Silly bonding question

[green.bubbly]

If I understand things correctly, in a main panel, the neutral bar and the ground bar should be bonded. Bonded meaning there is a common strap connecting both bars to the box as in the pic below.

Now, the sill question is this...

Why are the neutral conductors on one bus and the ground wires on another? Other than aesthetics, is there any other reason to keep them separated?

 

[mtne]

Only at the first point of service after the meter. Everywhere else they should be kept separate.

 

[green.bubbly]

So if this is the main panel (first point of service), the neutrals and grounds could be mixed together on one bar?

 

[sberry]

Yes, some mains do not even have separate bar, its often added just for more spaces.

 

[Alchymist]

They can and often are mixed together, but I feel it's bad practice. While in many cases it's not likely, there exists a chance that the main panel could some day become a subpanel, in which case having the ground wires separated and on their own bus bar would save a lot of effort when converting it to a subpanel. It would require only the removal of the bonding screw to convert.

 

[trbomax]

They can and often are mixed together, but I feel it's bad practice. While in many cases it's not likely, there exists a chance that the main panel could some day become a subpanel, in which case having the ground wires separated and on their own bus bar would save a lot of effort when converting it to a subpanel. It would require only the removal of the bonding screw to convert.

What he says,plus it just looks hokey.

 

[Aceman]

They can and often are mixed together, but I feel it's bad practice. While in many cases it's not likely, there exists a chance that the main panel could some day become a subpanel, in which case having the ground wires separated and on their own bus bar would save a lot of effort when converting it to a subpanel. It would require only the removal of the bonding screw to convert.

We do this when possible, it's hardly any more effort to seperate them when it's installed new, so why not?

 

[djjsr]

I'm getting the impression that something bad can happen if neutrals and grounds are on the same bar in a subpanel. Can somebody please explain? (in simple terms please )

 

[Alchymist]

Nothing "bad" will happen, just a lot more work for the next electrician if the panel is to become a subpanel. Plus it looks better - you know, workmanship, and all that. Also makes it easier to circuit trace.

 

[Aceman]

I'm getting the impression that something bad can happen if neutrals and grounds are on the same bar in a subpanel. Can somebody please explain? (in simple terms please )

Because you'll have current returning on the ground wire as well as the neutral.

 

[djjsr]

Because you'll have current returning on the ground wire as well as the neutral.

Ok, I understand that, but why doesn't that happen with a main panel if grounds and neutrals are on the same bar?

 

[mtne]

The electrical difference is that if the neutrals and grounds are maintained separate everywhere, there should be only one point where they are bonded. That one point is the reference for the breakers to recognize the difference in potential to trip. It also I think cleans up the potential for stray voltages by only allowing one pathway. From an engineering point of view the goal is to assure that there is only one pathway for fault and that all the voltages stay on a known and predictable path. This allows the various breakers and other electrical products to operate as they are designed to. This also allows engineers to design entire systems where fault current and arcfault potential can be predicted and accounted for in the hierarchy of whole building systems. So that a 20A breaker trips before the 400A main panel..................

The way it was explained to me is that if a subpanel has the neutrals and grounds on the same buss, the return voltage on the neutral can or will split among the available pathways going back to the main panel. If enough of the voltage bypasses the neutral, a breaker may not trip, or may not trip as fast.

Also there's something about stray voltage on grounds and how that can interfere with electronics........

I had a fair idea about these things but it really comes from my friend J the EE who I call with these kinds of questions.

From the electricians point of view it keeps things clean, easier to trace and troubleshoot. And of course it's code.......

 

[Naq]

Ok, I understand that, but why doesn't that happen with a main panel if grounds and neutrals are on the same bar?

I guess the simple answer is that it DOES happen and is expected from the service panel back to the utility transformer. The "grounded conductor" that the utility presents is the neutral from the service transformer and provides the final low impedance path for normal current, phase to neutral fault current and ground fault currents from the home electrical system.

The 2011 NEC handbook explanation for prohibiting re-gounding the neutral (grounded conductor) is as follows:

"Section 250.24(A)(5) prohibits re-grounding of the grounded
conductor on the load side of the service disconnecting
means. This requirement correlates with the requirement of
250.142(B), which is a general prohibition on the use of the
grounded conductor for grounding equipment. This requirement
prevents parallel paths for neutral current on the load
side of the service disconnecting means. Such parallel paths
could include metal raceways, metal piping systems, metal
ductwork, structural steel, and similar continuous metal
paths that are not intended to be a normal current-carrying
conductor."

mtne mentions some advanced subjects like ground fault coordination between protective devices so a fault in a single light fixture in the back warehouse doesn't bring down the whole plant and potential differences between grounds that can cause ground loops/current/voltage etc... They can be a nuisance in residential but disastrous in industry.
http://en.wikipedia.org/wiki/Ground_loop_(electricity)

 

[MrMark]

has nothing to do with any of that. Breakers trip on heat and/or magnetic field strength for short circuit current.

The reason they are kept separate is to protect against the danger of an open neutral between the sub panel and the main bonded point. In such a case the grounds would be unexpectedly and dangerously energized and open circuit.

 

[PRH44]

The ground wires and the neutrals are total different conductors and perform different functions in the electrical system.
The grounding conductors also known as equipment grounds are responsible to keep all equipment chassis at zero potential. Grounding wires are also responsible for facilitating the overcurrent protective devices AKA breaker This is accomplished by returning the fault currents to bond point creating large amount of fault current increasing the magnetic trip portion of the the breaker forcing it to trip.
The neutral AKA grounded conductor is a return path to the zero point at the transformer for the unbalanced current in a AC system.

The two are only bonded togather at one point. This keeps the both conductors at the same potential. This would be zero in an a correctly installed system under normal conditions.

As mentioned above mixing the two can cause stray currents causing interfence in electronics. the loss of an incoming neutral will cause return current to flow in the equipment grounding conductor if it is wired correctly. This can go on undeteced creating a very dangerous situation if one would decide to lift the infamous "ITS ONLY A GROUND WIRE" then all return current would flow through the individual to ground resulting in serious injury or almost certain death.

This can also affect fault current paths by splitting and reducing the speed at wich Over current protective devices trip. This will affect the fault current study and coordination of said devices. In an industrial and heavy commericial setting this could be disastrous to equipment and or personal in a large fault condtion.

Be advised currents can also flow away from the bond point in a branch circuit equipment ground if it became the path of least reisitance to earth ground.

The grounds terminated on the neutral bus is a very common and dangerous code violation. "ITS ALL THE SAME AT THE MAIN RIGHT" NO! Codes are written for protection of personal and equipment not to make things look good or easy.