[robertmee]

Reading several other threads there is the continual mention of avoiding parallel paths to ground. But in my new subpanel installation, I've been advised to run a 4 wire feeder (2/2/2/4) from the Main to the Sub AND a #6 from the sub to new ground rods on the shore line (sub is in a boathouse). Isn't that two paths to ground from the subpanel? Either through the ground back to the main (via #4) which is connected to ground rods at the house or through the #6 to the local ground rod on the shore line. I'm confused :confused1:

 

[Yoyizit]

Which diagram applies to you?

http://books.google.com/books?id=fB...=X&oi=book_result&ct=result&resnum=1#PPA79,M1

 

[ScottR]

I thought it was to avoid paralleling the neutral -- in other words, it's why you don't bond the neut. to the ground bar in the sub panel -- otherwise your grounding conductor ("the ground") between the sub and main panels would carry some current that should only be carried by the grounded conductor (neutral).

Multiple paths to earth are OK, but only 1 conducting path and 1 fault path to the main panel (and then to the center tap on the transformer). Your #4 grounding conductor back to the main panel is a ground fault path (short circuit), and the #6 to the rods is more for lightning protection.

(I've been having a bad day with electrical advice today, so don't be surprised if someone corrects me here).

 

[220/221]

Isn't that two paths to ground from the subpanel?

Yes but they are for different purposes.

Ground rods for lightning, grounding conductor for ground faults.

 

[robertmee]

Which diagram applies to you?

http://books.google.com/books?id=fB...=X&oi=book_result&ct=result&resnum=1#PPA79,M1

Figure 2.14(d) is exactly what I have (except two hots).

 

[robertmee]

Thanks guys (once again) for clearing it up!

 

[InPhase277]

We want to avoid parallel neutral paths. These can be established through our grounds if not careful. But when it comes to just grounds, we want as much redundancy as possible, this is why it is preferred (at least by me) to connect all grounds together in switch and junction boxes.

As 220 said, we drive rods in the dirt for mostly lightning protection.

 

[GL55]

Parallel groung paths

With a sub panel (feeder panel) you are required to have a grounding connection back to the main panel, this is your #4AWG conductor. This should be bonded to the cabinet by means of a grounding bus installed directly to the panel cabinet. You must not use sheet metal screws to install this bus (or grounding terminal strip), but must use an approved sheet metal screw identified for the purpose (green) to attach it to the cabinet, all of your other grounding conductors from the boat house must be attached to this ground bar, as well. Make sure you remove (or do not install) the Main Bonding Jumper that comes with the panel; that would create a parallel ground path. Remember to remove all paint in from behind the grounding buss, so you have full metal to metal contact.

The NEC also requires that any feeder panel containing 2 or more circuits have a grounding electrode (the ground rods) installed at the location of the feeder panel.

This installation does not have a parallel ground path because all you have done is continue the equipment ground path back to your main panel with the #$ AWG conductor. The ground rods (the NEC only requires one at a separate structure, but check eith your local AHJ, there may be amendments to the Code that require two in your location) serve as a dispersal path back into the earth for any voltage surges, either from the power company, or from lightning strikes.

 

[vsheetz]

• Multiple grounds paths are ok - and sometimes required.
• Multiple nuetrals paths are bad.

 

[AllanJ]

• Multiple nuetrals paths are bad.

Yes.

There must be exactly one return path from any load (light fixture, etc.) back to the panel and the path may not utilize any ground wires (grounding conductors) and the (neutral) conductors used must accompany the respective hot conductors in cables, conduits, etc. (Some very obscure and rare exceptions exist.)

 

[LyonsElecSupply]

Id like to clarify the purpose of ground rods.

Ground rods in normal residential structures are NOT for lightning protection or to provide a "Ground fault path"

ALL they do (during normal usage) is stabilize the voltage to ground. They do ABSOLUTELY NOTHING MORE. It also creates a network (between the xformer ground, system neutral, and your neighbors ground rods....) to stabilize voltage and provide a redundant return path....

Think of it this way: every point along the earths surface has a different voltage potential. Your outbuilding approx 100 feet away lets say has a different voltage potential. So therefore you run a separate ground wire (to provide a current return path) and a new ground rod (to prevent a voltage difference on your outbuilings ground system) Lets say you touch an object connected to your system ground. You are touching a grounded object with the voltage potential of your main system panel, NOT the geographic location you are at. Therefore by driving a ground rod it stabilizes this voltage difference between two different geographic locations. The same applies with your main structure ground rod, it stabilizes the voltage between the transformers ground wire and your structures physical location (a few hundred to a few thousand)

Now, in the event you lose your system neutral, They become a higher impedance ground that is a fail safe. It creates a path to ground, HOWEVER it is NOT a replacement, as it has a much higher resistance to get the current to ground.

THIS MEANS: if you lose your system neutral, and your ground rods/water system bonding become your neutral path, you most likely will get lightly shocked should you touch the metal case of something in your home......

Take an Ammeter to your copper bare ground wires.....get a decent amount of unbalanced loads going and meter your system neutral, bonding copper for your water system and your ground wire....the majority of your current should be going through your system neutral as it is the lowest impedance return path.

I hope this helps everyone understand the purpose of ground rods......

 

[vsheetz]

There must be exactly one return path from any load (light fixture, etc.) back to the panel and the path may not utilize any ground wires (grounding conductors) and the (neutral) conductors used must accompany the respective hot conductors in cables, conduits, etc. (Some very obscure and rare exceptions exist.)

Correct - Multiple nuetrals paths are bad.

 

[a7ecorsair]

Id like to clarify the purpose of ground rods.

Ground rods in normal residential structures are NOT for lightning protection or to provide a "Ground fault path"


I hope this helps everyone understand the purpose of ground rods......

So, if lightning hits the transformer...
The transformer case is grounded with some #6
The transformer case has a parallel path to ground through the neutral, the GEC, and the grounding electrode.

 

[Speedy Petey]

With a sub panel (feeder panel) you are required to have a grounding connection back to the main panel, this is your #4AWG conductor. This should be bonded to the cabinet by means of a grounding bus installed directly to the panel cabinet. You must not use sheet metal screws to install this bus (or grounding terminal strip), but must use an approved sheet metal screw identified for the purpose (green) to attach it to the cabinet, all of your other grounding conductors from the boat house must be attached to this ground bar, as well. Make sure you remove (or do not install) the Main Bonding Jumper that comes with the panel; that would create a parallel ground path. Remember to remove all paint in from behind the grounding buss, so you have full metal to metal contact.

The NEC also requires that any feeder panel containing 2 or more circuits have a grounding electrode (the ground rods) installed at the location of the feeder panel.

This installation does not have a parallel ground path because all you have done is continue the equipment ground path back to your main panel with the #$ AWG conductor. The ground rods (the NEC only requires one at a separate structure, but check eith your local AHJ, there may be amendments to the Code that require two in your location) serve as a dispersal path back into the earth for any voltage surges, either from the power company, or from lightning strikes.

This is a good reply. I see onyl two issues.

1) There is no designation that a detached structure only requires one electrode. The same rules apply here as a main structure. See 250.32

2) I know you are simply adding information, but this thread is almost two YEARS old.