Current Transformers (CTs) are the sensors that clip around the wires in your electrical panel and tell the Vue how much current is flowing. They're a core part of how the Vue monitors your home. This article covers how CTs work, the three types Emporia ships, how many you need, and when to use each type.

A CT is a split-core clamp built from ferrite, a magnetic material. When current flows through a wire, it creates a magnetic field around the wire. The ferrite core of the CT picks up that magnetic field and produces a small proportional current in the sensor, which the Vue reads.

Because the CT is split, you can open it up, slip it around an existing wire, and close it again — no need to disconnect or cut anything. The CT doesn't touch the conductor itself, so it's a passive measurement with no electrical connection to the monitored line.

Emporia offers three CT sensor types. Your Vue kit includes the first two; Flexible Sensors are sold separately.

200A CT Sensors (rigid, for main lines). Large clamp-on CTs that attach around the main conductors in your panel. They plug into the Vue's three main ports, labeled A, B, and C, and measure up to 200A per phase. These are the sensors used to measure your whole-home consumption.

50A CT Sensors (rigid, for branch circuits). Smaller clamp-on CTs for individual branch circuits (breakers). They plug into the 50A side ports on the Vue. They measure up to 50A per circuit — sufficient for most residential loads including HVAC, water heaters, EV chargers, and dryers.

Flexible Sensors (Rogowski coils). Long, flexible wire loops designed for panels where a rigid clamp-on won't fit. Most often used on main lines when the mains run through bus bars, tight spaces, or unusual configurations. Sold separately on the Emporia shop in 2-coil and 3-coil kits.

Unlike the rigid 200A and 50A CTs — which draw their power from the Vue itself — Flexible Sensors are paired with active integrator units that need their own power supply. Each kit includes one shared 120–277 V AC power supply that lands on a breaker in your panel. The coil window is 62.7 mm (the maximum conductor that fits inside the loop) and the max rated current is 200 A per coil.

A note on amperage and safety. Exceeding a sensor's rated amperage won't damage the Vue or the sensor — the measurement simply caps at the device port's maximum. Note though that the 50A branch CTs are fully accurate up to 63A and begin to saturate between 63A and 75A. See CT Accuracy: What to Expect from Your Measurements for a full breakdown.

200A main CTs: one per live phase on your main panel. Neutral and ground lines don't count as phases.

50A branch CTs: one per circuit you want to monitor individually. You can monitor up to 16 branch circuits per Vue. Customers typically start with their highest-consumption circuits — HVAC, water heater, EV charger, dryer, range — and add more over time.

200A CTs on smaller panels. The standard 200A CTs work fine on 100A panels and on sub-panels down to 30A. There's no need to buy lower-rated sensors.

Flexible Sensors come in two variants, and for most installations the red is the right choice:

Both variants measure current with equivalent accuracy. The difference is physical handling.

In unusual layouts, you may want to use a 200A CT on a 50A port (or vice versa). This is supported with a small wiring change and an app-side multiplier.

200A CT on a 50A side port. Re-attach the CT's sensor wires into the smaller 50A-sized terminal block. Match the black wire (positive) to the black/solid terminal indicator and the white wire (negative) to the white/empty indicator. Then apply a 4.0× multiplier on that circuit in the app.

50A CT on a 200A top port. Re-attach the sensor wires into the larger 200A terminal block, matching black and white to their corresponding indicators. Apply a 0.25× multiplier on that circuit in the app.

You'll need a small screwdriver for the terminal swap, or you can buy extra terminal blocks from the Emporia shop.

This knowledge base is continuously updated to provide the most helpful guidance for Emporia customers. If you found this article unclear or have suggestions for improvement, please contact our Customer Support team.