To ground, or not to ground...

[WesPete66]

So I am currently upgrading my electrical service in my garage/shop.
Previously what I had in the garage was 120V only. There was a simple dis-connect switch to control the lights and outlets. There is a 10/3 cable dug underground running from the main breaker panel to the garage. This cable has NO ground wire in it (wasn't important in the 50's..) Yes, I am just plain lucky that I have never had anything short out in the years I've lived here!
I am installing a basic sub-panel in the garage now so I can have 220V power on breakers. My dilemma is that I have no ground connection with the main breaker panel. Am I correct in thinking that I must run a ground wire back to the house, and connect to the ground bus in the main panel? I plan to place two earth ground rods in the process.
I think I know the answer but am hoping for a qualified opinion on this.
Thanks!

 

[Chipper5783]

You are correct that getting the grounding right is important. That would be a question for a qualified electrician or electrical inspector for your area (jurisdiction). Group opinions from areas outside your jurisdiction are pretty well irrelevant.

 

[rwdenney]

So I am currently upgrading my electrical service in my garage/shop.
Previously what I had in the garage was 120V only. There was a simple dis-connect switch to control the lights and outlets. There is a 10/3 cable dug underground running from the main breaker panel to the garage. This cable has NO ground wire in it (wasn't important in the 50's..) Yes, I am just plain lucky that I have never had anything short out in the years I've lived here!
I am installing a basic sub-panel in the garage now so I can have 220V power on breakers. My dilemma is that I have no ground connection with the main breaker panel. Am I correct in thinking that I must run a ground wire back to the house, and connect to the ground bus in the main panel? I plan to place two earth ground rods in the process.
I think I know the answer but am hoping for a qualified opinion on this.
Thanks!

Yes, you are correct. Code requires that subpanels are not grounded, but receive their safety ground from the main panel. Really, what that means is that the neutral is bonded to earth ground only at the main panel.

For 120V service 10/3 should provide a hot, a neutral, and a safety ground, so I’m not sure I’m understanding your description. The safety ground does not have to be a bare wire, it just has to exist (at least if we are only considering safety and not a picky inspector).

The problem is if you want to add 240VAC in the garage—for that you need four conductors if you also want 120VAC. You can power hard-wired 240VAC equipment, like compressors, using three wires: hot-hot-ground. But the equipment cannot have any 120VAC circuits, and it must load the two hots equally.

10-gauge isn’t big enough to run a bunch of stuff anyway. It would be limited to 30 amps per code.

Rick “put his shop on a separate meter to avoid these problems” Denney

 

[GeneT45]

Exactly what @rwdenney said. You need a ground, and it must terminate only in your main panel. You cannot have a separate earth ground for your shop. That much is National (U.S.) electrical code and will not vary with your locale within the U.S.

GsT

 

[f350ca]

Maybe different in the US than Canada but I ran overhead triplex 100 amp service to the shop then a separate ground at the shop panel with neutral bonded to ground at that panel. It's essentially a separate service fed from the house. Passed inspection fine.

Greg

 

[D.sebens]

Yes, you are correct. Code requires that subpanels are not grounded, but receive their safety ground from the main panel. Really, what that means is that the neutral is bonded to earth ground only at the main panel.

For 120V service 10/3 should provide a hot, a neutral, and a safety ground, so I’m not sure I’m understanding your description. The safety ground does not have to be a bare wire, it just has to exist (at least if we are only considering safety and not a picky inspector).

The problem is if you want to add 240VAC in the garage—for that you need four conductors if you also want 120VAC. You can power hard-wired 240VAC equipment, like compressors, using three wires: hot-hot-ground. But the equipment cannot have any 120VAC circuits, and it must load the two hots equally.

10-gauge isn’t big enough to run a bunch of stuff anyway. It would be limited to 30 amps per code.

Rick “put his shop on a separate meter to avoid these problems” Denney

New 10/3 would have 4 wires. 3 conductors and a ground. They don’t count the ground. Old 10/3 would only have 3 conductors since they didn’t run grounds. I believe the code also states that the ground can be separate but it has to be the same gauge or larger as what the circuit requires.

I am not an electrician but had to deal with this semi recently.

 

[rabler]

One issue with separate grounds is you are increasing the chances of a ground loop creating electrical noise. This gets really nasty if you happen to take a lightning strike anywhere in the vicinity. To put it crudely, a lightning strike creates a "splash" in the earth ground voltage potential. That will result in very brief but significant current getting coupled back through your neutral wire, and thus effecting all the electronics in both buildings and possibly blowing things out.

I worked in an aircraft test facility for a while where there was a "lightning" test chamber, we got to have all sorts of fun blowing things up

 

[WesPete66]

Yes, you are correct. Code requires that subpanels are not grounded but receive their safety ground from the main panel. Really, what that means is that the neutral is bonded to earth ground only at the main panel.

For 120V service 10/3 should provide a hot, a neutral, and a safety ground, so I’m not sure I’m understanding your description. The safety ground does not have to be a bare wire, it just has to exist (at least if we are only considering safety and not a picky inspector).

The problem is if you want to add 240VAC in the garage—for that you need four conductors if you also want 120VAC. You can power hard-wired 240VAC equipment, like compressors, using three wires: hot-hot-ground. But the equipment cannot have any 120VAC circuits, and it must load the two hots equally.

10-gauge isn’t big enough to run a bunch of stuff anyway. It would be limited to 30 amps per code.

What was installed decades ago was a 2-pole 30-amp breaker (main panel) feeding a 10/3 cable going to the garage, with no ground present in that cable. At the garage it separated into two 120V circuits, using a common neutral wire. No, it was not correct!
(edit: The new sub-panel breakers are 20 amp.)

Exactly what rwdenney said. You need a ground, and it must terminate only in your main panel. You cannot have a separate earth ground for your shop. That much is National (U.S.) electrical code and will not vary with your locale within the U.S.

Am I getting this right then? Not only should I not connect an earth ground rod to this wire, but it should use an insulated wire returning to the main panel?

 

[woodchucker]

just out of curiosity, why wouldn't an earth ground work? it provide a path to ground and in my house, my ground from the panel is to my water pipes, which is stupid, since it's a well, and black plastic pipe going out to the well pump.
I believe a copper rod in the ground is better, but hey , what do I know. And when I replaced my water heater , they told me to tie the hot and cold pipes together, and he looked at the ground to the well and was ok with it. I was thinking WHY?

 

[D.sebens]

What was installed decades ago was a 2-pole 30-amp breaker (main panel) feeding a 10/3 cable going to the garage, with no ground present in that cable. At the garage it separated into two 120V circuits, using a common neutral wire. No, it was not correct!
(edit: The new sub-panel breakers are 20 amp.)

Am I getting this right then? Not only should I not connect an earth ground rod to this wire, but it should use an insulated wire returning to the main panel?

They are referring to bonding. Your panel should have neutral and ground but they should not tie together in the panel. You will have one ground strip and one neutral strip. In your main panel they will be tied together with a buss bar