CT Calculation Helper

Shock Hazard: Unburdened CT's should never be handled with primary current applied!

Enter the information below to calculate the burden resistor required for Current Transformers that do NOT contain a built-in burden resistor.

Types of CTs

Current Transformers differ in the following ways:

Physical size: the size of conductor that they can encompass
Current rating: the maximum primary current they can sense
Core configuration:
- Solid-core: requires the primary conductor to be disconnected for installation
- Split-core: core can be opened for installation so that the primary conductor doesn't require disconnection
Electrical configuration:
- Voltage mode: produces an AC voltage that is proportional to the primary current
  - Example: 0.333V at rated current: this is the type of CT sold by Z3 Controls
- Current mode: produces an AC current that is proportional to the primary current
  - Example 1: 5A CTs produce 5A at the rated current
  - Example 2: CTs that are sold as a number of turns on the secondary winding
  - Current mode CTs require a burden resistor to be used with the NetMeter

Warning: the use of current mode CTs with the NetMeter can damage it. An appropriate burden resistor MUST be used with these CTs in order to be able to use them with the NetMeter.

Damage to the NetMeter caused by the use of current mode CTs is not covered under warrantee.

Do I need a burden resistor?

Some CT's contain a built-in burden resistor. If the CT is rated in Volts per Amp (for example 0.333V at 100A) then it already contains a built-in burden resistor.

If a CT is rated as a turns ratio, then it likely does NOT contain a burden resistor. The calculator below can be used to determine the desired burden resistor value.

If a CT is rated as a current ratio, then it does NOT contain a burden resistor.

5Amp CTs

A common CT type is 5A CTs that produce 5A at the rated primary current. 5A CTs can be very dangerous to work with when primary current is applied.

Option 1: Cascade a 5A to 0.333V CT

If existing 5A CTs are already in-circuit, it is easy to place a 5A to 0.333V CT onto the secondary of the original 5A CT. This is the safest way to attach.

Option 2: Burden Resistor

For 5A CTs to produce 0.333V at the rated current, the following burden resistor is required:

0.0666 Ohm
at least 1.665 Watt power handling

If this exact value is not available, then a slightly higher/lower value may be used. The voltage at rated current will be:

Vct = 5/R

Due to the fact that the R value is so low, wire resistance can be a factor. Consequently, the voltage leads should be placed very close to the resistor body.

CT Operation

A CT encloses a primary current (Ip) and produces a secondary current (Is) through the secondary winding of N turns (the turns ratio).

The voltage produced across the burden resistor (R) is given by the accompanying equation.

CT Calculator

The following calculator can be used to determine a suitable burden resistor for use with the Z3 NetMeter: