03 · Current Sensor Selection

rbAmp measures current through an external CT clamp (current transformer) and voltage through a built-in voltage transformer (on UI* SKUs). This chapter covers how to choose a clamp for your load and how to tell the module which model is installed via YAML.

Sensor family (sensor_class:)

The hardware revision of the module determines which current-sensor family is supported. Current SKUs ship with one option:

The class is set once and stored in the module's flash:

rbamp:
  id: meter1
  sensor_class: SCT_013   # defaults to SCT_013, may be omitted

On firmware v1.2+ this value becomes a precondition for writing the CT model: the module rejects the write if the class is not set. On earlier firmware the value is accepted by the schema and applied automatically on upgrade.

What "reserved" means for WIRED_CT and BUILTIN_CT — the component schema accepts both values today (the YAML validates, no cv.Invalid is raised). However, no current rbAmp hardware revision physically supports them: the module will accept the class write into the register, but without the matching front-end (WIRED_CT — a copper resistive shunt on the current-carrying bus; BUILTIN_CT — an on-board toroidal CT) the readings will not correspond to reality.

For production deployments today, choose SCT_013 only (or omit the key — it is the default). Declaring WIRED_CT / BUILTIN_CT without the matching hardware is an anti-pattern that the component cannot detect and will not warn about.

These classes will enter production documentation after the rbAmp-WiredCT / rbAmp-BuiltinCT SKUs ship — track Changelog.

Choosing an SCT-013 model

The module accepts any of the five models in the SCT-013 series. Their electrical outputs differ, so the module needs to know which one is installed — factory coefficients are loaded for the specific model.

The burden resistor is already fitted on the module PCB — no external resistors between the clamp output leads are required.

Selection tree

Peak continuous current through the conductor:
  |
  +-- <= 5 A    (LED, individual appliances, <= 1.15 kW)
  |       -> SCT-013-005
  |
  +-- 5..10 A   (PC, small loads, <= 2.3 kW)
  |       -> SCT-013-010
  |
  +-- 10..30 A  (whole-house UK/EU, <= 6.9 kW)
  |       -> SCT-013-030
  |
  +-- 30..50 A  (EV, large air conditioner, <= 11.5 kW)
  |       -> SCT-013-050
  |
  +-- > 50 A    (panels, commercial)
          -> SCT-013-100

Rule of thumb: pick the lowest-rated clamp whose maximum measured value covers the peak load with about 30% headroom. An oversized clamp on a small load reduces signal amplitude relative to noise — accuracy drops. A clamp rated below the peak saturates the core and produces grossly incorrect readings.

How to tell the module

There are two keys in YAML — pick one of them depending on whether you have the same clamp on every channel or different clamps. The syntactic shift is from a scalar value to a YAML list (array):

Step 1 — one clamp for every channel (ct_model:, scalar)

If you have UI1 (one current channel) or UI3 with identical clamps on all three channels, one line is enough:

rbamp:
  id: meter1
  ct_model: SCT_013_030      # ← scalar value, one model for every channel

This is the most common case. Most deployments start with a single clamp model and never move past this line.

Step 2 — different clamps on different channels (ct_models:, list)

If you have UI2 / UI3 and want to fit different clamps on different channels (for example a small clamp on a standby line and a larger one on the main feed), replace ct_model: with ct_models: (with a trailing s) and use a YAML list in square brackets:

rbamp:
  id: meter_ui3
  ct_models: [SCT_013_005, SCT_013_030, SCT_013_100]
  #         ↑                            ↑                           ↑
  #     channel 0                    channel 1                  channel 2
  #
  # This is a standard YAML list (1..3 elements).
  # Order matters: position in the list = channel index.

This is the spot people most often trip over when modifying multi-channel setups. Remember: ct_model: (no s) is a scalar value for a single model; ct_models: (with s) is a list of values in square brackets, one per channel. The list length must match the number of active module channels (1 for UI1, 2 for UI2, 3 for UI3).

⚠ Mutually exclusive. Declaring both ct_model: AND ct_models: at the same time is a schema validation error (cv.Invalid). Use only one of the two.

A UI3 install with different clamp sizes

The canonical scenario for a home with on-site generation or a mix of load types:

• Channel 0 — small loads (LED lighting, standby electronics). An SCT-013-005 (5 A) clamp gives maximum resolution on small currents.
• Channel 1 — main appliances (cooktop, water heater). An SCT-013-030 (30 A) clamp covers the typical household peak.
• Channel 2 — main inlet or a heavy consumer (EV, heat pump). An SCT-013-100 (100 A) clamp leaves headroom for peaks.

A single rbamp: block is enough:

rbamp:
  id: home_meter
  ct_models: [SCT_013_005, SCT_013_030, SCT_013_100]
sensor:
  - platform: rbamp
    rbamp_id: home_meter
    voltage:
      name: "Mains Voltage"
    current:
      name: "Lighting Current"     # SCT-013-005, channel 0
    current_1:
      name: "Appliances Current"   # SCT-013-030, channel 1
    current_2:
      name: "Main Inlet Current"   # SCT-013-100, channel 2
    # ...power_0/_1/_2, energy_0/_1/_2 declared the same way

The component installs the exact model on each channel at setup.

What the key does under the hood

On firmware v1.2+, writing ct_model: / ct_models: atomically loads the factory coefficients (noise floor + gain) for the selected model into the module's RAM and saves them to flash. No additional calibration steps are required on the user side — the module ships from the factory with every coefficient table preloaded.

On earlier firmware the value is stored as a model tag. Full auto-load of coefficients arrives with a firmware upgrade — no YAML changes will be needed.

Writing each model takes about 700 ms of flash-write time. For a UI3 with three different models (ct_models: [...]) total setup time is around 2-3 seconds at boot. The component feeds the watchdog during that window.

Verifying the install

After the first boot, check the readings on a purely resistive load — an incandescent bulb, kettle, or heating element. A resistive load gives a known signal shape:

• Power factor (power_factor) should be close to 1.00 (consistently > 0.95).
• Real power (power) should be positive and stable.
• RMS current (current) should match P / U with reasonable accuracy.

If the readings do not match: - current stays at ~0 with the load running — check polarity and that the clamp encloses a single conductor (see below) - power is steadily negative — flip the clamp on the conductor (arrow the other way) - power_factor at ~0.5..0.7 on an incandescent bulb — a reactive load may be nearby, or the clamp is installed incorrectly

Symptom-driven diagnostics are in Troubleshooting.

CT clamp polarity

The clamp body has a printed arrow indicating the "normal" energy flow direction (consumption, toward the load). With correct installation (arrow following current flow into the load):

• P_real > 0 — consumption.
• P_real < 0 — export (STANDARD / PRO only).

If real power is negative while a load is running, physically flip the clamp on the conductor. Do not correct polarity in software: that masks directional bugs in downstream automations.

The clamp must close completely around one conductor. Do not run L and N through the same clamp — their magnetic fields cancel and the sensor will read near zero.

Voltage channel

UI* SKUs contain a built-in voltage transformer. Its primary winding connects to mains L and N through the module's terminal blocks; its secondary is loaded into the board's burden network and fed into the ADC.

The voltage transformer's factory configuration is fixed by the board revision — there is nothing for the user to tune. The voltage sensor is ready to use out of the box.

I-only SKUs (I1 / I2 / I3) have no voltage channel. The voltage slot returns 0 on those and must not be declared alongside power: or energy: (the schema validator enforces the dependency).

Multi-channel modules

On UI2 / UI3 every channel is a separate ADC plus burden network, not a shared core. This lets you:

• Fit the same CT model on every channel (ct_model:) — the typical configuration for uniform loads.
• Fit different models on different channels (ct_models:) — to optimize accuracy on mixed installs (see above).
• Aim one channel at consumption and another at solar export — polarity on each channel is set independently by the physical clamp direction.

What's next

• Wiring — physical wiring, GPIO selection, multi-module bus
• Quickstart — first boot in 5 minutes with a pre-configured clamp
• Troubleshooting — symptom-driven install diagnostics

Source & issues: rb-amp/rbamp-esphome · this page in the repo: docs/03_sensor_selection.md