Use of metal Conduit as Ground path and Use of Sealing Locknuts

[choogenboom]

Can anyone help me reconcile these two quotes:

"Conduit (RMC, IMC or EMT) is an NEC-accepted ground path. So, if the wires are in steel conduit, no OTHER ground wire is required."

"Sealing Locknuts-Sealing locknuts are intended for use with rigid metal conduit and intermediate metal conduit with one sealing locknut in the outside or the inside and either an ordinary locknut or a sealing locknut on the opposite side of the enclosure for wet locations or liquid-tight applications."

Seems like one of them must have a qualifier to the effect of "unless it must be liquid tight" or "unless its relied on for conductivity". And seems like _both_ should have a requirement to verify they work as required. Ie "must test for the conduit to be a working low impedance electrical connection" and "must do a bubble test or similar to confirm a connection is liquid tight.". Or is there no need to test that required things actually work and which case why bother requiring them?

 

[ptonsparky]

Why?

 

[ActionDave]

Can anyone help me reconcile these two quotes:

"Conduit (RMC, IMC or EMT) is an NEC-accepted ground path. So, if the wires are in steel conduit, no OTHER ground wire is required."

"Sealing Locknuts-Sealing locknuts are intended for use with rigid metal conduit and intermediate metal conduit with one sealing locknut in the outside or the inside and either an ordinary locknut or a sealing locknut on the opposite side of the enclosure for wet locations or liquid-tight applications."

Don't know where these come from but they make sense.

Seems like one of them must have a qualifier to the effect of "unless it must be liquid tight"

Why? Both statements exclude liquid tight already.

or "unless its relied on for conductivity". And seems like _both_ should have a requirement to verify they work as required. Ie "must test for the conduit to be a working low impedance electrical connection"

You could say the same thing if you used conduit that didn't qualify as an equipment ground.

and "must do a bubble test or similar to confirm a connection is liquid tight.".

Conduit of any form isn't required to be water proof.

Or is there no need to test that required things actually work and which case why bother requiring them?

You could say the same thing about any part of the electrical installation. Every connection and component is at risk of damage, abuse, or failure. Insulation can fail. Connections can fail. About the only thing that can't fail is gravity.

 

[Fred B]

The Lock nut, water intrusion prevention, and GEC provision of the rigid conduit can seem like a pickle. The intent is not the creation of an air tight or to allow full submersion with no water ingress under pressure. That is a whole different thing.
AFA Ground path, you make that by use of bonding bushing and bonding wire connecting all conductive surfaces when the connection is not in direct metal contact or made within a concentric KO of an enclosure, among other conditions.
Unlike the GEC and GE requirement for under 25ohm tested unless additional requirements met, the wireways and raceways listed as acceptable for use as an EGC have no such requirements. Labs have tested these to find them as acceptable when compliant connections are made.

 

[choogenboom]

Why?

I am installing a transfer switch that, from left to right, is sistered up to what is currently a service disconnect breaker box which is in turn sistered up to a meter box. The service disconnect box's only electrical connection to ground is through a conduit nipple between it and the meter box to its right that is (1) rusty and (2) does not seem to use sealing lock nuts. I plan to connect the new transfer switch to its neighbor with a steel conduit nipple and I want to make sure I get it right. The two quotes I referenced came from this forum. Both make sense on their own but taken together are conflicting. My concern is that if a sealing lock nut is used to satisfy the liquid tight requirement that would impair the electrical connection function. Put another way the conduit must be both liquid tight and make a electrically low impedance connection between the boxes it connects which seem to be mutually exclusive. I am wondering if the current disconnect box to meter box conduit nipple was done wrong. Should it have been made liquid tight and should it not have been relied on as a electrical ground conductor and instead should there be a dedicated ground wire between the two boxes?

 

[ptonsparky]

Sealing locknuts are optional.
Pull an EG or use grounding bushings on both ends if you're worried.

 

[augie47]

I can't say the existing was "wrong" but in your situation if I were to use sealing locknuts I would likely pull a wire type EGC to assure a low impedance path.

 

[ActionDave]

.... Put another way the conduit must be both liquid tight and make a electrically low impedance connection between the boxes it connects which seem to be mutually exclusive. I am wondering if the current disconnect box to meter box conduit nipple was done wrong. Should it have been made liquid tight and should it not have been relied on as a electrical ground conductor and instead should there be a dedicated ground wire between the two boxes?

Liquid tight is a flexible conduit.

 

[augie47]

I was under the impression that the OP is using the term "liquidtight" to indicate "sealing against liquid entry" as opposed to a wiring method.

 

[suemarkp]

Doesnt the service disconnect also have the system bonding jumper? That should ground that enclosure to the chassis. Then, the conduit nipple is grounded from both ends, so you would need a lot of fail for it to be electrically isolated.

Will the transfer switch become the new service disconnect? If so, use a wire type EGC after it to ensure grounding and dont rely on the raceway.

 

[jaggedben]

By all means add a grounding bushing or two if you feel that standard or sealing locknuts aren't providing sufficient bonding for whatever reason, or won't be reliable in the future. But it's not code required if the locknuts are made up well enough to provide the bonding.

I agree with others you are not using the phrase 'liquid tight' appropriately. Your enclosures are most likely NEMA 3R and sealing locknuts or connectors would only be needed where required by 312.2.

 

[wwhitney]

Are sealing locknuts designed so that their teeth dig into the metal enclosure as well as a regular locknut, while still compressing the sealing material the correct amount?

Cheers, Wayne

 

[jaggedben]

Are sealing locknuts designed so that their teeth dig into the metal enclosure as well as a regular locknut, while still compressing the sealing material the correct amount?

Cheers, Wayne

Kind of. Sometimes.

 

[jaggedben]

The thing is, I don't know why you'd use a sealing locknut on both sides, so the inside is usually going to have a regular locknut that does the bonding.

 

[ptonsparky]

The thing is, I don't know why you'd use a sealing locknut on both sides, so the inside is usually going to have a regular locknut that does the bonding.

Try putting those things on backasswards a few times & you will know why they go on the outside of an enclosure.

 

[choogenboom]

Thanks all for taking the time to respond. I'll try to answer multiple questions here:

1. "Doesnt the service disconnect also have the system bonding jumper?"
   1. Assuming you mean the neutral to ground bond, the answer is yes but that's not the whole answer. Neutral is bonded to ground inside the meter box and is also connected to earth electrode and the meter's steel enclosure at that same location. To complete the picture there are 2 x 200 amp service disconnect boxes sistered up to the meter, one on the left and one on the right. Both rely on their steel conduit nipples to the meter box for electrical ground. Both have a isolated jumper connecting service neutral to load neutral. Both also have a smaller ground wire connected to that neutral jumper and that ground then enters the house and runs to the distribution panel. I believe this is incorrect and they should have run the grounds entering the house back to the bonding point inside the meter? Or is it allowed to bond the house's ground to neutral at a second/different point from the neutral ground bond in the meter? Finally there are 2 x 200 amp panels in the house and both of those also have a 200 amp service disconnect breaker.
2. "Will the transfer switch become the new service disconnect?"
   1. Yes it will. Followed by 2 x 200 amp breakers on the side of the house followed by 2 x 200 amp breakers in the panels in the house.

Given the topology I have described, do I need to keep the 2 x 200 amp service disconnect breakers that are currently in place on the side of the house adjacent to the meter? Seems like they are already redundant to the 200 amp disconnect breakers at the distribution panel and they will be really redundant with the addition of the 200 amp service disconnect breaker in the ATS.

Separate but related question. Which is better:

1. Keep the neutral to ground bond that currently exists in the meter box. In this case leave in place the existing neutral connections that go from meter to the disconnect boxes to house and then add a neutral + ground from ATS to meter box and do not bond neutral to ground in the ATS. Or
2. Remove the bonding in the meter, run a neutral + ground from meter to ATS, bond Neutral to Ground in ATS, and run Neutral and ground from ATS to the two service disconnects which feed the house, separate the existing neutral to ground bonds in the two service disconnects such that grounds going into the house originate from the ATS where ground and neutral are bonded. Or
3. same as #1 and but do bond neutral to ground in ATS

 

[jaggedben]

Do not remove bonding in the meter, it is required. See 250.80 or 250.92.

If your ATS is going to contain your new service disconnect then it must have a neutral-ground bond inside. The correct term for this bond is Main Bonding Jumper. Technically this bonds the neutral to the EGC(s) for the load side circuit(s). This is required to be in this location.

Also there must be no neutral-ground bond on the load side of the service disconnect, which means you must remove the existing neutral-ground bond in the 200A panel that you're backing up.

In other words...
On the line side (utility side) of the ATS you typically have hots and neutral, with metal parts such as meter socket bonded to neutral. (e.g. neutral lug in meter socket is not isolated from the can). Typically no green wire on the supply side of the ATS, although it's allowed to have bonding jumpers if necessary to bond something properly.

On the load side of the ATS you will have hots, neutral, and EGC. The EGC may be a green wire or the conduit if the latter is installed properly with locknuts or connectors that provide bonding.

 

[jaggedben]

You can leave the electrode connection (GEC) in the meter box if the utility is okay with that.

 

[jap]

Your service disconnects are not relying on the rigid nipples that attach them to the meter enclosure for the ground fault return path.
The Neutral from the meter should be bonded to the enclosure in both service disconnects.
If a fault occurs downstream of the service disconnect that fault travels back to the service disconnect via the equipment grounding conductor where it is connected to the Neutral conductor in the Service Disconnect.
The Neutral Conductor from the Service Disconnect to the meter serves as the ground fault return path although the fault will take all paths available.
The rigid nipples between the meter and the Service Disconnects should however have a bonding bushing on either the meter side or the service disconnect side of the nipple with a bonding jumper to the neutral in case there happens to be a fault in the nipple itself since it's a Service Conductor raceway.

JAP>

 

[choogenboom]

Neutral bonded to meter box and to earth electrode.

RCC200SE Breaker box/Service disconnect showing neutral to neutral, ground to neutral. And guessing the pan head slotted screw in the jumper is optional and when installed connects the otherwise isolated jumper to the case? Best I can tell the function of that screw is not documented on the label or the Eaton spec sheet or web site. If my guess is correct then my earlier statement about the breaker box relying solely on the steel conduit nipple for grounding is false.