Opened a metal outlet box, and noticed a ground wire coming out of it, connected to the water line. However, the receptacle inside is connected to older wire without ground. Can i connect the ground wire to the box then the receptacle to ground both?

 

The wire was connected to a water line and just stuck in a hole in the junction box ?

 

Only if the wire in the box connects to the water pipe within five feet of where the main breaker box ground wire connects to the water pipe.

 

It was screwed down with the clamp

 

This outlet hosts a dehumidifier in the basement. Does it need to be grounded? How would I do that?

And on the same note, I bought this house three years ago, and asked that all bathroom and kitchen outlets be grounded after inspection revealed that they were not. I see a green line coming down from the kitchen connected to the water line, so I’m assuming that whatever it is trying to ground is not either?

 

Sounds like the attempt at providing a ground was improperly done.

 

The ground wire cannot be under the clamp screw, it must be under a green grounding screw in the back of the box or it can be attached to the box with a ground clip. The ground wire should terminate inside the main electric service panel on the neutral bar. You need a pigtail from the box to attach to the grounding screw on the receptacle.

 

I’ve noticed several thin (maybe 14 gauge) copper wires attached to the plumbing throughout the basement. Hopefully something in this house is grounded correctly...

 

Probably not. As stated earlier unless it is within 5 feet of the main ground to the panel it isn't code compliant.

 

You can have ground wires (equipment grounding conductors) emerging from outlet boxes but the wires must continue down to the panel. Should such an EGC first reach a fat ground wire (grounding electrode conductor) running between the main panel and the main cold water pipe or a ground rod that EGC may end and be clamped on there.

(summary) Inside the outlet box the separate EGC must be:
1. Attached to the receptacle ground screw and that receptacle has an approved clip squashed between it and the (metal) box where it is attached, or,
2. Attached to the metal box using an approved clip or using a screw not used for something else such as a cable clamp in back where the receptacle also has an approved clip squashed as above or a jumper wire (pigtail) similarly attached to the box, or,
3. Attached to both the receptacle and the box.

If two wires want to go under one screw, use wire nuts and pigtails to get everything connected with one wire per screw. Also use pigtails if there is more than one receptacle, etc. in the box as each needs to be grounded.

Spiral metal sheathing for wires going between metal outlet boxes does not count as an EGC unless a bare wire or metal strip runs lengthwise inside, outside any paper lining for the circuit conductors inside. This strip does not have to come into each outlet box and be wire nutted with EGCs

All those #12 and #14 EGCs coming out of the basement ceiling and attached to water pipes may be extended to the panel to complete a proper ground.

No additions or extensions may be made to circuits where grounding is added later in this fashion. New circuits with EGC accompanying the circuit conductors would need to be installed for that.

Each outlet box (or group with EGC between them only) needing grounding needs its own EGC running down to the panel. However IMHO outlet boxes on the same branch circuit may share one separate EGC using either daisy chaining or tapping.

 

Might as well run new 14/2 and upgrade the house.