Reading a Transformer Nameplate

8 min · difficulty 3/10

A nameplate looks crowded, but four numbers carry most of the weight.

Annotated single-phase distribution transformer nameplate An annotated single-phase distribution transformer nameplate. Four fields are gold-highlighted as the most important: power rating 75 kVA, high-voltage 12470GrdY/7200, low-voltage 120/240, and impedance 1.8 percent. Supporting fields below the gold separator show BIL 95 kV, temperature rise 65 degrees C, mineral oil ONAN cooling, additive polarity, taps plus or minus 2 times 2.5 percent DETC, and connection HV GrdY with LV 120/240 3-wire. Callout leader lines on the right explain that kVA is rated power to divide by voltage for rated current giving 75000 divided by 240 approximately 312 A; HV VOLTS should match the distribution system; LV VOLTS should match the customer service; and impedance percent sets voltage sag under load and fault current, with lower percent meaning more fault current. DISTRIBUTION TRANSFORMER kVA 75 PHASE 1 Hz 60 HV VOLTS 12470GrdY/7200 LV VOLTS 120/240 IMPEDANCE 1.8 % Four fields that carry most of the weight BIL 95 kV TEMP RISE 65 °C INSULATING FLUID MINERAL OIL COOLING ONAN POLARITY ADDITIVE TAPS ±2 × 2.5 % (DETC) CONN HV GrdY · LV 120/240 3-wire Rated power — divide by voltage for rated current 75000 ÷ 240 ≈ 312 A Primary voltage — match to the distribution system Secondary voltage — match to the customer service Impedance % — sets voltage sag under load and fault current Lower % = more fault current
A real plate is crowded, but four fields carry most of the weight: the kVA rating, the high- and low-voltage ratings, and the percent impedance. Divide 75 kVA by the 240 V low side and you get the rated current, about 312 A. The rest — BIL, temperature rise, fluid, polarity, taps — supports those four.

The kVA rating is the continuous apparent power the unit can carry. Divide it by the rated voltage on a side to get the current that side can supply. That is usually the first thing a buyer checks against the load.

The primary and secondary voltages tell you where the unit sits and what it connects. Match these to the system on each side, including the way the windings are connected.

The impedance, given as a percent, is quieter but important. It sets how far the voltage sags when load is applied and how much current flows during a fault. A lower impedance lets more fault current through, which the protection downstream has to handle. When you compare two units that look alike, the impedance is often where they differ in ways that matter.

This is educational material, not engineering or procurement advice. Confirm any rating against the manufacturer documentation and a licensed professional before you specify equipment.

Nameplates and sizing decisions are the easy part; supplier capacity and lead time are what actually delay a transformer order. DistroForge report editions track transformer lead times, supplier capacity, and named-supplier scorecards for exactly this kind of procurement decision.

Question 1 of 2

What unit is a transformer's power rating given in? Give the abbreviation.

Educational material only. This is not engineering, safety, or procurement advice. Confirm any value against manufacturer documentation and a licensed professional before specifying equipment.