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.
Are there one or more books that actually coverf
*this* kind of stuff, ie that's being discussed in
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
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.
PS: eons ago I took some EE courses in college, but of course
that knowledge was pretty useless for this current subject.
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.
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
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
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.
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
Doug Miller (alphageek at milmac dot com)
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.
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.
Doug Miller (alphageek at milmac dot com)
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.
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.
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