75:5 Current Transformer (CT): Ratio Specifications & Selection Guide - Power Transformer Technical Guide & Educational Resources

Complete technical specifications for 75:5 current transformers. This guide covers accuracy classes for metering and protection, burden ratings, physical dimensions, and application recommendations for 75-Amp circuits.

⚡ 75:5 CT Quick Reference

CT Ratio
75:5

Turns Ratio
15:1

Metering Accuracy
Class 0.5S

Protection Class
5P20

Understanding the 75:5 CT Ratio

A 75:5 current transformer reduces 75 Amps of primary current to 5 Amps of secondary current, providing a 15:1 step-down ratio. This allows high-current circuits to be safely monitored using standard 5-Ampere meters and relays.

📐 Calculation Example

If your ammeter connected to a 75:5 CT reads 3.5 A:

Primary Current = 3.5 A × 15 = 52 Amps

Technical Specifications for 75:5 CTs

CT Ratio	75:5 (15:1)
Primary Current Rating	75 A	Secondary Current	5 A
Metering Accuracy Class	Class 0.5S	Protection Accuracy Class	5P20
Burden Rating	2.5 VA	Knee Point Voltage (est.)	~120 V
Window Size (typical)	0.75″ diameter	CT Type	Solid Core
Frequency	50/60 Hz	Insulation Class	0.6 kV (LV)

Accuracy Class Explained

Metering Class 0.5S:

Extended range metering (IEC Class 0.5S). Maintains ±0.5% accuracy from 20% to 120% of rated current. Ideal for variable loads.

Protection Class 5P20:

Protection class (IEC 5P20). Maintains 5% composite error up to 20× rated current for reliable fault detection.

Burden Rating: 2.5 VA

Low burden (2.5 VA). Suitable for modern electronic meters with minimal VA requirements.

⚠️ Critical Safety Note: Never open-circuit the secondary winding of an energized CT. This can generate dangerous high voltages (120V+ at rated current) and damage the CT due to core saturation.

Application Guide for 75:5 CTs

Category: Residential & Light CommercialSmall three-phase panels light commercial

Typical Use Cases

Single-phase residential main panels
Small retail stores and offices
HVAC unit monitoring
Branch circuit sub-metering

CT Selection Considerations

When to Choose 75:5 Ratio

Load Current: Use when expected maximum load is 45 – 75 Amps. The CT should be sized so normal operating current is 60-80% of CT rating for optimal accuracy.
Conductor Size: This CT has a 0.75″ window, suitable for conductors up to approximately 375 MCM or 1″ bus bar.
Turns Ratio: 15:1 reduction. Secondary current = Primary / 15.

When to Choose Adjacent Ratios Instead

Condition	Recommended Ratio	Reason
Load is typically below 37A	25:5	Operating at higher % of CT rating improves accuracy at lower currents
Load may exceed 75A occasionally	125:5	Prevents CT saturation during peak loads or motor inrush
Conductor won’t fit 0.75″ window	Larger window CT or Split-Core	Consider split-core for retrofit without de-energizing

Frequently Asked Questions about 75:5 CTs

❓ What does a 75:5 CT ratio mean?

A 75:5 current transformer has a turns ratio of 15:1. When 75 Amps flows through the primary conductor, the CT outputs 5 Amps in the secondary circuit. This step-down allows safe measurement with standard 5A meters.

❓ How do I calculate the primary current from the secondary reading on a 75:5 CT?

Multiply the secondary ammeter reading by the turns ratio (15).
Formula: Primary Amps = Secondary Amps × 15
Example: If the meter reads 2.5A, the primary current is 2.5 × 15 = 37 Amps.

❓ Can I use a 75:5 CT with a 1A secondary meter?

Yes, but you’ll need a 75:1 CT instead of 75:5. Alternatively, use a 5:1 auxiliary CT to step down the 5A secondary to 1A. The 1A secondary is common in IEC standards and allows for longer cable runs with less voltage drop.

❓ What happens if the primary current exceeds 75A?

The CT may saturate, causing non-linear output and inaccurate readings. For protection CTs (Class 5P20), the CT is designed to maintain accuracy up to 20× rated current (1500A) for fault detection. For metering, sustained overcurrent should be avoided.

Compliance & Standards

Current transformers should comply with the following industry standards for accuracy and safety. For official standard documents, visit IEC or IEEE.

IEC 61869-2: Instrument transformers – Part 2: Additional requirements for current transformers
IEEE C57.13: IEEE Standard Requirements for Instrument Transformers
ANSI C12.1: American National Standard for Electric Meters – Code for Electricity Metering
IEC 60044-1: Instrument transformers – Part 1: Current transformers (superseded by IEC 61869)
NIST Handbook 44: Specifications for Current Transformers Used in Metering

For more background on current transformer principles, see the Wikipedia article on Current Transformers.

Explore Related CT Ratios

← 50:5 CT

75:5 (Current)

100:5 CT →

Related Resources

📚 Current Transformer Guide
Complete CT fundamentals

📊 CT Ratio Selection Chart
Compare all CT ratios

📝 About This Guide

This technical specification guide is compiled by the Transformer4U Engineering Team, drawing from industry standards (IEC 61869-2, IEEE C57.13) and manufacturer datasheets.

Disclaimer: Specifications shown are typical values. Actual specifications vary by manufacturer and model. Always verify with the specific CT datasheet. Last updated: January 2026