Voltage Transformer (VT) Sizing and Accuracy Classes: A Practical Engineering Guide

A voltage transformer (VT), also called a potential transformer (PT), is a medium-voltage instrument transformer that steps down a high system voltage to a standardized low secondary voltage — typically used as a reference — so that protection relays, meters, and synchronizing equipment can measure it safely and accurately. This guide walks through the engineering logic behind VT sizing: how ratio, burden, and accuracy class are chosen together, with a fully worked numerical example and a comparison table for common accuracy classes. It is written for engineers and procurement teams specifying medium-voltage switchgear and substations across the Middle East and Gulf (including Egypt and Saudi Arabia), North Africa, CIS, and Sub-Saharan Africa, where correct VT selection directly affects metering revenue accuracy and protection reliability.

Before diving into the calculation method, it helps to recall that a VT is not sized by “capacity” in the same sense as a power transformer — it is sized by ratio, rated burden, and accuracy class, three parameters that must be matched to the actual connected load (meters, relays, wiring) rather than oversized “just in case.”

Why VT sizing matters

An incorrectly sized voltage transformer can silently distort billing data, delay protection tripping, or cause instability in automatic voltage regulation loops. Two failure modes are common in the field:

• Undersized burden capacity (too many devices connected on one VT secondary) pushes the VT beyond its rated accuracy class, causing ratio and phase-angle errors that are invisible until an energy audit or protection mis-operation occurs.
• Oversized VT relative to actual connected burden — using a VT rated for far more burden than the actual load — can also degrade accuracy at very light burden, because accuracy class is defined and guaranteed only within a specified burden range, not down to zero.

Because a VT typically feeds both metering circuits and protection relays from the same or separate windings, the engineer must reconcile two different accuracy requirements: metering needs high precision at rated voltage, while protection needs acceptable accuracy across a wider voltage range, including transient overvoltage conditions such as those following a fault or a system disturbance. This is why many VTs are specified with dual secondary windings, each with its own accuracy class — one core dedicated to metering, one to protection.

The method: how VT ratio, burden, and accuracy class are derived

Step 1 — Determine the rated primary voltage

The VT primary rated voltage is selected to match (or exceed) the system’s highest nominal voltage. For a phase-to-earth connected VT on an effectively earthed or resistance-earthed system, the primary rating is usually based on phase-to-earth voltage; for an isolated or resonant-earthed system, VTs are often rated for full line-to-line voltage on each phase to withstand sustained displacement during an earth fault.

Step 2 — Fix the rated secondary voltage

Secondary voltages are standardized (commonly 110 V or 100 V line-to-line for phase-to-phase connected VTs, and 110/√3 V or 100/√3 V for phase-to-earth connected VTs, per IEC practice) so that relays and meters designed to these standard values can be used interchangeably across different primary ratios.

Step 3 — Calculate the transformation ratio

The rated transformation ratio is simply:

Ratio = Rated primary voltage / Rated secondary voltage

Step 4 — Determine total connected burden

Every device connected to the VT secondary (protection relay voltage inputs, meters, synchronizing equipment, indicating instruments, and the burden of the interconnecting wiring itself) draws power (in VA) from the secondary circuit. The engineer sums the burden of all connected devices, including cable burden calculated from the wire resistance and secondary current, to obtain the total connected burden in VA.

Step 5 — Select a rated burden ≥ total connected burden

The VT rated burden (expressed in VA at rated voltage and rated power factor) must be equal to or greater than the calculated total connected burden. However, rated burden should not be excessively larger than the connected burden, because accuracy class is only guaranteed within a defined burden window — commonly between 25% and 100% of rated burden for many accuracy standards. A VT loaded far below 25% of its rated burden may exceed its accuracy class limits.

Step 6 — Select the accuracy class for each duty

Metering cores are typically specified in classes such as 0.2, 0.5, or 1.0 (percentage ratio error at rated voltage and rated burden), while protection cores are typically specified with a “P” designation (for example, class 3P or 6P), which defines permissible error over an extended voltage range rather than only at rated voltage — since protection VTs must remain reasonably accurate during voltage dips associated with faults.

Worked example: sizing a VT for an 11 kV switchboard

Consider a medium-voltage switchboard operating at a nominal 11 kV line-to-line, phase-to-earth connected, feeding one multifunction protection relay and one revenue meter from the same VT secondary.

Step 1 — Ratio. Primary rated voltage (phase-to-earth) = 11,000 / √3 ≈ 6,351 V. Using a standard secondary of 110/√3 V (≈ 63.5 V), the ratio is:

Ratio = 6,351 / 63.5 ≈ 100:1

Step 2 — Connected burden. Assume the protection relay’s voltage input draws 2.5 VA per phase, the revenue meter draws 1.5 VA per phase, and the secondary wiring (a run of copper conductor between the VT terminals and the panel) contributes an additional 1.0 VA per phase due to cable resistance at the secondary current.

Total connected burden per phase = 2.5 + 1.5 + 1.0 = 5.0 VA

Step 3 — Rated burden selection. If the VT is available in standard rated burdens of, say, 10 VA, 15 VA, 25 VA, and 50 VA, the engineer should NOT default to the largest option. Applying the “burden should fall within roughly 25%–100% of rated burden for guaranteed accuracy” principle:

• A 10 VA rated burden gives a loading of 5.0 / 10 = 50% → within the valid window. ✅
• A 50 VA rated burden gives a loading of 5.0 / 50 = 10% → below the 25% threshold, risking degraded accuracy at light load. ⚠️

Conclusion: the 10 VA rated burden VT, at class 0.5 for the metering core, is the correct engineering choice for this connected load — not the largest burden rating available.

Step 4 — Accuracy class check for protection. If the same VT also feeds the protection relay from a second secondary winding, that winding would be specified separately, for example class 3P, so its permissible error is defined over an extended voltage range (covering the reduced voltage seen during a nearby fault), rather than only at rated voltage — this is why metering and protection windings are rarely combined on a single core when both duties are critical.

Accuracy class comparison table

Note: this table reflects the general logic of IEC accuracy class families (metering “class” vs protection “P” class) rather than a specific manufacturer’s certified figures; actual class limits and burden ranges must be confirmed against the relevant IEC 61869-3 tables and the manufacturer’s type test certificate for the selected VT.

Selection criteria and common pitfalls

• Match primary connection to system earthing. Phase-to-earth VTs on isolated/resonant-earthed networks must be rated to withstand sustained line-to-line voltage across their insulation during a single-phase earth fault; using a VT rated only for phase-to-earth voltage on such a system is a common and dangerous sizing error.
• Separate metering and protection cores where both are critical. Sharing one winding between a revenue meter and a protection relay means the accuracy class must satisfy the stricter of the two duties, often forcing an unnecessarily expensive metering-grade class onto the protection function, or compromising metering accuracy.
• Do not oversize rated burden “for margin.” Unlike current transformers, where burden headroom is often harmless, VTs lightly loaded far below the lower bound of their accuracy burden range can produce ratio errors that defeat the purpose of a high accuracy class.
• Include cable burden in the calculation. Long secondary wiring runs, especially in outdoor substations or large switchrooms, add real VA burden that is frequently omitted from quick sizing estimates.
• Check ferroresonance risk. VTs connected phase-to-earth on ungrounded or high-impedance-earthed systems can be susceptible to ferroresonance with system capacitance; this is a design consideration separate from accuracy class but equally important during specification.
• Coordinate with the protection relay’s minimum voltage input burden, available from the relay manufacturer’s technical datasheet, before finalizing the VT’s rated burden.

Governing standards

Voltage transformers for medium-voltage systems are covered by IEC 61869-3 (Instrument transformers — Additional requirements for inductive voltage transformers), which is part of the broader IEC 61869 family covering instrument transformers generally (IEC 61869-1 for general requirements). These standards define rated voltage factors, accuracy classes for both metering and protection duty, rated burden ranges, and the extended voltage range testing used for protection-class windings. Where IEEE-based specifications are used instead (common in some export markets), the equivalent framework is IEEE C57.13, which defines analogous accuracy and burden requirements under a different classification scheme. Engineers specifying equipment for multi-standard export projects should confirm early in the design phase which standard family (IEC or IEEE) governs the contract, since accuracy class labels and burden test points are not directly interchangeable between the two.

For related equipment selection, see our hub pages on medium voltage switchgear, protection relays, and medium voltage transformers for how VT sizing fits into the broader substation design process.

Related guides

See also our guides on gas-insulated switchgear, ring main units and metal-clad switchgear.