CT & VT Sizing

Instrument-transformer ratios and burden screening, CT and VT tabs.

Before you calculate

Instrument-transformer ratio and burden screening

CT and VT tabs. A VA comparison never proves the accuracy class, the accuracy range or saturation performance.

Learn more — complete guide →

Step 1

CT and its secondary circuit

Rated primary current (A)

Rated secondary current

CT rated burden (VA)

Lead length, one way (m)

Copper lead cross-section (mm²)

Connected devices, total rated burden (VA)

Advanced: lead temperature and path, operating current, class, ALF (optional)

Lead temperature (°C)20 °C by default; warm leads have a higher resistance.

Lead pathWith a common return, earth faults see the loop (2 x).

Operating primary current (A)

Accuracy class (explanation only)

Rated ALF (protection CT)

Secondary winding resistance Rct (Ω)From the CT test report; without it the effective ALF is not evaluated.

Step 2

CT results

Check before relying on this result

The burden is a scalar sum of rated device VA and lead I²R: a screening, not an accuracy, accuracy-range or saturation check.

Burden at rated secondary current8.00 VA of 10 VA (80.0 %)Within entered nameplate VA burden (screening only)

Lead resistance and lead burden0.280 Ω · 7.00 VAMargin 2.00 VA

Nominal ratio20 : 1

Effective ALFALF not evaluatedNeeds the rated ALF and Rct.

Safety: never open-circuit a CT secondary while the primary carries current: dangerous voltages appear. Short it with the approved terminals and procedure. Qualified personnel only.

1. Lead resistance

Copper, ρ = 0.0175 Ω·mm²/m at 20 °C; two-wire loop.

R_{leads}=\frac{k\,\rho\,L}{A}

R_{leads}=\frac{2\times 0.0175\times 20}{2.5}=0.28\ \Omega

2. Burden at rated secondary current

Lead I²R plus the rated VA of the connected devices: a scalar screening sum.

S_{total}\approx S_{devices}+I_{sn}^2R_{leads}

S_{total}\approx 1+5^2\times 0.28=8\ \text{VA}

Calculations use full precision internally; the displayed numbers are rounded for readability.