Contactor Coil VA Calculator

A contactor coil VA calculation converts coil current into the apparent-power demand placed on the control circuit. The result is useful when reviewing a control transformer, power supply, relay output, fused control circuit, or multiple-coil load that must carry both normal sealed-coil demand and short-duration pickup demand.

The calculator produces four values:

• Steady-state VA — coil demand after the contactor has picked up and sealed.
• Inrush VA — coil demand during pickup, using the entered inrush current.
• Inrush-to-steady ratio — the relative increase from sealed-coil VA to pickup VA.
• Inrush excess — the additional VA required during pickup beyond the steady-state demand.

For a control-power review, the inrush VA is normally the number that exposes a pickup-capacity problem. A transformer or supply may carry the sealed coil load continuously but still allow a contactor to chatter, fail to pull in, or drop out if control voltage falls excessively during pickup.

Coil Load Inputs

Enter values from the contactor nameplate, manufacturer data, commissioning records, or an appropriate measurement.

Input	Unit	Electrical use
Coil voltage	V	The coil voltage used for both VA calculations
Steady-state current	A	Coil current after pickup or sealing
Inrush current	A	Pickup current, when known

Coil voltage must match the voltage applied to the coil during the current condition being evaluated. A 24 V control coil should be calculated using 24 V, not the nominal voltage of the branch circuit or feeder supplying the control transformer.

Steady-state current represents the coil load after the armature has closed. This is the continuous demand used when adding up the normal control-load burden of contactors, relays, solenoids, and similar control devices.

Inrush current represents the higher current required while the magnetic circuit is closing. It is especially relevant for AC contactor coils, where pickup demand can be substantially greater than sealed demand. If pickup current is not known, the worksheet cannot establish a valid inrush VA result from steady-state current alone.

VA Calculation

The calculator applies direct voltage-current arithmetic to each entered current value:

\(\displaystyle \text{steady VA} = \text{coil voltage V} \times \text{steady-state current A}\)

\(\displaystyle \text{inrush VA} = \text{coil voltage V} \times \text{inrush current A}\)

\(\displaystyle \text{inrush ratio} = \frac{\text{inrush VA}}{\text{steady VA}}\)

\(\displaystyle \text{inrush excess VA} = \text{inrush VA} - \text{steady VA}\)

The VA result is apparent power based on the entered voltage and current. It is not a calculation of coil watts, power factor, heat dissipation, conductor ampacity, voltage drop, transformer regulation, or protective-device sizing.

Calculation Example

A 24 V contactor coil has a documented sealed current of 0.2 A and pickup current of 1.5 A.

Field	Entered value
Coil voltage	24 V
Steady-state current	0.2 A
Inrush current	1.5 A

The resulting coil demand is:

\(\displaystyle \text{steady VA} = 24 \times 0.2 = \mathbf{4.8 VA}\)

\(\displaystyle \text{inrush VA} = 24 \times 1.5 = \mathbf{36 VA}\)

\(\displaystyle \text{inrush-to-steady ratio} = \frac{36}{4.8} = \mathbf{7.5 x}\)

\(\displaystyle \text{inrush excess} = 36 - 4.8 = \mathbf{31.2 VA}\)

Result	Value	Interpretation
Steady-state VA	4.8 VA	Normal sealed-coil demand
Inrush VA	36 VA	Pickup demand for this coil
Inrush-to-steady ratio	7.5 x	Pickup VA is 7.5 times sealed VA
Inrush excess	31.2 VA	Additional short-duration VA above sealed demand
Inrush comparison	Inrush VA exceeds steady-state VA	Pickup is the controlling demand for a basic control-power review

A panel containing several coils can have a modest total sealed VA while imposing a much larger momentary demand if multiple contactors energize at the same time. Sequential starting logic, interposing relays, and control-transformer capacity are often reviewed with that simultaneous pickup condition in mind.

Control-Power Application

Use the calculated values as an initial load entry when reviewing control power:

• Add steady-state VA for coils expected to remain energized together under normal operating conditions.
• Review inrush VA for coils that may pick up simultaneously.
• Compare the expected pickup demand with the capacity and performance characteristics of the control transformer or DC power supply.
• Investigate excessive control-voltage drop when a coil has high inrush VA, long control wiring, undersized control conductors, weak terminals, or multiple devices pulling in at once.
• Use manufacturer coil data when selecting or validating contactors, control transformers, and power supplies.

Control-circuit conductors may be small AWG sizes compared with branch-circuit or feeder conductors, but their resistance can still contribute to voltage drop during coil pickup. A coil that sees inadequate terminal voltage may hum, chatter, fail to seal, or release unexpectedly. The calculator identifies the electrical load represented by the entered current; it does not calculate voltage drop through the control conductors, terminals, raceway routing, or source impedance.

Field Verification

The calculation is arithmetic screening based on the supplied voltage and current values. Verify the actual coil type and operating conditions separately:

• Confirm whether the coil is AC or DC and use device-specific manufacturer data where available.
• Verify pickup timing and whether several coils can energize simultaneously.
• Check suppression devices, including coil diodes, RC networks, MOVs, and any effect they may have on circuit operation.
• Confirm control-transformer or power-supply capacity, available output voltage, and voltage regulation under pickup load.
• Measure control voltage at the coil terminals during operation when chatter, dropout, delayed pull-in, or overheating is suspected.
• Verify applicable equipment instructions, control-circuit design requirements, and AHJ requirements separately from the VA arithmetic.

Do not use steady-state VA alone to conclude that a control transformer or power supply will reliably pick up a contactor. The inrush VA result represents the higher coil demand that must be evaluated against the actual control-power source and installation conditions.