Size of Equip. Ground

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jamesoftn

Senior Member
Location
TN
Contractor is installing circuits in a 2in EMT conduit. For derating per 310.15 number of conductors at 40% = # 8 size of circuit conductors. The engineered drawings directed the EC to install 5, # 12 grounds for these circuits as the EGC. The drawings also directed them to install 5, #12 isolated EGC for these circuits. I believe that the EGC should be a # 8 to comply with 250.122 B. Does the code state a size required for the Isolated EGC or should it be sized the same as the EGC.
 
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roger

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Staff member
Location
Fl
Occupation
Electrician
The Isolated Grounds would have to comply with 250.122(B) as well.

Roger
 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I don't disagree with the answer as that is what the Code states. The last thing we need is another exception, but it would seem where the conductors are upsized due to derating, the equipment ground would not need be as it is seldom, if ever, a CCC. With upsizing for voltage drop, I can understand, but not for ambient. Oh well !
 

tryinghard

Senior Member
Location
California
...The engineered drawings directed the EC to install 5, # 12 grounds for these circuits as the EGC. The drawings also directed them to install 5, #12 isolated EGC for these circuits. I believe that the EGC should be a # 8 to comply with 250.122 B. Does the code state a size required for the Isolated EGC or should it be sized the same as the EGC.
Looks like 250.122(B) literally applies (EGC increase in size proportionally), but I'm with augie47 on this one.

Notice 250.118 "The equipment grounding conductor run with or enclosing the circuit conductors..." Will these "run with" the circuit conductors all the way or only part way? Why isolated grounds?

Think about this rhetorical question: how will fault current on the appliance plugging into an IGR route to its supply?
 
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jamesoftn

Senior Member
Location
TN
These (EGC) will run with the circuit conductors to the equipment installed. The isolated grounds are for computer equipment on theses circuits.
 

tryinghard

Senior Member
Location
California
If the equipment ground is isolated it most likely does not run with the phase conductors. The equipment ground only runs with the phase conductors if it terminates to the exact same equipment ground of the supply to the particular phase conductors, otherwise the EGC will not be the same length and may not operate the over-current device (it's real purpose).

The problem with isolated ground circuits is they are most often in violation. Engineers and electricians often do not know for sure if voltage drop will affect its function/purpose. If circuitry is installed correctly there is no need to "drain" stray current through power circuits for sensitive equipment. An auxiliary ground that bonds to the ground electrode system would be more effective.

In my opinion isolated ground circuits are not a remedy at all. Correct bonding is important as well as proper spacing from data/comm circuitry. Isolated grounds don't fix the problem they are an application that accepts bad wiring.
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I am curious as to why one would have to apply 250.122(B) to the EGC if the emt itself is a ground. I can see why you would need it for the isolated ground but not for the EGC.
 

tryinghard

Senior Member
Location
California
The EMT would need to be upsized to GRC [na] :cool:.

I agree with augie47, seems unnecessary to upsize for derating reasons. The conduit used as the equipment ground is just fine but an isolated ground circuit does not have the benefit of the conduits contribution to the equipment grounding of the item supplied.

With an isolated ground circuit I can visualize a short in the equipment, it will travel on this conductor all the way to the GEC?s termination then back upstream to its supply. Often the supply is a XFMR to an MDP/SWB to a panelboard then to the outlet. The circuits phase conductor is appropriately sized the entire route from supply to outlet but with IGC it?s often a #12 all the way.

I don?t see this as running with the phase conductors because it?s not sized accordingly. This is like ?A? phase routing one way around a building while ?B? comes a different way, and their not the same size while being the same circuit. :roll:

IGC?s are haywire!
 

Smart $

Esteemed Member
Location
Ohio
I am curious as to why one would have to apply 250.122(B) to the EGC if the emt itself is a ground. ...
You have to upsize the conduit wall, i.e. to IMC then RMC... IMC when upsizing one NEC-standard size, thereafter requires calculation :D








:grin: = Just kidding, of course!!!
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
So Gus what you are saying if one chooses to install an additional EGC in a recognized EGC per 250.118 then we can ignore 250,122(B)? Going back to OP that is.
The OP asked me that in a PM. :smile: I must msiread what I inteneded to write :grin: I don't think you can ignore 250.122(B) IF you pull a grounded conductor. My personal opinion, which is worthless, is that, when the condcutors are upsized for ambient only, it makes little sense that you have to increse the EGC. I undrstand for voltage drop. Obviously, the fact that it makes little sense to me, or that I have an opinion, changes nothing as far as the requirement.
 

dcspector

Senior Member
Location
Burke, Virginia
The OP asked me that in a PM. :smile: I must msiread what I inteneded to write :grin: I don't think you can ignore 250.122(B) IF you pull a grounded conductor. My personal opinion, which is worthless, is that, when the condcutors are upsized for ambient only, it makes little sense that you have to increse the EGC. I undrstand for voltage drop. Obviously, the fact that it makes little sense to me, or that I have an opinion, changes nothing as far as the requirement.
Thanks Gus at my age I can't figure anything out anymore.......:grin:
 

Smart $

Esteemed Member
Location
Ohio
The OP asked me that in a PM. :smile: I must msiread what I inteneded to write :grin: I don't think you can ignore 250.122(B) IF you pull a grounded conductor. My personal opinion, which is worthless, is that, when the condcutors are upsized for ambient only, it makes little sense that you have to increse the EGC. I undrstand for voltage drop. Obviously, the fact that it makes little sense to me, or that I have an opinion, changes nothing as far as the requirement.
Had you considered the reason for ambient and derating corrections also effects the grounding conductor. It is usually in very close proximity to the ones carrying the current and emitting heat as a byproduct... and as such, the grounding conductor's resistance is also affected.
 
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