400:5 Current Transformer (CT): Ratio Specifications & Selection Guide - Power Transformer Technical Guide & Educational Resources

Complete technical specifications for 400:5 current transformers. This guide covers accuracy classes for metering and protection, burden ratings, physical dimensions, and application recommendations for 400-Amp circuits.

⚡ 400:5 CT Quick Reference

CT Ratio
400:5

Turns Ratio
80:1

Metering Accuracy
Class 0.5

Protection Class
5P20

Understanding the 400:5 CT Ratio

A 400:5 current transformer reduces 400 Amps of primary current to 5 Amps of secondary current, providing a 80:1 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 400:5 CT reads 3.5 A:

Primary Current = 3.5 A × 80 = 280 Amps

Technical Specifications for 400:5 CTs

CT Ratio	400:5 (80:1)
Primary Current Rating	400 A	Secondary Current	5 A
Metering Accuracy Class	Class 0.5	Protection Accuracy Class	5P20
Burden Rating	10.0 VA	Knee Point Voltage (est.)	~300 V
Window Size (typical)	2.0″ diameter	CT Type	Solid Core
Frequency	50/60 Hz	Insulation Class	0.6 kV (LV)

Accuracy Class Explained

Metering Class 0.5:

Standard metering accuracy (IEC Class 0.5). ±0.5% ratio error at rated current. Common for energy management.

Protection Class 5P20:

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

Burden Rating: 10.0 VA

Standard burden (10.0 VA). Compatible with most analog and digital meters, plus short cable runs.

⚠️ Critical Safety Note: Never open-circuit the secondary winding of an energized CT. This can generate dangerous high voltages (300V+ at rated current) and damage the CT due to core saturation.

Application Guide for 400:5 CTs

Category: Commercial & Light IndustrialIndustrial main switchboards large motor feeders

Typical Use Cases

Commercial building service entrance
Motor control center (MCC) feeders
Data center rack PDU monitoring
Manufacturing floor sub-panels

CT Selection Considerations

When to Choose 400:5 Ratio

Load Current: Use when expected maximum load is 240 – 400 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 2.0″ window, suitable for conductors up to approximately 1000 MCM or 5″ bus bar.
Turns Ratio: 80:1 reduction. Secondary current = Primary / 80.

When to Choose Adjacent Ratios Instead

Condition	Recommended Ratio	Reason
Load is typically below 200A	300:5	Operating at higher % of CT rating improves accuracy at lower currents
Load may exceed 400A occasionally	500:5	Prevents CT saturation during peak loads or motor inrush
Conductor won’t fit 2.0″ window	Larger window CT or Split-Core	Consider split-core for retrofit without de-energizing

Frequently Asked Questions about 400:5 CTs

❓ What does a 400:5 CT ratio mean?

A 400:5 current transformer has a turns ratio of 80:1. When 400 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 400:5 CT?

Multiply the secondary ammeter reading by the turns ratio (80).
Formula: Primary Amps = Secondary Amps × 80
Example: If the meter reads 2.5A, the primary current is 2.5 × 80 = 200 Amps.

❓ Can I use a 400:5 CT with a 1A secondary meter?

Yes, but you’ll need a 400:1 CT instead of 400:5. 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 400A?

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 (8000A) 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

← 300:5 CT

400:5 (Current)

500:5 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