Greetings all. I am new here. Please forgive any initial ignorance to norms of these forums. Hopefully I will pick-up quickly. I have searched the forum archives regarding bonding but did not find a specific answer to my question so here goes.
I have been checking electrical codes and diagrams shown in various documents like "Code Check - Electrical." In the section that discusses sub-panels it says that they should have 4 wires (two hot, one ground, and one neutral) and that the neutral and ground must not be bonded together in the sub-panel. It also provides a diagram in the document that shows the neutral going to one buss bar and the ground going to another as the text describes. They aren't connected to each other by any bonding strap BUT they both appear to be screwed into the metal box, which would bond them together. Should the neutral buss bar be screwed into some form of non-conductive material that is then screwed into the box? What am I missing here?
I hope you will be patient with me as I have another question.
I have purchased a property that has and outbuilding. At the main service disconnect for the home on the property amoung all the OCPDs there is a single breaker for a 120V feed for the outbuilding. It connects to a 6 AWG black wire that is bundled with a white 6 AWG and a 10 AWG ground – the ground is not connected at either end. The 6 AWG white wire connects to the neutral buss bar in the main service panel. In the outbuilding the black 6 AWG appears and connects to a small sub-panel with two occupied circuit breakers. I notice that the building has a single grounding electrode that is attached to the ground buss bar in the sub-panel. The white 6 AWG is also connected to the same buss bar as are two neutrals for the little building’s two branch circuits. As it's wired the sub-panel seems to me to be wired like it is receiving a main service feed but instead of from the utility co. it's from the main house. Here's the three questions about this. 1) Is it wired correctly or should the existing ground between the two buildings be connected to the existing grounding bass bar?, and 2) Does the sub-panel require a service disconnect besides the one in the main house?, and 3) Should the out-building’s neutral and ground path’s be isolated?
Your neutral bar should be installed on some type of insulator; if it needs to be bonded you can use a bonding strap or a screw to bond it to the can. Your grounding bar may be mounted directly to the can or on insulators with a bonding strap or screw.
As far as your grounding, The NEC allows 2 scenarios:
1: You pull an equipment grounding conductor from the main panel with the supply (this is best IMO). In this case your neutral and ground are not to be bonded.
2: You pull no grounding conductor with the supply, you use a grounding electrode to supply your E.G. In this case you WOULD bond the neutral and ground at the sub panel.
Thank you for your kind and speedy response. Your answer was concise and easy to understand.
From what you said it leads me to another question. In both panels a ground conductor appears from both ends of the cable... I am curious why the previous installer chose not to use it can go through the expense of placing a ground electrode instead.
In any case... thanks.
[This message has been edited by leckemj (edited 12-05-2003).]
Note that the code requires a grounding electrode at the second building in both cases. Installing an EGC with the feeder condcutors does not eliminate the need for a grounding electrode at the second building. The only time that you can omitt the grounding electrode at the second building is when the supply to the second building is a single branch circuit. Don
Thank you for clarifying the issue regarding the GES for the building. I would like to ensure the sub-panel in the out-building was installed correctly and have it retrofitted if necessary. I believe I have read that each building should have a main power disconnect. Is this correct? If so, can a simple service interrupt (like those used for air conditioners) be used?
(Note: the below is based on lots of reading of theory, making assumptions, and extrapolating from what I know to what seems reasonable. I'd love corrections as appropriate.)
As noted above, there are two 'legal' scenarios for feeding a subpanel in a different building.
1) You wire as a normal sub-panel, running your grounded conductor, your non-grounded conductor(s), and your equipment grounding conductor. You keep separate 'neutral' and grounding buses in the sub-panel, and you do not bond the 'neutral'. You have to bond the EGC to the building ground (ground rod, structural metal, etc,), but the 'neutral' is _not_ bonded.
2) You run your grounded conductor ('neutral') and your ungrounded conductor(s) ('hot(s)'), but you explicitly do _not_ run an equipment grounding conductor. In fact you are required to have _no_ metallic pathways between the buildings. In this second case, you can treat the subpanel almost as a service, in the sense that you bond the grounded conductor to the local building ground.
The 'why' of selecting one method or the other falls to the level of theory and opinion.
The _problem_ with doing a subpanel incorrectly is that you get undesirable currents flowing in your grounding system. A proper grounding system inside a building is well bonded into a single conductive structure. Every bit of metal is connected by some path to every other bit. This means that if you connect the grounded conductor ('neutral') at multiple points to the grounding system, then you _will_ get current flow in the grounding system. Current flows in the grounding system are considered undesirable.
On the other side of the coin, the more frequently the electrical system is bonded to ground, the better defined the system voltage is with respect to ground. If you have a totally ungrounded system, then while the conductors have a well defined voltage _between_ them, the voltage to ground is quite undefined. A little 1.5V battery sitting on a 100kV static electricity generator still only has 1.5V between the terminals, but has 100kV to ground. Having a well defined voltage to ground is important for protecting the insulation system from transient overvoltages...if you walk across a carpet on a cold day and plug in your toaster, you want the static zap to have a good bonded path to ground that _doesn't_ involve going through the insulation.
The balance of the two requirements (good solid ground bonding to deal with overvoltage, and avoiding undesired current flow in the grounding system) is that inside a structure you are required to bond the grounded conductor at one, and only one, point. However outside structures _multiple_ bonding to ground is not simply allowed, but is in fact the norm.
Every house on the same transformer has its grounded conductor bonded to ground at the service. There is also a ground bond at the transformer itself. The distribution system to the transformer is grounded at multiple points. Current flow in the earth itself is _not_ 'considered' objectionable, and the desired protection from transient overvoltage means that grounding at multiple points is the way electrical distribution is done.
An outbuilding with a subpanel is somewhat halfway between a new service and a subpanel in the building, so you get to choose between using one method or the other. I personally would prefer wiring as a subpanel if the buildings were close together, and wiring as a service if they were far apart, but I don't have a good feel yet for what defines 'close' and 'far'. I'll let others weigh in on that topic.
Also, you should note that while currents in the earth itself are not _considered_ objectionable, the fact of the matter is that they _do_ cause problems. There can be detectable voltage gradients along the earth surface (step potential), which can disturb or harm livestock and humans. Additionally, the 'no continuous metallic systems' rule is not applied to POCO distribution to building services, and you often have multiple homes, each with a service, each with the neutral bonded to ground and the plumbing system, with significant current flow in the plumbing system. The problem of where to ground a system and weather or not to ground a system multiple times is _not_ as cut and dried as the code for a single building would seem to imply.
1) Is it wired correctly or should the existing ground between the two buildings be connected to the existing grounding bass bar?, and
Since an Equipment Grounding Conductor (EGC) was run with the circuit conductors it should be terminated to the bonded buss bar in each panel. In the service equipment at the supply end that buss bar will be the grounded circuit conductor "neutral" buss bar. In the panel at the outbuilding you will need to install a new EGC buss bar and remove the bonding screw or strap that bonds the existing grounded circuit conductor "neutral" buss bar to the panel cabinet.
2) Does the sub-panel require a service disconnect besides the one in the main house?, and
The answer here depends on the number of slots available in the panel in the outbuilding. If there are six or fewer slots for breakers than you do not need a separate main disconnect. If there are more than six slots but only six or fewer breakers are installed then you do not need a building disconnecting means YET! If there are more than six breakers installed then you need a single building disconnecting means. Since you have only two breakers installed presently you do not have to install a building disconnecting means.
3) Should the out-building’s neutral and ground paths be isolated?
Yes they should be isolated. Once you have installed the separate EGC buss bar and removed the bonding screw or strap that connects the grounded circuit conductor "neutral" buss bar to the outbuilding panel cabinet you will shift all of the bare or green EGCs to the new EGC buss bar. You will also shift the Grounding Electrode Conductor that connects the panel to the grounding electrode (ground rod) to the new EGC buss bar.
If this does not fully answer your question please post back or E-mail me direct by clicking on the envelope icon in this posting. -- Tom Horne
[This message has been edited by tdhorne (edited 01-05-2004).]
"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison