Grounding Electrode System - Sanity Check

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gh0st

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Hello. I'm hoping to have an informed discussion regarding designs I am evaluating for code compliance and VE opportunities. I consistently see specifications for a 4/0 AWG GEC to all grounding electrodes and other, what I would consider 'overkill', items. One engineer's stance is that "current will travel along the path of least resistance" and that it provides a 'better system.' :cautious:

Could I please get some eyes on this and confirm my understanding of it all? Thanks in advance for reviewing this wall of text.

#1 - The GES is not intended to clear faults. That is the role of the EGC and bonding conductors. Maximum GEC sizes are based on the physical limitations with 4 AWG (UFER) or 6 AWG (Rods) simply not being able to carry more current than the GE's capable of.

#2 - 4/0 AWG GEC is oversized. Period, always. I've seen it argued that 4/0 would be installed for longevity... I still don't buy it.

#3 - Connections to ground rods need not be larger than 6 AWG, provided that it doesn't extend to any other GEs.

#3A - In fact, we can call it done with just (2) 3/4" x 10' rods spaced 6' apart connected via 6 AWG. This is the most cost effective method.

#4 - There is no added benefit to installing a UFER in addition to any other grounding electrode. Connection is only required if a UFER exists and never needs to be larger than 4 AWG, provided that it doesn't extend to any other GEs.

#5 - A ground ring need not be smaller than 2 AWG and there need only be one connection back to the GES (MBGB). Connecting to 'every other' column offers no added benefit, excluding the situation where it would simply be easier for bonding of various utilities/ services entering at various locations in a larger building.

#6 - Rods are not required at light poles. The EGC is all that's necessary and it does not help in the event of lightning strike.

#7 - Grounding of concrete equipment pads are not code required, assuming all equipment is fed with EGC.

#8 - Triads aren't mentioned in the code nor does it provide any additional benefits to the GES, with the exception if maybe one of the points included an inspection well for ease of maintenance and testing.

#9 - Its fair to assume that modern water pipes will not be metallic and would not be considered a GE? Bonding to interior water piping would still be required. A sprinkler system is not likely to be energized.

#10 - The 'common GEC' is used between the main building ground bar and all other ground bars (ex. riser to connect mbgb to ground bars in sub-electrical rooms in multi-story facility) shall not be smaller than 3/0 AWG. If structural steel is available the riser is not required.

#11 - For static electricity, "the minimum size of the bonding and grounding wire is dictated by mechanical strength not by it current-carrying capacity....A resistance of 1 megaohm or less is generally considered adequate." In short, a 12 AWG bonding conductor to the GES is sufficient for electrostatic discharge, with use of grounding personnel straps.
 
Be sure to consider separately conductors and electrodes used only for NEC GES from those used by the lightning protection system (if any).
Size and short direct path requirements are very different.
 
I would say that for the most part you are more or less correct.

#4. However as far as Ufers go, you would not create one. It's either there as part of the foundation or it isn't. If it's there you're required to use it as part of the grounding electrode system. If it's not there you probably can't create it anyway since it's part of the foundation system and you're not a foundation contractor.

#3a. If I can recall correctly, ground rods are not required to be 10-ft long, just 8-ft in the Earth.

#1. I agree that the grounding electrode system is not there to clear faults. It just does not clear faults and it can't if it's installed properly. On the other hand I'm also not sure what the thinking is with the maximum required size of connections to concrete encased electrodes and ground rods is. It's not like the lightning which people think is going to flow through The conductor to the grounding electrode will be impeded either by the concrete and case electrode or the ground rod more than through a water pipe in the ground. Someone just decided this is the way it was going to be and I doubt there is any technical justification for it.
 
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petersonra said:
I would say that for the most part you are more or less correct.

#4. However as far as Ufers go, you would not create one. It's either there as part of the foundation or it isn't. If it's there you're required to use it as part of the grounding electrode system. If it's not there you probably can't create it anyway since it's part of the foundation system and you're not a foundation contractor.

#3a. If I can recall correctly, ground rods are not required to be 10-ft long, just 8-ft in the Earth.

#1. I agree that the grounding electrode system is not there to clear faults. It just does not clear faults and it can't if it's installed properly. On the other hand I'm also not sure what the thinking is with the maximum required size of connections to concrete encased electrodes and ground rods is. It's not like the lightning which people think is going to flow through The conductor to the grounding electrode will be impeded either by the concrete and case electrode or the ground rod more than through a water pipe in the ground. Someone just decided this is the way it was going to be and I doubt there is any technical justification for it.
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Bob, appreciate the evaluation.

#1 - Agree I would have liked to understand the origin of these maximum sizes. But from a VE standpoint, there are real savings between a 6 AWG and 4/0 AWG, especially if used for a ground ring for a large facility.

#3a - Looked back at this in the NEC. Per 250.52(A)(5)... "Rod and pipe electrodes shall not be less than 2.44 m (8 ft) in length and...Rod-type grounding electrodes of stainless steel and copper or zinc coated steel shall be at least 15.87 mm (5⁄8 in.) in diameter." 3/4"D is for pipe or conduit.

#4 - That's a great point and exactly right. I forgot that the electrode needed to be located in the concrete foundation or footing. Interestingly, in the comments under this section there is mention of footings being disjointed from the rest of the building. With that in mind, why couldn't 20' of 4 AWG in 2" concrete simply be buried and connected to the GES? Perhaps it is because the concrete is continuous with the building structure?
 
GoldDigger said:
Be sure to consider separately conductors and electrodes used only for NEC GES from those used by the lightning protection system (if any).
Size and short direct path requirements are very different.
Click to expand...

Understood. I've had this question regarding the ground rod at light poles come up a few times in the past year.
Never had the pleasure of designing a complete lightning protection system. In my consulting days, the company had a canned diagram illustrating intent. Would actually be nice to have that for review.
 
IE2EE said:
With that in mind, why couldn't 20' of 4 AWG in 2" concrete simply be buried and connected to the GES? Perhaps it is because the concrete is continuous with the building structure?
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It would probably make a great grounding electrode. It's just not something that whoever decided what grounding electrodes are legal decided to put in the code.

As for ground rings there is no code requirement to ever have a ground ring. It is a design choice.
 
I'd say the OP has it right but only from the specific perspective of the function of the GES with respect to faults in <600V building wiring.

The requirements for lighting protection, static dissipation, equipotential bonding, mechanical longevity, etc all need to be considered.

My bet is that if the reality of these installations were explored, that we'd find a bunch of things that are simply done because 'that's how it's always been'.

If you are running a bonding jumper buried in soil, you might need to use something larger than the electrical minimum in order to get the desired service life once things like corrosion are considered. But perhaps a solid tinned #2 would be even better than a stranded 4/0 if this is the concern.

Jon
 
winnie said:
I'd say the OP has it right but only from the specific perspective of the function of the GES with respect to faults in <600V building wiring.

The requirements for lighting protection, static dissipation, equipotential bonding, mechanical longevity, etc all need to be considered.

My bet is that if the reality of these installations were explored, that we'd find a bunch of things that are simply done because 'that's how it's always been'.

If you are running a bonding jumper buried in soil, you might need to use something larger than the electrical minimum in order to get the desired service life once things like corrosion are considered. But perhaps a solid tinned #2 would be even better than a stranded 4/0 if this is the concern.

Jon
Click to expand...
I should have clarified that I generally review <600V commercial designs, not including lightning protection systems.
You mentioned mechanical longevity...how would you deviate from Code in regards to the GES?
IMO the Code's bonding requirements should address concerns with equipotential bonding.
The Code refers to NFPA 77, per informational note, for Static Electricity. After thumbing through that, the recommended practice is to use a 1/0 'bus' connected to the GES via "1/4" bronze (bare) flexible ground cable". Guessing something similar to https://www.ganpatiengineering.com/braided-flexible-round-copper-wire.html
 
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