On 4/4/2011 10:49 AM, Dick Green WC1M wrote:
> Yes, and the neutral and ground must be isolated in any subpanels as well.
> However, there's one exception: if the subpanel is more than a certain
> distance from the main panel (not sure how far), such that the code requires
> a separate ground rod at the subpanel, the neutral and separate ground rod
> are connected together in the subpanel. This could be the case for a
> subpanel in a separate outbuilding.
The issue is NOT distance, but rather creating a separate SYSTEM. A
separate system is created by a TRANSFORMER, or by feeding another
building. A power system MUST have its neutral bonded to ground where it
is created, but ONLY at that point. The service entrance creates a
system, and is the ONLY permitted neutral-to-ground bond in most homes
and small buildings.
The GROUND that we're talking about here is the system of green wires,
and ALL grounds associated with the building must be connected to it.
The BOND we're talking about is the connection of NEUTRAL to that system
of grounds, which may occur at ONLY at the service entrance..
If power enters one building and is fed to another one (like a shack or
garage or workshop), there are two legal ways to do it. I used Method #1
to feed my shack, which is in the second building. In both methods,
power enters building #1, neutral is bonded to the steel enclosure at
the entry panel, one or more ground rods are bonded to that steel
enclosure, and all grounds associated with building #1 (CATV, telco,
structural steel, satellite dish, roof antennas, etc. coax entry panel
for your shack) must be bonded to that steel enclosure. In both methods,
you can have as many ground rods as you like, but they must all be
bonded together and to that system of green wires, and the best place to
make the connection is at a common point where they connect to the steel
enclosure.
In Method #1, power is fed from Building #1 to Building #2 with the two
240V phase (hot) conductors and the neutral. There must be NO GROUND
connection between the two buildings. In Building #2, there must be one
or more driven rods bonded to the steel enclosure of the entry panel,
all of the grounds associated with Building #2 must be bonded to that
steel enclosure, AND there must be a bond between neutral and ground in
that panel.
In Method #2, power is fed from Building #1 to Building #2 with the two
240V phase (hot) conductors, the neutral, AND there must be NO bond
between neutral and ground in Building #2. All of the grounds associated
with Building #2 must be bonded together, and to that steel enclosure.
Additional driven rods are certainly a very good idea adjacent to the
point where ham antennas enter the building and are bonded to ground.
With Method #1, you MUST also provide transformer isolation for any coax
lines running between buildings, and with Method #2 this is also a good
thing. I use isolators made by Jensen Transformers for my CATV feed
between buildings. Google to find them.
> I have this situation at my station, though not for an outbuilding. I have a
> motorized crankup tower with a 120VAC motor about 270 feet from the house.
> Four #10 wires buried in a 4' deep conduit (separate from the radio cables,
> of course) carry 240VAC from the main panel in the house to an outdoor-rated
> subpanel mounted next to the tower. The wiring and subpanel were installed
> by a licensed electrician.
>
> As I recall, the electrician did not bond neutral and ground together at the
> subpanel,
That's good.
> nor did he connect the subpanel to the tower ground.
That's bad.
> I'm sure that
> I had not built the tower ground system when he installed the subpanel, and
> he didn't bother to install a separate ground rod for the panel.
> Essentially, the installed it like a subpanel inside a house would be
> installed.
There is no requirement for a separate rod for that panel.
> After the subpanel went in, I built an extensive ground rod system for the
> tower: 12 rods spaced 16' apart in a radial pattern from the base, cadwelded
> with 1/0 wire. Several years ago, I read up on the code, and changed the
> wiring at the subpanel. I don't recall exactly what I did, but I do know
> that I connected the subpanel to the tower ground and bonded the local 120
> VAC neutral to the tower ground in the subpanel. I believe this is correct.
No, the neutral to ground bond is WRONG.
> What I don't remember is whether I disconnected the neutral and ground wires
> coming from the house. It seems to me that they should be disconnected in
> this case. In other words, ground and neutral for the 120VAC branches from
> the subpanel should use the local neutral and ground, not the ones coming
> from the house. Anyone care to comment on that?
See my summary above.
> What would be the specific
> hazard if the house ground and neutral wires are connected to the subpanel
> ground and neutral, and thus to each other?
It violates safety codes, can cause hum, buzz, and RFI, and is unsafe
for the reasons several have already cited.
> As soon as the snow melts, I'm going to take a walk to that subpanel and
> refresh my memory on how it's wired!
That would be a VERY good idea. If it's wired the way you think it is,
it's WRONG. :) The home I bought in California was wired by licensed
electricians. It is was WRONG, and I had to fix it.
All of this boils down to three fundamental principles.
1) Neutral and ground must be bonded (connected) at ONE, AND ONLY ONE
POINT, at the point where the system is created.
2) You can have as many ground rods as you want, more is better, but all
the rods (and other grounds) associated with a building MUST be bonded
together and to the power system ground (the service entrance steel
panel and the green wires).
3) The power system ground must extend to every outlet, to every piece
of equipment fed by the power system.
73, Jim Brown K9YC
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