Why Three Ground Rods?

[Revous]

This is just a general question, on some electrical drawings and specs I've notices that engineers tend to default to (3) ground rods for electrical services. But when I read the NEC at most are required to have (2) ground rods if you don't want to prove that (1) ground rod has a resistance less than 25 ohms. (I know that they are also supposed to another dedicated GEC system like the jumper at the water main or concrete-encased electrodes but I'm just focusing on the ground rod requiems for now.)

I know that lightning protection systems require the "claw" style ground rod configuration for their down conductors but I'm not sure why I see (3) ground rods on many engineer drawings for residential buildings and light-commercial spaces sometimes.

 

[LarryFine]

Welcome to the forum.

Why? Because they believe that, "If one is good and two are better, I'm gonna prescribe three."

 

[Revous]

Welcome to the forum.

Why? Because they believe that, "If one is good and two are better, I'm gonna prescribe three."

At my last job the PE basically said something similar when I brought it up to him. I told him it was silly for us to put it on our plans but it was "the way we have always done things".

Thanks for the response!

 

[ActionDave]

There is no practical reason. It's just a spec that gets written into most engineered plans. Engineers get fascinated with the Power of The Triangle, sometimes referred to as counterpoise.

Half of them know better but are afraid to break tradition so the boiler plate spec continues be passed down to each new generation of drawings.

 

[drcampbell]

Maybe it's because they know something unique about local soil conditions. Have you asked them?

The NEC is national in scope. Minimally complying with the NEC means doing the same thing in the dry, sandy, rocky soil of the desert southwest as you do in the damp, carbon-rich soil of the midwestern Corn Belt.

 

[Charged]

Fwiw the illustration in NEC 2020 250.106 shows three for lightning protection system before the bond to the GES. I know lightning protection is a different animal but thought it was interesting….

 

[Revous]

There is no practical reason. It's just a spec that gets written into most engineered plans. Engineers get fascinated with the Power of The Triangle, sometimes referred to as counterpoise.

Half of them know better but are afraid to break tradition so the boiler plate spec continues be passed down to each new generation of drawings.

Thanks for clearing that up!

Blessed be thy triangle and may your resistance always read less than 25 ohms.

 

[Revous]

Maybe it's because they know something unique about local soil conditions. Have you asked them?

The NEC is national in scope. Minimally complying with the NEC means doing the same thing in the dry, sandy, rocky soil of the desert southwest as you do in the damp, carbon-rich soil of the midwestern Corn Belt.

The project we did were mostly in the NJ and the soil conditions never came up in discussion. The PE I used to work for just always wanted (3) ground rods, when I pressed him on it he gave me a similar response to "We have always done it like this, all hail the triangle."

 

[Revous]

Fwiw the illustration in NEC 2020 250.106 shows three for lightning protection system before the bond to the GES. I know lightning protection is a different animal but thought it was interesting….

Yeah I know what you mean, the end of the down conductors seems to prescribe (3) ground rods but the NEC is pretty clear about having a minimum of 2

 

[Canton]

I remember getting jobs where they wanted a 500 kcmil GEC to all the electrodes and cadwelded. Don’t know about now, but there were no molds for a 500 to a ground rod back then.

 

[Charged]

I remember getting jobs where they wanted a 500 kcmil GEC to all the electrodes and cadwelded. Don’t know about now, but there were no molds for a 500 to a ground rod back then.

I admit I was guilty of doing similar things before I understood the difference between table 250.66 and 250.102 and applied the 12.5 percent thing when I shouldn’t. Also worked under someone that was a little over zealous with upsizing ground conductors and not using the NEC “minimum” which kind of messed me up for awhile.

 

[petersonra]

Maybe it's because they know something unique about local soil conditions. Have you asked them?

The NEC is national in scope. Minimally complying with the NEC means doing the same thing in the dry, sandy, rocky soil of the desert southwest as you do in the damp, carbon-rich soil of the midwestern Corn Belt.

what difference would the soil make? ground rods just don't serve much purpose at all.

I am of the opinion it is for the same reason that copper wire is speced over aluminum, or that a ground wire is required in RGS conduit.

 

[ActionDave]

And that is just the tip of the iceberg when it comes to wasteful specs that serve no purpose.
3/4" minimum conduit, 12AWG minimum conductor, dedicated neutrals for circuits, oversized feeder conductors, grossly oversized transformers, services, and generators .....
Ironically you find all this waste on projects that get a platinum LEED sticker on the design.

 

[infinity]

I agree more useless stuff that someone is paying for. Our last big project had miles of rebar in the footing that was use as a CEE and they still specified 3-10' X 3/4" rods with test wells every 100' in the basement. I wondered who is ever going to open the test well cover and actually do something.

 

[roger]

Remember, a tin foil cone hat and a triad are the same shape.

 

[Charged]

And that is just the tip of the iceberg when it comes to wasteful specs that serve no purpose.
3/4" minimum conduit, 12AWG minimum conductor, dedicated neutrals for circuits, oversized feeder conductors, grossly oversized transformers, services, and generators .....
Ironically you find all this waste on projects that get a platinum LEED sticker on the design.

I agree but I will say , it’s definitely falls under the category , no good deed goes unpunished. A lot of times when I get one dialed in to get the most cost effective design , are the ones that they forgot to tell me about a piece of equipment or realized they wanted to feed a storage building or something, meanwhile the oversized designs are looking like experts .

 

[drcampbell]

And that is just the tip of the iceberg when it comes to wasteful specs that serve no purpose.
3/4" minimum conduit, 12AWG minimum conductor, dedicated neutrals for circuits, oversized feeder conductors, grossly oversized transformers, services, and generators .....
Ironically you find all this waste on projects that get a platinum LEED sticker on the design.

At least the waste you cite is a mostly one-time thing.

LEED certification is so myopic that it's available to buildings located out in the exurbs that require the workers to drive their individual cars many miles to get there each day, and to factories cranking out a thousand sport-brutality vehicles per day, so long as it has a grass roof.

 

[hillbilly1]

I remember getting jobs where they wanted a 500 kcmil GEC to all the electrodes and cadwelded. Don’t know about now, but there were no molds for a 500 to a ground rod back then.

I had an inspector want a 3/0 to a ground rod, I told the inspector the #4 I ran was already oversized. He asked me to show him in the code where it was. Flipped to the page, he read it, and said “Your right, I learned something today” LOL!

 

[jaggedben]

Fwiw the illustration in NEC 2020 250.106 shows three for lightning protection system before the bond to the GES. I know lightning protection is a different animal but thought it was interesting….

Is that in the handbook? I don't see any illustration in my book.