# Summation current transformer

Canonical URL: https://eloragrid.ai/symbols/summation-current-transformer
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: ΣCT
Drawing use: Schematic, SLD
Reference families: IEC, Manufacturer

Auxiliary CT with several primary windings that adds the secondary currents of other CTs into one output.

## In plain English

### What does Summation current transformer mean?

A summation CT has several primary windings, each fed from the secondary of a main CT, and one secondary winding that carries their sum. It lets one meter or relay see the total current of several feeders, or in some relay designs combines three phase currents into a single weighted quantity.

### A simple example

All main CTs feeding a summation CT must have the same ratio, and their polarity must be consistent, otherwise currents cancel instead of adding. The symbol shows the summing arrangement but the burden it adds to each main CT has to be checked separately.

### What the drawings help you do

Check that every feeding CT has the same ratio. Verify polarity of each primary winding. A summation CT adds currents before measurement; a summation energy meter adds energy after measurement.

### This particular reference

Auxiliary CT with several primary windings that adds the secondary currents of other CTs into one output. A summation CT adds currents before measurement; a summation energy meter adds energy after measurement.

### Terms

- Winding: A coil of conductor inside an electrical machine or transformer.
- Coil: A wound conductor that can produce a magnetic field when energised.
- Phase: One of the related AC waveforms in a power system.

Concept diagram: Identify the device and form → Read the connections and state → Check the equipment reference
A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.

- [Current transformer symbol variants](https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-40AAEF4A-AC3F-4B3D-90A5-A9E0707C1241.htm)
- [Indoor current and voltage transformers](https://www.abb.com/gb/en/areas/electrification/medium-voltage/grid-components/core-components/indoor-instrument-transformers)


## What it represents

A summation CT has several primary windings, each fed from the secondary of a main CT, and one secondary winding that carries their sum. It lets one meter or relay see the total current of several feeders, or in some relay designs combines three phase currents into a single weighted quantity.
All main CTs feeding a summation CT must have the same ratio, and their polarity must be consistent, otherwise currents cancel instead of adding. The symbol shows the summing arrangement but the burden it adds to each main CT has to be checked separately.

## How to read it

- Check that every feeding CT has the same ratio.
- Verify polarity of each primary winding.

## Information to show or reference

- List feeding CTs
- Show polarity marks
- Check burden on main CTs

## Important distinction

A summation CT adds currents before measurement; a summation energy meter adds energy after measurement.

## Facts

- Inputs: Secondary currents of several main CTs
- Output: Sum of input currents
- Preview: Independent educational concept; consult the applicable standard and project legend for the exact graphic

## Public references

- [Current transformer symbol variants](https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-40AAEF4A-AC3F-4B3D-90A5-A9E0707C1241.htm) — Autodesk. Public AutoCAD Electrical current transformer family used for context; this variant is independently drawn.
- [Indoor current and voltage transformers](https://www.abb.com/gb/en/areas/electrification/medium-voltage/grid-components/core-components/indoor-instrument-transformers) — ABB. Public manufacturer overview of indoor current and voltage instrument transformers.

## Graphic scope

Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.