Why size for both sealed and inrush VA?

A control transformer must carry the continuous sealed load forever, but it must also survive the brief inrush when a large contactor energizes without the voltage sagging so far that other devices drop out. Sizing only on sealed VA can leave the panel unable to pull in a big contactor.

What is the difference between sealed and inrush VA?

Inrush (pull-in) VA is the high burden drawn for the first few cycles as a coil's magnetic field builds; sealed (holding) VA is the much smaller steady draw once it is energized. A contactor might draw 600 VA inrush but only 25 VA sealed.

Why does acceptable voltage dip change the size?

During inrush the transformer voltage sags. If you can only tolerate a small dip (say 5% for electronic loads), you need a larger transformer with stiffer regulation. If 15% dip is fine because only contactors are connected, a smaller unit works.

What are the standard control transformer sizes?

Common VA ratings are 50, 75, 100, 150, 200, 250, 350, 500, 750, 1000, 1500, 2000, and 3000 VA. The tool always rounds your calculated requirement up to the next standard size so you can specify a real part.

Should I add spare capacity?

Yes — leave at least 25% headroom for future additions and for the fact that coils age and panels grow. The calculator gives the minimum; specifying the next size up is common practice.

Control Transformer Sizing Calculator

The control power transformer (CPT) steps line voltage down to the control voltage (often 120 V or 24 V) for a panel’s relays, contactors, pilot lights, and PLC. Undersize it and a big contactor will not pull in; oversize it and you waste money and panel space. This calculator sizes the CPT for both the steady load and the inrush surge.

How it works

There are two sizing checks; the larger wins.

Steady-state (sealed) load. Add the holding VA of everything the transformer feeds:

sealed VA = relays/contactors + pilot lights + PLC and power supply

Inrush check. At the worst instant, one device is energizing (drawing its high inrush VA) while the rest of the panel is already sealed in:

inrush burden = sealed VA + largest single inrush VA

A transformer can deliver several times its nameplate VA briefly, but the voltage sags while it does. The allowable dip sets how much inrush headroom you need — a tighter dip (for electronics) demands a larger transformer than a loose dip (for contactors only). The tool converts the inrush burden into a required nameplate using a capability factor that depends on your dip limit, then rounds both results up to the next standard size.

Worked example

A motor control panel has 60 VA of sealed contactors, 10 VA of pilot lights, and a 50 VA PLC. The largest contactor draws 600 VA inrush, and you can tolerate a 5% dip.

Sealed load: 60 + 10 + 50 = 120 VA → next standard size 150 VA.
Inrush burden: 120 + 600 = 720 VA; at a 5% dip this needs a nameplate of roughly 720 / 5 ≈ 144 VA → 150 VA.

Both checks land on 150 VA, so a 150 VA CPT is the minimum — specify 250 VA for comfortable headroom.

Notes and tips

Get coil data from the contactor manufacturer; the ratio of inrush to sealed VA can be 10:1 or higher on large contactors.
If the panel feeds anything that does not like voltage dips (electronic relays, small drives, the PLC supply), use the 5% or 3% dip setting.
Always leave spare capacity. Standard sizes step up quickly, so the next size often costs little more.

Every calculation runs locally in your browser. No data is sent to any server.