaster does offer technical training courses. See the back of this guide or visit liftmastertraining.com for more
information.
D
Basic Electrical Information
Transformers
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