r/homeassistant • u/ChickenManager77 • 6h ago

Two wire through a CT

Hello, if i put two live wires through a Shelly Pro current transformer, is the reading the exact sum of the current flowing? The wires are both connected to the same terminal block (far away from CT)

Thanks

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u/wannebaanonymous 5h ago

2 wires: it adds the total at all times.

But it is directional as well, so putting e.g. the 2 wires (L and N) to e.g. a light bulb through it means you'll get 0. Reverse the direction of e.g. the N while going through it and you'll get twice what the bulb is using.

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u/neanderthalman 1h ago

Just to add. This is how GFCI’s work. Put a CT around line and neutral and it should always read nothing.

They don’t actually detect ground fault. Not directly. They detect a current imbalance between line and neutral. If the current isn’t balanced, the CT generates a signal and the GFCI trips. The unbalanced current must be taking another path, which is assumed to be a ground fault.

That’s how it can detect a ground fault on the neutral even though the neutral is already deliberately tied to ground.

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u/Mooisjken 5h ago

Both wires should be on the same phase as well. If not, result will still be the sum but not the sum you expect

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u/The_referred_to 5h ago

Yes, it sums both.