Sub Panel Installation

[Matthew Curtis]

I am installing a 100 amp sub panel in my detached garage. Do I need to have a separate grounding rod connected to the grounding bar? Or is the ground wire coming from the main panel in the house sufficient??

Also to make sure: I do not bond the ground and neutral in the new sub panel, Correct??

[Matthew Curtis]

Let me be more specific: I do not bond the ground and neutral together. Which terminal do I bond to the case?? Ground??

[David Helm]

Correct, on a subpanel the ground and neutral busses must be separate (not bonded to each other. On the ground rod, I don't think it is required, but it can't hurt. By todays standards you can never have too much grounding. Most new homes either have at least three ground rods or a Ufer ground (Grounding is connected to rebar in foundation).

[Will Rowland]

From what I understand, according to the NEC, you have to have 4 wires coming from the main panel to the subpanel (2 hots + neutral + ground). The sub panel is thus grounded back to the main panel ground. On my shop, I also added a ground point to the rebar in the shop slab (it is detached from the house).

However, when I had the electrical inspection on my shop in Houston, the inspector required me to also put in an external ground rod for the subpanel (wired to the ground bus in the subpanel). He claimed that was what the NEC required.

So I ended up with 3 ground points for my detached shop (back to main panel, rebar, ground rod in the dirt), which seems like overkill, but I guess you can never have too many ground points, as David noted.

[Rob Russell]

To the OP's question(s), the equipment grounding conductor is what would be connected to the grounding bus and electrically connected to the case of your panelboard. The other responses about not bonding the neutral and grounding busses are correct.

You need to check with your AHJ on what they want for ground rods at a detached structure. When I went to run a small subpanel out to a shed, our AHJ specifically did not want ground rods. Your AHJ may very well be different.

[Rollie Meyers]

If a rod is driven, unless you can prove 25 Ohms of resistance or less (The testing equipment is expensive, BTW). you need to drive a 2nd rod at least 6 feet apart per NEC section 250.56, here is a copy & paste from the 2008 NEC.

250.56 Resistance of Rod, Pipe, and Plate Electrodes.

A
single electrode consisting of a rod, pipe, or plate that does
not have a resistance to ground of 25 ohms or less shall be
augmented by one additional electrode of any of the types
specified by 250.52(A)(4) through (A)(8). Where multiple
rod, pipe, or plate electrodes are installed to meet the requirements
of this section, they shall not be less than 1.8 m
(6 ft) apart.

FPN: The paralleling efficiency of rods longer than 2.5 m

(8 ft) is improved by spacing greater than 1.8 m (6 ft).

[Anthony Whitesell]

I'm looking forward to the electrcian and building inspector tackling this one at my home next year when I change oout the main panel. My assumption is that they would normally be driven 3' to either side. Remembering that the first one was sunk 30 years ago under different rules, is directly below the main panel via a hole drilled through the concrete basement floor. To get 6 feet away would entail a 10' (4' vertical and 6' horizontal) run to the second ground bar. Which would be either in the walk way, the driveway, or around the corner of the house in the front (this one adding atleast 4' feet to the run). With all that copper wire exposed and suseptible to damage, I cannot see what benfit it will have. I can almost gaurantee it won't last but a few years before it cut by something.

[Andrew Howe]

Is this correct?
I am thinking of replacing my current sub panel which has the neutral bar (white) wires and the ground bar (bar copper wire) connected together

I do not think this is that safe but this is the way it was done.
I am planning on swapping out the sub panel tomorrow so please let me know if this is correct.

Correct, on a subpanel the ground and neutral busses must be separate (not bonded to each other. On the ground rod, I don't think it is required, but it can't hurt. By todays standards you can never have too much grounding. Most new homes either have at least three ground rods or a Ufer ground (Grounding is connected to rebar in foundation).

[Anthony Whitesell]

First check with the AJT (ie., building inspector).

The neutral should only be connected to the ground at the main service panel. All other ground-neutral pairs should be isolated from each other. If you do have a separate ground rod for the sub-panel (don't know if it is required, and I can't imagine that it would be against code), it should only connect to the ground; not to both the neutral and ground. The building inspector for your area should be able to clarify this.

[Chris Damm]

It depends on what the local code is. Here my subpanel had to have 2 ground rods in series 6' apart even though the main panel only has one. When I asked why (because I wanted to know the reason), I got the answer: "because I said so!".
That inspector has been fired because of many complaints. Check with your local inspector.

[Rollie Meyers]

It depends on what the local code is. Here my subpanel had to have 2 ground rods in series 6' apart even though the main panel only has one. When I asked why (because I wanted to know the reason), I got the answer: "because I said so!".
That inspector has been fired because of many complaints. Check with your local inspector.

That would be only required if it was a detached building, then grounding electrodes, & a main will be required, if attached then it is not required.