NEC 250.53 (B) requires a minimum of 6-ft between electrodes

[Cjmccarthy]

I am working with a contractor that is using a 20 foot ground rods, when they install the disconnect, they bond it to one ground rod 20 foot deep and then bond it to the "Ground Array" about 6 feet away. The contractor referenced NEC 250.53 (B) that requires a minimum of 6 feet separation. I told them that that is "Minimum" and each ground rod has its own zone of influence. The optimal spacing between rods should be twice the length of the ground rod. When the zones overlap, the net resistance of each rod increase, thus making the ground system less effective. I have found that when searching "Ground Rod" installation, but the Contractor keeps coming back to the NEC 250.53 (B). does NEC reference the zone of influence?

 

[infinity]

The minimum distance apart is 6' whether that's the best performance distance or not. Rod length and zones are not part of the distance requirement. Without looking at the book I don't believe that the 6' rule applies to different types of electrodes.

 

[roger]

I am working with a contractor that is using a 20 foot ground rods, when they install the disconnect, they bond it to one ground rod 20 foot deep and then bond it to the "Ground Array" about 6 feet away. The contractor referenced NEC 250.53 (B) that requires a minimum of 6 feet separation. I told them that that is "Minimum" and each ground rod has its own zone of influence. The optimal spacing between rods should be twice the length of the ground rod. When the zones overlap, the net resistance of each rod increase, thus making the ground system less effective. I have found that when searching "Ground Rod" installation, but the Contractor keeps coming back to the NEC 250.53 (B). does NEC reference the zone of influence?

I agree with you.

 

[Hv&Lv]

I am working with a contractor that is using a 20 foot ground rods, when they install the disconnect, they bond it to one ground rod 20 foot deep and then bond it to the "Ground Array" about 6 feet away. The contractor referenced NEC 250.53 (B) that requires a minimum of 6 feet separation. I told them that that is "Minimum" and each ground rod has its own zone of influence. The optimal spacing between rods should be twice the length of the ground rod. When the zones overlap, the net resistance of each rod increase, thus making the ground system less effective. I have found that when searching "Ground Rod" installation, but the Contractor keeps coming back to the NEC 250.53 (B). does NEC reference the zone of influence?

 

[Cjmccarthy]

I have produced multiple documents on this subject, they keep coming back with it is an NEC requirement, even though I tell them NEC requirement is the "Minimum" distance. I guess you can't fix stupid, not even with duct tape.

 

[jaggedben]

If there is really a reason you need to ensure more effective grounding than the NEC requires then it would need to be a job spec in the contract. Why are they using 20ft rods? That is not code required. What is the 'ground array'?

.... When the zones overlap, the net resistance of each rod increase, thus making the ground system less effective. ...

Nitpicking your language... The closer they are the more it's like you just have one instead of two. But two rods, collectively, will always have less resistance and thus be more effective than one. So it's more of a lost opportunity than a detriment. If the second rod is too close it can be fixed by driving a third at the right distance.

 

[CoolWill]

It's all voodoo anyway. 6 feet, 25 ohms, completely pulled out of thin air by someone long dead. A bolt of lightning doesn't care if the rod is 20 feet long or 20 centimeters. That #6 connected to it will be vaporized anyway.

 

[Cjmccarthy]

If there is really a reason you need to ensure more effective grounding than the NEC requires then it would need to be a job spec in the contract. Why are they using 20ft rods? That is not code required. What is the 'ground array'?



Nitpicking your language... The closer they are the more it's like you just have one instead of two. But two rods, collectively, will always have less resistance and thus be more effective than one. So it's more of a lost opportunity than a detriment. If the second rod is too close it can be fixed by driving a third at the right distance.

It is an FDOT spec. FDOT requires 5 ohm's at every device installed, if they get the 5 ohm's or less with one rod, that is all they need to install. The FDOT "array" is below.

 

[CoolWill]

If it's a job spec, then what can he say? Tell him to do it the way the spec says or hit the bread line.

 

[Cjmccarthy]

It's all voodoo anyway. 6 feet, 25 ohms, completely pulled out of thin air by someone long dead. A bolt of lightning doesn't care if the rod is 20 feet long or 20 centimeters. That #6 connected to it will be vaporized anyway.

That is why I hate when the contractor bonds all the equipment to the "array", but it is a requirement. When it gets hit by lightning, it goes everywhere and takes out our SPD's and sometimes even gets pass the SPD's and takes out our equipment

 

[jaggedben]

That is why I hate when the contractor bonds all the equipment to the "array", but it is a requirement. When it gets hit by lightning, it goes everywhere and takes out our SPD's and sometimes even gets pass the SPD's and takes out our equipment

Did a lightning protection professional design the grounding array spec? I'm no lightning expert but it seems like perhaps the array is either 'voodoo' like coolwill says, or else the spec is incomplete in not specifying exactly where and how contractors are allowed (and prohibited) to bond anything to it. My understanding is there should only be one grounding electrode conductor to the array and everything else should be bonded to it above ground, but I'd consult the LPS professional.

On the other hand, SPDs giving themselves up to protect equipment is something they are supposed to do, so maybe that's just par for the course in Florida and/or you need higher rated SPDs.

In any case, contractors should understand that grounding designed for lightning protection has nothing to do with the NEC and they can't fall back on NEC requirements when installing anything having to do with it. It doesn't surprise me there are contractors who don't get this, but they should.

 

[Cjmccarthy]

Did a lightning protection professional design the grounding array spec? I'm no lightning expert but it seems like perhaps the array is either 'voodoo' like coolwill says, or else the spec is incomplete in not specifying exactly where and how contractors are allowed (and prohibited) to bond anything to it. My understanding is there should only be one grounding electrode conductor to the array and everything else should be bonded to it above ground, but I'd consult the LPS professional.

On the other hand, SPDs giving themselves up to protect equipment is something they are supposed to do, so maybe that's just par for the course in Florida and/or you need higher rated SPDs.

In any case, contractors should understand that grounding designed for lightning protection has nothing to do with the NEC and they can't fall back on NEC requirements when installing anything having to do with it. It doesn't surprise me there are contractors who don't get this, but they should.

That is a good point, I will investigate. Thank you.

 

[Fred B]

For the Grounding Electrode System, Nothing says you can't be farther apart on the electrodes just not any closer than 6 ft. 250.53(B), (Contractors reference). Your reference to 20ft of Rod has me puzzled as code only calls for min length of 8ft for rod electrodes, the only reference to a 20ft requirement is related to the Concrete encased electrode 250.52(A)(3) or ring electrode 250.52(A)(4).

Now if the system you are speaking of is related to lightning protection (primary code consideration is found in NFPA 780), the code term is Strike Termination Devices, that has nothing to do with the Grounding Electrode System and cannot be used in lieu of the required Grounding Electrodes (250.60). But it is permitted to be bonded into the Grounding Electrode Systems, and (250.106) would indicate that it is required.

2023 NEC 250.60 Use of Strike Termination Devices.
Conductors and driven pipes, rods, or plate electrodes used for grounding strike termination devices shall not be used in lieu of the grounding electrodes required by 250.50 for grounding wiring systems and equipment. This provision shall not prohibit the required bonding together of grounding electrodes of different systems.

2023 NEC 250.106 Lightning Protection Systems.
The lightning protection system ground terminals shall be bonded to the building or structure grounding electrode system.

Could you and the contractor be comparing Apples and Oranges? If you are designing the lightning protection system refer the contractor to NFPA 780.

 

[Cjmccarthy]

For the Grounding Electrode System, Nothing says you can't be farther apart on the electrodes just not any closer than 6 ft. 250.53(B), (Contractors reference). Your reference to 20ft of Rod has me puzzled as code only calls for min length of 8ft for rod electrodes, the only reference to a 20ft requirement is related to the Concrete encased electrode 250.52(A)(3) or ring electrode 250.52(A)(4).

Now if the system you are speaking of is related to lightning protection (primary code consideration is found in NFPA 780), the code term is Strike Termination Devices, that has nothing to do with the Grounding Electrode System and cannot be used in lieu of the required Grounding Electrodes (250.60). But it is permitted to be bonded into the Grounding Electrode Systems, and (250.106) would indicate that it is required.

2023 NEC 250.60 Use of Strike Termination Devices.
Conductors and driven pipes, rods, or plate electrodes used for grounding strike termination devices shall not be used in lieu of the grounding electrodes required by 250.50 for grounding wiring systems and equipment. This provision shall not prohibit the required bonding together of grounding electrodes of different systems.

2023 NEC 250.106 Lightning Protection Systems.
The lightning protection system ground terminals shall be bonded to the building or structure grounding electrode system.

Could you and the contractor be comparing Apples and Oranges? If you are designing the lightning protection system refer the contractor to NFPA 780.

just for clarity, we have multiple devices along the highway, each device has "lightning protection" to a ground array. FDOT standards has the ground array at 5ohms or less. Each location has 600v coming into a disconnect, then to a stepdown transformer that steps it down to 120/240v or 24o/480v depending on the device being powered. The transformer at each site is bonded to the ground rod, ground rod "A". So if I am understanding you correctly, the ground rod for the transformer "can", "should", or "must" be a separate ground rod, and then it can be bonded to the array? I guess my question is, can the transformer be directly bonded to the ground rod A in the array? Below is a typical site ground array setup for each site. The picture is of a site with the red circle is the location of ground rod A. The transformer and disconnect are mounted on the pole, and are bonded to ground rod A of the lightning protection with an aerial on top of the pole.

 

[Fred B]

Sounds as if you will have 2 separate systems, a GES (Grounding Electrode System) and a lightning protection system Grounding Strike Terminations (GST). Each must be considered separately, and meet their own system's criteria. Also remember your GES and your GST serve two very different purposes.

Your reference to a primary ground rod point A if you intend this as the GES, Your GES must meet criteria of article 250 with verified 25ohm or less or 2 rods at least 6ft apart (among other requirements).

Then your Grounding Strike Termination GST (lightning protection) as shown as Rods B,C,D connected per requirement of NFPA 780, and a single point of bonding to the GES.

As mentioned in both the NEC and 780 They are interconnected or bonded to each other, but neither will serve the minimum requirement of either separate system. So the GST being connected via Rod A (per your diagram) would not be compliant as Each system must be treated separately. Your GST system Rods B,C,D, should be interconnected separate of the GES.

Additionally looking simply at the difference between the 2 systems as related to the GEC (Grounding Electrode Conductor) sizing would show the separation of the 2 systems. Article 250 of the NEC will dictate the GEC size based on the amperages, voltages, and GE type used (250.66) for the GES. Whereas article 4.1.1 of NFPA 780 will dictate the wire size and type for the conductors of the GST system, and they are not even the same type of conductors between the two systems. GST is protecting for very high voltage and very high frequency compared to the GES thus the mechanism for dispersion of the current related to each is very different from each other.

 

[Cjmccarthy]

Sounds as if you will have 2 separate systems, a GES (Grounding Electrode System) and a lightning protection system Grounding Strike Terminations (GST). Each must be considered separately, and meet their own system's criteria. Also remember your GES and your GST serve two very different purposes.

Your reference to a primary ground rod point A if you intend this as the GES, Your GES must meet criteria of article 250 with verified 25ohm or less or 2 rods at least 6ft apart (among other requirements).

Then your Grounding Strike Termination GST (lightning protection) as shown as Rods B,C,D connected per requirement of NFPA 780, and a single point of bonding to the GES.

As mentioned in both the NEC and 780 They are interconnected or bonded to each other, but neither will serve the minimum requirement of either separate system. So the GST being connected via Rod A (per your diagram) would not be compliant as Each system must be treated separately. Your GST system Rods B,C,D, should be interconnected separate of the GES.

Additionally looking simply at the difference between the 2 systems as related to the GEC (Grounding Electrode Conductor) sizing would show the separation of the 2 systems. Article 250 of the NEC will dictate the GEC size based on the amperages, voltages, and GE type used (250.66) for the GES. Whereas article 4.1.1 of NFPA 780 will dictate the wire size and type for the conductors of the GST system, and they are not even the same type of conductors between the two systems. GST is protecting for very high voltage and very high frequency compared to the GES thus the mechanism for dispersion of the current related to each is very different from each other.

Thank you for that information. I will pass that along to my engineer.

 

[Cjmccarthy]

Sounds as if you will have 2 separate systems, a GES (Grounding Electrode System) and a lightning protection system Grounding Strike Terminations (GST). Each must be considered separately, and meet their own system's criteria. Also remember your GES and your GST serve two very different purposes.

Your reference to a primary ground rod point A if you intend this as the GES, Your GES must meet criteria of article 250 with verified 25ohm or less or 2 rods at least 6ft apart (among other requirements).

Then your Grounding Strike Termination GST (lightning protection) as shown as Rods B,C,D connected per requirement of NFPA 780, and a single point of bonding to the GES.

As mentioned in both the NEC and 780 They are interconnected or bonded to each other, but neither will serve the minimum requirement of either separate system. So the GST being connected via Rod A (per your diagram) would not be compliant as Each system must be treated separately. Your GST system Rods B,C,D, should be interconnected separate of the GES.

Additionally looking simply at the difference between the 2 systems as related to the GEC (Grounding Electrode Conductor) sizing would show the separation of the 2 systems. Article 250 of the NEC will dictate the GEC size based on the amperages, voltages, and GE type used (250.66) for the GES. Whereas article 4.1.1 of NFPA 780 will dictate the wire size and type for the conductors of the GST system, and they are not even the same type of conductors between the two systems. GST is protecting for very high voltage and very high frequency compared to the GES thus the mechanism for dispersion of the current related to each is very different from each other.

I was also asked about the GES. The power feed comes into the disconnect (2 hots and a ground), then it goes to the transformer, we do it this way so we can service the transformer onsite. The question was, should the transformer be directly connected to the GES? The site in the picture has the wire to the GES from the transformer, but it also gets bonded at the disconnect, rigid conduits, and the cabinet.

 

[jaggedben]

See 250.60 and 250.106. The NEC wants you to treat the NEC GES and the lightning protection system as separate things, but bond them together.

 

[tom baker]

It's all voodoo anyway. 6 feet, 25 ohms, completely pulled out of thin air by someone long dead. A bolt of lightning doesn't care if the rod is 20 feet long or 20 centimeters. That #6 connected to it will be vaporized anyway.

Not vodoo at all. Testing was done on ground rods to detetmime why 2, why 6 ft apart…I have info from AEMC I can post in a few dsys