A clamp measures the magnetic effect of current, so what it reads depends entirely on which conductors are inside the jaws.
It is the one instrument that measures without breaking into the circuit, which is exactly why people use it without thinking about what they have enclosed.
Clamp one conductor and you read the current in it. Clamp the outgoing and returning conductors together and their magnetic effects oppose -- in a healthy circuit they very nearly cancel, and the meter reads close to nothing while full load current is flowing through both. The current did not change. What you enclosed did.
Check yourself: You clamp a cable containing both conductors and read almost zero. Is the circuit dead?
Answer: No. Enclosing outgoing and returning conductors can cancel their magnetic effects.
The clamp responds to the net magnetic effect inside the jaws. Two conductors carrying the same current in opposite directions produce effects that oppose, so a near-zero reading is what a healthy loaded circuit looks like when you clamp the whole cable.
No card. No account needed to start. Score by topic against the 70% mark.
Written from the owner's poster for this lesson. Nothing added beyond what the poster states.
Written for Texas apprentices as free study material. The Texas apprentice licence is an application, not an exam (Occupations Code 1305.161), and nothing here counts as apprenticeship hours or continuing-education credit. VoltMark is not affiliated with, endorsed by, or sponsored by NFPA, PSI, TDLR, or any state or local licensing authority. Always follow your employer's safety programme and your supervising electrician.
Electrician licensing exam prep: practice questions, timed exam simulations, and step-by-step help finding every answer in the NEC.