[WolfeMacleod]

T-slot duplex socket.

US combination duplex socket


The parallel and tandem socket accepts normal parallel NEMA 1‐15 plugs and also tandem NEMA 2‐15 plugs. Both pairs of sockets are fed internally by the same supply.

A more recent and fairly common version of this type is the T-slot socket, in which the locations of the tandem and the parallel slots were combined to create T-shaped slots. This version also accepts normal parallel NEMA 1‐15 plugs and also tandem NEMA 2‐15 plugs. Incidentally, a NEMA 5‐20, NEMA 6-15, or NEMA 6‐20 plug with a missing earth pin would fit this socket. This receptacle type has been unavailable in retail shops since the 1960s but still available from the manufacturer Leviton (model 5000-I) for replacement only and not for new installations.

Harvey Hubbell had patented the parallel blade plug in 1913, and patented a polarized version in 1916. He also patented the T-slot single outlet in 1915, and a duplex T-slot outlet in 1916 both meant to take his older 1904 tandem and newer parallel plug design. (Single: US Pat. 1146938; Duplex: US Pat. 1210176). Prior to the 1930s, when Hubbell's parallel blade plug received its official rating of 125v (and became the United States' general service electrical plug standard), all of his electrical connectors were used interchangeably on either 120v or 240v. The two blade tandem configuration plug received its official rating of 250v in the 1950s but has been "banned" from use since the 1960s because of its lack of a ground or neutral.

Note: See the NEMA 1-15 ungrounded (Type A) section of this page for the parallel blade patent reference numbers.