CT Ratio Explained: 200/5, 1200/5 and 4000/5 — How to Read and Select a Current Transformer

When an electrical contractor or metering engineer specifies a CT, the ratio number — 200/5, 1200/5, 4000/5 — is the first piece of information a supplier needs. Yet it is also one of the most frequently misread specifications on purchase orders. This guide explains exactly what those numbers mean, how to choose the right one, and what additional parameters must accompany the ratio for a complete, accurate specification.

What a CT ratio number actually says

A CT ratio is written as primary current / secondary current, both in amperes. A 200/5 CT produces 5 A at its secondary terminals when 200 A flows through the primary busbar or cable. A 4000/5 CT produces the same 5 A secondary output when 4000 A flows through the primary. The secondary is always standardised at 5 A (or 1 A in some IEC metering systems) so that the meter, relay or recorder can be a universal 5 A input device regardless of the primary current level.

Choosing the primary rating: 200/5, 1200/5 or 4000/5?

The primary rating must equal or exceed the maximum continuous current the circuit will carry. Select the next standard ratio above your maximum load:

• 200/5 — sub-distribution boards, residential services, small commercial feeders (up to 200 A)
• 600/5 or 800/5 — medium commercial loads, distribution transformers up to 400 kVA at 415 V
• 1200/5 — industrial feeders, main incomer boards, 800–1200 A circuits
• 2000/5 or 3000/5 — large industrial plant, bus ties
• 4000/5 — high-current main feeders, generators, utility supply points

Never select a CT whose primary rating is below the maximum circuit current — the CT core will saturate on overloads and both accuracy and safety are compromised.

Accuracy class: what each rating means for your application

IEC 60044-1 defines accuracy classes for metering CTs: 0.1, 0.2, 0.2S, 0.5, 0.5S, 1, 3 and 5. The number is the maximum percentage composite error at rated current. The suffix S (Special) means accuracy is maintained from 1% to 120% of rated current — essential for billing meters that must be accurate at light loads.

Class	Max error at In	Typical use
0.2S	0.2%	Utility revenue billing, tariff meters
0.5S	0.5%	Commercial billing, sub-metering
1	1.0%	Panel instruments, industrial monitoring
3	3.0%	Indication only, protection CTs

Our MSQ series is available in Class 0.2S, 0.5 and 1. When in doubt, specify 0.5S — it satisfies most commercial metering requirements without the cost premium of 0.2S.

Burden VA — the parameter most buyers forget

Burden is the maximum VA load the secondary circuit can impose on the CT while maintaining accuracy. A typical CT burden is 2.5 VA, 5 VA or 10 VA. Calculate your secondary circuit burden: measure the resistance of the connecting leads (R_lead × I²) and add the input impedance of the connected meter. If the total exceeds the CT rated burden, either increase the conductor cross-section or select a higher burden CT.

MSQ-60 series: how it covers 200/5 to 4000/5 in one family

The MSQ-60 outdoor low-voltage CT family uses a 60 mm × 60 mm window aperture to accept flat busbars up to 60 × 10 mm or cables up to 50 mm². The same housing accommodates primary ratings from 200/5 to 4000/5, with the ratio achieved by varying the number of primary and secondary turns. Compliance: BS 3938/76 and IEC 185 (superseded by IEC 60044-1), rated for 0.66 kV / 2 kV impulse. Suitable for indoor panel and outdoor enclosure installation.

FAQ

What does 200/5 CT ratio mean?

A 200/5 CT ratio means the primary winding carries up to 200 A and the secondary winding outputs 5 A at full load. A 200 A line current becomes 5 A at the secondary terminals — safe for meters and relays rated at 5 A input.

What is the difference between 1200/5 and 4000/5 CT?

Only the primary rating differs. A 1200/5 CT is used on circuits carrying up to 1200 A; a 4000/5 CT on circuits up to 4000 A. Both produce a 5 A secondary output at full load.

What accuracy class should I specify for a metering CT?

For revenue metering specify Class 0.2S or 0.5S (IEC 60044-1). Class 1 and Class 3 are acceptable for monitoring and indication only.

What does CT burden VA mean?

Burden VA is the maximum impedance the CT secondary circuit can drive while maintaining its accuracy class. If the connected load exceeds the rated burden, the CT error increases beyond the class limit.

Can I use a 200/5 CT on a 150 A circuit?

Yes. The CT will output 3.75 A at full load. Program the meter with the 200/5 ratio so it scales the reading correctly. Never run a CT above its primary rating.