PT/VT Ratio Calculator
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Calculate
Line-to-line voltage of the electrical system (bus voltage, not phase-to-neutral).
Auto mode: PT Selection when only voltage entered; Burden Verification when burden entered; Voltage Calculation when ratio + measured voltage entered.
Connection type, secondary voltage, application, accuracy class, burden, BIL, frequency — all optional (IEEE C57.13-2016 defaults).
Overview
Use this calculator to select a PT/VT ratio from system voltage, secondary voltage, burden, and accuracy class. It implements IEEE C57.13 standard ratios, burden classes (W, X, M, Y, Z, ZZ), and accuracy classes (0.15, 0.3, 0.6, 1.2). The calculator picks the lowest standard PT primary voltage rating that satisfies both the nominal system voltage and the 110% continuous-capability rule per IEEE C57.13, then computes the resulting PT ratio, classifies the connected burden, and verifies the chosen accuracy class can be maintained at the calculated burden.
Potential transformers, also called voltage transformers (VTs), reduce high primary voltage to a standardized secondary voltage suitable for metering and protective relaying. Standard ANSI secondary voltages are 120 V (line-to-line) and 69.3 V (line-to-neutral, 120/√3). IEC standards use 110 V and 63.5 V, or 100 V and 57.7 V. The calculator supports all of these.
Three calculation modes cover the common selection decisions. PT selection is the default: given a system voltage, secondary voltage, and connected load, the tool recommends a standard PT primary rating with appropriate accuracy and burden class. Burden verification is the reverse: given an existing PT ratio and the actual connected devices, verify the combination meets accuracy and burden requirements. Voltage calculation converts between primary and secondary voltage using an installed PT ratio; when both measured voltages are supplied, the calculator performs a consistency check and flags deviations above 2%.
Used in substation design, revenue metering verification, protective relaying coordination, and commissioning checks. Results are based on IEEE C57.13 standard ratios and burden classes. For revenue-grade billing compliance, manufacturer test certificates remain authoritative.
How to Use This Calculator
Enter system primary voltage — the line-to-line nominal system voltage at the PT installation point. Select kV or V from the unit dropdown. Examples: 480 V (LV switchgear), 4.16 kV (MV motor), 13.8 kV (distribution), 138 kV (sub-transmission).
Select secondary voltage — 120 V ANSI L-L is the default for North American installations. Select 69.3 V for L-N grounded wye. Select IEC values (110/63.5 V or 100/57.7 V) for IEC regions. Use Custom for non-standard secondaries.
Click Calculate — the calculator infers PT Selection mode automatically from the entered fields. The recommended PT primary rating, ratio, and accuracy class appear in the results.
Review the result — the result is classified NORMAL (green) or a specific failure mode (amber). The combined badge shows voltage class for NORMAL results (e.g., NORMAL / MEDIUM-VOLTAGE-2).
For burden verification — enter the existing PT ratio and connected burden in VA. The calculator automatically switches to Burden Verification mode.
For voltage conversion — enter only the existing PT ratio and a measured primary or secondary voltage. The calculator switches to Voltage Calculation mode.
Open advanced parameters — expand to set connection type, application, accuracy class, connected burden, burden power factor, BIL, frequency, and primary fuse protection.
Results are based on IEEE C57.13 standard ratios and burden classes. Accuracy compatibility uses common nameplate patterns — specific manufacturer nameplates may extend or restrict the combinations. For revenue-grade billing compliance, manufacturer test certificates remain authoritative. The calculator does not model harmonics, ferroresonance, CVT transient response, or voltage factor for fault conditions.
Formula
Core Calculation Steps
Step 1: Winding Voltage by Connection Type
Line-to-Line or Open Delta:
V_winding = V_LL (line-to-line system voltage)
Line-to-Neutral or Wye-Wye:
V_winding = V_LL / √3
Step 2: PT Primary Rating Selection (110% Rule, IEEE C57.13)
Check 1: V_PT_rated ≥ V_nominal
Check 2: V_PT_rated × 1.10 ≥ V_max_continuous
Select the smallest standard PT primary rating satisfying both checks. Default V_max_continuous = V_nominal × 1.10.
Step 3: PT Ratio
PT_ratio = V_PT_primary_rated / V_secondary
Display format: "V_primary : V_secondary (ratio:1)"
Step 4: Burden Classification (IEEE C57.13)
| Designation | VA Rating | Power Factor |
|---|---|---|
| W | 12.5 | 0.10 |
| X | 25 | 0.70 |
| M | 35 | 0.20 |
| Y | 75 | 0.85 |
| Z | 200 | 0.85 |
| ZZ | 400 | 0.85 |
Select lowest designation where VA_rated ≥ B_total. PF match within ±0.15 preferred.
Step 5: Accuracy Class Compatibility (typical nameplate screening)
| Accuracy Class | Burden Designations Typically Supported |
|---|---|
| 0.15 (IEEE C57.13.6) | W, X |
| 0.3 | W, X, M, Y, Z |
| 0.6 | W through ZZ |
| 1.2 | W through ZZ |
Step 6: Voltage Calculation Mode
Forward (primary → secondary):
V_sec = V_primary / PT_ratio
Reverse (secondary → primary):
V_primary = V_sec × PT_ratio
Consistency check (both measured):
deviation = (V_sec_measured − V_sec_expected) / V_sec_expected × 100%
Flag if |deviation| > 2%.
What is a PT/VT Ratio
A potential transformer (PT) or voltage transformer (VT) ratio is the relationship between the primary winding voltage rating and the secondary winding voltage. It determines how much the high-voltage primary is stepped down to the standardized secondary voltage used by metering and protection equipment. A 14400:120 PT has a ratio of 120:1 — when 14.4 kV appears on the primary, 120 V appears on the secondary.
The PT ratio is not simply the system voltage divided by 120 V. The correct PT primary rating must satisfy two IEEE C57.13 requirements simultaneously: the rating must be at least the nominal system voltage, and the PT must operate continuously at 110% of its rated voltage without exceeding thermal limits or accuracy degradation. These checks determine the standard primary rating the calculator selects.
IEEE C57.13 burden classes (W, X, M, Y, Z, ZZ) define the VA load the PT can supply while maintaining its rated accuracy. A PT nameplate reading "0.3 W X M Y Z, 1.2 ZZ" means the PT maintains 0.3% accuracy at burdens through the Z class, and relaxes to 1.2% at the heaviest ZZ burden. Full burden class and accuracy class details are in the dedicated sections below.
Basic Insulation Level (BIL) specifies the PT's impulse withstand capability. Every voltage class has a typical minimum BIL per IEEE C57.13 — 10 kV for 600 V class, 60 kV for 5 kV class, 95 kV for 15 kV class, and so on. The selected BIL must equal or exceed the typical minimum for the installation voltage class.
PT VS CT — IMPORTANT SAFETY DISTINCTION
Potential transformers and current transformers require opposite secondary-circuit handling. CTs are current-source devices — their secondary must always be connected to a burden or shorted; an open-circuit CT secondary generates dangerous high voltages. PTs are voltage-source devices — their secondary must never be short-circuited while the primary is energized.
A shorted PT secondary creates a fault that can damage the PT, blow primary fuses, and generate arcing. Protect PT secondary circuits with fuses or breakers and always de-energize the primary before rewiring secondary connections.
LINE-TO-LINE VS LINE-TO-NEUTRAL PT SELECTION
Connection type is the most common source of PT selection errors. The voltage applied to each PT primary winding depends on how the PT is wired into the three-phase system.
For line-to-line (L-L) connections, each PT primary winding sees the full line-to-line voltage: V_winding = V_LL. This applies to single-phase L-L PT installations and to open-delta (V-V) configurations. For line-to-neutral (L-N) grounded and wye-wye connections, each PT primary winding sees the phase voltage: V_winding = V_LL / √3. This applies to single-phase L-N PT installations and to wye-wye PT banks.
Practical effect for a 13.8 kV system:
- L-L PT: V_winding = 13.8 kV → PT primary rated 14.4 kV (with 110% margin) → PT ratio 14400:120 = 120:1
- L-N PT (grounded wye): V_winding = 13.8 / √3 = 7.97 kV → PT primary rated 8.4 kV → PT ratio 8400:69.3 ≈ 121:1
Open-delta uses two PTs and provides L-L voltages adequate for L-L metering and basic relaying but not phase-to-neutral or zero-sequence voltage. Wye-wye uses three PTs and provides all three phase-to-neutral voltages plus zero-sequence — required for ground-fault protection.
PT/VT STANDARD RATIOS REFERENCE
Standard PT primary voltage ratings (V) per IEEE C57.13 and common manufacturer catalogs:
Low-voltage class (600 V and below): 120, 240, 277, 480, 600 V. Exact-match PT ratings (e.g., 480:120) are standard industry practice for LV installations.
Medium-voltage 5 kV class: 2400, 4200, 4800 V. Common in industrial motor control and distribution metering.
Medium-voltage 15 kV class: 7200, 8400, 12000, 14400 V. 8400 V is standard for L-N PT installations on 13.8 kV grounded wye systems.
Medium-voltage 25 kV class: 20125, 24000 V.
Medium-voltage 35 kV class: 27600, 34500 V.
High-voltage class: 46000, 67000, 69000 V.
High-voltage transmission class: 115000, 138000, 161000 V.
Extra-high-voltage class: 230000, 345000, 500000 V.
For L-N connections, divide primary by √3 and use 69.3 V secondary (or the corresponding IEC L-N value). Catalog ratings such as 8400 V (15 kV class L-N) reflect common manufacturer practice for grounded wye systems on 13.8 kV nominal.
HOW TO CALCULATE PT RATIO
The PT ratio is the primary rated voltage divided by the secondary rated voltage:
PT_ratio = V_PT_primary_rated / V_secondary
For a 14400:120 PT, the ratio is 14400 / 120 = 120:1. The ratio is fixed by the PT's winding turns — it does not change with the actual operating voltage. When measured primary is 13800 V on a 120:1 PT, the secondary reads 13800 / 120 = 115 V.
For voltage calculation, rearranging: V_secondary = V_primary / PT_ratio, or V_primary = V_secondary × PT_ratio. For consistency checking during commissioning: deviation = (V_sec_measured − V_sec_expected) / V_sec_expected × 100%. A deviation exceeding ±2% warrants investigation of the PT nameplate, primary tap, or measurement.
IEEE C57.13 ACCURACY CLASSES EXPLAINED
IEEE C57.13 defines four standard PT accuracy classes:
| Class | Application |
|---|---|
| 0.15 | High-accuracy revenue (IEEE C57.13.6) |
| 0.3 | Revenue billing, commercial metering |
| 0.6 | Commercial metering, general accuracy |
| 1.2 | Protective relaying, indication |
Lower number means tighter accuracy. Class 0.15 was added in IEEE C57.13.6 specifically for electronic revenue metering where light burdens require verified low-end accuracy. The accuracy class applies across a range of burdens defined on the PT nameplate. A typical nameplate reads "0.3 W X M Y Z, 1.2 ZZ" — the PT maintains 0.3 accuracy at W through Z burdens and relaxes to 1.2 at the heaviest ZZ burden.
Revenue billing utilities typically specify class 0.3 or better, and may require independently certified test certificates documenting RCF and phase angle at multiple burdens. Per IEEE C57.13 Section 5, for class 0.3 the Ratio Correction Factor (RCF) must remain within 0.997 to 1.003 (±0.3%) across 90% to 110% of rated primary voltage. Class 1.2 allows 0.988 to 1.012 (±1.2%).
WHAT IS A PT BURDEN
Burden is the volt-ampere (VA) load that connected secondary devices present on the PT at rated voltage. Sum the VA ratings of each device: electronic relays typically 0.1–0.5 VA, digital meters 0.2–1 VA, analog voltmeters 1–3 VA, watt-meters 3–5 VA, electromechanical relays 5–15 VA.
IEEE C57.13 defines six standard burden designations by VA rating and power factor:
| Designation | VA Rating | Power Factor |
|---|---|---|
| W | 12.5 | 0.10 |
| X | 25 | 0.70 |
| M | 35 | 0.20 |
| Y | 75 | 0.85 |
| Z | 200 | 0.85 |
| ZZ | 400 | 0.85 |
The PT accuracy class is verified at the calculated burden. Tighter accuracy classes apply only at lighter burdens — a 0.3 class PT typically maintains accuracy at W through Z but requires a premium PT to cover ZZ burden at 0.3 accuracy. Cable burden is often small for modern electronic loads but can matter on long secondary runs.
PT SELECTION FOR REVENUE VS PROTECTION APPLICATIONS
Revenue metering typically requires 0.3 accuracy class or 0.15 for high-accuracy applications per IEEE C57.13.6. The PT must maintain accuracy across the burden range from 90% to 110% of rated voltage. Manufacturer test certificates are typically required for billing compliance. Modern electronic meters present very light burdens (often under 1 VA total) — verify low-burden accuracy curves with the PT manufacturer.
Protective relaying typically accepts 1.2 accuracy class for simple protection functions. Directional, distance, sync-check, V/Hz, and sensitive schemes may justify tighter accuracy (0.3 or 0.6 class) where ratio and phase accuracy affect sensitivity or coordination. PTs must withstand transient overvoltages during faults — verify voltage factor capability from manufacturer data.
For applications combining metering and protection on the same PT, the accuracy class must satisfy the more stringent metering requirement, and the burden must accommodate both metering devices and protective relays simultaneously.
PT/VT APPLICATIONS IN POWER SYSTEMS
Revenue Metering. Utility billing metering typically requires 0.3 class accuracy or better, or 0.15 per IEEE C57.13.6 for high-accuracy applications. The PT must maintain accuracy across the burden range. Manufacturer test certificates are typically required for billing compliance. Apply Ratio Correction Factor (RCF) where specified by utility.
Protective Relaying. Distance, directional, voltage, and synchronism protective relays use PT inputs. 1.2 accuracy class is often adequate for simple protection functions; directional, distance, sync-check, V/Hz, and sensitive schemes may justify tighter accuracy (typically 0.3 or 0.6 class). PT must withstand transient overvoltages during faults — verify voltage factor capability from manufacturer data.
Commercial and Sub-Metering. Commercial metering for sub-billing, energy management, or process monitoring uses 0.3 or 0.6 class PTs. Modern electronic meters present light burdens (often under 1 VA total). Verify low-burden accuracy with manufacturer data when burdens are very light.
Synchronization. Synchronization PTs supply voltage references for synchroscopes and synchrocheck relays. Phase angle accuracy matters as much as ratio accuracy. Use matched PTs (same manufacturer and design) for both bus and incoming generator references.
Power Monitoring. Power monitoring uses 0.3 or 0.6 class PTs. Harmonic distortion can degrade PT accuracy at non-fundamental frequencies; IEEE C57.13 does not cover harmonic response. Verify with PT manufacturer if harmonic measurement accuracy is critical.
Key Facts
- A PT nameplate reading '0.3 W X M Y Z, 1.2 ZZ' means the device maintains 0.3% accuracy at W through Z burdens and only relaxes to 1.2% at the heaviest ZZ burden.
- IEEE C57.13 requires PTs to operate continuously at 110% of rated voltage — for a 13800 V PT, that is 15180 V continuous capability.
- The winding voltage for L-N or Wye-Wye connected PTs is V_LL / √3. A 13.8 kV grounded-wye system requires PT windings rated for 7.97 kV, not 13.8 kV.
- BIL (Basic Insulation Level) for 15 kV class PTs is typically 95 kV — the PT must withstand this peak impulse without failure.
- PT secondary circuits are voltage-source circuits. Shorting the secondary while the primary is energized creates a fault — the opposite of CT behavior where the secondary must never be open-circuited.
- The six standard burden designations (W, X, M, Y, Z, ZZ) range from 12.5 VA to 400 VA with varying power factors from 0.10 to 0.85.
- Ratio Correction Factor (RCF) accounts for the small difference between the actual PT ratio and the nameplate ratio — critical for revenue-grade billing accuracy.
- PT (Potential Transformer) and VT (Voltage Transformer) are two names for the same device — parallel-connected instrument transformers that step down system voltage to a standardized secondary value for metering and protection.
- Standard ANSI secondary voltages: 120 V (line-to-line) and 69.3 V (line-to-neutral) in North America. IEC standard values: 110 V / 63.5 V or 100 V / 57.7 V.
- IEEE C57.13-2016 is the governing North American standard for PT ratios, burden designations, and accuracy classes. IEEE C57.13.6 adds the 0.15 high-accuracy class for electronic revenue metering.
- PT secondary circuits must be solidly grounded at one designated point per IEEE C57.13 for safety and to prevent capacitive coupling hazards.
- For installations at 600 V and above, primary fuse protection for the PT is standard practice per IEEE C57.13 Section 6.3.
- Modern electronic relays present very light burdens (0.1 to 0.5 VA each). Very light total burdens can challenge PT accuracy at the low-burden end of the class — verify manufacturer low-burden accuracy data for revenue applications.
Applications
- Substation PT selection for new switchgear and distribution systems.
- Revenue metering PT sizing for utility billing accuracy compliance per IEEE C57.13.
- Protective relaying PT burden verification for distance, directional, and differential relay inputs.
- Commissioning ratio consistency check using measured primary and secondary voltages.
- IEC vs ANSI secondary voltage comparison for international projects.
- BIL selection for PT installations in switching-transient-prone environments.
- Grounded wye L-N PT selection with correct winding voltage division by √3.
- Open-delta PT configuration assessment for three-wire three-phase systems.
Example Calculation
Example 1 — 13.8 kV Distribution PT Selection
System primary voltage: 13.8 kV | Secondary voltage: 120 V (ANSI L-L) | Connection: Line-to-Line | Burden: 30 VA at 0.85 PF
Result: NORMAL / MEDIUM-VOLTAGE-2 — Recommended PT rating: 14400 V, ratio 120:1, accuracy class 0.3 at Y burden (30 VA actual / 75 VA capacity, 40% utilization), BIL 95 kV.
Winding voltage = 13.8 kV (L-L). Max continuous = 13.8 × 1.10 = 15.18 kV. Smallest standard rating with 110% capability ≥ 15.18 kV: 14400 V (provides 15.84 kV). Ratio = 14400 ÷ 120 = 120:1. Burden 30 VA at 0.85 PF → Y class (PF match). 0.3 accuracy class supported at Y.
Example 2 — L-N Grounded Wye 13.8 kV PT for Protective Relaying
System primary voltage: 13.8 kV | Secondary voltage: 69.3 V (ANSI L-N) | Connection: Line-to-Neutral, grounded primary | Burden: 25 VA at 0.70 PF
Result: NORMAL / MEDIUM-VOLTAGE-2 (with HIGH-BURDEN-NOTICE) — Recommended PT rating: 8400 V, ratio 121:1, accuracy class 1.2 at X burden (25 VA actual / 25 VA capacity, 100%).
Winding voltage = 13.8 ÷ √3 = 7.97 kV. Max continuous = 7.97 × 1.10 = 8.77 kV. Smallest standard rating: 8400 V (provides 9.24 kV). Ratio = 8400 ÷ 69.3 ≈ 121:1. Burden at 100% of X rating triggers HIGH-BURDEN-NOTICE — consider reducing burden or upgrading to Y-rated PT.
Example 3 — ACCURACY-MISMATCH: 4.8 kV Revenue Metering with Heavy Burden
System primary voltage: 4.8 kV | Secondary voltage: 120 V | Accuracy class: 0.3 (revenue metering) | Burden: 250 VA at 0.85 PF
Result: ACCURACY-MISMATCH (amber) — Recommended PT rating: 7200 V, ratio 60:1, burden ZZ class (250 VA actual / 400 VA capacity).
4800 V winding, max continuous 5280 V. Standard 4800 V PT at 110% = 5280 V — no margin above assumed max. Next standard: 7200 V (provides 7920 V cap). Ratio = 7200 ÷ 120 = 60:1. 250 VA at 0.85 PF → ZZ class. Standard nameplates do not extend 0.3 accuracy to ZZ burden → ACCURACY-MISMATCH. Options: reduce burden, accept 1.2 class, or specify a premium PT.
Example 4 — Voltage Calculation with Ratio Consistency Check
Existing PT ratio: 120:1 | Measured primary: 13.8 kV | Measured secondary: 110 V
Result: RATIO-MISMATCH-NOTICE — Expected secondary = 13800 ÷ 120 = 115.0 V. Measured = 110 V. Deviation = −4.35%, exceeds ±2% tolerance. Verify PT nameplate ratio, check for incorrect primary tap, or investigate PT condition before relying on secondary measurements.
Standards & References
- IEEE C57.13-2016 — Standard Requirements for Instrument Transformers. The primary North American standard for PT ratios, accuracy classes (0.3, 0.6, 1.2), burden designations (W, X, M, Y, Z, ZZ), and BIL ratings. Covers electromagnetic voltage and current transformers used in metering and protective relaying.
- IEEE C57.13.6-2005 — Standard for High-Accuracy Instrument Transformers. Adds the 0.15 voltage transformer accuracy class and corresponding low burden definitions for electronic revenue metering applications. Supplements IEEE C57.13 for high-accuracy installations.
- IEEE Std 242-2001 — IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (IEEE Buff Book). Provides PT application guidance for protective relaying coordination, voltage factor considerations, and coordination with other instrument transformers.
- ANSI/IEEE C12.10 — American National Standard for Electric Meters — Safety Requirements. Related metering safety context for revenue installations.
- IEC 61869-3 (reference context) — Instrument transformers, Part 3: Additional requirements for inductive voltage transformers. International standard equivalent to IEEE C57.13 for IEC regions; burden class designations and accuracy classes differ from IEEE practice. Reference for international users; calculator uses IEEE-based burden designations.
- IEEE Std C37 series (context only) — Protective relay application guides relevant to PT selection for relaying.
- IEEE Std C57.19.01 (context only) — Standard performance characteristics and dimensions for outdoor apparatus bushings. Relevant for PT primary terminations in MV/HV applications.
Units
- System primary voltage — kV or V
- Secondary voltage — V (standard: 120, 69.3, 110, 63.5, 100, 57.7 V)
- Burden — VA (volt-amperes)
- Power factor — dimensionless (0–1.0)
- BIL — kV
- PT ratio — dimensionless (e.g., 120:1)
- Results — kV for MV/HV primary voltages; V for LV and secondary voltages All internal calculations use SI units.
Limitations
- Standard PT primary ratings list covers common ANSI/IEEE catalog values. Manufacturer-specific intermediate ratings may exist — verify specific catalog before ordering.
- Accuracy class compatibility uses common nameplate patterns (typical '0.3 W X M Y Z, 1.2 ZZ' configuration). Specific manufacturer nameplates may extend or restrict these combinations.
- BIL selection uses typical minimum values per voltage class. Project-specific insulation coordination studies and switchgear BIL requirements may specify higher values.
- Burden calculation uses steady-state VA at rated secondary voltage. Transient burden during faults or switching is not modeled.
- Harmonics analysis is not included — IEEE C57.13 does not specify PT accuracy at harmonic frequencies.
- Ferroresonance risk is not assessed. Installations with long cable runs, lightly loaded cable systems, or ungrounded systems may require ferroresonance analysis.
- Capacitor Voltage Transformer (CVT) selection is out of scope — this calculator covers electromagnetic PTs only.
- Three-phase unbalance corrections are not applied. Single-phase PT selection assumes balanced system voltages.
- Ratio Correction Factor (RCF) is not computed — revenue metering requires RCF from the PT manufacturer's test certificate.
Common Mistakes to Avoid
- Ignoring the 110% continuous rule — selecting a PT rated exactly at nominal system voltage leaves no margin for sustained over-voltage.
- Applying L-L system voltage to L-N winding selection — for grounded wye PTs, divide V_LL by √3 before selecting the primary rating.
- Shorting the PT secondary — PT secondaries are voltage-source circuits; a shorted secondary creates a fault (opposite of CT behavior where open secondary creates dangerous voltage).
- Applying revenue-grade accuracy to relaying — 0.3 or 0.15 class PTs add cost without benefit for simple protection functions where 1.2 class is adequate.
- Paralleling PT secondaries — this creates circulating current risk from ratio and phase mismatch; split burdens across separate secondary circuits instead.
- Omitting cable burden — on long secondary runs, cable resistance can contribute meaningfully to total burden; verify cable length and conductor size.
- Using L-L standard ratio for L-N application — 14400:120 (L-L) and 8400:69.3 (L-N) produce similar line-to-line ratios but require different winding insulation and secondary connections.
- Forgetting to ground the PT secondary — PT secondary must be solidly grounded at one designated point per IEEE C57.13. An ungrounded secondary creates capacitive coupling hazards and a floating voltage reference for protective relays.
- Treating peak voltage as nominal — the system voltage entered to the calculator should be nominal RMS line-to-line. Peak values (1.414 × RMS) are not the same input; use the dedicated maximum continuous voltage field for worst-case sustained voltage.
- Selecting BIL at the typical minimum without margin — standard BIL ratings are typical minimums for the voltage class. Installations with significant switching transients (capacitor banks, large motor starting) or coastal pollution exposure may require next-higher BIL.
- Ignoring power factor when matching burden designation — burden designations have both VA and PF characteristics. A burden of 30 VA at 0.85 PF fits the Y designation (75 VA at 0.85 PF) rather than M (35 VA at 0.20 PF). VA ceiling alone is not sufficient.
Frequently Asked Questions
How do I calculate the PT ratio?
What is a 120:1 PT ratio?
What is the PT ratio for 13.8 kV to 120 V?
What is the PT ratio for 4160 V to 120 V?
What is the PT ratio for 480 V to 120 V?
What is the difference between PT and VT?
What PT primary rating should I use for a 13.8 kV system?
How does connection type affect PT ratio selection?
What is the burden of a PT?
What accuracy class should I use for revenue metering?
Is it safe to short the PT secondary?
What is BIL and how do I select it for a PT?
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Calculate
Line-to-line voltage of the electrical system (bus voltage, not phase-to-neutral).
Auto mode: PT Selection when only voltage entered; Burden Verification when burden entered; Voltage Calculation when ratio + measured voltage entered.
Connection type, secondary voltage, application, accuracy class, burden, BIL, frequency — all optional (IEEE C57.13-2016 defaults).