Hi folks. I'm returning to the reflector to learn some more about amps
before I buy a second one. I've been reading the archives and see that the
discussion certainly has been lively!
I read the original thread on Line Wiring in the archives. Very informative.
My question is, should the ground and neutral be tied together at subpanels?
Based on the comments I read, my guess would be no -- because if there's a
break in the neutral conductor running from the main panel to the subpanel,
then the ground lead becomes a conductor. Right?
It's not an idle question. Eighteen months ago, I had a subpanel installed
by an electrician next to my motorized crankup tower. There's a dedicated
250-foot run of 1" conduit containing four #8 insulated wires carrying the
two phases, neutral and ground to the subpanel. The subpanel contains a
220VAC 20A main breaker and two 110 VAC 20A surge-protecting circuit
breakers, one for the tower motor and one for a weatherproof utility outlet
adjacent to the subpanel. The tower and panel are in a location well
isolated from any person other than me.
I was concerned that a lightning strike on the tower could travel through
the motor and associated wiring to the subpanel, and then to the house via
the four AC wires in the conduit. I already had surge protectors on all the
coax and control cables, so I installed the 110VAC surge protecting breakers
on each leg of the circuit and connected the tower ground system (12 8' rods
with 1/0 radials) to the subpanel ground lug. I can't think of any reason
why there would be any problem connecting the tower ground to the subpanel,
and it seemed like the right thing to do, but I'd like an opinion from the
experts out there.
Unfortunately, the 110VAC surge protecting breakers shunt the surge to
neutral, not to ground. I think they must have been designed to go in main
panels where the ground and neutral are tied together. I realized that a
surge shunted to the neutral wire would travel back to the house to find the
ground at the main panel, so I'm pretty sure that I tied the ground and
neutral together at the subpanel (I can't remember now for sure -- it was a
while ago and I'd have to trudge 250' through two feet of snow to find out.)
If I did tie them together, then I guess the primary danger would be a break
in the neutral conductor running from the house to the subpanel. If that
happened, the ground lead in the conduit would carry the AC return for the
tower motor. Luckily, it's insulated #8, just like the neutral wire, but
that ceases to be true once the run enters the house (it's #10 bare copper
back to the main panel.) How dangerous is it for the ground lead to carry
the AC return and what are the specific shock hazards, both at the tower and
in the house?
I'm hoping that if it is technically not correct to tie the ground and
neutral togther at the subpanel, that the danger inherent in that is
outweighed by the potential safety provided by shunting lighting strikes on
the AC lines directly to the tower ground system (wishful thinking?) What
are your opinions about the proper configuration for the subpanel, both from
a code and lightning protection perspective? Should I disconnect the neutral
from ground at the subpanel, remove the 110VAC surge-protecting breakers and
just install MOVs between the three AC lines and ground?
By the way, for protection against surges induced along the 250-foot run of
wire, I put 200V MOVs from Polyphaser on the two hot wires and the neutral
just after they enter the house from the conduit (hey, I've got an 87A on
that same circuit, which justifies as much protection as possible and
ensures that this post is not off-topic!) Sound OK?
73, Dick, WC1M
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