CT101 vs CT303: which Milesight current transformer for your circuit?

Short answer: Both are self-powered Milesight LoRaWAN current transformers that report RMS current. The CT101 is a single clamp rated to 100 A with a 16 mm opening, for one conductor. The CT303 has three clamps rated to 300 A each with a 24.2 mm opening, and reads all three phases at once on one transceiver. The conductor size, the current and whether the circuit is three-phase decide it.

Which one fits the circuit?

Choose the CT101 for circuits up to 100 A where the conductor passes through a 16 mm opening: lighting, small power, a kitchen circuit or a single machine. One clamp goes round one conductor.

Choose the CT303 for three-phase loads and larger feeders. Its three clamps are each rated to 300 A with a 24.2 mm opening, and it detects all three phases simultaneously, so one device covers a three-phase supply that would otherwise need a CT101 on every phase.

What about small currents?

Both power themselves from the conductor they measure, and that sets a floor. The CT101 needs 1.5 A at a 1 minute interval, or 1.4 A at 10 minutes, to keep reporting. The CT303 needs 3 A at 1 minute, or 2.5 A at 10 minutes, and draws its power from the phase A wire only. Both can run from 5 V over USB-C instead, which is the answer for circuits that are often lightly loaded.

How accurate are they?

The CT303 is rated ±1%. The CT101 is ±1% above 5 A and ±3% at 5 A and below. Both sample at 3.3 kHz, resolve 1 mA and include an NTC temperature sensor. Both measure current, not energy: kWh is calculated with an assumed voltage and power factor, so treat it as management data rather than a billing reading.

Installation

The CT101 is one compact unit of 86.5 × 31 × 37.4 mm. The CT303 is a small transceiver with three clamps of 250 g each, the clamps rated from -40°C to 55°C and the transceiver from -20°C to 70°C. Both are IP30, so they belong inside a distribution board or panel, and fitting either is work for a competent electrician with the board made safe.