In article , Wayne says...

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).

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.

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

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?

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.

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.

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.

Jim


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