Careful With That 40VA Transformer

September 5, 2018

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

#overload #transformer #uv #va

Check out our most recent video

How to Evacuate an AC system, Full Vacuum Procedure

Our latest Podcast

HVAC School – For Techs, By Techs

Real training for HVAC ( Heating, Ventilation, Air Conditioning and Refrigeration) Technicians. Including recorded tech training, interviews, diagnostics and general conversations about the trade.

In this episode, Bryan and Bert dive into the often misunderstood world of compressor testing, focusing on common mistakes technicians make when using ohmmeters, compressor testers, and megohm meters. The conversation highlights how diagnostic errors can lead to unnecessarily condemned compressors and wasted time and resources.

The hosts begin by discussing fundamental concepts like the difference between “open” and “short” conditions, emphasizing that a path doesn’t necessarily indicate a short circuit—it simply means there’s a low-resistance path. They explore how many technicians get confused when testing terminals, mistakenly thinking that normal winding-to-winding measurements indicate a problem. Through practical examples from their field experience, they illustrate why understanding normal readings is crucial before taking any measurement.

A significant portion of the discussion centers on proper testing techniques for scroll compressors, which often show different measurements to ground compared to other compressor types due to their design, with windings submerged in refrigerant and oil. Bryan and Bert explain why standard motor testers can lead to false diagnoses when they indicate “bad” at threshold levels like 20 megohms—a reading that would be normal for many functional scroll compressors. They recommend proper megohm meters calibrated to appropriate voltage levels for accurate testing.

Key Topics Covered:

The difference between “open” circuits and “shorts” and how to properly interpret ohm readings
Common mistakes when testing compressor terminals, especially misinterpreting normal winding-to-winding measurements
How to properly check for shorts to ground and the importance of clean connections
Why scroll compressors naturally show different resistance-to-ground readings due to their design with submerged windings
Problems with inexpensive compressor testers that give false “bad” readings on functional equipment
The “redneck test” for isolating a compressor to determine if it’s truly the source of breaker trips
The importance of understanding meter auto-ranging and correctly interpreting readings in ohms, kilohms, and megohms
Why a reading below 0.5 megohms to ground is the general threshold for concern with scroll compressors
Differences between megohm meters, high pot testers, and standard multimeters
The misconception that winding-to-winding shorts are common without corresponding shorts to ground

Have a question that you want us to answer on the podcast? Submit your questions at https://www.speakpipe.com/hvacschool.

Purchase your tickets or learn more about the 6th Annual HVACR Training Symposium at https://hvacrschool.com/symposium.

Subscribe to our podcast on your iPhone or Android.

Subscribe to our YouTube channel.

Check out our handy calculators here or on the HVAC School Mobile App for Apple and Android

Search Episodes

Compressor Ohm Mistakes w/ Bert

05/22/2025

Bryan Orr

Motor Ambient Rating – Short #241

05/20/2025

Bryan Orr

Superheat Talk w/ JD Kelly

05/15/2025

Bryan Orr

How to Deal w/ Too Much – Short #240

05/13/2025

Bryan Orr

Delta T Talk w/ JD Kelly

05/08/2025

Bryan Orr

Fire Sprinklers – Short #239

05/06/2025

Bryan Orr

The PATH to High Performance HVAC w/ David Richardson

05/01/2025

Bryan Orr

Estimating Tips and Software

04/30/2025

Bryan Orr

Nylog, Oil? Where does it go? – Short #238

04/29/2025

Bryan Orr

Growing your HVAC Business the Smart Way w/ Copeland

04/24/2025

Bryan Orr

Search Results placeholder

Comments

Mike byrne @bryanorr

Is that the actual formula

4/3/17 at 05:18 PM

Is that the actual formula

Anonymous @bryanorr

Tadalafil sans ordonnance en ligne: Acheter Cialis 20 mg pas cher – Cialis sans ordonnance 24h tadalmed.shop

4/25/25 at 04:52 PM

Tadalafil sans ordonnance en ligne: Acheter Cialis 20 mg pas cher – Cialis sans ordonnance 24h tadalmed.shop

Anonymous @bryanorr

kamagra en ligne achat kamagra or kamagra oral jelly
https://www.britishchambers.org.uk/customer/login?redirect_url=https://kamagraprix.shop kamagra gel
[url=https://cse.google.com.mt/url?sa=t&url=https://kamagraprix.shop]kamagra oral jelly[/url] Kamagra Oral Jelly pas cher and [url=https://domod.click/home.php?mod=space&uid=76331]kamagra oral jelly[/url] Acheter Kamagra site fiable

4/25/25 at 01:41 PM

Anonymous @bryanorr

acheter mГ©dicament en ligne sans ordonnance: pharmacie en ligne – pharmacie en ligne france fiable pharmafst.com

4/25/25 at 12:38 PM

Anonymous @bryanorr

cialis sans ordonnance: Cialis generique prix – Acheter Cialis 20 mg pas cher tadalmed.shop

4/25/25 at 12:16 PM

cialis sans ordonnance: Cialis generique prix – Acheter Cialis 20 mg pas cher tadalmed.shop

Anonymous @bryanorr

cialis prix Cialis sans ordonnance pas cher or Cialis en ligne
http://historico.cubainformacion.tv/enlazar.php?video=leccionesdemanipulacion077ITA.flv&id=17321&ur=https://tadalmed.com/ Cialis sans ordonnance 24h
[url=https://maps.google.cz/url?sa=t&url=https://tadalmed.com]cialis prix[/url] Achat Cialis en ligne fiable and [url=https://forum.beloader.com/home.php?mod=space&uid=1751936]Acheter Cialis[/url] Achat Cialis en ligne fiable

4/25/25 at 11:05 AM

Anonymous @bryanorr

olympe casino: olympe casino cresus – olympe casino en ligne

4/23/25 at 06:15 PM

olympe casino: olympe casino cresus – olympe casino en ligne

Anonymous @bryanorr

Hello! I know this is kind of off-topic but I needed to ask. Does managing a well-established blog such as yours take a massive amount work? I am completely new to running a blog however I do write in my diary daily. I’d like to start a blog so I will be able to share my own experience and views online. Please let me know if you have any kind of recommendations or tips for brand new aspiring blog owners. Appreciate it!

4/23/25 at 07:32 AM

Michael Rodriguez @bryanorr

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 ?

4/3/17 at 09:13 PM

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 ?

David Watkins @bryanorr

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?

9/28/21 at 06:40 AM