Three-way switch
One of a pair of switches that control the same light from two places. Plans mark it S3.
This explains what a symbol represents on a drawing. Conventions vary between firms and standards, and a drawing's own legend takes precedence.
How to read it
The Army's print-reading manual describes a three-way switch circuit as one that controls a single outlet from two locations, and says the switch appears on an electrical plan as the symbol S3. NECA 100 lists the three-way switch after the single and double pole switches and before the four-way switch. The Navy's Engineering Aid course gives the typical case: one switch at the head of a stairway and the other at the foot.
Also searched as: 3-way switch, S3.
Look-alikes
A plain S is a single-pole switch, which controls a light from one place.
More blueprint symbols
Sources
- Symbols for Electrical Construction Drawings, NECA 100-2013 (ANSI), National Electrical Contractors Association, sec. 4.0, symbol 4.003
- Construction Print Reading in the Field, TM 5-704 / AFM 85-27, Departments of the Army and the Air Force, para. 3-27c
- Engineering Aid 3 (NAVEDTRA 14069), Nonresident training course, April 1995, Naval Education and Training Command (copy hosted by Navy Tribe), Ch. 9, switches
Last reviewed September 24, 2026 by Rory Hansen. How we research