Current transformers (CTs) are sensors used to measure alternating current. They are particularly useful for measuring whole building electricity consumption (or generation for that matter).

Like any other transformer, a current transformer has a primary winding, a magnetic core, and a secondary winding.

In the case of whole building monitoring, the primary is the live or the neutral wire (not both!) coming into the building itself, and is passed through the hole in the CT. The secondary winding is made of many turns of fine wire housed within the transformer casing.

The alternating current flowing in the primary produces a magnetic field in the core, which then induces a current in the secondary winding circuit.

The current in the secondary winding is proportional to the current flowing in the primary winding:

Isecondary = CTturnsRatio x Iprimary
CTturnsRatio = NoTurnsPrimary / NoTurnsSecondary

If the number of secondary turns in the CT is 2500, so the current in the secondary is one 2500th of the current in the primary.

Normally this ratio would be written in terms of currents e.g. 100:4 (for a 4A meter scaled 0 – 100A). The CT above would normally be written as 100:0.04.

Burden resistor

A current output CT needs to be used in conjunction with a burden resistor. A burden resistor completes (closes) the CT secondary circuit. The burden value is chosen such that it provides a voltage proportional to the secondary current. The burden value needs to be low enough to prevent core saturation.


The secondary circuit is galvanically isolated from the primary circuit. (i.e. has no metallic contact)


In general a CT must never be open-circuited once installed. A CT is potentially dangerous if open-circuited.

If open-circuited with current flowing in the primary the transformer secondary will attempt to continue driving current into what is effectively an infinite impedance. This will produce a high and potentially dangerous voltage across the open secondary.

Some CT’s have built-in protection. If the CT is a voltage-output type, it will have a built in burden resistor and so cannot be open-circuited.

Installing a CT

The primary winding of the CT is the wire carrying the current you want to measure. If you clip your CT around a two or three core cable that has wires carrying the same current but in opposite directions, the magnetic fields created by the two wires are equal and opposite and will cancel each other. Your CT will have no output.