Depth of Ground Ring

[Jin Kim]

NFPA70(2020) requires 30" whereas NFPA780(2020) states 18" minimum depth.
Which do you concur with in design?
Does a property not having a lightning protection system differs from the opposite?

 

[mayanees]

I think I'd have to go with NFPA 70 and specify 30". I suppose that if the lightning protection system was not used as the grounding electrode system, then you could get away with 18".

 

[paulengr]

NFPA70(2020) requires 30" whereas NFPA780(2020) states 18" minimum depth.
Which do you concur with in design?
Does a property not having a lightning protection system differs from the opposite?

You are mixing systems. NFPA 780 is for lightning protection and it’s a structural ground. If you read the rest of the Code it should also be separate from the power system. NEC is system grounding and part of the power system. It has only incidental lightning protection and primarily is intended to absorb indirect effects such as a lightning strike to the incoming service wiring.

In addition in most jurisdictions NFPA 70 is legally required. NFPA 780 is an engineering standard.

As far as the engineering of the system ground rings are a ridiculous waste of money. The only advantage is that it makes it easy for the dirt diggers and the steel monkeys to understand without screwing it up. The effectiveness of a system is a combination of factors but the deity is linear with conductance while the diameter produces diminishing returns. Multiple electrodes also produce diminishing returns. Effectively a ring is just a super shallow very large diameter ground rod. A single 10 foot deep ground rod will typically produce the same resistance as the hundreds of feet of buried wire used in most rings. The only purported (and questionable) value is that since it is usually a short distance from every building column the surge impedance of the jumper is low. But this is more than made up for by the higher impedance of the grounding electrode

So in short unless you have a ufer ground condition (bad soil) just sink two rods and call it a day. Tie your J bolts into the rebar. Ignore inspections that only look for ground straps outside the structure. A slab is much more ground rod than you can possibly ask for. Mines are required to measure every ground every year. I’ve done thousands of tests. You quickly get to know what good grounding is that way.

 

[Jin Kim]

You are mixing systems. NFPA 780 is for lightning protection and it’s a structural ground. If you read the rest of the Code it should also be separate from the power system. NEC is system grounding and part of the power system. It has only incidental lightning protection and primarily is intended to absorb indirect effects such as a lightning strike to the incoming service wiring.

In addition in most jurisdictions NFPA 70 is legally required. NFPA 780 is an engineering standard.

As far as the engineering of the system ground rings are a ridiculous waste of money. The only advantage is that it makes it easy for the dirt diggers and the steel monkeys to understand without screwing it up. The effectiveness of a system is a combination of factors but the deity is linear with conductance while the diameter produces diminishing returns. Multiple electrodes also produce diminishing returns. Effectively a ring is just a super shallow very large diameter ground rod. A single 10 foot deep ground rod will typically produce the same resistance as the hundreds of feet of buried wire used in most rings. The only purported (and questionable) value is that since it is usually a short distance from every building column the surge impedance of the jumper is low. But this is more than made up for by the higher impedance of the grounding electrode

So in short unless you have a ufer ground condition (bad soil) just sink two rods and call it a day. Tie your J bolts into the rebar. Ignore inspections that only look for ground straps outside the structure. A slab is much more ground rod than you can possibly ask for. Mines are required to measure every ground every year. I’ve done thousands of tests. You quickly get to know what good grounding is that way.

Thank you for the clarification.
It seems that both NFPA780 and 70 allow separately or bonded GES.

NFPA 780(2020)
4.13.1.1* Each down conductor shall terminate to one of the following:
(1) A grounding electrode dedicated to the lightning protection system
(2) A grounding electrode system of a building, structure, or facility that has multiple grounding electrodes bonded together with a ground ring electrode meeting the requirements of 4.13.4

NFPA 70(2020)
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.

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