On a Federation starship, a power transfer conduit (abbreviated PTC) was an advanced type of plasma conduit, used for carrying warp plasma from the vessel's warp core to the drive nacelles. The conduits were magnetically shielded against the superheated warp plasma, and were roughly 0.6 to 1 meter in width.
Characteristically, the conduits emerged from the equatorial region of the matter-antimatter reaction chamber of the warp core, and from there traveled aft through the ship until they turned 90 degrees, to port or starboard, through the nacelle pylons, before terminating in the plasma injectors, transferring the warp plasma they were carrying into the warp coils.
Along with carrying the warp plasma, the power transfer conduits also contained manifold links to the electro-plasma system (EPS), where electronic energy was bled off from the warp plasma in the form of microwaves, providing electrical energy for the vessel.
In the USS Enterprise-D, (Star Trek: The Next Generation) the power transfer conduits could be entered for inspection. Chief Engineer Geordi La Forge performed an inspection in the company of Dr. Leah Brahms from the Daystrom Institute during her technological inspection of the Enterprise, which she helped design. (TNG: "Galaxy's Child") During such an inspection, safety concerns applied, of course.
In the warp drives of Intrepid-class starships, including the USS Voyager, the power transfer conduits were not immediately visible in engineering, so it could be assumed that they were below the deck. However, they seem to have been easily accessible, since a transwarp-mutated Lieutenant Tom Paris once managed to use a hand phaser on one of the transfer conduits of Voyager, causing power failures all over the ship. (VOY: "Threshold")
When the magnetic constrictors lost alignment on Voyager in 2372, B'Elanna Torres asked Michael Jonas if he adjusted the power transfer conduits. Doing so, to no effect, Torres noted to Jonas to not let the PTC temperature rise above 3.2 million Kelvin. (VOY: "Investigations")
Aboard the space station Deep Space 9, power conduits were also used to channel the station's energy. In 2369, the computer located an independent tritanium source in the power conduit in level twelve, section eight. Miles O'Brien ordered the computer to reroute the power flow. After checking the power conduit, O'Brien found the remains of his EJ7 interlock as well as the organic remains of Ensign Aquino, who was missing for a few hours. (DS9: "In the Hands of the Prophets")