Why does the (expletive Deleted) transformer fuse keep popping?!

I’ll bet you’ve changed more than a few 24vac transformer fuses in your career.

Sometimes, the cause was obvious…other times, not so much.

The first question when the fuse blows is “how many amps is this transformer good for?”

The second question is “How many amps should the circuit draw?”

Maybe you have a zoning system and the transformer is powering a bunch of zone dampers. How many dampers can the transformer handle?

Answer to the first question: How to calculate rated transformer amps

Divide the transformer’s VA rating by the secondary voltage.

Example; the transformer secondary is 24 volts rated at 75va.

Question; How many amps can it handle?

75va÷24vac = 3.12 amps

Answer to the second question: How to calculate how many amps the circuit can draw?

First, add up the loads. If they have a va or watt rating (va and watts are kinda the same thing) it’s easy; add up the individual load’s watt ratings and divide by 24 (sound familiar?).

Example; 10 damper actuators. Each one is rated at 0.5 watts.

10 actuators X .5 watts each = 5 watts. Obviously 5 watts is way below the transformer’s rated 75va.

So, for troubleshooting purposes, what amp draw should we see?

5va ÷ 24volts = .20 amps if all actuators are running at the same time.

In this case, if you're drawing more than .20 amps,there's a problem.


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