Sorry, the safe way to do this is to run the green or bare wire all the
way through from the main box to the sub panel to the VFD to the Lathe
to make sure all are at the same potential. then fix your ground loops
as seen by the VFD. I doubt if you will have any troubles, VFD's are
designed to work in some very ugly environments.

Wayne wrote:

Thanks for the terminology corrections.
I know about the stuff, but not always the right words.

On 22 Mar 2005 18:06:05 -0800, jim rozen wrote:


OK, from what I gather, you have a sub-panel (what you
call a "local box" which is fed from your service entrance
(what you call the 'main breaker box') and that
subpanel has hot legs and an unbonded neutral.


correct.


This is correct as one is not supposed to bond
neutrals in a sub-panel.


That was my understanding also.


From what you say, the subpanel has the ground*ing*
conductor (green safety wire ground) supplied by
the conduit from the service entrance to the subpanel.
This is not uncommon.

So your question is, how do you connect your new VFD
to the sub-panel, correct?


Yes, and VFD to lathe also, from a grounding point of view.
Hot wires aren't an issue.


The hot legs of course are wired through just as you
would imagine, and if there is a neutral required
by the VFD (probably not) then you would bring that
in to the VFD housing as well.


No neutral required.


The grounding, or green wire safety ground can be supplied
only by the conduit to the VFD, that is probably OK. If you feel
peculiar about this then you could run a similar sized
green wire inside the conduit and connect the VFD chassis
to the body of the sub-panel. This would create a tiny
ground loop but it would exist only inside the conduit
and would be unlikely to cause any high frequency problems.


So the VFD is only supposed to be grounded by conduit and nothing else.

This also implies that the lathe is only grounded through the conduit
and nothing else.

I shouldn't also have a green ground wire run through the conduit from
the service
to the VFD and lathe in addition to the conduit.



Do NOT connect the neutral in the sub-panel to the ground
in that enclosure. That is the same thing as 'bonding' it
at that point and is against code.


No connection here!


Jim



Wayne D.