Instrument Transformer Burden Calculator

An instrument transformer burden calculation determines the voltage developed across a known resistive burden and the apparent burden imposed on a current-transformer secondary circuit. The primary outputs are Burden voltage in volts and Resistive burden in VA.

This calculation is used when reviewing a CT secondary circuit that includes a meter input, relay input, test switch, terminal blocks, secondary conductors, and other connected resistance. The resulting burden helps screen whether the connected circuit may be compatible with the transformer’s intended metering or protection duty before completing the manufacturer-specific accuracy, saturation, phase-angle, and protection review.

Enter Secondary current and Burden resistance to calculate the transformer burden.

Secondary Circuit Burden

A current transformer supplies a secondary current through its connected circuit. Any resistance in that circuit produces a voltage drop and consumes burden. For a purely resistive screen, the burden is calculated from the secondary current and total resistance entered.

The Secondary current field is the current through the burden, expressed in amperes. A nominal 5 A CT secondary is common in existing metering and relay applications, although the entered value should represent the actual secondary current being evaluated rather than an assumed nameplate value.

The Burden resistance field is the resistive burden used for the arithmetic screen, expressed in ohms. It can represent a known equivalent resistance for the connected secondary circuit or a selected resistive portion of that circuit.

Resistance may come from components such as:

• Meter or relay input circuits
• Test switches and terminal blocks
• CT secondary leads and splices
• Disconnecting links and connection hardware
• Purpose-built burden resistors

Long secondary conductor runs can add material resistance that affects both voltage drop and burden. Conductor size, conductor material, total loop length, terminations, and circuit routing should be evaluated separately when establishing the actual installed resistance.

Burden Voltage and VA

The calculator applies Ohm’s law to calculate Burden voltage:

\(\displaystyle \text{Burden voltage V} = \text{Secondary current A} \times \text{Burden resistance ohm}\)

It then calculates Resistive burden as apparent burden in volt-amperes:

\(\displaystyle \text{Burden VA} = (\text{Secondary current A})^2 \times \text{Burden resistance ohm}\)

For a resistive circuit, the VA result is numerically equivalent to real power in watts. In an actual CT secondary circuit containing inductive or capacitive device inputs, the circuit impedance and power factor can affect the burden relationship. The worksheet uses the entered resistance only.

Result	Electrical meaning	Practical use
Burden voltage	Voltage developed across the entered burden at the entered secondary current	Screens the CT secondary voltage requirement and indicates the effect of circuit resistance
Resistive burden	VA imposed by the entered resistance at the entered secondary current	Supports preliminary comparison with transformer burden capability or application requirements
Burden resistance used	The resistance value applied in the calculation	Confirms the resistance basis for the screen
Secondary current used	The current value applied in the calculation	Confirms whether the calculation was based on the intended secondary current

Calculation Example

Enter the following values:

Input	Entered value
Secondary current	5 A
Burden resistance	1 ohm

The burden voltage is:

\(\displaystyle V = 5\text{ A} \times 1\text{ ohm} = 5\text{ V}\)

The resistive burden is:

(displaystyle VA = 5^2 times 1 = 25text{ VA})

The calculated results are:

• Burden voltage: 5 V
• Resistive burden: 25 VA
• Burden resistance used: 1 ohm
• Secondary current used: 5 A

At 5 A secondary current, every additional ohm of purely resistive burden adds 5 V across the burden and adds 25 VA of burden. Because burden rises with the square of current, a change in secondary current can materially change the VA result even when resistance remains constant.

For example, the same 1-ohm resistive burden at 1 A would produce 1 V and 1 VA. At 5 A, it produces 5 V and 25 VA.

CT Application Limits

This calculation performs instrument-transformer burden arithmetic only. It does not determine whether a current transformer will meet its metering accuracy or protection performance requirements in the installed circuit.

Verify the following separately:

• CT or instrument-transformer burden class and manufacturer application data
• Secondary frequency and non-resistive circuit impedance
• Metering accuracy and phase-angle performance
• Protection relay duty, fault-current behavior, and saturation performance
• Secondary conductor resistance, loop length, splices, terminations, test switches, and device inputs
• CT ratio, rated secondary current, polarity, grounding arrangement, and wiring configuration
• Commissioning, insulation, continuity, polarity, and functional test procedures
• Project specifications, utility requirements, and AHJ requirements where applicable

Do not open-circuit an energized CT secondary. A current transformer can develop hazardous secondary voltage when its secondary circuit is open while primary current is flowing. Perform CT secondary work under the applicable electrical safety procedure and the equipment manufacturer’s instructions.