The Stanley S300, covered by the FMHT77407/STHT77407 instructions, is an electronic locator with stud, metal and live-AC functions. It can assist a mounting project, but it cannot certify that drilling at a mark is safe or that a fixing will support the intended load.
This is a manual-based review. We have not tested its accuracy across different wall assemblies or compared it with a magnetic finder under controlled conditions.
The depth figures describe different modes
The Stanley S300 manual lists up to 1-1/2 inches for wood or metal studs through drywall, two inches for live 120-volt AC wiring and three inches in metal-scan mode. A three-inch claim therefore does not mean three-inch stud detection through any wall.
The manual excludes several surfaces, including masonry, foil-faced material and ceramic tile, and says it is not designed to locate plastic pipes. Thickness, density and moisture affect sensing. Read the limitations for the actual wall rather than treating the deepest advertised number as a guarantee.
Calibration and repeat readings
The S300 uses a nine-volt battery. Follow the calibration sequence on a suitable area, hold the sensor flat and scan as directed. Starting over a stud or unsuitable material can produce an unhelpful calibration.
Mark and compare repeat passes instead of acting on one isolated beep. Where the pattern does not make sense, stop and investigate the construction. A wide indication near a door can reflect framing details; a signal by plumbing or electrical fittings needs particular caution.
An AC warning is not a clearance certificate
The manual warns that shielded wiring and some dense or metallic assemblies can prevent detection. Absence of an alert does not establish absence of a cable. Do not use the S300 as the sole method for identifying concealed services or checking that a circuit is de-energized.
Before invasive work, identify likely services and arrange appropriate isolation and verification. If the route remains uncertain, obtain competent help. A screw long enough to reach a stud can also reach something behind or alongside it.
For persistent inconsistent readings, our stud-finder false-positive guide explains why the wall assembly matters.
Verdict
Consider the S300 for compatible interior wall scanning when its modes answer the job’s needs. For a shelf or TV mount, separately verify wall structure, bracket instructions, fasteners and load support. Choose another inspection method where the surface is excluded or the readings cannot be explained; buying a locator does not remove those checks.