OT - Electrical question. Sub panel grounding?

I think my electrician is wrong!

Building a new workshop 250' from main house breaker panel. Panel upgrade finished to 200 amp and 250' of 2AWG, four conductors run to sub panel on shop. Before slab was poured. 24' of copper grounding rod put in ground and is also bonded to the re-bar.

When sub panel was put in and wired up the electrician bonded both neutral and ground to the sub panel box. he did not use my earth ground at all and stated "it was not required by code and the ground wire back to the main panel would take care of the earth ground".

I was surprised when I saw him do this but he stated what he did was to code. He has been in business for years.

As far as I know you never want the return current on the ground wire at all. And if the neutral has a fault, all return current is now on the ground wire and hence anything metal in the shop that has a ground back to the panel, including the panel itself could have voltage on it.

Should be easy to research, but what I have found is depending on who you ask and where in the country you are at you will get a different answer.

I have read:

Don't need a ground from main panel at all and use the grounding rod at the shop. Some say to bond the neutral and ground at the shop some say not to.

Some say what my electrician did is OK.

Some say neutral and ground should not be bonded and ground from main panel should connect to ground on shop panel and then to earth ground rod at shop. So the only time the ground will see current is if hot touches something metal. This is what I think is correct.

So who is correct?

I am not an electrician and am not an expert on the NEC. But I ran into a similar situation when installing a new sub panel at work in Iowa. That sub panel was in the middle of a large building and it fed all the electronics and some of the lights in the equipment area. The equipment was audio and video recording, duplicating, and editing equipment and a good, solid, LOW RESISTANCE ground was absolutely necessary. We had experienced previous problems with hum creeping into our audio signals so I went all out on that ground system.

Directly below the new panel I drilled through the concrete slab and installed three copper clad ground rods. They were tied together with 0000 bare wire and all connections were soldered. A length of 0000 bare wire ran up to the panel and the licensed electrician who installed it tied it in to the ground buss. The cable from the building's main switch gear had three phases and a neutral. The conduit served as the required ground connection between the two locations. I really do not recall weather he connected the neutral and the ground together at the sub box. However he did assure me that it was completely in compliance with the NEC. It was a small town and there were no local inspectors or local codes to worry about.

My point is that there should be no problem with attaching a local ground to the ground bus in a sub panel. Personally, I would recommend it. And follow the code with respect to the neutral/ground connection or lack of it.

PS: that low resistance ground also ran from those three ground rods to every location where we had equipment. The ground itself ran along the floor level, never raised above it and consisted of either 0000 gauge wire or six inch wide copper strap with a similar cross section to the 0000 wire. Again, all connections were soldered. The equipment racks and other chassis were all connected to it with heavy gauge wire, 6 or 8 gauge IIRC.The connections there were the only ones that used mechanical connections (screws). I never heard any new complaints about hum after installing that ground system.

I did consider it absolutely essential that the standard, safety ground of the NEC and the low resistance, signal ground that I designed, be solidly connected to each other at the point of entry to the area (the sub panels). My one worry with that system was that there was the potential of picking up something from the air due to the numerous but short ground loops created by the combination of two ground connections to each piece of equipment: the line cord's ground (which we retained) and the low resistance ground plane formed by the heavy gauge wires and copper straps.

It is not according to NEC. Electrician would only be correct if the Authority Having Jurisdiction overrides the National Electrical Code. I would go further and congratulate you in your summation as most electricians just don't get it. It isn't safe for man, or equipment.

Your electrician has installed a NEC code violation. You can only bond the neutral and ground at your primary over current protection location, which a sub panel is not.
Proper connection will be to connect the ground wire from the main to sub panel and isolate ground wires from the neutrals in the sub panel. Also get the ground field and rebar bonded to the ground in your sub panel.
You need to find an electrician that installs to code standards.

Laverda said:

When sub panel was put in and wired up the electrician bonded both neutral and ground to the sub panel box. he did not use my earth ground at all...

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There are two things here.
First is the statement of "bonded both, the Neutral and Ground to the subpanel"
If that means that he had tied both together at the subpanel, then it is not correct!
Ground and Neutral can only be tied (bonded) at the very first, mains panel. All subsequent installations must treat Ground and Neutral to be separate conductors, never to
be at equipotential.

IOW the subpanel's Ground bus and the Neutral bus must not be electrically connected. There is typically a cross bar on the top or bottom of the panel's guts
that connects the left side to the right. In this case that bar should be removed, and one side should become Ground, the other is Neutral and connected accordingly.


As far as the ground bars in the slab, I really don't know ( and believe me, recently had reasons to dig into this and could not come up with anything decent from the NEC )
but I also do not see why one would not connect ground to the slab grid at the panel.
Any metallic conduit will do it in it's own way anyway, so while it may not be required, it obviously won't harm either.
But!!!
In no case can or should the slab-grid be relied upon as proper grounding in and of itself !!!
Any and all circuits ( equipment, outlets, lights etc ) must have it's very own independent and separate ground conductor that's running back all the way to the subpanel.
IOW you cannot just run two pieces of THHN to something from the panel to supply hot and neutral ( or single phase if it may ), and then bond said something to the
slab-grid as a ground.

It depends on local code.

Safety ground and NEUTRAL are to be connected at only service entrance and often is called "meter ground".

Sub panel safety ground is questionable in that connecting back to meter ground and or adding independent ground is dependent on a few different variables including distance and intended use.

ALWAYS contact the local building inspector as it is their responsibility to advise how code applies to your specific site.

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SeymourDumore said:

There are two things here.
First is the statement of "bonded both, the Neutral and Ground to the subpanel"
If that means that he had tied both together at the subpanel, then it is not correct!
Ground and Neutral can only be tied (bonded) at the very first, mains panel. All subsequent installations must treat Ground and Neutral to be separate conductors, never to
be at equipotential.

IOW the subpanel's Ground bus and the Neutral bus must not be electrically connected. There is typically a cross bar on the top or bottom of the panel's guts
that connects the left side to the right. In this case that bar should be removed, and one side should become Ground, the other is Neutral and connected accordingly.


As far as the ground bars in the slab, I really don't know ( and believe me, recently had reasons to dig into this and could not come up with anything decent from the NEC )
but I also do not see why one would not connect ground to the slab grid at the panel.
Any metallic conduit will do it in it's own way anyway, so while it may not be required, it obviously won't harm either.
But!!!
In no case can or should the slab-grid be relied upon as proper grounding in and of itself !!!
Any and all circuits ( equipment, outlets, lights etc ) must have it's very own independent and separate ground conductor that's running back all the way to the subpanel.
IOW you cannot just run two pieces of THHN to something from the panel to supply hot and neutral ( or single phase if it may ), and then bond said something to the
slab-grid as a ground.

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Absolutely agree - you can't have the neutral and ground connected at more than one point. The sub-panel needs to have them separated.

As far as bonding your ground rods/rebar to the sub-panel ground, I honestly don't know if its required. I do not know what local codes say, but I do believe when I researched it years ago, NEC did not require it.

As an EE with equipment reliability grounding experience (not a grounding code expert but know a bit), I have a serious issue with bonding a second ground to the sub-panel ground from the main supply. There is always a potential difference between the two grounds which causes circulating currents. For many reasons circulating currents are bad. In addition, in a fault or lightning strike condition, you can get significant voltage differential between the two grounds, potentially putting people and equipment in harms way. You always try to keep all equipment at equal potential, and that is very difficult to do in a lightning strike.

In an industrial facility, we try very hard (when people know what they are doing) to have all grounds in a star pattern grounding scheme. This means that your ground wire starts at one point and is distributed from there - you do not ground equipment with various grounds. This helps to minimize circulating currents.

One thing that I learned recently is that you can't blindly use the ground conductor charts in the NEC as gospel. They are just a minimum - larger grounds may very well be required. I would personally oversize your grounds whenever possible, especially feeding a a garage sub-panel 250' away and I did not do a full grounding conductor ampapacity requirement study.

I have a good friend that sits on the NEC ground panel. I will ask his perspective next time I see him.

Personally, unless a local inspector forced me to do it otherwise, I would separate the neutral and ground that is fed from the main panel at the sub-panel and leave it at that. I would not bond the ground rod/rebar to the sub-panel.

This is simple. If you connect the neutral and the ground together in a subpanel there will ALWAYS be current flowing on the ground conductor, because they are now parallel conductors! Whichever one has the least resistance will carry the most current. That is why you don’t bond a neutral to ground in a subpanel. It has nothing to do with interpretation of the NEC.

Yes you do want to connect the ground rod, rebar grid, and building steel to the ground conductor. That is so they can never be energized by a hot conductor going to ground. The ground rod grid can not carry away fault currents because the resistance will always be way to high. However in a lightning strike it can keep a massive surge from going back to your main service on the ground conductor.

Grounded rebar is often called an "ufer ground" but more correctly "concrete-encased grounding electrode". Done correctly it's perfectly legal (subject to local AHJ), often required, and can be far more effective than a ground rod.

On Solid Ground: Defining and Understanding UFER Ground | Electrical Contractor Magazine

OK, I think I have it figured out. Thanks, all.

I don't trust my electrician anymore!

Local jurisdictions have differed on local ground rods tied to the EGC from main panel.

NOBODY I know of differs about connecting the neutral and EGC at a sub panel. Not done.

If the sub panel is far enough away, it may just need its own service.

Electrician test:
Hand him a piece of 3 conductor power cord with no ends and a male cord end. Ask him which end the cord end goes on.

Having separate ground rods is usually considered bad - can create ground loops. Definitely do not connect neutral and ground anywhere other than main panel.

From what I've been told and seen if the "sub" is in another structure then you need a ground rod at that site.

As long as you still have the ground wire from the main panel it's fine, and I think it's in the new code to have both. From what I understand a ground rod should never be installed at a subpanel without the ground wire also going back to main panel. Like was said before, certainly can't hurt in normal situations and could definitely help in a lightning strike.

Alocal ground used to be limited to milking barns, etc, to prevent critters from getting shocks due to ground differences. I think it has been made less restrictive, Refer to the current accepted code version for your area (might not be the latest).

I don't understand your test. Which end the cord end goes on? The "cord end" of what? A piece of 3 conductor power cord has TWO ends, by simple geometry. And both of them are "cord ends". Every one I have ever seen was like that.

It is possible to choose the end of a 3 conductor cord to allow a standard plug or socket to be installed on it without criss-crossing the conductors. Is that what you mean?

And I don't understand what it has to do with the present discussion here.

garyhlucas said:

Electrician test:
Hand him a piece of 3 conductor power cord with no ends and a male cord end. Ask him which end the cord end goes on.

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EPAIII said:

I don't understand your test. Which end the cord end goes on? The "cord end" of what? A piece of 3 conductor power cord has TWO ends, by simple geometry. And both of them are "cord ends". Every one I have ever seen was like that.

It is possible to choose the end of a 3 conductor cord to allow a standard plug or socket to be installed on it without criss-crossing the conductors. Is that what you mean?

And I don't understand what it has to do with the present discussion here.

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This is critically important if you have $100+ per foot Monster-type directional power cords.

Yan Wo said:

This is critically important if you have $100+ per foot Monster-type directional power cords.

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If it is a shielded cable that has been prestripped for you then you need to check that one end has ground bonded to the shield and that the other does not, and use the bonded side as the source, but that's oddly specific and not at all in the scope of extension cords.

more than you ever wanted to know about grounding and bonding:

Bonding and Grounding