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Wayne
 
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Default Grounding, Ground loops

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.
  #2   Report Post  
Ned Simmons
 
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In article ,
says...
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?


I'm not sure I follow everything you've described, but the short answer
is don't worry about ground loops when you're bonding enclosures and
conduit runs. It's likely you can't avoid potential ground loops and
wire to code without going to great lengths anyway, and redundancy is
better than something remaining unbonded. If you want to be conservative
and run shielded cable for the VFD control wiring, ground the shield at
one end of the cable only and try to connect all signal returns to a
single point. The VFD is unlikely to care either way; I've installed
dozens in nasty environments and can't remember ever having a noise
problem other than the noise created by the VFD interfering with other
sensitive devices.

Ned Simmons
  #3   Report Post  
Jerry Foster
 
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Default


"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


  #4   Report Post  
jim rozen
 
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Default

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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  #5   Report Post  
Wayne
 
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Portions snipped out...

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.


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.


This is the part that confuses me.
....The two are tied together in the main box, nowhere else...
But it is ok to ground through the conduit and a ground (green) wire?
Am I not tying it together somewhere else?
Or maybe that refers to adding something like another ground rod
by the lathe.

Wayne D.


  #6   Report Post  
Wayne
 
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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.
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RoyJ
 
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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.

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jim rozen
 
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In article . net, RoyJ says...

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.


As I said, it would not hurt at all to have a green wire go all the
way from the machine, to the VFD, to the sub-panel, and back to
the service entrance.

Many local codes allow the conduit to be used as the grounding
conductor. Because he does not have a green wire all the
way back to the service entrance, his green wire would stop
at the sub-panel - unless he continuted it on to the service
entrance.

This is probably how I would do it, personally.

Jim


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please reply to:
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  #9   Report Post  
jim rozen
 
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In article , Wayne says...

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.


To make it clear, you most likely would not run into any
ground loop issues by running a green wire all the way
back to the service entrance.

I would personally do this if it were my installation.

Many codes allow the conduit itself to be used in place
of the green wire as a grounding conductor. I don't think
any codes prohibit putting a green wire in the pipe as well.

This means you would have to run that wire back from the
sub-panel to the service entrance, as it does not exist there
at the moment.

Jim


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==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
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Bob May
 
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The thing here is that the low side of the power is supposed to be ground
but doesn't have to be at any point that you can see.
There is a seperate safety ground which does get connected to ground at
several points and eventually gets connected to earth ground. This line is
for the safety of the operator of the equipment and isn't designed to carry
any of the power used by the equipment. It's purpose is to make the
equipment chassis at the ground potential so that you don't get shocked by
operatiing the equipment.

--
Why isn't there an Ozone Hole at the NORTH Pole?




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Wayne
 
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Thanks all, I understand now.
I woke up in the middle of the night and figured out part of it.
The neutral is only bonded to ground at the service (which I knew).
If it gets bonded elsewhere, then the conduit also becomes a conductor
(bad).
For some reason, in my brain, I was equating the neutral bonding
issue with that of adding a ground wire and I was thinking that was
bad also. That's where most of my confusion came from.

The last part you guys answered today that it is ok to add
an additional ground wire. That is what I wanted to do in the
first place. That will get added because I'd trust the ground wire
more than the conduit.

The control wires for the VFD will be shielded & grounded properly per
VFD instructions and only @ the VFD. So there won't be any
ground loop problems with them.

Wayne D.

On 23 Mar 2005 10:38:16 -0800, jim rozen wrote:

In article , Wayne says...

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.


To make it clear, you most likely would not run into any
ground loop issues by running a green wire all the way
back to the service entrance.

I would personally do this if it were my installation.

Many codes allow the conduit itself to be used in place
of the green wire as a grounding conductor. I don't think
any codes prohibit putting a green wire in the pipe as well.

This means you would have to run that wire back from the
sub-panel to the service entrance, as it does not exist there
at the moment.

Jim



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ATP*
 
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"Wayne" wrote in message
news
Thanks all, I understand now.
I woke up in the middle of the night and figured out part of it.
The neutral is only bonded to ground at the service (which I knew).
If it gets bonded elsewhere, then the conduit also becomes a conductor
(bad).


Particularly if you lose the neutral conductor. The conduit would then
return the unbalanced load and it would tend to heat up at fittings.



  #13   Report Post  
RoyJ
 
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My electrican was in wiring for my new TIG welder yesterday and the
subject of conduit grounding came up. He said that the local code for
commercial spaces allows a maximum of 6' flex conduit before you have to
run a seperate groud wire inside.

Wayne wrote:
Thanks all, I understand now.
I woke up in the middle of the night and figured out part of it.
The neutral is only bonded to ground at the service (which I knew).
If it gets bonded elsewhere, then the conduit also becomes a conductor
(bad).
For some reason, in my brain, I was equating the neutral bonding
issue with that of adding a ground wire and I was thinking that was
bad also. That's where most of my confusion came from.

The last part you guys answered today that it is ok to add
an additional ground wire. That is what I wanted to do in the
first place. That will get added because I'd trust the ground wire
more than the conduit.

The control wires for the VFD will be shielded & grounded properly per
VFD instructions and only @ the VFD. So there won't be any
ground loop problems with them.

Wayne D.

On 23 Mar 2005 10:38:16 -0800, jim rozen wrote:

In article , Wayne says...

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.



To make it clear, you most likely would not run into any
ground loop issues by running a green wire all the way
back to the service entrance.

I would personally do this if it were my installation.

Many codes allow the conduit itself to be used in place
of the green wire as a grounding conductor. I don't think
any codes prohibit putting a green wire in the pipe as well.

This means you would have to run that wire back from the
sub-panel to the service entrance, as it does not exist there
at the moment.

Jim



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jim rozen
 
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In article . net, RoyJ says...

My electrican was in wiring for my new TIG welder yesterday and the
subject of conduit grounding came up. He said that the local code for
commercial spaces allows a maximum of 6' flex conduit before you have to
run a seperate groud wire inside.


Flex conduit (greenfield) and rigid are two kinda different
animals. Whereas I might consider not running a green wire
inside rigid, I would always run one inside of greenfield
or liquid-tight, given the nature of the connectors at the
ends.

Jim


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ATP*
 
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"RoyJ" wrote in message
ink.net...
My electrican was in wiring for my new TIG welder yesterday and the
subject of conduit grounding came up. He said that the local code for
commercial spaces allows a maximum of 6' flex conduit before you have to
run a seperate groud wire inside.

That's been in the NEC for quite a while.


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