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I'm trying to add a 20 amp circuit breaker into my existing main panel. The main panel has one vacant slot to put the breaker, but the neutral and ground bars have no more vacant slots. My question is this -- can I hook up the new neutral and ground wires to an existing slots? Meaning if the slots are already being used by another breaker, can I also hook up my new neutral and ground wires into these slots since there are no more available slots? I also see that the neutral and ground wires are all mixed up on the two bars. I'm saying that neutral wires and ground wires are not hooked up seperately from the two bars. I guess they're okay if done in the main panel.

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I'm having real difficulty understanding the problem. Can you attach a photograph? -- Just link to photobucket or some other provider and someone here will edit it into your question text. – Chris Cudmore Jun 26 '12 at 20:02
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Why not install a sub-panel and move a few breakers to that instead? As long as it is done correctly then there is no question about it being to code or not. – maple_shaft Jun 27 '12 at 13:15
@maple_shaft A subpanel is a lot of work/money just to avoid using a wire nut. If you really want to avoid a wire nut, though, you can ask the manufacturer about drilling and tapping an additional bus bar. Almost all manufacturers allow you to do this. – Michael Jun 28 '12 at 12:42

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NEC 2008

110.14 Electrical Connections. (A) Terminals. Connection of conductors to terminal parts shall ensure a thoroughly good connection without damaging the conductors and shall be made by means of pressure connectors (including set-screw type), solder lugs, or splices to flexible leads. Connection by means of wire-binding screws or studs and nuts that have upturned lugs or the equivalent shall be permitted for 10 AWG or smaller conductors. Terminals for more than one conductor and terminals used to connect aluminum shall be so identified.

So if more than one conductor can be placed under a lug, it must say so somewhere on the panel (or in the panel documentation).

408.3 Support and Arrangement of Busbars and Conductors. (D) Terminals. In switchboards and panelboards, load terminals for field wiring, including grounded circuit conductor load terminals and connections to the equipment grounding conductor bus for load equipment grounding conductors, shall be so located that it is not necessary to reach across or beyond an uninsulated ungrounded line bus in order to make connections.

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If you have a bus bar on each side of the panel like in the diagram above, you should not connect the neutral to one bus and the ground to the other. Neutral and ground from all circuits on the left should go to the left bus bar, and Neutral and ground conductors from the right should go to the right bus bar.

408.41 Grounded Conductor Terminations. Each grounded conductor shall terminate within the panelboard in an individual terminal that is not also used for another conductor.

Exception: Grounded conductors of circuits with parallel conductors shall be permitted to terminate in a single terminal if the terminal is identified for connection of more than one conductor.

So if the panel allows multiple conductors under a lug, you can terminate multiple equipment ground conductors under a single lug. However, you cannot use a single lug for multiple grounded conductors (neutrals), or a mix of equipment ground and grounded (neutral) conductors.

Here's why you can't have multiple neutrals in a single terminal.

Multiple neutral conductors in a single termination create a significant problem when the circuit needs to be isolated. In order to isolate the circuit, the branch breaker is turned off and the neutral is disconnected by removing it from the terminal. If the terminal is shared with another circuit, the connection on the other (still energized) circuit will be loosened as well. Loosening of the second neutral (loss of neutral) under load is a safety hazard, and may establish an overvoltage condition on lighting and appliances if the neutral is part of a 120/240 Vac multi-wire branch circuit. Source

And this is why you can't have a neutral and a ground in a single terminal.

The connection of a neutral and equipment-grounding conductor in the same termination creates a similar issue. One of the objectives of the particular arrangement of bonding jumpers, neutrals and equipment grounds is to allow circuit isolation while keeping the equipment grounding conductor still connected to the grounding electrode (see UL 869A - Reference Standard for Service Equipment). When the neutral is disconnected, the objective is to still have the equipment ground solidly connected to the grounding electrode. If both the neutral and grounded conductor is under the same terminal, this cannot be accomplished. Source

You can , however, have both grounded conductors (neutrals) and equipment ground conductors connected to the same bus bar in the main service panel if the grounded conductor bar/bus (neutral bar) is bonded to the equipment ground bar/bus (it's different if you are dealing with a sub-panel, since the bar/bus will not be bonded).

Long story short

You'll have to check the panel documentation to determine if multiple conductors can terminate under a single lug. If they can. You can connect a couple equipment ground conductors to a common terminal, which should free up enough space to add the breaker.

NOTE: This only applies to bus bar terminals, most breakers are not rated to be "double tapped". So you should never have two conductors under a single breaker lug.

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"You should not connect the neutral to one bus and the ground to the other... right should go to the right bus bar." Not all service panels are set up like the diagram, and even in the diagram, you can loop the neutral and/or ground around the bottom (assuming the service entrance conductor is at the top.) Great job on finding those references, though. I took a look at 250 and a few other sections (including 408, but I was half asleep by then) and found nothing, unfortunately. – Michael Jun 27 '12 at 13:25
@Michael It's true not all panels are set up this way. But why would you run a wire all the way down and around the inside of the panel just to separate the ground and neutral? Seems like a huge waste of wire, and does not seem like it would look "tidier". If you terminate the neutral and ground on adjacent terminals, it looks clean and it makes it easier to remove or modify the circuit later. – Tester101♦ Jun 27 '12 at 14:11
Wouldn't be tidier in the case of your diagram, but you might still loop one around to the other side if one bus bar is already full for whatever reason. – Michael Jun 27 '12 at 15:10
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Yes, it's fine that the neutral and ground wires are intermixed. The ground and neutral bar and both bonded inside the service panel, so even if all of the ground wires go to the ground bar and all of the neutral wires go to the neutral bar, they are still electrically connected via the bonding jumper. It is only once they exit the service panel that they must remain separated for various reasons. For appearance reasons, however, it definitely looks tidier if the neutral and ground wires go to their designated bars.

To answer your main question, no, it is not usually acceptable to have two wires go to the same screw on the neutral or ground bus bar. The only exception to this case is if the bus bar is UL-listed for that purpose. Call the manufacturer if you want to find out, or you can just play it safe and use wire nuts to combine the two wires into one wire, and then connect that single wire to the bus bar.

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Hmm - I'd like to really know the code regarding your last paragraph. My box has several "shared screws" on the G/N bars. All installed by (what I thought were) licensed electricians. – Steve Fallows Jun 26 '12 at 20:39
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@SteveFallows I sometimes use the wrong words, and this is the case here. It is indeed true that you will often see two wires under a screw, but nine times out of ten, it is a violation. You are only allowed to put an extra wire under the screw if the part is UL-listed for that purpose. – Michael Jun 26 '12 at 22:42
@Michael - I plan to remove 2-3 ground wires from their existing lugs on the bus bar to open up spaces. Then hook up the new neutral wire to one lug. Then wire nut the ground wires and pigtailed to the remaining lug/s. In this case, I have put the new neutral wire in one seperate lug. Is this okay. In addition, I checked the door of the main panel and found out about wiring on the bus terminal screws. It says that it allows wiring one (1) #14-4 wires or 2-3 #14-10 wires per terminal screws. Please tell me what this means? Thank you. – Rolando Jul 9 '12 at 15:26
@Rolando 12/2 and 14/2 are common types of Romex electrical wire, but that is not what they are talking about. In this case, they are referring to what sorts of wires you can put under the lugs. The first number is the gauge (in AWG [lower numbers are bigger wires]) of the wire and the second is the number of them: one (1) wire of size AWG14 to AWG4 (#14-4); or two to three (2-3) wires of size AWG14 to AWG10 (#14-10) It sounds like your busbars are capable of handling more than one wire as most of your wires will be AWG12 and AWG14. (Use some wire of known size as a reference.) – Michael Jul 10 '12 at 2:23
As far as your plan goes, it's fine, but make sure you have wire nuts to fit the wires that are being pigtailed. Wire nuts will also have a description on their package explaining what and how many wires they can accommodate, so you should select appropriately sized wire nuts. Also, make sure that none of the GND/N wires goes flailing around as you work on the other wires, as they could make contact with live electrical parts. It would be safest if you cut off all electricity prior to servicing the panel. I am not responsible for anything that might go wrong! – Michael Jul 10 '12 at 2:29
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