After you attach a faucet to the sink, you can attach the water supply lines that will eventually be connected to the shut-off valves on the main water supply pipes. Regardless of what your supply line is made of, it probably uses a coupling nut to secure it to the faucet tailpiece. Simply screw the coupling nut onto the tailpiece until it’s snug and then give it a couple of final snugs with groove-joint pliers.
Don’t rush when attaching the supply lines. Faucet tailpieces are usually either brass or plastic, depending on the quality and the manufacturer. But no matter which material is used, the threads can be easily stripped if the coupling nut is started unevenly. Finger-tightening the nut onto the tailpiece helps ensure that it’s going on straight. Eyeball it to make sure it looks straight; if it doesn’t, back off the nut and start over.
Three kinds of tubing are used as water supply lines:
Factory-attached tubes: Some faucets come with factory-attached soft-copper supply lines on both the hot- and cold-water tailpieces, which means the only attaching will be directly to the shut-off valves. You should, however, do a little preshaping of the soft-copper before setting the sink into position in the countertop.
Measure the distance between the water supplies under the sink and then gently bend the soft-copper supply tubes until they’re about the same distance apart as the water supply. They don’t have to be exact, just close.
Be very, very careful when shaping the soft copper. You don’t need to be forceful with copper. Copper is very fragile and kinks relatively easily. And after it’s kinked, you won’t be able to get rid of the kink, which restricts water flow and will eventually begin to leak.
The best way to shape the copper into position is to gently slide it through your hands as you gradually move it into position. Don’t try to shape it in one shot. Make two or three passes through your hands for best results.
Flexible copper supply tubes: Flexible copper supply tubes are similar to the factory-attached soft-copper supply tubes found on some faucets. The same care is needed to bend and shape the copper tubes that you install. Try to shape the tube into position before attaching it to the sink’s tailpieces. After the tube has been shaped, secure it to the tailpiece with the coupling nut.
Although these copper supply tubes are somewhat flexible, they can’t be looped or twisted around if they’re too long for your supply setup. You need to cut them to length to fit into the open end of the shut-off valve.
Braided tubes: One of the best new plumbing products to come along is the line of braided steel supply lines. They’re constructed of a rubber supply (like a hose) wrapped in a steel-braided outer jacket. And what’s really great about them is their flexibility. You can take the excess length and simply put a loop in it and then connect it to the shut-off valve.