100:5 Current Transformer (CT): Ratio Specifications & Selection Guide - Power Transformer Technical Guide & Educational Resources

Complete technical specifications for 100:5 current transformers. This guide covers accuracy classes for metering and protection, burden ratings, physical dimensions, and application recommendations for 100-Amp circuits.

⚡ 100:5 CT Quick Reference

CT Ratio
100:5

Turns Ratio
20:1

Metering Accuracy
Class 0.5S

Protection Class
5P20

Understanding the 100:5 CT Ratio

A 100:5 current transformer reduces 100 Amps of primary current to 5 Amps of secondary current, providing a 20:1 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 100:5 CT reads 3.5 A:

Primary Current = 3.5 A × 20 = 70 Amps

Technical Specifications for 100:5 CTs

CT Ratio	100:5 (20:1)
Primary Current Rating	100 A	Secondary Current	5 A
Metering Accuracy Class	Class 0.5S	Protection Accuracy Class	5P20
Burden Rating	5.0 VA	Knee Point Voltage (est.)	~150 V
Window Size (typical)	1.0″ 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: 5.0 VA

Low burden (5.0 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 (150V+ at rated current) and damage the CT due to core saturation.

Application Guide for 100:5 CTs

Category: Residential & Light CommercialResidential main service 100A panels

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 100:5 Ratio

Load Current: Use when expected maximum load is 60 – 100 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 1.0″ window, suitable for conductors up to approximately 500 MCM or 2″ bus bar.
Turns Ratio: 20:1 reduction. Secondary current = Primary / 20.

When to Choose Adjacent Ratios Instead

Condition	Recommended Ratio	Reason
Load is typically below 50A	50:5	Operating at higher % of CT rating improves accuracy at lower currents
Load may exceed 100A occasionally	150:5	Prevents CT saturation during peak loads or motor inrush
Conductor won’t fit 1.0″ window	Larger window CT or Split-Core	Consider split-core for retrofit without de-energizing

Frequently Asked Questions about 100:5 CTs

❓ What does a 100:5 CT ratio mean?

A 100:5 current transformer has a turns ratio of 20:1. When 100 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 100:5 CT?

Multiply the secondary ammeter reading by the turns ratio (20).
Formula: Primary Amps = Secondary Amps × 20
Example: If the meter reads 2.5A, the primary current is 2.5 × 20 = 50 Amps.

❓ Can I use a 100:5 CT with a 1A secondary meter?

Yes, but you’ll need a 100:1 CT instead of 100:5. 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 100A?

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 (2000A) 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

← 75:5 CT

100:5 (Current)

150:5 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