Sub panel gets its own ground rod?

[the_tool_man]

Hi all:

Six years ago, I built my workshop about 150ft from the house with a

100A service. I ran a four-conductor cable to the subpanel in the shop, and kept the ground and neutral conductors isolated from each other. I did not bond the sub-panel ground to its own ground rod, thinking it might cause a ground loop and/or noise in the intercom circuit bewteen the buildings. More recently, when I put in a spa with it's own GFCI breaker, I had several people advise me to drive a separate ground rod for it, and that I should have done the same for my workshop. So far, I have had no issues, but I want to make sure I did the right thing.

My searches here have yielded many debates on the need to keep the ground and neutral bonded only in the main panel and not the subs (which is how I did it), but nothing about the ground rod question. Does a remote subpanel need its own ground rod or not? Thanks in advance, John.

[Greg]

Yup you need a ground rod and that gets connected to the ground bus, The neutral still stays isolated

[newsgroups01REMOVEME]

2002NEC 250.32(b)2

Paraphrase, if you have no grounding method between your main panel and the garage panel, then you bond your sub panels grounded(neutral) and grounding conductors in the sub panel.

2002NEC 25.32(A) Exception

Paraphase:if you have only one branch circuit supplies your subpanel, and it has an equipment grounding conductor, then you are ok without the ground rod.

No debate, all in black and white in the blessed book. :-P

hth,

tom @

[SP]

[DaveG]

Unless you have a connection between your garage and main dwelling such as water pipe, you need to install a ground rod for your sub panel. You don't need to put in a ground rod to your spa. You do, however, need to bond any grounded metal within 5' of the spa water to the bonding lug on your spa panel, using bare solid copper.

[SQLit]

Supplemental grounding gets tricky pretty fast. Excellent choice for the 4 wire service to the garage. Now if you drive a ground rod at the garage then the grounding conductor back to the service must be of the same size as your service ground. If not you COULD have a problem with an fault.... note could.. As for the spa driving another ground rod is just complicating the issue. I assume that you took a grounding conductor to the spa, if it is connected correctly you do not need a ground rod there. Try reading the Soars book on Grounding. Lots of pictures and easy text to understand. Heck I probably do not understand what your doing cause I can not see it.

[John Grabowski]

Since you ran a grounding conductor with the feed to the subpanel, you do not need to install a ground rod at that location.

If you feel the need to install a ground rod it should be at the main panel. What is your current main ground?

John Grabowski

[zxcvbob]

If he ran a 100A feeder to the garage panel, the grounding wire is at least 6 gauge, and maybe 4. IIRC, #6 is the largest wire you are ever required to use to connect a ground rod to the grounding electrode system.

I would probably drive a ground rod at the garage. I don't know if it is required or not (but I think it is.) Since there's a seperate grounding wire already, I don't think it's all that important.

Bob

[newsgroups01REMOVEME]

thought 6 was small, so went looking. 2002nec table 250.66 for copper

3/0 is the largest, and for aluminum 250 kcmils. They are a little bigger than 6awg.

Good point, it doesn't seem to be anything against driving another grounding rod, but then, if he wired it correctly, nothing seems to indicate he needs to.

imho,

tom @

[zxcvbob]

(I wish I could find my code book, even though it's 10 years old.) I don't think NEC ever requires a conductor that big to connect a ground rod to the electrode grounding system. In fact, I think there is a specific exception that says it never has to be larger than #6 copper or #6 copper-clad aluminum. It probably does require an equipment grounding conductor (ground wire), or grounded conductor (neutral wire) that big for 200A or larger feeder circuits, but that's not what we are talking about.

Bob

[Greg]

Simply not true. Anything beyond a single branch circuit to a remote building requires a ground electrode system. The only thing a 3 wire vs 4 wire feeder affects is regrounding the neutral at the remote building.

[Greg]

Right on Bob The GEC to a "made electrode" (rod or plate) does never have to be bigger than #6 copper

[John Grabowski]

Greg, I think you misinterpreted the pictures as indicating a ground rod for each panel. The grounding symbol is used to only illustrate a common ground. Read the text and you will see that an additional grounding electrode is not required if a grounding electrode conductor was run with the service feeder.

The original poster did the job correctly by running four wires and keeping his grounding conductor separate from his grounded conductor (Neutral).

[HorneTD]

Sorry John but that is bad information.

Every building that has more than a branch circuit to supply it has to have a grounding electrode system. That can be as little as a single ground rod if it has an impedance to ground of twenty five ohms or less. If the impedance of the single rod is higher you have to add a second rod. Most electricians do not bother to measure it and just add a second rod. If the building already has an electrode such as a concrete encased electrode then you only have to bond the service equipment enclosure to it and your done. In all cases each separate building gets a grounding electrode system. There are no exceptions under the US NEC. The objective is to "limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines" and "stabilize the voltage to earth during normal operation."

250.50 Grounding Electrode System. If available on the premises at each building or structure served, each item in 250.52(A)(1) through (A)(6) shall be bonded together to form the grounding electrode system. Where none of these electrodes are available, one or more of the electrodes specified in 250.52(A)(4) through (A)(7) shall be installed and used. Copyright 2002 National Fire Protection Association.

In short every building

1) needs a grounding electrode system if it is supplied with more than a branch circuit. 2)The building disconnecting means gets bonded to the grounding electrode system.

-- Tom H

[HorneTD]

John The debates on this topic are endless and I won't try to settle them. I will say that the US NEC requires that every building that has an electrical supply that is more than a branch circuit must have a grounding electrode system. There are no exceptions.

What you have done so far is great. Since you have an intercom between the two buildings the code requires you to run an Equipment Grounding Conductor (EGC) with the feeder conductors which you did. All that is left to do is to connect the building disconnecting means to any grounding electrodes at the garage / shop building or in the absence of existing electrode you must install an electrode system. The point of termination of the Grounding Electrode Conductor (GEC) to the disconnecting means enclosure is usually done on the buss bar within the enclosure that is bonded to the enclosure. In a three wire feeder the bonded buss bar is the neutral buss. In a four wire feeder the bonded buss bar is the EGC buss.

So Locate the existing electrodes or install one or more new ones and connect them to the disconnecting means using an appropriately sized GEC.

-- Tom H

[John Grabowski]

Tom, I agree that each building needs to be connected to the grounding electrode system. However my interpretation is that the grounding electrode system originates at the main service entrance and can be extended to secondary buildings along with the power. Therefore a ground rod at each secondary building is not necessary as long as the building components (Metal piping, steel, subpanel, etc.) are bonded to the grounding electrode system that originates from the main service.

I seem to recall an article from a few years ago in one of the trade magazines that discussed this issue of multiple grounding along a distribution system. I think one of the problems is that it is possible that the ground rod in a secondary building could have less resistance than the primary ground rod (Or water pipe). Since electricity follows the path of least resistance it is possible that the loss of a neutral could have current travel away from the main service to a secondary building's ground rod. Anyway, my memory isn't that great. I could be mistaken.

John G.

[Dan Bolt]

This subject gets hashed around among inspectors quite a lot. It breaks down to the definition of a branch circuit. The NEC Handbook shows an illustration and a statement that a sub-panel requires an electrode connected to the ground from the main panel. You'll never be wrong to install the ground provided it is connected to the ground from the main. An individual inspector may interpret in different ways so sometimes it is easier to install it and be sure.

Dan

[newsgroups01REMOVEME]

Might be a can of worms, without the ground rod, you could pass. Now that you put one in you could fail till you put a second in, since now you might not meet the 25 ohm requirement.

Just saying, if you don't follow what is in black and wihite, and can interpet it to be anythign, what was the reasoning behind of writing it down in black and white. The nec chould have have been one line, Article1: Do what you feel is the max to ensure safety. End.

later,

tom @

[Takoma Park Volunteer Fire Dep]

John We can talk what ifs until the cows come home. The requirement in the US NEC is plain. If the building is supplied by a feeder rather than by a branch circuit it must have a grounding electrode system and the building disconnecting means must be bonded to it. It makes no difference whether there is or is not an Equipment Grounding conductor run with the feeder a grounding electrode system is still required at each building. Which conductor of the feeder gets bonded to the Grounding Electrode System at each building is dependent on whether there is an EGC in the feeder. Either way you must bond the building disconnecting means to a grounding electrode at the building that the building disconnecting means serves. The US NEC does not allow any other course of action.

-- Tom H

[John Grabowski]

Tom, I believe in this particular instance that the feed to the subpanel is a branch circuit even though the owner is calling it a service.