How To Calculate Current Transformer Burden
Burden is the single specification most often got wrong, and it is the one that quietly pushes a Class 0.5 transformer outside Class 0.5.
What Burden Actually Means
Burden is the apparent power, in VA, that the secondary circuit draws at rated secondary current. It is not the load on the primary. A current transformer is a current source: it will drive whatever voltage it needs to push its rated secondary current through the connected impedance, and the point at which it can no longer do so is where the ratio error runs away.
The Three Contributions
Add the VA of the instruments and relays in the loop, the loss in the secondary wiring, and the resistance of the terminations. On a 5 A secondary the cable loss dominates surprisingly quickly, because the loss goes as the square of the secondary current.
Instrument burden
Taken from the instrument datasheet, in VA at the rated secondary current. Modern electronic meters are often below 0.5 VA; electromechanical instruments and relays are considerably higher.
Cable burden
Cable burden in VA equals I² × R, where I is the rated secondary current and R is the total loop resistance of the run out and back. At 5 A, one ohm of loop resistance is 25 VA. At 1 A the same ohm is 1 VA, which is exactly why long runs are specified with a 1 A secondary.
Worked Example
A 400/5 A CA-62/30 feeds a digital meter of 0.5 VA through 30 m of 2.5 mm² cable, run out and back, so 60 m of conductor. At roughly 0.0074 Ω per metre that is about 0.45 Ω, and 5² × 0.45 gives about 11 VA of cable burden. Add the meter and the total is around 11.5 VA, so a 15 VA rated CT is the correct selection and a 10 VA unit would be under specified.
What Happens If You Get It Wrong
Over burdening pushes the transformer past the voltage it can develop, and the ratio and phase errors leave the declared class. Under burdening is safe for accuracy, but a CT running far below its rated burden is simply an over specified purchase. Never leave the secondary open circuit: an unloaded current transformer on a live primary develops a dangerous voltage.
Practical Rules
- Specify the next standard burden above your calculated total, not several steps above
- Use a 1 A secondary when the run between CT and instrument is long
- Count every device in the loop, including test blocks and transducers
- Fit short circuit links, as the DX double terminal entry body allows, before disconnecting an instrument
Products Referenced In This Guide
ABO Current Transformer
Primary current 5 A to 5000 A
Secondary current 5 A, 1 A, 0.5 A, 0.25 A, 0.1 A
Still Not Sure Which Part To Specify
Describe the circuit and the instrument. An engineer will name the frame.