Transformer VA Budget 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.
Devices on the secondary
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
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 Astandard 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.