I hope this isn't too dumb for the forum, but here goes:
I worked with an electrician who was big on directing process and not too big on explaining theory. Most directions were explained by "Because I said so."
I've noticed on forums here repeatedly the concept that you do not terminate ground wires and neutral wires in a panel on the same terminal strip. The old boss is retired and moved away, so maybe one of you pros can explain to me what the deal is here.
I should mention that we worked exclusively on residential.
Last edited by Retired_Helper; 08/25/0702:50 PM. Reason: Duh, left out words!
The ground and the neutral are permitted/required to be connected at 1 place only. That place is where the power first enters the building and is referred to as the Service. From that point forward, those 2 wires are never supposed to be connected together ever again.
The reason behind that stems from the behavior of circuit breakers and (also) fault currents. The system is designed to be installed with that single point of commonality between the ground and the neutral and everything that you can buy and install has been designed and tested with that constraint.
If the ground and neutral get connected together in more places, problems can develop that are very difficult to find and fix.
Connecting them together willy-nilly as the old boss said to can cause ground loops, strange behavior of electronic equipment and a host of other potential problems.
As long as we are talking about panel that comprises the main service, then it's perfectly fine to terminate the ground wires and "neutrals" on the same bar. Some people prefer to keep them separated on different bars, and that's fine as long as both bars are connected to the service neutral conductor. An example would be some panels that have neutral bars that run down both sides of the panel. Now if this were a sub-panel, then no, the ground wires and "neutrals" must be kept separated all the way back to the main service panel.
This is because if, for some reason, the neutral between the sub and main panels came loose or apart, the voltages that the neutral should be carrying now end up going back out over the ground wires for every circuit in the sub panel. This also means that ground pins in receptacles, face plate screws, appliance frames, just about anything metal could and probably would become energized.
By keeping neutrals and grounds separated at sub panels, a failed neutral becomes simply an inconvenience, not a shock hazard.
Let me elaborate / expand this topic, with a few examples.
EGCs (Equipment Grounding Conductors) and Grounded Conductors (AKA "Neutrals") - Terminating to Busses
* A: Basic Concepts -
In Panelboards / Service Equipment, where the System's Grounded Conductor (AKA "Neutral") is connected directly to the Grounding Electrode Conductor (the GEC is connected to the "Neutral Bus"), is one of the conditions where the Equipment Grounding Conductors - along with any other Bonding Conductors terminating at that location, will be terminated to the Grounded "Neutral" Conductor's Bus Bar.
These EGCs will terminate on the same Bus, in conjunction with any Grounded Circuit Conductors ("Neutrals") for Branch Circuits, Feeders and if applicable, the Service Feeder's Grounded Conductor.
The "Neutral" Bus in this situation is the "System's Bonding Point", where the Grounded Conductor is bonded to:
* The "Local" Grounding Electrode System (via a Grounding Electrode Conductor),
* The Metallic Equipment (Panel Enclosures) via the _MAIN BONDING JUMPER_,
* Equipment Bonding Conductors - if any,
* Any "Supplemental" Bonding Conductors - EGCs would fit this description.
So, in a Panelboard such as this one, the "Neutrals", "Grounds" and Bonding Jumpers will all terminate to the same Bus Bar.
*** NOTE: Unless a Bus Bar / Bus Kit is listed for more than one Conductor under one Terminal Screw, only "Land" one wire per screw terminal. Do not place - for instance, one bare Grounding Conductor + one White (stripped end, of course) under the same screw.
Terminate one wire per screw terminal (hole). It does not matter where on the Bus you place certain wires (like all white wires at the top, all bare / green wires at the bottom). For the sake of Workmanship, you may terminate them in a certain fashion.
FYI: If you need additional termination spaces (not enough on the Neutral Bus), install a separate Ground Bus, and land the EGCs to it instead of on the Neutral Bus. When applying this option, the Grounding Bus will be attached to the Panel's enclosure, using the pre drilled/tapped holes, which the Manufacturer puts in most Panels, along with the proper type Screws to mount the bus kit (these normally are supplied with the bus kit).
Install a Bonding Jumper between the new Bus kit and the existing Neutral Bus kit - size the Jumper accordingly (see NEC Article 250 for details).
The Above Text Covers Typical Service Equipment
*** OTHER PANELS WHERE THE EGCs + THE "NEUTRALS" LAND ON THE SAME BUS KIT:
* 1: In some cases - especially for very long runs, a remote Panelboard (AKA "Sub Panel") located in a _DETACHED_ Building may have the Grounded "Neutral" Conductor re-bonded to a local Grounding Electrode System at that Building / Structure.
In this case, the Feeders are only the Ungrounded ("Hot") Conductors, and the System's Grounded "Neutral" Conductor. There is no Equipment Grounding Conductor included with the Feeder, and the Feeder's Raceway is not "Electrically Continuous" between the remote Panelboard and the Panel / Equipment where it was fed from. Along with this, there will be no other metallic paths, between the two points of connection - this includes Metallic Water Pipes, Metallic Gas Pipes, Rebar, or "Messenger Wires" used with Telco / CATV drops.
For the Panelboard described above, the "Neutrals" and the Equipment Grounding Conductors terminate to the same Bus; and this Bus will _NOT BE ISOLATED_ from the Panelboard's Enclosure - it will be bonded directly to the Enclosure.
A Local GES is created at the remote building, and a Grounding Electrode Conductor is run from it to the remote Panelboard - and is terminated to the same Neutral Bus.
Additionally, a Main Bonding Jumper (or screw) is installed, which bonds the Panel to the Neutral Bus. Any other supplemental Bonding Conductors will also terminate to the Neutral Bus.
*** OPTION FOR AN SDS: BONDING AT THE FIRST PANELBOARD / DISCONNECT ***
Another place you would find "Neutrals" and "Grounds" terminated to the same Bus Kit would be for a Separately Derived System - where the installers chose to Bond the System at the First Panelboard (or Disconnect Switch), rather than do all the Bonding at the Transformer.
Personally I prefer to do all the Bonding at the Transformer, but it's just a matter of choice - as if installed correctly, either method is safe.
In all other cases, the Grounded "Neutral" Conductors are terminated to an Isolated Bus Kit (not directly bonded to that Panelboard), and Equipment Grounding Conductors are terminated to a separate Bus Kit - which IS directly bonded to the Panelboard.
The primary reason to perform the Isolation of a System's Grounded Conductor in remote Panelboards is to keep "Unbalanced Current" from flowing on Grounded / Bonded Metallic Equipment (Conduits, Metallic Sheathing of MC and AC Cables, Panelboards, Steel Studs, etc.) and on Equipment Grounding Conductors between Panelboards.
If the Grounded Conductor is "re-bonded" in a remote Panelboard - AND there are any L-N Loads (Line to Neutral Loads), the Load Current on the Grounded Conductor feeding that Panelboard will divide - part running on the Grounded Conductor Feeder, the remainder on the Metallic pathways (conduits, etc.). This is hazardous in many ways - as it creates heat on the metallic pathways, and at points along the system will result in a Ground Fault hazard for Personnel.
EGCs land on Neutral Bus Kits, where that Bus Kit is ALSO "Correctly" Bonded to a Grounding Electrode System (GES) and to the Service / Panelboard Equipment. This is typically done in Service Panels / Equipment, but may also be done for remote Panelboards in detached buildings, or for the first Panel / disconnect for an SDS.
In all other cases, the System's grounded Conductors are kept Isolated from Conductors which are physically Bonded to Metallic Equipment (typical Equipment Grounding Conductors), and is additionally isolated from Metallic Equipment.
There is an AC Power System which has no System Grounded Conductor, yet has Equipment Grounding Conductors in use, which bond Metallic Equipment together, but these Conductors - and the Local Grounding Electrode System, are _NOT_ Connected to the System in any physical way.
This would be an UNGROUNDED AC SYSTEM Typically would be a 480V 3 Phase 3 Wire Ungrounded Delta, but there are a couple other flavors out there.
There is no Grounded System Conductor on this System, but there is a Voltage to Ground!!!
If you are interested in this type system, perform a topic search in the Electrical Theory and Applications area for "Ungrounded Systems"
Let us know if the examples in this post do not address your questions properly. Go ahead and let us know if it did help out with your questions.
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!
With the type of grounding arrangement known in the U.K. as PME or TN-C-S, we have the similar "one point" rule about neutral and ground meeting, but we achieve it in a slightly different way.
Over here, the main service panel always has separate neutral and ground bars, with the neutral completely isolated from the case. The main bond from the ground bar is bonded to the incoming neutral at the service cut-out ahead of the meter.
But we have other systems in which there is no neutral-ground bond whatsoever. These would not be permitted under the American NEC.
In my part of the world here is how it works: In new work the ONLY place neutural and ground come together is in the meter socket. The cable from the meter socket to the main service panel is 4 conductors with the neutural conductor being insulated. The neutural is not allowed to be derated, and must be the same as the hot conductors. In the panel the branch circuit neuturals and grounds are landed os seperate terminal strips, the can is bonded to the GROUND, but NOT the neutural, same as a sub panel. Theory being that in a three wire service (ground and neutural on same conductor) the neutural/ground conductor would not be big enough to carry the neutural load and any ground fault current that might occur, at least that is the way it was told to me.
Life is tough, Life is tougher when you are stupid
Thank you all, my understanding is much enhanced. I find this issue illustrative of the other current thread about education/experience for upcoming Journeymen: I think this would have become clear instantly if I was standing in front of a panel with a pro instructing.
I have never seen a 4-wire service such as Jimmy described, with an insulated neutral kept completely separate. I would be interested in opinions on such a service.