Abstract: An extension box for electrical condulets or junction boxes can be used by itself as a basic junction box in an electrical conduit system or as an auxiliary junction box coupled to a basic junction box to increase the interior volume of or extend a branch conduit system from the basic junction box. The body of the extension box has coupling hubs leading through the body walls to the body interior, for receiving ends of electrical conduits. Complementary male and female coupling formations are provided at the open front and back of the extension box body to permit two or more extension boxes to be coupled in stacked relation to a conventional junction box such as a condulet. Removable covers close the open back of the extension box.