PT Ratio Calculator

PT ratio example: an 11,000 V primary system with a standard 110 V secondary gives a PT ratio of 11000:110, or 100:1. A potential transformer (PT), also called a voltage transformer (VT), scales a high system voltage down to a small, standardized secondary voltage that meters, relays, and instruments are built to accept — conceptually similar to how a CT scales current, but working on voltage instead, and connected in parallel across the voltage being measured rather than in series with current flow.

Formula used: PT Ratio = Primary (System) Voltage ÷ Secondary Voltage. Primary voltage is the actual system nominal voltage (11kV, 33kV, 66kV, and so on); secondary voltage follows a standard value, most commonly 110V internationally.

Worked example: an 11,000V (11kV) system uses a PT with a 110V secondary standard. PT Ratio = 11,000 ÷ 110 = 100, expressed as 11000:110 or simplified to 100:1. For a 33kV system with the same 110V secondary standard, PT Ratio = 33,000 ÷ 110 = 300:1.

Why PT primary voltage doesn't get a sizing margin the way CT current rating does: PT primary voltage should be selected to match the actual system voltage and the applicable equipment/insulation class it's designed for — there's no equivalent to "oversizing for future load growth" the way there is with CT current rating, since a PT rated for a specific voltage class has insulation and turns ratio engineered for that class, not a margin above it. Ratio alone does not fully specify a PT: an 11kV system calls for an 11kV-class PT, and using a PT rated for a different, incompatible voltage class (even one seemingly "close" like 6.6kV or 22kV) isn't a matter of accuracy trade-offs the way CT margin selection is — it's a mismatch against the equipment's actual voltage and insulation rating.

Line-to-line versus line-to-neutral PT connection: for a three-phase system, PTs can be connected either line-to-line (measuring the full voltage between two phases, with a 110V line-to-line secondary) or line-to-neutral/phase-to-ground (measuring a single phase's voltage relative to neutral or ground, typically with a secondary of 110÷√3 ≈ 63.5V for a system whose line-to-line secondary would be 110V). Both configurations serve different metering and protection purposes — three-phase power metering typically needs line-to-line (or a full three-phase PT set) measurement, while some protection functions (like earth fault or unbalance detection) specifically need phase-to-neutral or phase-to-ground voltage information that a line-to-line-only connection wouldn't provide.

Standard PT primary voltages follow the system voltage series: unlike CT primary ratings (which follow their own dedicated standard current series), PT primary voltage ratings simply match the standard nominal system voltage series used across power distribution and transmission — 415V, 3.3kV, 6.6kV, 11kV, 22kV, 33kV, 66kV, 132kV, 220kV, and so on, varying somewhat by country and utility practice. A PT is manufactured for a specific system voltage class (or sometimes a small family of closely related classes), with insulation, creepage distance, and turns ratio all engineered around that specific voltage — selecting the correct voltage class PT for your actual system is a straightforward, near-exact match exercise, unlike CT current rating selection which involves the accuracy-versus-future-growth trade-off discussed for CTs.

Standard PT Ratio Chart (110V Secondary)

The table below gives the PT ratio for common system voltages with the widely-used 110V secondary standard, including the 11kV and 33kV levels common in HT distribution systems — if your installation uses a 100V or 120V secondary standard instead, use the calculator above to get the exact ratio for your actual secondary voltage.

Ratio = System Voltage ÷ 110V, matched to the standard nominal voltage series — actual PT primary voltage rating and insulation class must still be confirmed against the manufacturer's specification for your exact system voltage and installation.

Why insulation level matters as much as ratio for PT selection: beyond the turns ratio calculated here, a PT's insulation must be rated for the actual system voltage class it's connected to, including margin for transient overvoltages the system might experience (switching surges, lightning-related transients on outdoor systems). This insulation rating is a separate specification from the voltage ratio itself — two PTs might have an identical 100:1 ratio but be rated for different maximum system voltages and insulation levels, so ratio alone doesn't fully specify a PT; insulation class must also match the actual installation's voltage class and environment.

Three-phase PT configurations: for full three-phase voltage measurement, installations commonly use either three single-phase PTs (one per phase, connected phase-to-neutral, giving complete individual phase voltage information), a single three-phase PT unit (integrating three windings in one housing, more compact but sometimes less flexible), or the open-delta (V-connection) arrangement using just two PTs where a neutral reference isn't practically available. The right configuration depends on what voltage information is actually needed (just line-to-line magnitudes, or full per-phase detail for unbalance/earth-fault detection) and practical installation constraints like available space and whether a system neutral point exists to reference against.

Summary: use PT Ratio = System Primary Voltage ÷ Standard Secondary Voltage, select a PT/VT whose rated primary voltage and insulation/equipment ratings are appropriate for the system voltage and installation requirements (not a margin above it, unlike CT current sizing), confirm both ratio and insulation level match your installation, and remember the critical safety distinction: never short-circuit an energized PT secondary.

Connecting PT selection to overall metering and protection design: PT ratio, alongside CT ratio, feeds directly into how meters and relays interpret raw secondary readings back into actual primary system values — a power meter reading PT and CT secondary voltage and current needs both ratios correctly configured to calculate accurate real-world power, energy, and other derived quantities. Getting PT ratio right is therefore just as fundamental to accurate metering as CT ratio, even though the specific selection considerations (fixed voltage class match vs current-based utilization optimization) differ meaningfully between the two instrument transformer types.

PT selection in medium and high voltage switchgear: in medium-voltage systems (commonly covering roughly the 1–35/36kV range, though the exact boundary varies by applicable standard or regional classification) and high-voltage switchgear — often referred to as HT (High Tension) switchgear in Indian and subcontinent practice, as distinct from LT (Low Tension) — PTs are often a specifically ordered, factory-fitted component of the switchgear assembly rather than a separately field-installed device, given the insulation coordination and safety clearances required at these voltage levels. Understanding the ratio and configuration principles covered here remains useful for specifying requirements to a switchgear manufacturer, verifying delivered equipment matches specification, or troubleshooting an existing installation, even when the PT itself isn't something typically installed as a standalone field retrofit at these voltage classes.

Whether you're specifying a new switchgear order, verifying an existing installation's documentation, or simply building foundational understanding of how voltage measurement scales down safely from high system voltages to instrument-friendly levels, the same ratio principle covered here underlies every one of these tasks.

1. Applying CT-style current margin thinking to PT voltage rating selection. PT primary voltage is fixed by the actual system voltage class, not selected with an accuracy-driven margin the way CT current rating is — there's no equivalent "utilization percentage" consideration for PT primary voltage selection.

2. Confusing which secondary voltage standard (110V, 100V, or 120V) applies to your specific installation. Using the wrong standard secondary voltage assumption in a ratio calculation gives an incorrect ratio — confirm the actual standard used by your region, utility, or specific meter/relay manufacturer before calculating.

3. Applying CT secondary safety practice (safe to short) to a PT. This is a genuinely dangerous mistake — a PT secondary should never be short-circuited while energized, the opposite safety rule from CT secondaries, since a PT behaves like a voltage source and a short circuit drives high, damaging current.

4. Not distinguishing line-to-line from line-to-neutral PT secondary voltage when checking meter compatibility. A meter expecting a 110V line-to-line input won't read correctly if fed a 63.5V line-to-neutral PT secondary output, or vice versa — confirm which connection type both the PT and the connected instrument actually use.

5. Ignoring PT burden compatibility when connecting multiple meters or relays. Like CT burden, connecting devices whose combined VA burden exceeds the PT's rated burden can degrade accuracy below its nameplate specification — check total connected burden, not just ratio selection alone.

6. Assuming a single PT automatically provides adequate accuracy for both metering and protection without checking accuracy class. While PTs are less prone to the dramatic metering-vs-protection accuracy conflict that CTs experience at fault current levels, critical or high-voltage applications may still specifically require a protection-class (3P/6P) PT rather than assuming a general metering-class unit is adequate for both purposes.

7. Overlooking insulation level as a separate specification from ratio. Two PTs can share an identical voltage ratio while being rated for different maximum system voltages and insulation levels — confirm insulation class matches your actual system voltage and environment, not just the calculated ratio number.

8. Choosing PT configuration (three single-phase, one three-phase unit, or open-delta) without considering what protection functions actually need. Some protection functions specifically require full per-phase voltage information that an open-delta or line-to-line-only configuration doesn't provide — confirm your configuration choice supports every metering and protection function the installation actually needs, not just line-to-line magnitude measurement.

9. Ignoring terminal polarity when connecting for power or power-factor measurement. Reversing the secondary polarity relative to the primary (mixing up 1a and 1n, or P1 and P2) doesn't change the ratio, but it can invert the sign of a power or power-factor reading — always verify polarity dot markings against the wiring diagram, not just the ratio, before commissioning any metering or protection scheme.