How the current transformer burden calculator works
A current transformer reproduces the primary current at a smaller secondary current, 1 A or 5 A, and drives it through whatever is connected: the cable to the panel and the meters, relays or transducers at the end. That circuit is the burden, stated in volt-amperes at rated secondary current. A CT keeps its accuracy class only while the burden stays within its rated range.
The calculator adds the burden of the two cable conductors, I² × 2 ρ L / A with copper’s resistance at the conductor temperature, to the devices’ burden, and compares the total with the CT’s rated burden. It also gives the longest cable for the rated burden and the accuracy limit factor under the actual burden.
Why cable is the usual problem
At 5 A every ohm in the secondary circuit costs 25 VA. Twenty metres of 2.5 mm² copper there and back is 0.275 Ω, or 6.9 VA, more than a typical 5 VA CT can carry before any meter is connected. With a 1 A secondary the same cable is 0.28 VA. This is why CTs with long cable runs, from switchgear to a remote panel, normally use 1 A secondaries, and why 5 A secondaries stay close to their meters.
The 25 to 100 % range
IEC 61869-2 specifies the ratio and phase error of measuring classes such as 0.2, 0.5 and 1 for secondary burdens between 25 % and 100 % of the rated burden. Above it the CT reads low and saturates earlier; below it the error is not covered by the class and may exceed it. Modern electronic meters take a fraction of a volt-ampere, so an oversized rated burden can leave a CT below 25 % with the cable alone. The datasheet states the range a particular CT is specified for.
Accuracy limit and security factor
Protection CTs have an accuracy limit factor, ALF, the multiple of rated current up to which the composite error stays within the class. Measuring CTs have an instrument security factor, FS, which limits how much current reaches the meter during a fault. Both depend on the actual burden: the usual approximation is ALF’ = ALF × (Rct + Rrated) / (Rct + Ractual), where Rct is the secondary winding resistance. A light burden raises the factor, which helps a protection core but lets more fault current through to a meter.
For the voltage drop on cables powering field sensors, use the sensor supply voltage drop calculator; for measuring current with a resistor instead, the current shunt calculator.
Frequently asked questions
Can I add VA and ohms directly?
Convert first: R = VA / I². The calculator treats cable and devices as resistive and adds them arithmetically, which overstates the total slightly for inductive devices. That errs on the safe side.
Must the secondary never be opened?
Yes. An open secondary under load drives the CT into saturation and can produce dangerous voltages. Short the secondary terminals before disconnecting a meter.
Does the conductor temperature matter?
Copper’s resistance rises by 0.393 % per kelvin. A cable at 60 °C has 16 % more resistance than at 20 °C, and the same rise in cable burden.