" I thought I could use the same grounding bar as a neutral bar also." - - No, The neutral (groundED) conductors need to be electrically isolated/insulated from the groundING conductors in a subpanel.

NEC (2008?)

250.32

(B) Grounded Systems. For a grounded system at the separate building or structure, an equipment grounding conductor as described in 250.118 shall be run with the supply conductors and be connected to the building or structure disconnecting means and to the grounding electrode(s). The equipment grounding conductor shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. The equipment grounding conductor shall be sized in accordance with 250.122.

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Any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrode(s).

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Exception: For existing premises wiring systems only, the grounded conductor run with the supply to the building or structure shall be permitted to be connected to the building or structure disconnecting means and to the grounding electrode(s) and shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded where all the requirements of (1), (2), and (3) are met:

(1) An equipment grounding conductor is not run with the supply to the building or structure.

(2) There are no continuous metallic paths bonded to the grounding system in each building or structure involved.

(3) Ground-fault protection of equipment has not been installed on the supply side of the feeder(s).

Where the grounded conductor is used for grounding in accordance with the provision of this exception, the size of the grounded conductor shall not be smaller than the larger of either of the following:

(1) That required by 220.61

(2) That required by 250.122

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http://freenec.com/T79.html

You must have a neutral conductor that goes from that sub-panel back to the main panel in order to have a 120v circuit. A ground is NOT the same as a neutral.

Sub panel is connected to a 100 amp breaker at the main panel in the house. Black wire, white wire and copper ground. Black and white are hot and neutral?

Are you saying that you have a 2 pole, 100 amp breaker in your main panel that feeds the subpanel, and the black and white are landed on the 2 pole breaker, and you only have 3 conductors (black, white, and copper ground) from the main panel to the sub? If that's the case, then you don't have a neutral at the subpanel.

Both Hank and Yankee are correct. You're missing the grounded (i.e., neutral) conductor which is required if you have 120V loads in the subpanel and a lesser issue of an improperly marked ungrounded feeder conductor.

How can a main panel have only three wires but a sub panel has to have four?

I think I am starting to understand. I didn't mention the ground rod I installed at the outbuilding. Will this serve as the ground? Where do I connect it? I only have three connection points in the sub panel. They are all being used. Thanks for all your help, sorry I am no electrician.

First, how are you dealing with the lack of a neutral conductor coming from the main panel? The bare wire can only be a ground.

After you correct that, the additional bus bar can be purchased as an accessory and added to the panel, perhaps from an electrical supply rather than a big-box store.

I think I will hire an electrician to come out and tell me exactly what is wrong and how to fix it.

"As Randy indicated, your subpanel is missing a neutral bus bar as well as a neutral wire."

"There is only one grounding bar and it is not bonded with the green screw."

The neutral (groundED) bar is the one that comes with the panel and is isolated/insulated from the metal parts of the panel unless you bond it with the green screw. The groundING bar (often just called a "ground bar") is the one that is missing (purchased separately), and is screwed directly into the metal of the panel into the pretapped holes provided. You can't use the bar that is screwed directly to the metal panel for the neutrals. The neutrals must go to the bar that is already in the panel, and without adding the bonding screw.

Fred...that depends very much on the panel. Your arrangement describes SQUARE D's configuration. Other manufacturers already have multiple bus bars and all you need do is remove the bond between them (no additional bar is required and in many cases, not even available).

I was just correcting the misuse of terms so that nobody got confused....

The point I was making is that NO panel for residential wiring comes without a neutral bar, and only with a ground bar bonded to the panel. And if something is missing, it is the bonded ground bar, not the neutral bar. If there is only one bar, you would use it for the neutral and add a simpler bonded ground bar.

Well I fixed my electrical problem thanks to all of you for your great advice. I bought a roll of 6/3 wire made for direct burial and ran it through my buried conduit. Hooked everything up and Voila', it works. Thanks for all your help.