To understand a Square D writing diagram, follow its respective key and recognize basic schematic symbols. A standard key for Square D diagrams can be downloaded at Schneider Electric's website.
Major Square D schematic icons include resistors being depicted as a rectangle surrounding the text "RES" (except for heating elements, which are designated with "H"), inductors being indicated by wide-curved lines with tight loops between arches, and capacitors being depicted as a straight line that is perpendicular to a curved line. Alternating-current motors are shown as circles, while direct-current motors are either groups of loops similar to the capacitor icon or, in the case of armatures, circles with lines extending from each side. Wiring diagrams that run through multiple rooms may also have a variety of unique symbols depicting electric appliances such as lights.
A power circuit is generally shown in wiring diagrams as a solid line that starts and terminates at a power source, but is broken up by icons that indicate resistors, switches and other disruptions. These disruptions often correspond to devices such as "on/off" buttons and light bulbs. On Square D schematics, power circuits are indicated with bold lines.
Control circuits are specialized to carry data rather than raw electric power, and have convoluted paths that define certain technological behavior. Printed circuit boards, commonly found inside computer chassis, are a common example of control circuitry. On Square D diagrams, a control circuit is indicated by thin lines.