I am helping family with finishing and upgrading the service to a separate garage 30ft from the house. Currently, there is 12-2 NM w/ground on a 20 amp breaker cord-capped and plugged in to a receptacle under the deck. The pvc is just stubbed up and unfinished under the deck and outside the garage. I'm going to j-box it up and pipe it all in. I intend to pull #6 THHN and install a two pole 60 amp breaker. My question is can I use the EGC there at the 200 amp panel in the basement or do I need to get my own out there at the garage? Is this treated as a separately derived system?
Pull an EGC and put a ground electrode in
You'll need to pull an equipment grounding conductor (although it doesn't have to be a 6AWG -- a 10AWG bare copper wire will do as per Table 250.122, and also saves on conduit fill) with the garage feeder from the main panel, and put a grounding electrode such as a ground rod in at the garage since it's a separate structure.
The EGC will provide a conductive bonding path to the main panel for fault currents, while the grounding electrode makes sure that the garage wiring doesn't get elevated to a high common-mode voltage relative to the earth itself (say by a nearby lightning strike). Tie the garage grounding electrode to the EGC at the garage disconnect/subpanel (which will need its bonding strap/screw pulled) with more 10AWG copper wire (250.32(B) says this connection is an extension of the EGC).
To confirm, you indeed need to pull four wires (if you want 120/240V):
- an equipment safety ground wire
- an insulated Neutral
- a 120V "hot" wire
- the other 120v "hot" wire on the other pole, presuming you want 120/240V at the site.
If you only want 240 (no 120) you can omit neutral.** If you only want 120V you can omit one of the hots.
Some types of metal conduit can serve as the ground wire. A ground wire must be copper. You can consider aluminum for the conductors.
In addition to the ground cable running back to the house, you also need to have a local grounding sustem, i.e. Ground rods. The ground rods take lightning and ESD to earth. The ground wire assures that fault current can return to source, and flow enough current to assure a breaker trip. (Dirt alone is too poor a conductor to be relied on for this.)
In your sub-panel, you must have separate neutral and ground buses which must be isolated from each other. Typically you need to buy an accessory ground bus for your panel, and remove the screws and straps that connect the neutral bus to the metal box. If you're used to working in main panels where they all (can) go on the same bus, that can be a real habit-breaker.
** often people run /2 cable to a remote location where they have lights, then later, they want to add a 240V device, typically a pump. Many tell them to trench a new /3 cable. I tell them to change the lights to multivoltage 120/240 LED types and reclassify the circuit to 240V-only.
This is an interesting question. Article 250.32 (B) covers this and actually it is one of the few times it gives you the ability to either to connect the grounding conductors in the main building service equipment or install a separately derived ground at the remote building.
The separately derived system was originally accepted and used, but fell out of favor during the 1980's. Then the grounding from the main building system seemed to become the standard. What is important is that you do not do both as it will violate the single point of ground rule.
If you want to know more about grounding I recommend the "Soares Book on Grounding" which is the International Association of Electrical Inspectors "Bible" on grounding. It's direct and to the point and an easy read.