feeder ground: conductor vs. conduit

Status
Not open for further replies.
apologies if this seems too elementary but I have not been able to find a definitive answer or code reference. If I am running a feeder to a subpanel in EMT, do I need to pull a grounding conductor or can the conduit serve as ground as it would for a branch circuit? I'm worried about ground impedance and OCD clearing in the event of a fault (not sure what the cross-section of EMT metal is or how it compares to the current-carrying conductors).

If a ground conductor is not required, is it bad practice to run one anyway? Can it be bare copper or is insulation required? If it is a supplemental ground (i.e. not required by code) is there minimum conductor size?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
250.118 is pretty much your answer. EMT is recognized as an equipment grounding conductor as is a properly sized conductor, insulated or bare; solid or stranded.
As to the advisability of having both, you will get varied opinions. Most feel both is "better", but obviously not required. An argument can be made that the conduit is the better of the two, but, of course that depends on the integrity of the system.
If you do install a conductor, it must be sized per 250.122.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
The EMT is a listed EGC so it may be used without a conductor type EGC.

The impedance of the EMT will be lower than that of the conductor type EGC required by 250.122.

If you install an EGC it must be sized according to 250.122 and keep in mind that 250.122(B) may apply.

There are some here who can't sleep at night if they run EMT without an EGC, I'm not one of them. :)
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
apologies if this seems too elementary but I have not been able to find a definitive answer or code reference. If I am running a feeder to a subpanel in EMT, do I need to pull a grounding conductor or can the conduit serve as ground as it would for a branch circuit? I'm worried about ground impedance and OCD clearing in the event of a fault (not sure what the cross-section of EMT metal is or how it compares to the current-carrying conductors).
By code, EMT can generally serve as the EGC. There are a few exceptions, but in general it is code "legal".

Usually, the impedance of the EMT is going to be lower than the copper EGC, so in that respect the EMT is a "better" EGC.

If a ground conductor is not required, is it bad practice to run one anyway?
It adds cost. That's bad. However, often the extra cost is not that much.

Can it be bare copper or is insulation required?
For an EGC insulation is not required. It can also be AL. Copper is not required.

If it is a supplemental ground (i.e. not required by code) is there minimum conductor size?
That is an interesting question. The code requires an EGC of the appropriate size. It does not prohibit additional EGCs from being run that are smaller, as far as I can recall.

My biggest issue with EMT is that I have seen a fair amount of it in field installations where the fittings have come apart. Usually this is the dimpled style, although I have seen a few set screw types that have come loose as well. If it bothers you, use the compression style fittings. They don't come apart as long as the nuts are tight.
 
Last edited:
Thanks for the replies! Interesting discussion.

A related question that comes to mind then is, if you are running a full-sized EGC, why would you use metal conduit, especially EMT, rather than PVC? I always considered that one of the design trade-offs: EMT costs more but saves the cost of the grounding conductor, but wasn't sure the same trade-off applied to feeders as branch circuits.

I can't think of cases where EMT would be preferable over RNC/PVC, but maybe there are some?
 

skeshesh

Senior Member
Location
Los Angeles, Ca
That is an interesting question. The code requires an EGC of the appropriate size. It does not prohibit additional EGCs from being run that are smaller, as far as I can recall.

My biggest issue with EMT is that I have seen a fair amount of it in field installations where the fittings have come apart. Usually this is the dimpled style, although I have seen a few set screw types that have come loose as well. If it bothers you, use the compression style fittings. They don't come apart as long as the nuts are tight.

Interesting question indeed. I would think it makes no sense to run a conductor as EGC and not size it to the minimum allowed. I don't know if running a seperate conductor as EGC is "additional" ground but rather "redundant". If the purpose is to make sure a path to ground exists should the EMT fitting integrity be compromised then it does not make sense to not size it properly. Is it not legal though? I'd say so but you may in fact be able to convicne an inspector otherwise.
 
would an inspector take a dim view of a "redundant" ground, especially if fully sized? Reason being, it is essential for a metal raceway to have a low-impedance to ground and able to carry fault current of conductors inside of it, to prevent the raceway from being at a potential above ground. A redundant ground wire is not truly redundant then--it is a secondary equipment ground but not for the conduit itself (unless you count a bare wire making some contact inside).

Seems like an additional ground wire may be a red flag that the installer has limited confidence in the ground integrity of the conduit installation.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Seems like an additional ground wire may be a red flag that the installer has limited confidence in the ground integrity of the conduit installation.

More likely that the installer was just following a spec. that an engineer wrote. I think there are enough installations with the additional ground wire that inspectors probably don't think twice about it.

Steve
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
If you choose to run a wire EGC it must meet the NEC requirments.

There is no such thing as a reduced size EGC.

I am not sure what you mean by this.

Suppose I had a metallic cabletray being used as the EGC for (3) 3/0 conductors.

Are you saying you could not also run a multi-conductor #14 TC that had an integral ground inside the same cabletray?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
would an inspector take a dim view of a "redundant" ground, especially if fully sized? Reason being, it is essential for a metal raceway to have a low-impedance to ground and able to carry fault current of conductors inside of it, to prevent the raceway from being at a potential above ground. A redundant ground wire is not truly redundant then--it is a secondary equipment ground but not for the conduit itself (unless you count a bare wire making some contact inside).

Seems like an additional ground wire may be a red flag that the installer has limited confidence in the ground integrity of the conduit installation.

I think the opposite would be true for most inspectors. Good grounding is considered important by EIs and your choice to include an additional grounding means only shows you share that belief.
As far as the conductor being smaller, the policy in this area is not to accept a conductor smaller than required by 250.122 even if it is redundant.
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
Technically, Article 250 only discusses “supplemental” and “auxiliary” grounding conductors in terms of grounding electrode conductors(GEC). See Art 250, PART III, starting in Section 250.50. Also technically there is no such thing as a “ground conductor” - period. There are various “grounded” and “grounding conductors.” This usually isn’t a problem in the vernacular, but it can be hairy when trying to read the Code and interpreting it properly.

SO – what we are basically discussing is equipment grounding conductors (EGC) as discussed in Art 250, PART VI, starting in Section 250.110. Section 250.118 discusses the various Types of acceptable EGCs. My opinion is they would be more properly called Equipment Bonding Conductors or Equipment Bonding Jumpers (on the Load Side of the Service), because it is effectively what they are, but years of tradition have prevented the clear and unambiguous use of any of the terms. [My very first Code Proposal was to rename Article 250 to “Bonding and Grounding.” At the time, it was known simply as “Grounding.” Since I got the order wrong, it was rejected; but some one got the order right the next cycle :D]

In any case, from an engineering standpoint, once you design the intentional “Effective Ground-Fault Current Path” of a circuit you can use any combination of Section 250.118 Types that are desirable, the only provision is that if one or more happens to be a wire, it has to meet 250.122. This is one reason I happen to like raceways and cable trays alone – even in Classified locations. CMP 3 and 6 have spent a good bit of text stepping on CMP 5’s toes. In some cases it is impossible to meet some of the literal text of Arts 300 and 310 with respect to EGC’s
 

iwire

Moderator
Staff member
Location
Massachusetts
In some cases it is impossible to meet some of the literal text of Arts 300 and 310 with respect to EGC?s

That was pretty much what I came up with while thinking about petersonra's question above.

It seems he has to run an EGC with his cable but it does not meet 250.122.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
That was pretty much what I came up with while thinking about petersonra's question above.

It seems he has to run an EGC with his cable but it does not meet 250.122.

If this was the case, how could you ever use TC in a cable tray unless whatever EGCs where in the TCs where all the same size?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...
My biggest issue with EMT is that I have seen a fair amount of it in field installations where the fittings have come apart. Usually this is the dimpled style, although I have seen a few set screw types that have come loose as well. If it bothers you, use the compression style fittings. They don't come apart as long as the nuts are tight.
I have seen more compression type EMT connectors or couplings that come apart than I have the set screw type. While both are listed as suitable for being part of the grounding path, it is my opinion, that the set screw type provides a superior grounding connection as compared to a compression type.
 

220/221

Senior Member
Location
AZ
Keep in mind that local codes vary and may require an EGC. We have been using EGC's since the 80's. I was surprised to learn (here, I think) that it wasn't an NEC requirement.

As far as using PVC VS EMT?


EMT looks better/straighter.
 
EGC is always required, that is not being debated.

What is debated is the practice of using a metallic raceway as EGC, vs. running a wire in the raceway.

It is also required to bond a metallic raceway to ground. Using the raceway as EGC serves both functions.

However, running a grounding wire through a metal raceway serves as EGC for the supplied panel, but does not bond the metal raceway to ground.

This is where the argument gets tricky and I wonder how inspectors might be suspect: If you say that your conduit is solid, conductive, and bonded to ground at every point, then it is very hard to justify an additional wire for EGC.
 
Status
Not open for further replies.
Top