We do not supply these. This page is here because the question comes up and the answer is hard to find written plainly. What we do supply is three phase and single phase pad mounted distribution transformers from 25 to 2500 kVA.
What it is for
Machine control circuits run at 120 volts rather than the 480 volts feeding the machine, because 120 volts is safer to work on and because contactor coils, relays, pilot lights and PLC power supplies are built for it. The control transformer is what makes that 120 volt circuit, and it usually lives inside the panel it serves.
Inrush is what sizes it
A contactor coil draws several times its holding current at the instant it picks up. A control transformer sized only for steady state load will sag on that inrush and the contactor will chatter or fail to pull in. Control transformers are therefore rated with an inrush VA figure alongside the continuous VA, and the inrush figure is usually the one that governs.
Not the same as a 480 to 120 distribution transformer
A common and expensive mistake. If you need 120 volt receptacles and lighting through a building you want a 480 to 208Y/120 three phase transformer, which gives 120 volts line to neutral on all three phases. A control transformer gives one small single phase circuit sized in hundreds of VA. Ordering the second when you meant the first wastes a delivery.
Common questions
What is a control transformer used for
Producing a 120 volt control circuit from a higher voltage supply, normally 480 volts, so contactor coils, relays, pilot lights and control electronics can run at a safer and more standard voltage.
How do you size a control transformer
By inrush, not steady load. Contactor coils draw several times their holding current when they pick up, so the transformer must hold voltage through that surge. Manufacturers publish both a continuous VA and an inrush VA, and the inrush figure usually decides the size.
What is the difference between a control transformer and a 480 to 120 distribution transformer
Capacity and purpose. A control transformer is a few hundred VA feeding one circuit inside a panel. For 120 volt power through a building you need a 480 to 208Y/120 transformer, which gives 120 volts line to neutral across three phases.
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