Control Transformer VA Load Calculator
Add up the thermostat, contactor, gas valve and relay loads that can be on at the same time and see how much of your 24 V transformer they use.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
Within rating
- Total connected load
- 20VA
- Share of transformer rating
- 50%
20 VA on a 40 VA transformer (50 %).
- Coils draw more current for a moment while they pull in. If a device lists an inrush VA, check that too.
- Secondary current
- 0.83A
- Rated secondary current
- 1.67A
- Spare capacity
- 20VA
Show the working
- Total = sum of the loads energized together5 + 7 + 8 + 0 + 0 = 20 VA
- Share of rating = total ÷ rating20 ÷ 40 = 50 %
- Secondary current = VA ÷ volts20 ÷ 24 = 0.833 A
VA, volts and amps
A control transformer is rated in volt-amperes (VA): its secondary voltage times the current it can supply. A 40 VA, 24 V transformer — a common size for residential HVAC controls — can deliver about 1.67 A.
Add the VA of every load that can be energized at once, from each device’s label or data sheet. If you only have a clamp-meter reading at a device, VA = amps × volts.
When the total is too high
An overloaded transformer runs hot and its voltage sags, which can make contactors chatter or relays drop out. Coils also draw more for a moment while they pull in, so a load close to the rating leaves little room. If a device lists an inrush VA, check that too.
Replacement or added transformers must follow the equipment manufacturer’s wiring diagram and the circuit’s Class 2 limits; the primary side is line voltage and is work for a qualified technician.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Transformer rating
- 40
- Secondary voltage
- 24 V
- Loads entered as
- VA from each device’s label or data sheet
- Load 1 (e.g. control board or thermostat)
- 5
- Load 2 (e.g. contactor coil)
- 7
- Load 3 (e.g. gas valve or relay)
- 8
- Load 4
- 0
- Load 5
- 0
Result
- Total connected load
- 20 VA
- Share of transformer rating
- 50 %
- Secondary current
- 0.83 A
- Rated secondary current
- 1.67 A
- Spare capacity
- 20 VA
20 VA on a 40 VA transformer (50 %).
Assumptions
- VA values add directly (worst case: all loads in phase).
- Only loads that can be energized at the same moment are entered.
Limitations
- Inrush current while coils pull in is not included.
- Transformer replacement must match the equipment manufacturer’s specification and the Class 2 limits of the circuit; work on line-voltage wiring requires a qualified technician.
Frequently asked questions
How many amps can a 40 VA transformer supply?
About 1.67 A at 24 V.
Do I add loads that never run together?
No. Add only the loads that can be energized at the same moment, such as a call for heat with the gas valve, inducer relay and thermostat all on.
Can I add a smart thermostat?
Check its VA rating and add it to the total. Most use only a few VA, but zone systems and accessories add up.
Sources
- University Physics Volume 2, §9.4 Ohm’s Law — OpenStax, Rice University (CC BY 4.0). V = IR; electrical power P = IV.
- Resideo AT140A1000 control transformer (distributor product listing) — Habegger Corporation. A Class 2 control transformer with a 24 VAC secondary and a 40 VA capacity for thermostats, relays, zone controls and gas valves; multi-tap 120, 208 or 240 VAC primary.
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