Careful With That 40VA Transformer


A 40VA transformer is rated for 40 VA or volt-amps on the secondary.

For a typical 24 volt secondary, this is simply using Watt's law to calculate amperage. (For more information on Watt's or Ohm's laws, please check out this article HERE.)

40 ÷ 24 = 1.666666 ∞ (round up to 1.67 already)

So, you cannot place more than 1.67 amps of constant load on the transformer without overloading it.

Many accessories use 24-volt power, such as zoning systems, UV lights, alarms, etc.


The common UV light pictured above uses 0.65 amps. If you connected to the existing transformer, that would leave only 1.01 amps for the system controls.

It is generally a safer bet to either add in an additional accessories transformer or upsize the existing transformer to 60 or 75 VA.

When adding an additional transformer, make sure it either has an internal fuse or that you add a fuse inline to protect the transformer and connected devices.

—Bryan

Related Tech Tips

Lifting Techniques Part 2 - Pulleys
This is Part 2 series by Senior Refrigeration Tech (and prolific writer) Jeremy Smith. Pay attention to this one, folks. I know rigging and safe lifting practices may be boring to some of you, but it could very well save your back or your life. Disclaimer This article is written by a technician representing his […]
Read more
Why Discharge Line Temperature is a Useful Reading
Since I started in the trade, we would take discharge line temperature in the winter on a heat pump system. You can easily check the discharge line in the winter, while suction superheat and even subcool can be more difficult to access. The old-timers who trained me would say that the discharge temp will be […]
Read more
Reach-In Temperature Controls
  This article is written by Christopher Stephens of JVS Refrigeration in California, with just a few additions by me (Bryan) in italics. Thanks, Chris! Reach-in refrigerators are an interesting side of our industry, often looked at as frustrating and troublesome. Since we usually see reach-ins in kitchens or convenience stores, the refrigerators are never […]
Read more

3 responses to “Careful With That 40VA Transformer”

  1. Would you still need to block off the bottom portion that hangs over with either sheet metal or duct board I’d say yes not sure if it’s already plated off ?

    • I am not an HVAC pro or tech but I only want to know how you reset a Mitsubishi HVAC 2 room or dual cabin unit compact model? It’s on the backside of my Grammys house and it just stopped working all a sudden and the internal thermostat just says..not responding so I have no clue what to do but I’m sure there’s a reset somewhere right?

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from