CT ratio and burden equations
The CT secondary loop includes both outgoing and return conductors, and its burden increases with secondary current squared.
Rlead = 2ρL/AVAlead = Is² × RleadVAtotal = VAlead + ΣVAdevicesCT primary ratio >= Iprimary × Fmarginwhere:
- ρ
- Copper conductor resistivity used by the calculator[Ω·mm²/m]
- L
- One-way secondary lead length[m]
- Is
- Rated CT secondary current[A]
- VAtotal
- Total connected secondary burden[VA]
A burden pass does not establish metering accuracy or protection-CT saturation performance.
How to calculate CT ratio and burden
Enter the maximum primary current, desired ratio margin, 1 A or 5 A secondary, one-way copper lead length, conductor area, and all connected device burdens. The calculator selects a reference primary ratio and adds the complete lead-loop burden.
- Choose a ratio above the maximum intended primary current.
- Add both outgoing and return secondary lead resistance.
- Calculate lead VA at rated secondary current.
- Add meter, relay, and other burdens.
- Compare total burden with the CT rated burden.
CT lead burden and the 1 A versus 5 A choice
Secondary wiring burden follows I squared R. At the same loop resistance, a 5 A CT produces 25 times the lead burden of a 1 A CT. This makes 1 A secondaries attractive for long cable runs, subject to the connected equipment and project standard.
Rlead = 2ρL/A
VAlead = Is² × Rlead
Metering CT and protection CT limits
A VA burden pass alone does not establish suitability. Metering CTs need the required accuracy range and security behavior. Protection CTs need saturation analysis using class, accuracy limit factor or knee point, winding resistance, relay demand, maximum fault current, and system X/R ratio.
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CT burden example
A 5 A CT connected through 20 m one-way of 2.5 mm² copper has about 0.28 ohm loop resistance and 7 VA lead burden. Adding 3.5 VA of instruments and relays creates 10.5 VA total burden, which fits a 15 VA CT before accuracy and saturation checks.
Assumptions
- Copper leads at approximately 20°C with resistivity 0.0175 ohm mm²/m
- Lead length is one way and the calculation includes the complete outgoing-and-return loop
- Connected devices are represented by their stated burden at rated secondary current
Important Warnings
- Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions.
- Ratio and VA burden do not establish accuracy class, instrument security factor, knee-point voltage, saturation, ALF, transient performance, or protection suitability.
- Never open-circuit an energized CT secondary.