How Do I Size a Control Transformer for DC Inrush?

The panel powers up, the 120 VAC control bus dips, contactors chatter or fail to pull in, and sensitive instruments reset or hang. Start here: separate continuous load from short-duration inrush, then decide which loads can occur at the same time. A 25 A, 8 ms input pulse is not a 3,000 VA continuous load, but ignoring it can still produce an unacceptable secondary-voltage dip.

Reject the fixes that waste time

Several shortcuts produce either an oversized transformer or a panel that fails during switching.

• Do not multiply every electronic load by an arbitrary inrush factor. A generic 2-to-5-times or 20-times multiplier discards the stated 25 A and 8 ms limits. Use the published maximum pulse and the transformer manufacturer's short-duration selection data.
• Do not size the transformer at 3,000 VA continuous. For the 1764-24AWA, 120 V multiplied by 25 A equals 3,000 VA at the specified maximum-inrush operating point. That arithmetic describes instantaneous input volt-amperes, not a 3,000 VA nameplate requirement.
• Do not rely on 10% spare capacity as an inrush calculation. Ten percent can be a working allowance above the established steady load, but it does not predict the voltage dip caused by a high, brief pulse.
• Do not assume startup inrush is harmless. A contactor that is still de-energized may not care about the first dip, but an instrument already powered from the same transformer can reset or hang. A power supply that is switched during production creates a different operating case from one energized only with the panel.
• Do not install a massive shared transformer before checking segregation. A separate source for sensitive electronics can solve voltage-dip and transient problems more directly than increasing the shared transformer's rating.

Separate continuous VA from pulse demand

The panel has 480 VAC three-phase incoming power, but the control-transformer secondary is 120 VAC single-phase. Size that secondary with the single-phase relationship:

VA = V × I

The 100-C09 contactor has 8 VA sealed demand and 70 VA inrush demand. Its 70 VA requirement applies during coil pull-in; its 8 VA requirement remains after the armature closes. Repeated contactor operation makes both values relevant to normal machine operation.

The 1764-24AWA has 70 VA power consumption and a maximum power-supply inrush current of 25 A for 8 ms. At a nominal 120 V:

120 V × 25 A = 3,000 VA

Keep the result labeled as a short input pulse. Do not convert it to RMS current, average VA, or an equivalent continuous load. That conversion would require the current waveform, off-state current, and repetition period. None of those quantities is supplied.

The transformer's winding resistance, leakage impedance, upstream source impedance, and secondary conductors limit the actual pulse. The same impedance produces the visible voltage sag. The design question is therefore not whether the transformer can deliver an ideal 25 A indefinitely; it is whether its secondary stays above every connected load's acceptable voltage during the 8 ms event.

Match each symptom to the active load

Build the simultaneous-load cases

Sequence changes which inrush values belong in the same case. Document actual switching states instead of adding every maximum indiscriminately.

• Panel cold start: Include the 70 VA continuous demand of the 1764-24AWA and its 25 A for 8 ms startup pulse. Add other electronic inputs that energize from the same secondary at that instant. Add contactor inrush only if a contactor is commanded during that same interval.
• Normal contactor operation: Add 70 VA for each 100-C09 pulling in at that moment, 8 VA for each already sealed 100-C09, and the operating demand of the DC-fed system and other 120 VAC loads.
• Switched accessory operation: Include the accessory's stated input inrush whenever it can be plugged in or switched during operation. The fact that it resembles a small adapter does not remove its charging pulse.
• Restart after a brief outage: Determine which devices re-energize together. If logic or relays immediately command contactors while electronic supplies restart, this can be more demanding than an orderly cold start.

Sequencing reduces transformer stress only when the events are actually separated. Confirm that separation under power restoration, manual operation, and fault recovery. Do not credit an undocumented operator habit or an assumed controller delay.

Size the transformer with a voltage-dip limit

1. Total the continuous load. Add the stated running or sealed VA of every load that can remain energized. For the cited devices, use 70 VA for the 1764-24AWA and 8 VA for each sealed 100-C09.
2. Create one row for each switching case. Record which loads are steady, which contactor coils are pulling in, and which electronic input is starting.
3. Keep pulse duration attached to pulse magnitude. Record the 1764-24AWA event as 25 A for 8 ms at the 120 VAC input. Do not replace it with 3,000 VA continuous.
4. Set the allowable secondary-voltage minimum. Use the strictest connected device's published input range, ride-through behavior, and contactor pickup/dropout requirements. About 85% of nominal is a useful field screening point for possible 120 V contactor dropout or chatter, not a substitute for the actual coil data.
5. Select from transformer performance data. Check the candidate transformer's regulation or inrush-selection information at the required continuous VA and short-duration load. If the manufacturer presents separate tables or curves for control-device inrush, use the case that matches the pulse and permitted voltage dip.
6. Check conductors and protection. Excessive secondary wiring impedance can create a local dip even when the transformer is adequate. Select protection from the transformer's documented application requirements and applicable panel rules; do not enlarge protection merely to conceal repeated trips.
7. Add an engineering allowance after the calculation. A modest spare allowance, such as the cited 10%, can cover expected continuous additions. It cannot replace the pulse and voltage-dip checks.

Choose between sharing and segregation

Use one transformer when measured voltage remains acceptable through every switching case and the connected electronics tolerate the control-bus environment. This keeps the architecture simple and avoids adding hardware solely because the arithmetic produced 3,000 instantaneous VA.

Separate the 24 VDC power supply when sensitive instruments reset, hang, or lose data while large contactors switch. Segregation also prevents a DC-supply startup pulse from pulling down the AC coil bus and reduces exposure of instrumentation to contactor-generated transients. Size the separate transformer for the power supply's continuous demand and its own startup pulse using the same method.

A larger shared transformer remains a valid choice when simultaneous operation is unavoidable and the voltage-dip calculation supports it. Compare that solution with a separate source on secondary-voltage performance, panel space, protection, wiring, and future load growth—not on nameplate VA alone.

Verify the result at the panel

1. Measure steady conditions first. Record primary voltage, transformer secondary voltage, and secondary current with all normal sealed and electronic loads operating.
2. Capture the minimum 120 VAC voltage. Use an instrument with sufficient transient capture capability across the secondary while energizing the panel. A slow display may miss an 8 ms dip.
3. Repeat every credible sequence. Test cold start, individual and simultaneous contactor pull-in, switched accessories, and restoration after a brief loss of supply.
4. Measure at the affected device. A good reading at the transformer does not rule out conductor or terminal voltage drop between the transformer and the load.
5. Watch device behavior. Check for coil chatter, failed pickup, PLC restart, instrument reset, communication loss, or a supply status change. Compare the captured minimum voltage with each device's published limits.
6. Retest after segregation or resizing. The fix passes only when the worst sequence produces acceptable voltage and no connected device changes state unexpectedly.

If the initial measurement shows a steady low secondary voltage, correct the continuous loading, tap, source, or wiring problem first. That is not an 8 ms inrush fault.

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