How many ground rods?

C

Charlypt

Member
Location
Florida
Occupation
Electrician
We have a little discussion at my company about how many ground rods are necessary. Many old installers talk about formulas that if you have more than x amps you need 2 or 3, if it is three-phase you need 3 in a triangle. We compare different projects, some ask for a single rod and others up to three 20-foot rods in a triangle.
I have tried to look for a justification in the NEC, even in the service installation manuals of the utility companies and nothing.
Is there any code that defines this, other than that you have to have 25 ohms or less? How do engineers calculate the ground system?
 
If you install a single rod it has to be tested to be 25 ohms or less. If you install more than one rod no testing is required and the two rods are considered to be a grounding electrode. The two rod electrode can be used on any service.
 
See 250.53(A)(2)


(2) Supplemental Electrode Required​


A single rod, pipe, or plate electrode shall be supplemented by an additional electrode of a type specified in 250.52(A)(2) through (A)(8). The supplemental electrode shall be permitted to be bonded to one of the following:
  1. Rod, pipe, or plate electrode
  2. Grounding electrode conductor
  3. Grounded service-entrance conductor
  4. Nonflexible grounded service raceway
  5. Any grounded service enclosure
Exception: If a single rod, pipe, or plate grounding electrode has a resistance to earth of 25 ohms or less, the supplemental electrode shall not be required.

A triad is never required and a waste of time
 
roger said:
See 250.53(A)(2)

(2) Supplemental Electrode Required​


A single rod, pipe, or plate electrode shall be supplemented by an additional electrode of a type specified in 250.52(A)(2) through (A)(8). The supplemental electrode shall be permitted to be bonded to one of the following:
  1. Rod, pipe, or plate electrode
  2. Grounding electrode conductor
  3. Grounded service-entrance conductor
  4. Nonflexible grounded service raceway
  5. Any grounded service enclosure
Exception: If a single rod, pipe, or plate grounding electrode has a resistance to earth of 25 ohms or less, the supplemental electrode shall not be required.

A triad is never required and a waste of time
Click to expand...
 
Those engineers always try to kill us.
They have asked us for 3 - 20 foot rods in a triangle 10 feet apart where there is no space.
We had to join them with a coupling and install them at an angle due to the hardness of the terrain.


Thanks for making my point.
 
Charlypt said:
Those engineers always try to kill us.
They have asked us for 3 - 20 foot rods in a triangle 10 feet apart where there is no space.
We had to join them with a coupling and install them at an angle due to the hardness of the terrain.


Thanks for making my point.
Click to expand...
Engineers like to over design. Code requires one rod tested 25 ohms or less or two rods, 6' apart. Nothing more.
 
I recently had a project where a vendor required the resistance at their equipment to be 2 ohms or less. It's obviously difficult to add rods and connections after the fact. For this facility we had a 4/0 ground ring around the entire thing with occasional connections to columns, whips to large equipment, and rods every ~50'.

As for how you design to a goal like 2 ohms, I've only really heard rules of thumb.
Like how you always see 4/0 ground rings because it's the minimum size that can handle the average lightning strike supposedly. Space rods at least 2x the length apart to minimize the resistances "overlap". And more rods = less reactance for a lightning strike, which is high frequency. The triads supposedly filter noise; the whip from the triad can be brought to a lab or IT room with sensitive equipment.

(I have no backup for any of that, just things I've heard... Wish I knew for sure)
 
HarrySlother said:
I recently had a project where a vendor required the resistance at their equipment to be 2 ohms or less. It's obviously difficult to add rods and connections after the fact. For this facility we had a 4/0 ground ring around the entire thing with occasional connections to columns, whips to large equipment, and rods every ~50'.

As for how you design to a goal like 2 ohms, I've only really heard rules of thumb.
Like how you always see 4/0 ground rings because it's the minimum size that can handle the average lightning strike supposedly. Space rods at least 2x the length apart to minimize the resistances "overlap". And more rods = less reactance for a lightning strike, which is high frequency. The triads supposedly filter noise; the whip from the triad can be brought to a lab or IT room with sensitive equipment.

(I have no backup for any of that, just things I've heard... Wish I knew for sure)
Click to expand...
I'm not sure if any of that is true but what do I know about 2 ohms. I do know that the NEC allows a single rod @25 ohms or less and if that were grossly inadequate it would likely be changed in the NEC.
 
For the purpose of low voltage system grounding, the NEC requirements are probably excessive. 25 Ohms, 250 Ohms, it probably doesn't make a difference.

For lightning protection systems you certainly have different requirements.

Radio antenna grounding yet another set.

Single wire telegraph stations...

If the engineer has a design basis for whatever configuration, then you install it. If the engineer just threw some boilerplate in for an excessive configuration, then you complain and install it. But maybe if you are the paying customer you ask the engineer why...
 
winnie said:
If the engineer has a design basis for whatever configuration, then you install it. If the engineer just threw some boilerplate in for an excessive configuration, then you complain and install it. But maybe if you are the paying customer you ask the engineer why...
Click to expand...

What happens if you have some rubbish plans, and in the same place in a previous project you made a NASA ground system, and in another part of the same complex, another engineer designs a single rod for a similar service (it is a real case), then the contractor and the client ask you why on one side you have a complex system and the other a simple one? Finally we had to install the same over designed system.
 
Charlypt said:
Those engineers always try to kill us.
They have asked us for 3 - 20 foot rods in a triangle 10 feet apart where there is no space.
We had to join them with a coupling and install them at an angle due to the hardness of the terrain.


Thanks for making my point.
Click to expand...
Depends on the application. NEC for most every occupancy the 2 rods and done is all that is required. But what you are describing sounds as if you are describing Cell or radio Tower, large utility scale Solar, utility infrastructure or similar applications. Then they have there own requirements that typically do have what you describe. Also most times will also have equipotential planes designed in addition to the triad rods to reduce high voltage gradients.
 
Charlypt said:
Those engineers always try to kill us.
They have asked us for 3 - 20 foot rods in a triangle 10 feet apart where there is no space.
We had to join them with a coupling and install them at an angle due to the hardness of the terrain.


Thanks for making my point.
Click to expand...
Most times those type of engineers wear tin foil triangle hats as well.
 
Fred B said:
Depends on the application. NEC for most every occupancy the 2 rods and done is all that is required. But what you are describing sounds as if you are describing Cell or radio Tower, large utility scale Solar, utility infrastructure or similar applications. Then they have there own requirements that typically do have what you describe. Also most times will also have equipotential planes designed in addition to the triad rods to reduce high voltage gradients.
Click to expand...

No, one was a baseball field (field lights, scoreboards, bathrooms), 600 A, grounding system: 3 - 20' rods in triangular configuration, 3/0 GEC
and the other, modular classrooms (trailers), 1000A.

We had to match the first one at the client's request.
 
Charlypt said:
No, one was a baseball field (field lights, scoreboards, bathrooms), 600 A, grounding system: 3 - 20' rods in triangular configuration, 3/0 GEC
and the other, modular classrooms (trailers), 1000A.

We had to match the first one at the client's request.
Click to expand...
The field lights are nice lightning rods, so I can see that, the classrooms though have a much lower chance of lightning.
 
Charlypt said:
No, one was a baseball field (field lights, scoreboards, bathrooms), 600 A, grounding system: 3 - 20' rods in triangular configuration, 3/0 GEC
and the other, modular classrooms (trailers), 1000A.

We had to match the first one at the client's request.
Click to expand...
Who exact is "the client"? Usually it's an engineer or designer that is wasting the clients money and the owner should fire him. If it is truly the owner making the decision then all you can go is tell him he's wasting is money but you are happy to do what he wants as long as you get paid.
 
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