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Solid-Core Current Transformer

Solid-core current transformers (CTs) are approximately toroidal in shape and require the circuit being measured to be disconnected during installation. They are more accurate than split-core CTs, but are often not a practical solution for residential circuit breaker panels because it is necessary to physically disconnect each current line to install the CT. Solid-core CTs can be a good choice for permanent installation if you have the opportunity to install the CTs while the circuits are being wired up for the first time.

Solid core current transformer
Credit: Lawrence Berkeley National Laboratory