I am installing a 100 amp sub panel next to the main panel at our office. There is an existing 100 amp sub panel by the main 400 amp panel and who ever did the work did not run a ground wire from the main to the sub panel. There are mant green ground wires landed on the ground bar but I'm thinking it's not really doing anything since the panel is mounted on a sheet of plywood. I need to run a Grnd from the main to the sub correct?

Unless there has been a very recent change in the wiring code, the subpanel neutral is not grounded with a ground conductor and the bonding screw is not installed. The neutral is left floating. The neutral bus bar and therefore the neutral wires are grounded only at the service panel. There should be a ground wire to the ground bar of the subpanel.

First, of all if this is a commercial facility you need to make absolutely sure that an unlicensed electrician can do the work.

Typically, only homeowners are allowed to do electrical work on their residence. You need to have this conversation with the AHJ for your area.

Next, feeds from a service panel to a subpanel require four conductors. Ground from the panel will connect to the ground bus bar. Since you are in the same building, you are able to take advantage of the existing grounding in the service panel.

The neutral is not left floating. It is isolated from the grounding system in a sub panel.

Most importantly is to make sure that you are allowed to do the work.

Any conduit doing the proper connection?

Sub panels are required to be grounded. Neutrals are isolated!Either a wire or conduit should be completing the ground path from the sub panel to main. If the ground wires are run to the neutral bar in the sub panel then this is a violation.

Joe,

Only a subpanel in a remote bldg. is required to have a grounding rod. Please see Article 250.4 (A)(1).

If the subpanel is in the same bldg. it uses the existing grounding system.

Don,

Is 250.4 another typo? There is a Section 250.4 (A), however (1) does not exist.

Lets have a lesson on Code book, there Chapters, Articles, and Sections.

Article 250 covers all of Grounding and Bonding.

It resides in Chapter 2, one Article of Wiring And Protection.

Each Article contains Sections. If 250.4 was correct, it is called Section 250.4.

Section 250.4 is not relevant to ground rods.

Mike, just because you do not see an equipment grounding conductor does not mean one does not exist. Metal conduit is allowed to act as an equipment grounding conductor.

Photos of what you have will help greatly.

TK,

No it is not a typo. In the response I provided, Article 250.4 was provided to reference the "General purpose" for grounding.

Also missing was the reference to Article 250.32 and the further reference to Part III of Article 250.

I typed this offline in an editor that is supposed to allow "drag and drop". It looks like it did not take the whole paragraph. Cra* it chopped the devil out of the response. I'm not using that one anymore.

Do you want me to re-write the post?

Joe,

Only a subpanel in a remote bldg. is required to have a grounding rod. Please see Article 250.4 (A)(1).

If the subpanel is in the same bldg., it uses the existing grounding system.

I never mentioned a ground rod, or that there is a separate building!

TK,

Okay, I figured out the reason my response got chopped up. It has to do with font calls.

The editor I used had a font call in it that the HTML editor could not match, so the text got dropped.

I guess I'll go back to using MS Word.