CT Burden Calculator (VA)
By Santaji GadepatilinUpdated October 28, 2026
A current transformer (CT) can only hold its accuracy while the load on its secondary — the burden — stays within its VA rating. Long leads and thin cable add more burden than people expect, especially with 5 A secondaries. This CT burden calculator adds up leads and devices; see CT burden calculation and the current transformer working principle.
How to use the CT burden calculator
Choose the CT secondary rating, enter the one-way lead length and cable size, and the burden of the connected devices from their datasheets. The calculator gives the lead loop resistance, lead burden and total burden. Select a CT whose rated burden is at least the total — ideally 25–100% of it, since accuracy is specified within that band.
CT burden formula
Rleads = 2 × L × ρ ÷ A · VAleads = Is² × Rleads
with copper resistivity ρ = 1.72 × 10⁻⁸ Ω·m at 20 °C.
Lead burden: 5 A vs 1 A (2.5 mm² copper)
| One-way length | Loop resistance | 5 A lead burden | 1 A lead burden |
|---|---|---|---|
| 5 m | 0.069 Ω | 1.72 VA | 0.069 VA |
| 10 m | 0.138 Ω | 3.44 VA | 0.138 VA |
| 25 m | 0.344 Ω | 8.60 VA | 0.344 VA |
| 50 m | 0.688 Ω | 17.20 VA | 0.688 VA |
| 100 m | 1.376 Ω | 34.40 VA | 1.376 VA |
| 200 m | 2.752 Ω | 68.80 VA | 2.752 VA |
Typical device burdens
| Device | Burden |
|---|---|
| Digital multifunction meter | 0.1–0.5 VA |
| Numerical protection relay | 0.1–0.5 VA |
| Electromechanical overcurrent relay | 3–10 VA |
| Moving-iron ammeter | 1–3 VA |
Standard CT burdens
2.5, 5, 10, 15 and 30 VA. Metering CTs are specified as e.g. 0.5 / 5 VA; protection CTs as e.g. 5P20 / 15 VA.
Safety
Never open-circuit a live CT secondary — short it first with the test block or shorting link.
Frequently asked questions
What is CT burden?
The total load on the CT secondary — relays, meters and leads — usually expressed in VA at rated secondary current. VA = I² × R.
Why are 1 A CTs preferred for long cable runs?
Lead burden rises with current squared, so a 1 A secondary has 25 times less lead burden than a 5 A secondary over the same cable.
What happens if the burden exceeds the CT rating?
Accuracy degrades and protection CTs may saturate during faults, causing relays to under-reach or operate slowly.
Is it dangerous to open a CT secondary?
Yes. An open-circuited CT secondary can develop thousands of volts. Always short the secondary before disconnecting a meter or relay.