Why must ground & neutral be seperate in subpanel?

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wrote:

Right answer to the wrong question. The GFCI will work, but unless there's a capacitor or something in there that I don't know about, the test button won't.
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Yes, it will. The test button has nothing to do with ground. It creates an imbalance by routing some current from the hot side of the outlet around the GFCI to the neutral wire connected to it. This is the same sort of imbalance caused by a wet person touching hot.
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27 days until the winter solstice celebration

Mark Lloyd
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A resistor, I believe. It shunts a small amount of current around the balance circuit simulating an imbalance in the two sides, so if it's working right the GFCI will interrupt the current.
-D
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"What do *you* care what other people think?" --Arline Feynman

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Yes.
The one I examined connects a resistor between neutral on the line side and hot on the load side. Ground is not involved.

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27 days until the winter solstice celebration

Mark Lloyd
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OK that makes sense, and the test will work without a ground and without a load...
thanks
Mark
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Are there one or more books that actually coverf *this* kind of stuff, ie that's being discussed in *this* thread?
Not some home-improvement book you might find at HD, but something that really, tutorially, gets down to the depth of *this* stuff, this "more complex" stuff than you usually see covered in the home-improvement-style electrical-books.
Something so that when you finish a subject, you really understand it, not so that you can merely "do" some home-job, but even would let you (correctly!, and usefully) contribute to eg this current thread.
Ideas?
Thanks!
David
PS: eons ago I took some EE courses in college, but of course that knowledge was pretty useless for this current subject.
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snipped-for-privacy@verizon.net wrote:

It's not that they're to be isolated at the pony panel (we call it up here), it's that there is to be a single connection to "ground", and it's to be at the main panel ... kind of like it's just not right to connect the bare copper and white in a receptacle :-) Or, that receptacle box if wired that way could float above ground if there's sufficient current through that bare copper ... a "potential" for ... geez, that smarts.
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All household grounds must meet only at a common point for reasons similar to why ground loops create hum in a stereo system. Neutral, equipment ground, and earth ground that connect at a common point (main breaker box safety ground) avoids 'ground loops' and other adverse or surprise currents.
To explain same using a different perspective, first, all wires are electrically different at both ends. That difference increases as more current flows. To better explain this, we express that difference as a separation or electrical distance.
A three prong appliance is powered from wires that are distant from the breaker box (because they carry current). A separate safety (equipment) ground wire connects directly (shorter) to breaker box safety ground because it carries no current. Appliance connected electrically shorter to breaker box means greater human safety.
Again, if safety ground and neutral wire were connected anywhere (other than in breaker box), then that safety ground wire would be electrically farther from breaker box (because it carries current). Another perspective that explains why NEC demands separate neutral and ground wires.
Another situation: suppose neutral and safety ground wire were both carrying current. Suddenly that common wire breaks. What happens to appliance connected to third prong safety ground? It suddenly becomes electrically hot - directly connected to black hot wire. AND no safety ground exists to protect human and trip circuit breaker. We want neutral wire separated from safety ground so that any neutral wire break always leaves appliance still connected directly to breaker box safety ground and not connected to a neutral wire that is no longer connected to breaker box. Just another reason why those two wires always remain separate.
Home has its own single point safety ground inside breaker box. Power wires connect that system to another system that has its own single point ground - pole transformer. Pole transformer connects primary (high voltage) ground, secondary neutral, and earth ground to a common point. Lightning strike to primary (high voltage) wire simply gets conducted safely to earth at transformer which is but one reason why that primary wire can be highest on pole.
Meanwhile, household single point ground in breaker box is one ground system centered at a single point. Transformer has its own single point ground system. How far apart are those two grounds? As current increases on neutral wire (transformer to house), then both grounds become electrically more separated. Again, using a perspective of electrical distance to explain a concept.
snipped-for-privacy@verizon.net wrote:

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So, what is the proper way to connect a 220v sub-panel that has a single bus bar for neutral and ground to a main panel with the neutral bar bonded to the ground bar?
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There isn't one.
To make a Code-compliant connection, you must install a second bar so that you can separate the neutral and ground conductors for the various circuits to separate busses. The neutral bus must be electrically insulated from the ground bus and from the panel chassis, and the ground bus must *not* be insulated from the chassis.
*Also* you must connect the subpanel to the main panel using *four* conductors, e.g. black, red, white, and bare (or green). White goes from the neutral bus bar in the main panel to the neutral bus bar in the subpanel. Bare (or green) goes from the ground bus bar in the main panel to the ground bus bar in the subpanel. Black and red go from the two lugs on the circuit breaker in the main panel which feeds the sub, to the lugs on the main breaker in the subpanel.
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Regards,
Doug Miller (alphageek at milmac dot com)
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Doug Miller wrote:

Thanks everyone, especially Doug for this post which spells it out clearly. Please comment on this proposed "fix": I run another wire back to the main panel (I'll probably use some 12/2 with ground) and attach all three conductors to the ground bar in the main panel. Then, at the sub panel, I will connect all grounds to the new cable but not to the neutral bus bar. Now all grounds will be grounded back at the main panel, and the neutral in the sub will be isolated from the grounds and from the sub panel chassis. (as long as I remove the grounding screw from the neutral bus bar) Thanks again for your responses. Joe
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12/2 wire to power a 40 amp breaker? Not acceptable. 12/2 when, as Doug said, four wires are required? 12/2 is not four wires and not sufficient gauge.
snipped-for-privacy@verizon.net wrote:

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He's talking about using that entire cable as the grounding conductor for the subpanel. Still not safe, still not Code-compliant... but not quite as bad an idea as you assumed.
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Doug Miller (alphageek at milmac dot com)
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snipped-for-privacy@verizon.net wrote:

Code violation -- all conductors for a circuit must be in the same cable, raceway, conduit, etc. You need to run a new cable, 3 conductors plus ground as I described.

Another violation -- you *need* a grounding bus bar in the subpanel. Among other things, doing what you describe leaves the subpanel chassis ungrounded.

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Doug Miller (alphageek at milmac dot com)
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Doug Miller wrote:

This is great but it doesn't mention wire size. If the ground is smaller than the current-carrying conductors, doesn't it become a fire hazard?
-tg

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snipped-for-privacy@earthlink.net wrote:

No. Under normal circumstances, the grounding conductor doesn't carry current anyway. In circuits 40A and above, the grounding conductor is permitted to be smaller than the circuit conductors; see NEC Table 250.122 for details.
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Doug Miller (alphageek at milmac dot com)
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Doug Miller wrote:

But the ground is meant to protect against a short to the 'case', so if a short happens, the ground will not be protected by the circuit breaker---it will overheat.
I went through this exercise in running a circuit to an outbuilding. I never quite figured out what the code meant with respect to a ground for that building as well. There was something about livestock, but I don't remember it now.
-tg

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snipped-for-privacy@earthlink.net wrote:

The breaker will trip long before the wire will melt.
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Doug Miller (alphageek at milmac dot com)
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wrote:

Specifically, because it is a "short" condition, rather than an overload, which can last much longer.
Bob
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Doug Miller wrote:

Again, Thanks for the input. To clarify what I should have written, my plan was to tie my new cable into a ground bussbar which will be secured to the metal of the subpanel. The current neutral bar either floats or is grounded depending on one screw which makes the ground connection. I will remove that to make it float. Regarding the guage and being seperate from the supply, well sometimes something is better than nothing. 3 #12 conductors all tied to the main panel ground bus and to the sub panel ground bus is better than the current situation. Each #12 conductor is good for 20 Amps, so in theory my ground wire(s) can carry 60A combined, and it is only a 40A breaker. A lesser evil than the current situation which has not been problematic so far anyway. When I shop for the bussbar I will price a length of #4 bare copper and consider running that to the main instead. The issue then will be finding a lug in the main large enough to bond it to. Regards, Joe
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