How to Wire a Transformer

Identify the windings first

H terminals are the high voltage winding, X terminals the low voltage winding. That never changes. If you are stepping down, the supply lands on H and the load leaves from X. Get this backwards and you will either destroy the unit or produce a voltage nobody expected, because the turns ratio works in whichever direction you drive it.

What happens if you wire one backwards

You step up instead of down. Feed 480 volts into the X terminals of a 480 to 120 volt transformer and the H side produces roughly 1,920 volts. That is lethal and it will usually destroy the insulation immediately. Reverse feeding as a deliberate design choice is a different matter, done knowingly with a unit rated for it, but doing it by accident is one of the more dangerous mistakes in the trade.

Sizing the primary conductors

Size the primary from the primary current, not the secondary. Primary current is the kVA divided by the primary voltage, times the square root of three on three phase. A 75 kVA unit at 480 volts draws about 90 amps on the primary and delivers about 208 amps at 208 volts on the secondary. Sizing the primary feeder for 208 amps wastes money, and sizing the secondary for 90 amps is dangerous.

Checks before you energise

Confirm the tap position matches the actual supply voltage rather than the nominal. Confirm the neutral to ground bond exists at one point and only one point. Confirm phase rotation if anything downstream is a motor. Megger the windings if the unit has been in storage or outdoors. On a medium voltage pad mount, none of this is a job for anyone who is not qualified and authorised by the utility.

What happens if I wire a transformer backwards

It steps up instead of down. Feeding 480 volts into the low voltage side of a 480 to 120 volt transformer produces roughly 1,920 volts on the other side, which is lethal and will normally destroy the insulation. Deliberate reverse feeding is a different matter and needs a unit rated for it.

How do I size the primary wire for a transformer

From the primary current, which is kVA divided by the primary voltage, and divided again by the square root of three on a three phase unit. A 75 kVA transformer at 480 volts draws about 90 amps on the primary. Sizing it from the secondary current will oversize it badly.

Which side of a transformer do I connect the supply to

The H terminals for a step down, which are the high voltage winding. X terminals are the low voltage side and feed the load. The nameplate states which is which.

Can you connect a transformer to an outlet

Only a small plug connected unit designed for it. A distribution transformer is hard wired, and a pad mounted unit is connected by a utility through medium voltage cable and loadbreak elbows. It is not equipment that plugs in.