Understanding 250.166(c)

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Hi all,

This is my first posting to this forum, so please be gentle.

I'm trying to reconcile two different interpretations of 250.166(c) and am hoping someone here can weigh in. Specifically, I am trying to understand what is meant by : "that portion of the grounding electrode conductor that is the sole connection to the grounding electrode shall not be required to be larger..." ?

I have gnereally read that to mean that a GEC running to a ground rod doesn't have to be larger than 6 AWG in any case (I assume that is because the contact resistance with the ground is naturally going to limit the fault current to a level where 6 is ample in size). But what is meant by 'the sole' connection in that section? If there is more than one wire landing on ground rod (i.e. the DC/AC bonding jumper) does it no longer apply?

For example, in a grid tied system if we're driving a new, dedicated DC Grounding Electrode according to 690.47(C) (1) and then bonding it to the existing AC electrode as required, can we still size the GEC according to 250.166(c)? Or is the GEC now no longer the 'sole connection' to the electrode and thus we need to size according to 250.166(A) or (B)?

That seems to be the interpretation in Mike Holts' book where his note says that (C) and (d) only apply in stand alone DC systems. But other interpretations (including, I think, John Wiles) differ and say basically that as long as your just connecting to ground rods (not building steel etc) the sizing of the GEC can be done according to 250.166(C) (and then the DC/AC bonding jumper gets sized at the larger of the AC or DC GEC).

Anyone have an opinion about that?

Thanks
~Fortunat
 

Smart $

Esteemed Member
Location
Ohio
Sole connection means that section of GEC connects only one electrode to the GES. There cannot be a bonding jumper to another electrode. You can have more than one GEC connected to the single electrode and each GEC can be the sole connection to the electrode, IMO.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Sole connection means that section of GEC connects only one electrode to the GES. There cannot be a bonding jumper to another electrode. You can have more than one GEC connected to the single electrode and each GEC can be the sole connection to the electrode, IMO.

So let's say I have a GEC going to a ground rod, and then a bonding jumper to a second rod. The bonding jumper is not required to be bigger than 6awg, but the first GEC (that connects both rods) may be required to be bigger?

I have never been able to make sense of this either.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Think of a weak link in the chain analogy. If your service size requires say a #2 grounding electrode conductor in general.
The exception allows you to install a #6 to the ground rod. For routing you decide to connect your metallic water pipe to the ground rod (another electrode) rather than back to your service. This becomes a violation as the #6 is a reduction in size for the water pipe electrode conductor.
 

kwired

Electron manager
Location
NE Nebraska
So let's say I have a GEC going to a ground rod, and then a bonding jumper to a second rod. The bonding jumper is not required to be bigger than 6awg, but the first GEC (that connects both rods) may be required to be bigger?

I have never been able to make sense of this either.

I question that also. 250.53(A)(2) requires a supplemental electrode for rod, pipe and plate type electrodes, with exception being if the first electrode is 25 ohms or less.

So if in Augies example, say we need a 2AWG GEC, and the only available electrodes is ground rod(s), and we use 2 rods to satisify 250.53(A)(2), are we supposed to run 2AWG to the first rod if we install a bonding jumper from first rod to second rod? I think it is pretty clear if we install separate conductor to each rod they both can be 6 AWG.
 

Smart $

Esteemed Member
Location
Ohio
So let's say I have a GEC going to a ground rod, and then a bonding jumper to a second rod. The bonding jumper is not required to be bigger than 6awg, but the first GEC (that connects both rods) may be required to be bigger?

I have never been able to make sense of this either.
I question that also. 250.53(A)(2) requires a supplemental electrode for rod, pipe and plate type electrodes, with exception being if the first electrode is 25 ohms or less.

So if in Augies example, say we need a 2AWG GEC, and the only available electrodes is ground rod(s), and we use 2 rods to satisify 250.53(A)(2), are we supposed to run 2AWG to the first rod if we install a bonding jumper from first rod to second rod? I think it is pretty clear if we install separate conductor to each rod they both can be 6 AWG.
That is my interpretation. GEC connecting one rod can be the #6. If a second rod is connected via bonding jumper to the first, the GEC to the first is not a sole connection to a single electrode.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
It has been allowable here to use the #6 to the 1st ground rod and to the supplemental rod if needed. We look at it as one electrode...perhaps incorrectly.
 

wirebender

Senior Member
Think of a weak link in the chain analogy. If your service size requires say a #2 grounding electrode conductor in general.
The exception allows you to install a #6 to the ground rod. For routing you decide to connect your metallic water pipe to the ground rod (another electrode) rather than back to your service. This becomes a violation as the #6 is a reduction in size for the water pipe electrode conductor.

This is how I have always thought of it.
 
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