does this make any sense

[jburton32]

I am installing a "sub" circuit breaker panel in my garage. I recently discovered I have to install a second, isolated bus bar in the sub panel for the nuetral line seperate from the ground bar. ironicly the source for both bars can/will come from the same place. (the bus bar in the maine panel)
i spoke to a licensed electrician who addmitted it makes no sense to him either.

[jwelectric]

I am installing a "sub" circuit breaker panel in my garage. I recently discovered I have to install a second, isolated bus bar in the sub panel for the nuetral line seperate from the ground bar. ironicly the source for both bars can/will come from the same place. (the bus bar in the maine panel)
i spoke to a licensed electrician who addmitted it makes no sense to him either.

If the electrician said that it made no sense to him either then I am saying that he ain't much of an electrician and maybe you should not ask him/her any more questions.




Although this is not the best installation it does show how the equipment grounding and grounded (neutrals) MUST be installed.

Note that the grounded (neutrals) do not land on the installed bar for the equipment grounding conductors. That would be a hazard in so much as the panel enclosure would then be part of a current carring conductor.

[Mikey]

I think his puzzlement (mine, too) is that if you trace everything back to the main panel, the neutrals and grounds are tied together, so why not do it here, too? There have been repeated references to an article that explains why, but I can't find it right now.

[Bill Arden]

The rule is there for several reasons.

1. if the neutral and ground are tied together at the sub panel and then the connection to the main is lost... all the grounds connected to the sub panel will become hot.

2. Due to the current going threw the neutral there is a tiny amount of voltage on the neutral and thus tying it to ground can cause problems, ground loops ect.

3. They love to make you spend more money.
You have to run 4 wires instead of 3

[Billy_Bob]

Also note that electrical panels have a "bonding screw" which connects the neutral bar to the metal case of the panel. For subpanels with a separate ground bar (which would be screwed into the metal case), the bonding screw should be removed from the separate neutral bar.

And why should you do this separate ground / neutral bar thing with a subpanel?

In general every single electrical code rule is in place because someone's home burned down or someone was electrocuted due to an electrical problem. Like those nightclub fires where many people die and then there is a big outcry from the public to prevent these things from happening again. Well they determine the cause of these things and then revise the building and electrical codes.

Anyway if everything electrical worked properly forever, then a lot of things required in the electrical code would not be needed.

BUT things break, wires come loose with time. Insulation on old wiring can melt due to heat or become brittle and fall off. This can be the case with old appliances.

You can get a situation where a hot electrical wire is touching the metal case on an appliance.

You can get a situation where the neutral wire on the main service panel comes loose. Actually this is not all that uncommon. Wires which carry a lot of amperage like a main service wire can become warm or hot if they are not torqued down correctly when installed (torque to foot pounds per label on panel). Then as you use and don't use electricity, the connection is hot/cold, hot/cold. The wire expands and contracts. Then works its way loose.

Another common situation is for the main service panel ground wire to become disconnected. Plastic water pipe may be installed to replace metal pipe and the ground wire is not transferred to ground rods (required on new installs). A lawn mower may knock a ground wire loose. I saw a situation where a car scraped the side of a building and cut the ground wire.

When the above situations occur or combinations of the above situations occur, modern electrical wiring installed to code will still protect you. Then there will be no tragedy and no need for a public outcry...

[Mikey]

I understand and accept the possibility of the cases you mention, but other than IR-drop issues on the neutral and/or ground between the main and sub panels, I still fail to see why separating the ground and neutral at the subpanel is of any advantage. The only difference between 6 breakers in a subpanel, and the last 6 breakers on the main panel is, after all, the greater resistance of the "bus conductors" connecting these 6 breakers to the others.

I'm not the sharpest crayon in the box, though, so I'll find the article I mentioned that goes into lots of detail in these cases, spent some time on Mike Holt's forum, study some more, and hopefully get it someday. (This is not to say I don't separate the ground and neutral in my subpanels -- I just don't know why I do, other than the Code says to.)

[kencaz]

I understand and accept the possibility of the cases you mention, but other than IR-drop issues on the neutral and/or ground between the main and sub panels, I still fail to see why separating the ground and neutral at the subpanel is of any advantage. The only difference between 6 breakers in a subpanel, and the last 6 breakers on the main panel is, after all, the greater resistance of the "bus conductors" connecting these 6 breakers to the others.

The reason that you cannot bond the Neutral to the subpanel is simple. The Neutral is a Current Carrying Conductor and the Equipment Grounding Conductor, (EGC), should only carry current for fault clearing.

So, if you Bond the Neutral to the Equipment ground at a Sub panel, you have essentially made the ECG, the enclosure and all the conduit, (if any), current carrying. This could be very deadly if someone were to touch a hot wire and any metal enclosure on that panel.

KC

[Mikey]

Aha - I see now. Any neutral current would be divided in inverse proportion to the resistance of the neutral and ground conductors between the main and sub. I guess my next question would be why not use an independent ground at the sub instead of running a ground conductor back to the main?

[Chris75]

I guess my next question would be why not use an independent ground at the sub instead of running a ground conductor back to the main?

Look at this picture and see if it makes any sense... See how ALL current from faults MUST go back to the utility transformer, if it didnt, it would NOT open the overcurrent device.

[Mikey]

Thanks for that diagram -- it helps a lot. I guess what they're saying is there's no guarantee that an independent ground (or the utility ground, for that matter) is effective enough to carry enough current to open any of the circuit breakers in the hot path -- thus the need to provide a path to the utility neutral. GFCIs seem to solve this problem in a much better way at the individual circuit level.

[Billy_Bob]

Ok let's take this idea of not having a separate ground wire further up the line to the outlet...

You have a grounded outlet. Why bother with the separate ground wire? Afterall the ground connects to the neutral in the breaker panel! So why not use the white neutral wire and not run the separate ground wire?

So we just have a white and black wire. We connect the black to the hot side of the outlet then the white to the large prong neutral side of the outlet and also to the ground connection.

Then the white wire is cut when someone drills a hole in the wall and drills through just the white wire...

Then we plug in an appliance with a metal grounded cover. There is only a black wire (hot) connection now. We turn on the appliance and touch the cover. ZAP! The path of the black wire (hot) electricity goes to the appliance and back out through the white wire, then goes to the ground prong and back up the appliance cord to the metal case of the appliance, then to your hand.

Had there been a separate ground wire conected to the ground of the outlet, you would not have been shocked. The only thing which would have happened would be that the appliance would not work. Much safer to have a separate ground wire all the way back to the main panel.

[Mikey]

OK, let's look at another case... the guy with the drill gets both the neutral and the ground, or maybe just the ground, and a chafed wire in the appliance applies the hot side to the case. Now an independent ground might be a good thing. Interesting to imagine an independent ground with 0 resistance. As far as helping the overcurrent device, all the help in the world won't open it in time or at a level to protect the guy holding on to the appliance, who will die, on average, when 0.07A flows through the right parts of his body.

I'm not trying to be a smartass, just commenting that no matter how well-planned, code-compliant, and well-built, we can imagine a scenario where the guy holding on to the appliance gets zapped -- although I'm having trouble coming up with a plausible case that defeats a GFCI.

I do understand now, though, why the Code says what it says; thank you.

[Chris75]

GFCIs seem to solve this problem in a much better way at the individual circuit level.

Not really, A EGC trumps a GFCI every time.

[leejosepho]

I guess my next question would be why not use an independent ground at the sub instead of running a ground conductor back to the main?

I might be wrong, but I believe that is actually required when the subpanel is in a detached building. When I ran power out to a garage a few years ago, I was told to run 220 (2 hots) and a neutral, and to connect the subpanel to a ground rod at the garage and with the subpanel not being bonded.

[Mikey]

Not really, A EGC trumps a GFCI every time.

Not sure what you mean by that. As far as I can see, no matter what kind of circuit problems we dream up, only the GFCI will recognize that some of the current supplied to the load is not returning and will open the circuit before that "lost" current reaches a potentially fatal level. You don't need an EGC for that, and adding an EGC doesn't help. It doesn't matter if the GFCI is in a main panel, sub panel, or outlet, with or without an EGC, ground rod, or what-have-you.

This is the kind of discussion that needs a blackboard and beer.