Pigtail
Pigtail turns up in 4 fields: electricians, HVAC, boilers, fiber optics. Here is what it means to the people in each one.
Pigtail in electricians
Short length of wire spliced to several conductors so only one lead goes to a device terminal.
A pigtail is a short jumper, usually 6 to 8 inches, spliced together with the circuit conductors so a single wire lands on the device. Pigtailing keeps the rest of the circuit intact when a device is removed, and code requires it for grounds and for the neutrals of multiwire branch circuits. Pigtails also tie the ground to a metal box with a green screw, and the word covers factory leads on fixtures and cord-and-plug sets on appliances.
"Pigtail the neutrals on that shared circuit, don't feed through the receptacle, the inspector checks for it."
Pigtail in HVAC
A short jumper wire spliced into a group of conductors so they can share a single terminal.
Terminals and device screws are rated for one or two wires, so techs splice several conductors together with a wire nut or crimp and run one short lead, the pigtail, to the terminal. In HVAC panels and thermostat wiring, pigtails keep common and power connections tidy and code-compliant. A loose or overheated pigtail is a classic intermittent no-heat or no-cool fault.
"Three commons on one screw at the board, make a pigtail and land one wire, that's why it keeps dropping out."
Pigtail in boilers
A looped siphon pipe between a steam boiler and its pressure gauge or control, filled with condensate to block live steam.
Steam condenses in the loop and forms a water seal, so the gauge or pressure switch sees boiler pressure but not steam temperature, which would ruin its bellows or diaphragm. The pigtail must be kept clear, because sludge or scale plugging it leaves the control blind and the boiler free to overpressure. Standard drawings call it a siphon; everybody in the boiler room calls it a pigtail.
"Pressuretrol isn't seeing pressure, pull the pigtail and blow it out, it's probably packed with crud."
Pigtail in fiber optics
A short fiber with a factory connector on one end and bare fiber on the other, spliced onto a cable to terminate it.
A length of fiber, usually 1 to 3 m of 900 µm buffered or jacketed fiber, with one factory-polished connector. The bare end is fusion spliced to a cable fiber and the splice stored in a tray, giving the cable a factory-quality termination without field polishing. It is the standard way to terminate loose-tube outdoor cable in panels.
"Splice 48 LC/UPC pigtails onto the feeder in the panel and label them by tube color before you close it up."