Building structural frame for grounding of subpanel?

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markusius

Member
Hi everyone,

I hope I'm not thread jacking but this seemed the closest thread to my question and I do not see any activity since February. Pardon me if I should have started a new topic.

Here is the scenario: I have an all metal warehouse with large 14'inch I-beams for structural support all connected together via metal supports along the roof line. The building earth ground connects this structure to the grounding grid which is buried under the parking lot. I'm assuming that it is sufficient however if need be I can do the IEEE Fall of potential method to evaluate it further. It has come to my attention that a sub-panel is not grounded.

My question is if the NEC allows for grounding the sub-panel to the structural steel it is mounted on via a sufficiently current capable conductor.

I have a quote from a licensed electrician for two scenarios:

A. install a correctly sized ground conductor from the sub-panel to the earth ground located by the service entrance.

OR...

B. create a new earth ground near the sub-panel. Apparently this is less costly than option A.

Forgive me for maybe asking perhaps an obvious question but I'm not an electrician. I don't have knowledge of NEC.

My first question: it appears to me that without bonding the proposed option "B: new earth ground" to the existing earth ground we can have serious safety implications regarding ground loops since the two different earth grounds see a different impedence (resistance).

My second question: Can the building structure since it is bonded to the earth ground be used as the bonding conductor between the sub panel and the existing earth ground?

Any advice is greatly appreciated.

thx,

Markus
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
You cannot use earth ground nor the building steel to ground the sub panel. If there is metallic conduit then the conduit may be used as a ground. If it is an existing install then no equipment ground is necessary if there are no other metallic pathways back to the building.

Can he install another wire in the conduit?
 
augie47

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
250.2 and 250.136(A) and 250.134 would be some of your '08 Code references.
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
It seems to me there is no need to ground the subpanel at all, if by ground you mean connect it to earth. The power system is either connected to earth at the service point, or is never connected to earth at all.

It does need to have an EGC run to it from the source. You can't just run another wire to replace an EGC that is missing.
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
markusius said:
The tenant is using welding equipment and the electrician said the subpanel was insufficiently grounded.
Click to expand...

Just what did the electrician mean by Insufficiently grounded"?

Does it not meet code, or is there some problem they are having the electrician believes would be alleviated by "proper grounding"?

I might buy into the former, but the later is highly unlikely.
 
M

markusius

Member
Dennis Alwon said:
You cannot use earth ground nor the building steel to ground the sub panel. If there is metallic conduit then the conduit may be used as a ground. If it is an existing install then no equipment ground is necessary if there are no other metallic pathways back to the building.

Can he install another wire in the conduit?
Click to expand...

I'm not sure what the clearance is in there but couldn't the conduit itself be used as the ground. BTW...the building was built in the 70'- early 80's and it used to house a stone saw mill operation before I leased the space.

Markus
 
M

markusius

Member
petersonra said:
It seems to me there is no need to ground the subpanel at all, if by ground you mean connect it to earth. The power system is either connected to earth at the service point, or is never connected to earth at all.

It does need to have an EGC run to it from the source. You can't just run another wire to replace an EGC that is missing.
Click to expand...

Maybe a dumb question but if there is running 208V 3-phase to the sub-panel then the EGC is bonded to the neutral back at the service entrance? If so can it bonded again at the sub-panel?

thx

Markus
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
markusius said:
I'm not sure what the clearance is in there but couldn't the conduit itself be used as the ground. BTW...the building was built in the 70'- early 80's and it used to house a stone saw mill operation before I leased the space.

Markus
Click to expand...
As I said earlier, if it is metallic conduit then you can use that as the equipment ground.
 
M

markusius

Member
petersonra said:
Just what did the electrician mean by Insufficiently grounded"?

Does it not meet code, or is there some problem they are having the electrician believes would be alleviated by "proper grounding"?

I might buy into the former, but the later is highly unlikely.
Click to expand...

I looked back in my email correspondence and the electrician states that the panel is NOT grounded...yikees!
 
M

markusius

Member
Ok I did check the sub-panel and it has a conduit made of galvanized steel (checked with a magnet) that runs all the way back to the main panel. The conduit appears to be about 3 inches in diameter.

Not a comment about NEC however I went ahead to check the ground path to make sure there was a safety ground in place using an ohm meter. I checked the ground supplied from said panel and the 110V wall outlet and it is indeed grounded to the sub-panel case. I checked this ground to the building structure as well as all other grounds in wall outlets. They are all grounded together with less than 0.2 ohms resistance. I did not check if the building metal structure was grounded to the EGC. I do have a picture of the nearly 1 inch thick copper ground diving into the parking lot outside the building to what I presume is the EGC.

sub-panel in question (the conduits are all metal but had been painted white by the painters):

98175abd.jpg



129bdcd8.jpg


6828ea02.jpg


6828ea02.jpg


6b3fb126.jpg
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Okay I think it is time to ask why you are asking all these questions. Is the electrician not capable of doing this job? If there is a problem with the grounding then find another electrician who knows what to do. For us to try and tell you if the electrician is right or wrong would be unfair.

BTW, that is probably not an EGC in the picture but rather an GEC.
 
M

markusius

Member
markusius said:
Hi everyone,

I hope I'm not thread jacking but this seemed the closest thread to my question and I do not see any activity since February. Pardon me if I should have started a new topic.

Here is the scenario: I have an all metal warehouse with large 14'inch I-beams for structural support all connected together via metal supports along the roof line. The building earth ground connects this structure to the grounding grid which is buried under the parking lot. I'm assuming that it is sufficient however if need be I can do the IEEE Fall of potential method to evaluate it further. It has come to my attention that a sub-panel is not grounded.

My question is if the NEC allows for grounding the sub-panel to the structural steel it is mounted on via a sufficiently current capable conductor.

I have a quote from a licensed electrician for two scenarios:

A. install a correctly sized ground conductor from the sub-panel to the earth ground located by the service entrance.

OR...

B. create a new earth ground near the sub-panel. Apparently this is less costly than option A.

Forgive me for maybe asking perhaps an obvious question but I'm not an electrician. I don't have knowledge of NEC.

My first question: it appears to me that without bonding the proposed option "B: new earth ground" to the existing earth ground we can have serious safety implications regarding ground loops since the two different earth grounds see a different impedence (resistance).

My second question: Can the building structure since it is bonded to the earth ground be used as the bonding conductor between the sub panel and the existing earth ground?

Any advice is greatly appreciated.

thx,

Markus
Click to expand...

I'm asking these questions because I'm trying to help my tenant out.

My background is in electrical imaging of the subsurface. It is very similar to an MRI that a medical doctor uses to look inside of you before operating. I'm a Geophysical engineer (geophysicist) by training and at my day job (www.agiusa.com) I'm part of a team that designs equipment and 3D software that is used in imaging the subsurface of the ground using electrical fields. Yes we inject DC directly into the ground. In some cases our clients use up to 15kW energy or more (3000V/5A) to create the electric fields. This same equipment (in a much less powerful 50 watt model) is also used for the ASTM G57 and the IEEE Fall of Potential Method to check grounding grids. I'm familiar with electricity in a sense of the physics and designing electronics but not so much in the sense of commercial/residential electrical work in regards to the NEC. That's why I'm turning to the experts on this forum so I can learn more.

In my spare time I have a classic car restoration shop (www.WolfenTECH.com). Its nothing fancy. I lease a warehouse and I sublease a separate part of my warehouse to a group of fire fighters whom manufacture heavy duty doors used by fire departments to practice breaking into industrial doors. The welders who work for the firefighters brought in their electrician friend to have some lights put in. When he was there he told the welders that they didn't have a safety ground installed and then wrote up an estimate for two scenarios to correct the problem. Needless to say, the welders were very concerned that their lives are in danger.

In their lease they have to pay for any electrical work. I want to save them money.

Here is what the welders electrician friend said on their estimate:

"
We hereby submit the estimate to install the grounding for the warehouse as followed:

1. Install a #6 ground from the main meter loop. $2,154.00

2. Install a #6 ground from a ground plate. $1,940.00

This price includes:

Regular time
EMT pipe
THHN wire

This does not include:

Permit
Taxes
"

Looking at the estimate again I get the feeling that the two options are not options at all but two separate line items of the same job. This is the reason why I was very interested because I deal with ground loops all day long and that is where I saw a problem. It makes sense now on a second glance.

Ok that was the long version. I want to thank everyone on this forum for helping me understand more.
 
acrwc10

acrwc10

Master Code Professional
Location
CA
Occupation
Building inspector
markusius said:
Hi everyone,

I hope I'm not thread jacking but this seemed the closest thread to my question and I do not see any activity since February. Pardon me if I should have started a new topic.

Here is the scenario: I have an all metal warehouse with large 14'inch I-beams for structural support all connected together via metal supports along the roof line. The building earth ground connects this structure to the grounding grid which is buried under the parking lot. I'm assuming that it is sufficient however if need be I can do the IEEE Fall of potential method to evaluate it further. It has come to my attention that a sub-panel is not grounded.

My question is if the NEC allows for grounding the sub-panel to the structural steel it is mounted on via a sufficiently current capable conductor.

I have a quote from a licensed electrician for two scenarios:

A. install a correctly sized ground conductor from the sub-panel to the earth ground located by the service entrance.

OR...

B. create a new earth ground near the sub-panel. Apparently this is less costly than option A.

Forgive me for maybe asking perhaps an obvious question but I'm not an electrician. I don't have knowledge of NEC.

My first question: it appears to me that without bonding the proposed option "B: new earth ground" to the existing earth ground we can have serious safety implications regarding ground loops since the two different earth grounds see a different impedence (resistance).

My second question: Can the building structure since it is bonded to the earth ground be used as the bonding conductor between the sub panel and the existing earth ground?

Any advice is greatly appreciated.

thx,

Markus
Click to expand...

Just what does, "design electrical resistivity equipment" mean?
 
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