"Wayne" wrote in message
news
Hi all,

I've tested out my new VFD and it will operate
like I want it to. So now I ready to permanently
mount it.

Power will come from a local circuit breaker box
which the contains the hot wires, a wire from the
main breaker box which is from the neutral/ground
bus, and the local box is grounded by conduit. The
ground wire in the local box is not tied to
the box (but could be).

The VFD has 1/2" knockouts to cable to. The
physical connection is metal. It is connected to
the place where they also have ground terminals.

So if I run conduit from the local breaker box
to the VFD, then connect the ground wire to
the VFD am I going to run into problems
(ground loops, against code to connect)?

Same issue would apply connecting the lathe.
(grounding by conduit & wire).

I could use insulated connectors to connect
the conduit. But they need to grounded somewhere
for shielding purposes.

I suppose everything (VFD, lathe) could be
grounded only by conduit, but this doesn't
seem right/safe.

Anyone know the correct procedures for this?

Thanks,
Wayne D.

It is always tricky to ask about "code," since it has subtle variations from
place to place. But, if you are concerned about the integrity (safety) of
grounding only through the conduit, run a ground wire through the conduit
along with the power wires. The ground wire must be either green or bare
copper.

I'm a little confused by your statement that "the ground wire in the local
box is not tied to the box."

There are two grounds: the power ground (which should be white) and the
safety ground (which should be green or bare). In a properly installed
conduit system, the conduit may also serve as the safety ground. The two
should be tied together in the Main box, but nowhere else. The ground bus
in the Main should also have a wire to a ground stake.

If I understand you correctly, your sub panel (local box) is grounded to the
Main via the conduit (for safety purposes) and only the power ground (some
people refer to it as the neutral) is run on a wire.

Should you run a "backup" safety ground through the conduit, it should
connect to the boxes (sub panel to VFD box) and to the frame of the lathe.
The purpose of the safety ground is to trip the breaker if something shorts.
Other than that, the safety ground should NEVER conduct power.

Conduits should never be insulated. Should the insulation of a hot wire
fail, the conduit could go hot, which is highly ungood for a variety of
reasons.

If you are unsure, consult your friendly electrician, lest you find yourself
bounded upon your backside in a most ungentlemanly manner... The risk here,
from the way you describe it, is that the sub panel may not be properly
grounded. If you are sure it was properly installed, it should be OK. But,
if it was installed by someone who didn't know what he was doing.....

Jerry