Build a selection brief for an AC metering CT, or check a sensor you are considering. Start with the current range, the space around the conductor and the meter input.
Minimum round opening, with your allowance
14mm
Check the values to see a result.
For AC load metering. Aperture, rating and nominal output checks do not establish accuracy, insulation suitability or protection performance.
Our engineers will match a CT to this conductor.
Talk to an engineerRound opening = conductor envelope diameter + 2 × clearance per side
Allow a round opening of at least 14 mm, or a rectangular opening of 14 × 14 mm. Investigate a primary rating of at least 100 A and verify accuracy down to 10 A.
Tip: Measure the insulated conductor or same-phase bundle at the installation point. Cable area in mm² is not an outside diameter.
A current transformer must fit the installation, cover the current range and work with the meter. Those are separate checks. This tool produces the dimensions and operating range to put in a selection brief; it can also compare a candidate sensor against those requirements. It does not select an accuracy class from current alone.
Start with the meter specification. A 1 A input, a 5 A input, a voltage-output CT input and a raw Rogowski input require different signals. Then measure the insulated conductor at the installation point and establish the lowest and highest currents you need to record.
D = d + 2c
For a cable of outside diameter d, add your installation allowance c on each side. A 10 mm insulated cable with 2 mm allowance needs a 14 mm round opening. The allowance is a mechanical planning input, not a universal electrical clearance rule.
D = √(w² + h²) + 2c; W = w + 2c; H = h + 2c
Measure the complete width and height, including insulation and spaces between parallel conductors. A round window must clear the diagonal of that rectangle. A rectangular window can be checked in either orientation. Verify corner radii and the real shape against the drawing; a flexible coil also has bend and closure constraints.
Operating fraction = measured current ÷ rated primary current × 100%
Investigate a rating that covers the maximum operating current, then check the error limits at the lowest current of interest. The candidate check shows both ends as percentages of the rating. Those percentages are not an accuracy assessment: obtain the ratio and phase-error specification at the intended burden and frequency.
Use the candidate panel with any manufacturer’s CT datasheet. A pass for aperture means only that the stated geometric opening and allowance fit. A nominal output match does not establish burden, insulation or accuracy. Check the outside dimensions of the case, mounting arrangement and access to its latch before ordering.
With conventional 1 A or 5 A secondaries, the meter and connecting leads contribute burden. Use the CT ratio and burden calculator to calculate it, then compare with the CT manufacturer’s declared operating range. A higher maximum VA rating alone does not prove that the chosen accuracy applies at a very small burden.
The minimum round opening is 14 mm. Investigate a primary rating of at least 100 A with a 5 A output, then verify its accuracy from 10 A to 100 A and check the connected burden. A larger aperture may be convenient, but it does not require a larger current ratio.
Minimum round opening, with your allowance 14 mm Open in the calculator
The diagonal is 50 mm, so the minimum round opening is 54 mm with the allowance. A rectangular opening can instead be 34 × 44 mm. Investigate a rating of at least 400 A; the meter input still has to be confirmed. Neither answer specifies the outside size of the CT case.
Minimum round opening, with your allowance 54 mm Open in the calculator
No. Cable area describes the metallic cross-section. Insulation, construction and the manufacturer determine the finished diameter. Measure it or use the cable’s dimensioned datasheet. For parallel conductors of one phase, measure the whole bundle rather than multiplying a single cable diameter.
No. Its opening construction can avoid disconnecting the conductor, but isolation and safe installation still need to be planned by a qualified installer. A solid-core CT also needs an installation path over the conductor. Never open an energised conventional current-output CT secondary.
Only with a suitable signal path. A raw coil needs integration and the correct sensitivity and phase treatment. A matched integrator can provide a 333 mV output; a raw coil and a 333 mV CT are not directly interchangeable. Confirm the complete coil, integrator and meter combination.
Open Optional EpiSensor matching to compare this brief with the current catalogue. The result is a separate meter-and-sensor option, not a recommendation for a third-party meter. For a ZDR mini-CT connection, enter the existing CT’s rated secondary and secondary lead diameter separately from the primary circuit. The mini-CT reads that secondary lead; do not pass the primary conductor through it.
No. It covers AC load-metering geometry and nominal current/output checks. Protection applications need fault-current and saturation requirements, while DC needs a suitable DC sensing method. Billing requirements, frequency response and installation category also require their own specification checks.