The Fluke I40S-EL3X/3PK 40A AC Clamp-On Current Transformer (3 Pack). Current Clamp-on Transformer is used for low current measurement; suitable for making measurements up to 40A, and also measures accurately on 1A and 5A secondary current transformers. The i40s-EL jaw is capable of being clamped on current conductors up to 15mm in diameter