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SLDResources / SLD symbolsUpdated 29 Oct 2026

Transformer symbols for single line diagrams

Power transformers change the voltage between two parts of the system. Instrument transformers, current transformers (CTs) and voltage transformers (VTs), step current and voltage down for meters and protective relays.

SymbolANSI / IEEEIEC 60617What it is
TransformerS00841A power transformer changes voltage between two parts of the system, for example 11 kV to 690 V.
Three-winding transformerS00844A transformer with three windings, for example a primary and two secondaries at different voltages, or a delta tertiary for stability.
Current transformer (CT)S00850A current transformer steps the line current down to a small secondary current (1 A or 5 A) for meters and protective relays.
Voltage transformer (VT)S00841A voltage transformer steps the system voltage down, usually to 110 V or 120 V, for meters, synchronising and voltage relays.

Transformers: ANSI and IEC

The main difference between the standards is the power transformer. IEC draws two overlapping circles (S00841). US one-lines draw either two coils across a core or the same two circles. Instrument transformers differ more: IEC threads the conductor through a circle for a CT (S00850), while US drawings use a loop beside the conductor with a polarity dot.

Every CT on the drawing feeds a relay or a meter, so its ratio and accuracy class sit next to it. Reading them is the first step in checking that the protection sees what it should.

Frequently asked questions

Which transformers appear on a single line diagram?

This legend covers transformer, three-winding transformer, current transformer (CT), voltage transformer (VT). Project legends may add more.

Are these symbols different in IEC and ANSI?

3 of the 4 are drawn differently. Each symbol page shows both forms side by side.

Can I download these symbols?

Yes. Each symbol page has free SVG files, and the hub has printable PDF charts.

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