Current transformer selection and sizing for metering and protection: CT ratio, burden, accuracy class, wiring and how to specify a CT for your panel.
A current transformer (CT) is the device that lets a standard ammeter, energy meter or protection relay measure a current far larger than it could ever pass through its own terminals. It wraps a core around the primary conductor and induces a precisely scaled-down current in its secondary winding — typically 5 A or 1 A — that the measuring instrument reads instead of the real load current. Get the CT wrong and the meter reads nonsense, the protection relay under- or over-trips, or worse, the open secondary flashes over. This guide covers what a buyer or panel builder should actually specify.
The CT ratio is written as primary current : secondary current, e.g. 100/5 A or 300/1 A. The secondary is almost always 5 A (common in legacy panels) or 1 A (preferred for long lead runs because it loses less voltage in the wiring). Pick the primary rating at or just above the maximum continuous load current you expect on that conductor — never size it to the breaker rating alone. A 250 A feeder that normally runs at 180 A wants a 200/5 or 250/5 CT, not a 400/5, because a CT operating near the bottom of its range reports poorly.
| Primary current | Secondary | Typical use |
|---|---|---|
| 50 – 200 A | 5 A | small distribution, sub-metering |
| 200 – 1000 A | 5 A | main incomers, panel meters |
| 1000 – 3000 A | 1 A | large boards, long cable runs |
| any (retrofit) | 5 A / 1 A | split-core / clip-on for existing busbar |
These are planning values; always confirm the continuous rating against the conductor's own ampacity and your enclosure's derating.
The burden is the total impedance the secondary sees — the meter, the wiring and its connections — expressed in volt-amperes (VA). A CT is only accurate if its actual burden stays below the rated burden stamped on the nameplate. A 15 VA CT feeding a 2 VA meter on 30 m of thin wire can drift badly. Accuracy classes split into two families: metering classes (0.2, 0.5, 1.0) for billing-grade measurement, and protection classes written as an accuracy limit factor, e.g. 5P10 or 10P10 — the “10” meaning the CT stays within 10% error at ten times rated current, so the relay still sees a fault.
A metering CT is designed for the normal load range and saturates (deliberately flattens) above rated current so a fault current does not damage the meter. A protection CT is the opposite: it must stay linear far into the fault range so the relay gets a true signal. Do not swap them — a metering CT on a protection circuit will under-report a fault and let equipment cook; a protection CT on a billing meter will overstate energy and annoy the customer.
Wound CTs have marked terminals (often S1/S2 or P1/P2). The instrument must connect to the polarity-marked pair or the reading reverses. Bar-type and split-core (clip-on) CTs are common for retrofits: the split core opens, clamps around the live conductor, and closes — no need to break the circuit. For multi-circuit panels, a single ring-type CT slipped over each phase conductor inside the enclosure is the cleanest install. Keep all three phase CTs the same ratio and the same orientation, or the summed reading is wrong.
Never leave a CT secondary open while current flows in the primary. With no load to absorb it, the core drives the secondary voltage to thousands of volts — enough to arc, shock and destroy the winding. If you must disconnect a meter, either short the secondary first with a purpose-made shorting block or de-energize the primary. This single rule causes more damaged CTs than everything else combined.
ELEHPD stocks ring-type, split-core and bar-type current transformers in 5 A and 1 A secondaries with metering and protection classes, matched to our ammeters, multifunction power meters and transducers for turnkey panel builds.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.