Control Transformer VA Calculator

A control transformer VA calculator establishes the electrical burden that a control power transformer must support under normal energized conditions and during the highest simultaneous pickup event. The primary sizing outputs are Continuous VA with margin and Combined inrush VA.

These values support transformer selection and control-circuit review. The continuous VA result represents the expected sealed burden with an added planning allowance. The combined inrush VA result represents the calculated transformer demand when the maximum simultaneous inrush burden occurs with the sealed control load.

The resulting VA requirement is used when comparing available control transformer ratings, reviewing the practical capacity of a 120 V control circuit, and evaluating whether relay, contactor, solenoid, pilot-light, and other control-device loads can operate without an excessive secondary-voltage dip during pickup.

Load Inputs

The calculation uses four field inputs:

Input	Electrical use
Total sealed VA (VA)	The simultaneous steady-state burden after devices have picked up and remain energized
Maximum simultaneous inrush VA (VA)	The highest simultaneous pickup or inrush burden based on device data
Control voltage (V)	The transformer secondary voltage used to place sealed VA in secondary-current context
Planning margin (%)	Additional allowance applied to the continuous sealed VA burden

Total sealed VA is not necessarily the sum of every control component installed in a panel. It is the load expected to remain energized at the same time in the operating condition being evaluated.

Maximum simultaneous inrush VA also depends on the actual control sequence. A contactor coil that picks up only after another coil has completed its action may not belong in the same simultaneous-inrush total. Device data should distinguish sealed VA from pickup, inrush, or VA-at-pickup values.

The Control voltage (V) input does not change either VA burden in this calculation. It produces the steady-state secondary-current result, which is useful for control-conductor, terminal, and voltage-drop review.

VA Calculation

The calculator applies the planning margin to the sealed control burden:

\(\displaystyle \text{Continuous VA with margin} = \text{Total sealed VA} \times \left(1+\frac{\text{Planning margin}}{100}\right)\)

It combines sealed VA and maximum simultaneous inrush VA by root-sum-square calculation:

\(\displaystyle \text{Combined inrush VA} = \sqrt{ (\text{Total sealed VA})^2+ (\text{Maximum simultaneous inrush VA})^2 }\)

The calculator also reports the sealed-load secondary current:

\(\displaystyle \text{Steady-state secondary current} = \frac{\text{Total sealed VA}}{\text{Control voltage}}\)

The reported Margin multiplier is:

\(\displaystyle \text{Margin multiplier} = 1+\frac{\text{Planning margin}}{100}\)

The margin affects the continuous VA calculation. The combined inrush VA calculation uses Total sealed VA and Maximum simultaneous inrush VA as entered.

Calculation Example

For a 120 V control transformer secondary with these inputs:

Input	Value
Total sealed VA	80 VA
Maximum simultaneous inrush VA	250 VA
Control voltage	120 V
Planning margin	20%

The planning multiplier is:

\(\displaystyle 1+\frac{20}{100}=1.2\)

The continuous burden with margin is:

\(\displaystyle 80 \text{ VA} \times 1.2 = \text{96 VA}\)

The combined inrush burden is:

\(\displaystyle \sqrt{80^2+250^2} = \sqrt{68{,}900} = \text{262.4881 VA}\)

The steady-state secondary current is:

\(\displaystyle \frac{80 \text{ VA}}{120 \text{ V}} = \text{0.6667 A}\)

For this load condition, the transformer selection review must account for both 96 VA continuous capacity and 262.4881 VA combined inrush demand. A transformer that appears adequate based only on 80 VA sealed load may not provide acceptable pickup performance when the 250 VA simultaneous inrush condition occurs.

Secondary Current and Control Wiring

The Steady-state secondary current result provides a current reference for the control circuit. In the example, 80 VA at 120 V produces 0.6667 A of sealed secondary current.

That current can be used during a control-wiring review to evaluate practical conductor selection, terminal capacity, branch-circuit protection coordination, and voltage drop. Long control runs can experience enough voltage drop to affect coil operation, particularly when a device is attempting to pick up. The sealed current result alone does not represent the momentary current associated with the maximum simultaneous inrush VA condition.

Control conductors are commonly much smaller than feeder or motor branch-circuit conductors, but conductor selection is still an installation decision. Raceway fill, conductor insulation temperature rating, terminal rating, available ampacity, control-circuit overcurrent protection, and the installed routing can affect the final wiring method. AWG or kcmil conductor size is not produced by this VA calculation.

Field Verification

Use actual device information for every energized load included in the operating sequence. Coil VA can vary between pickup and sealed states, and one manufacturer’s contactor or relay data cannot be substituted for another device without confirming its ratings.

Verify these items separately from the calculator arithmetic:

• The transformer’s published continuous VA rating and its applicable inrush or regulation capability.
• The actual simultaneous operating sequence, including all contactors, relays, solenoids, control relays, timers, pilot devices, and other energized loads.
• The transformer primary voltage, secondary voltage, frequency, and any control-power configuration requirements.
• Secondary conductor ampacity, AWG size, insulation temperature rating, terminals, overcurrent protection, and voltage-drop conditions.
• Raceway fill and physical panel or enclosure layout when control wiring shares a raceway with other conductors.
• Applicable NEC requirements, equipment instructions, project specifications, and AHJ requirements.

The calculator provides load arithmetic from the entered VA values. Final control transformer selection must be based on the transformer manufacturer’s published performance data and the installed control-circuit conditions.

Related workflows: Transformer KVA Calculator and Motor Starter Calculator.