Control transformers: Watt’s a VA?

stock-photo--transformer-dc-voltage-appliances-on-a-white-background-339882341It’s 4:50pm. The unit you’re working on has a blown control fuse. The supply houses close in 10 minutes, and it’s 20 minutes away.

You scour every shelf, box, and bin in your truck looking for the ever elusive 3amp fuse…but no luck.

You glance at the fuse panel in your service truck trying to decide what you don’t need…tail-lights, horn…radio (no, not the radio, need tunes for the ride home) only to find it’s loaded with those new-fangled micro-fuses.

But wait! What’s that in your cup holder, sitting among the spare change, crimp connectors and pocket lint? Its a 5 amp fuse!

Now the big question; It had a 3 amp fuse, but you’ve got a 5 amp. Is it too big? If it’s too big you’re gonna let the smoke out of the transformer…then you’re really hosed.

Second big question; what caused it to blow in the first place? Was it the 2 dampers you added to a transformer that already had 3 connected to it? Are 5 dampers too much for a 75va transformer?

So many questions, so many uncertainties! If only you knew the max amp draw of the transformer, or how many amps the dampers will draw.

The Solution

Divide the transformer VA rating by the secondary voltage.

The Answer

Example; the transformer is 24 volt secondary voltage, rated at 75va.

75va ÷ 24vac = 3.12 amps

Now you add up the loads.

Example; 5 damper actuators rated at 5 watts each= 25watts, or 25va.

Obviously 25va is well below the transformers rated 75va, but what is that in amps?

25va ÷ 24volts = 1.04 amps (all actuators were stroking at the same time)

The answer- the transformer is big enough, but now you have to find the short that blew the fuse.

Also, the fuse you found is too big, you need a smaller fuse.

We never said it was going to be a happy ending…

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Patrick is Zen HVAC’s diagnostic and training guy.

Patrick started in the trade the day he left technical school and never looked back.

He’s served in various technical and training roles in the HVAC industry but specializes in system troubleshooting and diagnostics, retro commissioning, and technical training.

His moto: If I can understand it, anybody can.

Patrick uses the Zen common sense approach to teach

Patrick’s Likes- His Wife, kids and dog. Old pickup trucks. Hiking. The industrial Revolution.

Patrick’s Dislikes- Taking work too seriously. Anything unintuitive. Emoticons :(

Patrick’s Favorite famous person- Theodore Roosevelt “I am only an average man, But I work harder at it than the average man”

Famous Patrick Quote- “Well, that was stupid of me”

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