Required Ohms per grounding system

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kfenn22

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Can some please tell me where in the NEC it states that the grounding system needs to be less then 25 ohms?

I typically design for 5 ohms, but I'm fighting a contractor right now, and he is saying that we only have to have 25 ohms.

Thanks.
 
If you are the design engineer and it is YOUR DESIGN you (and your customer) get to set the rules (usually). No where in the NEC does it require grounds to be 25 ohms or less, basically what it says if one rod does not achieve 25 ohms drive a second rod.

If the first rod has a resistance reading to earth of 1000 ohms drive a second rod and walk away.

Why are you designing to 5 ohms?
 
Therer is no such requirement for 25 ohms or less. 250.56 states that a single rod of 25 or more shall be augmented with one additional rod. However once that second rod has been added the NEC could careless what the impedance is as it has no importance or purpose. It could be 1000 and it is acceptable to the NEC.
 
brian john said:
If the first rod has a resistance reading to earth of 1000 ohms drive a second rod and walk away.

Why are you designing to 5 ohms?
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dereckbc said:
It could be 1000 and it is acceptable to the NEC.
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You guys the same person?:rolleyes:
 
kfenn22 said:
I typically design for 5 ohms, but I'm fighting a contractor right now, and he is saying that we only have to have 25 ohms.
Thanks.
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WHY? What do you expect to achieve by having a 5 ohm reading as opposed to a 100 ohm reading?
 
The 25 ohm requirement is the reason we have to use two grounds in CT. Just drive two rods and forget about it, no measurement required.
Lou
 
IEEE 142 requires 5 ohms or less for large commercial or industrial systems and 1 ohm or less for generating or transmission stations (Unless otherwise specified by the owner)
 
You ain't gonna find a requirement for 25-ohms in the NEC because it doesn't exist. The reference to 25-ohms is ONLY for ground rods and then only says that one additional electrode must be installed. As others have said, there is NO requirement that the resistance of the 2 rods be 25-ohms or less. There is no mention of a resistance measurement being required for any of the other allowed grounding electrodes, such as a UFER ground (CEE), or an underground water pipe.
 
I am sick of having this discussion of ground grid requirements every week, cant we just make a link to where this was discussed before, its like beating a dead horse.

A ground rod at a house or Taco Bell should not be discussed in the same post as a ground grid at a power plant switchyard. Different rules, different protection schemes, different blah blah blah....
 
5 ohms is what the customer wanted, Following the IEEE.
This is a build/design; I mean Design build project.

Long story short, the EC got caught putting in #1 awg instead of 2/0 for the ground loop. Now he wants me to write letter approving his VE from the orginal design.

I'm sticking with the orginal design.
Thanks for the help.
 
kfenn22 said:
5 ohms is what the customer wanted, Following the IEEE.
This is a build/design; I mean Design build project.

Long story short, the EC got caught putting in #1 awg instead of 2/0 for the ground loop. Now he wants me to write letter approving his VE from the orginal design.

I'm sticking with the orginal design.
Thanks for the help.
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Then your EC is correct, using a # 1 vs. a 2/0 will not change a thing in respect to impedance or performance, all it effects is cost. He saved you some $$$ assuming he subtrated the cost difference. On the other hand if the EC bidded and charged you for for 2/0 well... IMHO I would give him two options.

1. Subtract the material cost difference on the bill, and write him the VE. (IMO the best option)
2. Tough, dig it up, replace at EC expense per contract.
 
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zog said:
IEEE 142 requires 5 ohms or less for large commercial or industrial systems and 1 ohm or less for generating or transmission stations (Unless otherwise specified by the owner)
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That is not enough reason for justification. There has to be a specific reason to justify that requirement. For most commercial applications, you cannot justify a low impedance for any technical or performance reasons. It is only when you cross the line of medium to high voltage, or special purpose requirements like communications facilitieswith integrating lightning protection schemes. Otherwise there is no purpose or justification.
 
I mentioned nothing (Nor did the IEEE reference) about commercial appliactions. Or course we are talking MV/HV applications here.
 
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