Step Up and Step Down Transformers

There is no such thing as a step up transformer

A transformer has a high voltage winding and a low voltage winding. Feed the high side and it steps down. Feed the low side and it steps up. The nameplate says which winding the manufacturer expects you to energise, and everything else follows from that. When a supplier sells you a step up transformer they are selling a unit whose ratings and taps are arranged for that direction, not a different kind of device.

Can you reverse feed a standard transformer

Often, but not always, and it is not free. Taps sit on the high voltage winding, so reverse feeding puts the taps on your output where they no longer correct incoming voltage. Inrush current on a reverse fed unit is higher, and on smaller units the output voltage runs low because the design compensates for internal drop in one direction only. Below about 15 kVA it is usually fine. Above it, ask the manufacturer in writing.

What the buyer is usually actually asking

Most people searching for a step up transformer have 208 or 240 volts available and a machine that wants 480. Most people searching for step down have 480 and need 208Y/120 for receptacles and lighting. Both are ordinary distribution transformers. The unusual cases are 110 to 220 for imported appliances, which is a small single phase unit and not our product, and 480 to 400 for European machinery, which is.

The catch nobody mentions on step up

Stepping up means the low voltage side carries the current. A 150 kVA step up from 208 to 480 draws about 416 amps on the 208 side. That feeder, its breaker and the panel space it needs are frequently more expensive than the transformer. Check the existing service can carry it before ordering anything.

Can you step up 240 three phase to 480 three phase

Yes, with a 1 to 2 ratio transformer, commonly 15 to 150 kVA for this conversion. The 240 volt side carries twice the current of the 480 volt side, so size the feeder and protection on the 240 side to the full load current, not to the 480 side figure.

Can you step down 480 to 208

Yes, and it is the most common step down in North American commercial buildings. The ratio is 2.31 to 1 and the secondary is normally 208Y/120 so you get 120 volts line to neutral for receptacles and 208 volts line to line for larger single phase loads.

What is a 240 to 208 step down transformer

A small ratio unit used where equipment rated 208 volts is being fed from a 240 volt supply. Because the ratio is close to one, a buck boost transformer is often cheaper than a full isolation transformer for the same job, as long as isolation between windings is not required.

Is a step up transformer the same as a step down transformer

Physically, usually yes. The difference is which winding is energised and how the taps and ratings are arranged. A unit sold as step down can frequently be reverse fed, but taps then sit on the output and inrush is higher, so confirm with the manufacturer rather than assuming.