CT Ratios: How to Spec a Current Transformer

An applied extra to Cobler's Electricity Fundamentals course.

Open any commercial meter cabinet and you will find them: fat metal rings clamped around the incoming busbars, each one stamped with two numbers and a slash. 800/5. Or 400/5, or 1600/5. The same pair turns up on every sub-metering quote you will ever read, and every energy-monitoring project starts with the same question from the contractor: "what CT do we need?" Most people nod along, sign the quote, and never find out what the ring on the busbar is or why its two numbers were chosen. The CT ratio is one of those things that looks like an expert's private code and turns out, once someone says it plainly, to be almost embarrassingly simple.

What does a CT ratio like 800/5 actually mean?

It means that when 800 amps flow in the busbar, exactly 5 amps flow out to the meter. Everything in between scales in proportion.

A CT, or current transformer, is a transformer worn like a ring on a cable. It runs on the same physics as the green box outside your condo: a changing current in one winding induces a proportional current in another. The busbar threaded through the ring is the primary, usually a single pass. Wound inside the ring is a secondary of many turns, and it produces a small, faithful copy of the big current. The electricity meter can only drink from a thin, polite trickle, so the CT hands it 5 amps that mirror the 800 amps in the bar. The meter never touches the firehose; it reads the copy and multiplies back up by the ratio to display the real amps.

So 800/5 is a 160-to-1 gear-down. At 800 A in the bar you get 5 A out. At 400 A you get 2.5 A. At 80 A you get 0.5 A. The meter is programmed with that ratio and does the arithmetic for you. Read the stamp and you already know what the ring is doing.

How do you spec a CT? Five plain questions

Choosing a CT is not a dark art. It is five decisions, and a good sub-metering contractor is quietly answering all five when they ask about your breaker sizes and cable runs.

1. Primary rating: pick the next standard size above the circuit's maximum current. The primary number should sit just above the circuit's maximum continuous current, or the main breaker rating feeding it. CTs come in a standard series (100/5, 200/5, 400/5, 800/5, 1600/5, with intermediate 250, 500, 600, 750, 1000 and so on), so you round up to the next one. A common rule of thumb is full-load amps times 1.25, then up to the next standard ratio. What you must not do is oversize wildly "to be safe." A CT is at its most honest when the normal load sits around 40 to 100 percent of its rating. Fit an 800/5 on a circuit that only ever pulls 30 A and the CT spends its life loafing near 4 percent, below the floor where a plain metering class stays specified at all (CT selection guide, ctlsys.com).

2. Secondary: 5 A as standard, 1 A when the meter is far away. 5 A is the traditional secondary and fine when the CT sits close to the meter, which is most panels. Choose 1 A when the meter or relay is a long cable run away. Power lost in the secondary leads rises with the square of the current, so a 1 A secondary wastes 25 times less in the wire than a 5 A one over the same run. That is why large substations and switchyards, where leads travel far, tend to run on 1 A (Janitza, selecting current transformers).

3. Accuracy class: 0.5 for metering, 0.5S if the load runs light. The class is the CT's honesty rating. Class 0.5 keeps the ratio error inside its band over a specified range, roughly 5 to 120 percent of rated current, and pins the tightest 0.5 percent only near full load; below 5 percent it is unspecified and error grows. Class 0.5S is the better animal for billing: it extends that specified band all the way down to 1 percent of rating. That extended low-end honesty is exactly what the "S" buys you, and it is exactly where an oversized ordinary CT falls apart (IEC accuracy classes, daulma.com). Protection classes like 5P10 are a different creature entirely. They are built to stay linear during a fault (5P10 means it stays within 5 percent up to ten times rated current, so a relay can still see a short circuit clearly). They are not built to bill accurately at normal load. You meter on a metering core and protect on a protection core; you do not swap them.

4. Burden: the VA the CT has to drive. Burden is the total load the CT secondary must push its current through: the meter coil plus the connections plus the resistance of the leads themselves. It is rated in VA. Old electromechanical meters and long leads needed a lot of it, which is why legacy CTs were spec'd at 15 or 30 VA. Modern electronic meters draw almost nothing, often well under 1 VA, so a huge-VA CT is now carrying a featherweight and can actually drift out of its accuracy band from being under-loaded. Match the burden to the real meter and leads rather than buying the biggest number on the shelf: spec around 1 VA for a modern electronic meter, not the 15 VA the legacy sheet still lists.

5. Physical form: solid ring for new builds, split-core for retrofits. A solid (ring) core is a closed doughnut, so the busbar has to be disconnected and threaded through it. That is fine on a new build where the panel is open and dead. A split-core CT opens on a hinge, so you can clamp it around a live-but-isolated busbar without dismantling the switchboard. That is the workhorse of retrofit sub-metering, where shutting the whole building down to fit a ring is not an option. Revenue-grade split-cores now exist at Class 0.5, so you rarely trade away accuracy to get the convenience (Accuenergy AcuCT-R).

The one rule everyone in the trade knows and no layman does

Never open the secondary of a CT while its primary is carrying current. This is the rule electricians learn on day one and the public has never heard.

In normal running, the secondary's 5 A produces a magnetic flux that cancels most of the primary's flux, keeping the core calm and the secondary voltage low. Disconnect the meter with the busbar still live and that cancelling flux vanishes. The full primary current now drives the core hard into saturation, and with nowhere for the 5 A to go, the CT tries to force it through the open gap the only way it can: by cranking its voltage up and up. On a large-ratio CT that open-circuit voltage can reach several kilovolts, enough to arc across the terminals, break down insulation, and start a fire (Schneider Electric, FA125574). It is a pump slammed against a shut valve, building pressure until something bursts.

This is why metering panels have shorting blocks. A short circuit is a CT's happy, safe state: current keeps circulating, the secondary voltage stays near zero, nothing builds up. Before anyone removes a meter, they short the CT terminals first. A CT's natural idle state is shorted, the exact opposite of a voltage transformer, which must never be.

What you can read now

Pick up a metering schedule and the codes have stopped being a private language. You know that the ring stamped 800/5 gears 800 amps down to 5, that its primary number was chosen just above the breaker rating, that the 0.5S in the accuracy column means it stays honest even when the circuit runs light, and that the contractor asked about your breaker sizes and lead lengths because those two answers set questions one, two and four. You can look inside your own meter cabinet, the same one you have walked past for years, and read what it is measuring you with. That same cabinet, decoded further, is what a single-line diagram draws as a page.

CT selection is step one of every CobiNeural sub-metering deployment, and specifying Class 0.5S rather than a loose ordinary CT is a large part of why our platform's numbers stand up when you lay them next to TNB's own meter. The measurement is only ever as good as the ring on the busbar.

This is an applied extra to Cobler's Electricity Fundamentals course. It builds directly on How Transformers Work and How Electricity Meters Work, where the CT first appears as the meter's firehose tap.

Want your sub-meters spec'd and read well enough to argue with TNB's bill? Book a demo and we will show you what accurate metering looks like on your own busbars.