A hardwired doorbell includes a small transformer that converts the household alternating current (AC), to direct current (DC) for the doorbell chimes. A small, 16 awg wire runs from the door button to the chimes. When pressed, the button will send the transformer output through the chimes, sounding the bell until it is released.
The white wire on the switch loop is used to carry current from the source to the switch and it is marked with black tape or paint to label it as hot. The black on the switch loop runs from the top switch terminal to the top half of the receptacle. With this arrangement the top half of the duplex is controlled with the switch and the bottom half is always hot.
This drawing shows the wiring for adding a new light fixture using an always-hot receptacle outlet as the source. New cable is run from the receptacle to the new fixture location and a switch loop cable is run from there to the new switch location.
The neutral wire in this circuit is taken from the receptacle and spliced to the white on the new cable, and to a pigtail that runs back to the receptacle. The hot is spliced to the black cable wire and a pigtail back to hot on the receptacle.
A special isolated-ground receptacle is require for this circuit and can be identified by the orange color and a small triangle imprinted on the face. When connecting the wires, the isolated ground wire (the red wire pictured here) is marked with green tape or paint on each end and connected to the grounding bar in the service panel, and to the grounding terminal on the receptacle.
This page contains wiring diagrams for adding a new light fixture using an existing receptacle as the source. Diagrams include adding a light from a switched receptacle and tapping an always-hot outlet. Different wiring arrangements are included to allow for either the light or the switch to come first in the circuit.
The neutral and ground wires for the circuit are connected to a bar along the side of the service panel box. The neutral and grounding bars in the box may be separate or, in the case of older service panels, the same bar may be used for both purposes.
This page contains several diagrams for wiring a switch to control one or more receptacle outlets including a split receptacle and multiple outlets wired together.