Tools · Transformer Sizing

Control Transformer Sizing Calculator Electrical

“Does this transformer have room for one more actuator?”

The most-retrofits question with the least paperwork. List what’s hanging on the 24 VAC secondary, pick the transformer, and read the percent loaded, the headroom, and a secondary fuse suggestion.

Inputs are seeded with an example — edit them to your numbers.

Devices on the secondary

Transformer

Rows with no VA are skipped — leave the spares blank. Get the VA from the device label or datasheet, not from memory; the reference table below is for sanity-checking, not for filling in.

Output

Enter device loads to see the verdict.

Total connected load —

Percent loaded —

Headroom —

Suggested secondary fuse —

Worked example

The seeded panel: a controller (14 VA), two damper actuators (7 VA each), a valve actuator (10 VA), and an interposing relay (2 VA) on a 75 VA transformer.

Total: 14 + 7 + 7 + 10 + 2 = 40 VA.
Loading: 40 ÷ 75 = 53 % — 35 VA of headroom, comfortably under the 80 % planning line.
Fuse: rated secondary current is 75 ÷ 24 = 3.1 A; 3.1 × 1.25 = 3.9 → 4 A standard size.

The fuse is sized off the transformer rating, not the connected load — it protects the transformer and the wiring, and it shouldn’t need touching when you add a device later.

Typical 24 VAC loads — sanity-check table

Device	Typical VA	Notes
DDC controller	4 – 14	Grows with onboard I/O count.
Damper actuator, non-spring	2 – 5	Holding; more while driving.
Damper actuator, spring-return	5 – 10	Holds against the spring continuously.
Valve actuator	3 – 10	Size class matters more than type.
Interposing relay coil	1 – 2	Inrush ~5–10× for tens of ms.
Thermostat / sensor with display	1 – 3	Loop-powered transmitters draw from DC, not this budget.

Read the budget in two tiers. Above 80 % loaded, it could be a problem — fine on the bench, but spring-return actuators holding against their springs and relay pull-in draw more than the label’s resting number, and the next add-on device has nowhere to go. Above 100 %, it is a problem — the transformer runs hot, sags under inrush, and the panel reboots on the worst mornings.

This tally is steady-state — resting and holding VA. The other failure is transient: several spring-return actuators stroking at once on power-up, plus relay pull-in (inrush ~5–10× for tens of ms), can momentarily exceed the transformer’s VA rating even when the steady-state sum passes. The 80 % line doesn’t cover that coincident surge — so a panel that only reboots on cold mornings, when everything drives home together, is failing on the transient, not this number.

The fuse row is a starting point, not a spec. It applies the standard 125 % convention to the nameplate VA as a cross-check; the transformer manufacturer’s fusing table, the panel drawing, and the listing govern what actually gets installed. A protection device is the one place a sizing shortcut can cost the equipment it was meant to protect.

Transformer sizing FAQ

How do you size a 24 VAC control transformer?

Add up the sealed (holding) VA of every device on the secondary, then pick a standard transformer — 40, 75, 100 VA and up — whose rating leaves headroom, aiming to load it to no more than about 80%. Class 2 transformers are inherently current-limited.

How many actuators can one 24 V transformer power?

Divide the transformer's VA by each actuator's holding VA and keep ~20% headroom — a 40 VA transformer running 3 VA holding actuators handles roughly 40 × 0.8 ÷ 3 ≈ 10. Check inrush too, since spring-return actuators pull far more VA while driving than holding.

What is inrush versus holding VA?

Inrush is the momentary high draw when an actuator, relay, or contactor first energizes; holding (sealed) VA is the steady draw once it's positioned. Size the continuous load on holding VA, but confirm the transformer and secondary fuse tolerate the combined inrush.

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