🎉 Free launch period — every calculator, every feature unlocked, no account needed, through mid-November 2026. PDF reports carry a watermark for everyone during this period.
AAJapps
Browse calculators

VT Sizing Calculator

IEC 61869-3 voltage transformer ratio, rated voltage factor by system earthing, itemized burden against standard rated outputs, and secondary lead voltage drop. Protection accuracy classes, rated thermal limiting output and open-delta residual voltage are subscriber features.

AAJappsEngineering Calculation Report

VT Sizing Calculator

IEC 61869-3

Generated

For preliminary engineering use only — always verify results against project-specific standards and a qualified engineer's review before use in a real design.

Connection & system earthing

kV

Secondary burden

Every meter, relay or indicating instrument on the VT secondary contributes its own VA at rated secondary voltage.
VA
VA
VA

Secondary leads (to instrument/relay panel)

m
mm²

Protection accuracy class (3P / 6P)

Subscriber

Rated thermal limiting output

Subscriber
VA

Open-delta (broken-delta) residual voltage

Subscriber

VT sizing result

Rated ratio
100.0 : 1
6.351 kV / 63.51 V
Voltage factor Vf
1.2× (Continuous)
Phase-to-earth, effectively (solidly) earthed neutral system
Connected burden
8.00 VA
80% of 10 VA rated output
Recommended standard size
10 VA
Smallest standard size ≥ burden
✅ Burden is within the 25–100% of rated output range — accuracy class 0.5 is maintained (also requires 80–120% rated voltage, per IEC 61869-3).

Burden breakdown

DeviceVA @ rated secondary voltage
Protection Relay5.00 VA
Multifunction Meter3.00 VA
Total8.00 VA

Secondary lead voltage drop

Lead resistance
0.420 Ω
Secondary loop current
0.126 A
Voltage drop
0.053 V (0.08%)
Approximate — assumes the connected burden draws current in phase with the secondary voltage. Long lead runs to a remote relay panel can meaningfully erode metering/protection accuracy even when the VT itself is correctly sized.