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The Electrical Code (USA) requires that a single ground rod have a maximum resistance to ground of 25 ohms (NEC 250.53(A.2)). If this isn't met, an additional ground rod is required, but no resistance test is required. Where does the value of 25 ohms come from?

From my reading of various sources on the internet, the test for ground resistance is infrequently done for residential installs. What kind of issues might I experience if my resistance to ground is above this value with a single ground rod?

This article has a seemingly informative title, but lacks actual details: Who Cares About 25 Ohms or less?

The basis for this question is that my ~30 year old house only has a single ground rod. I am willing to install a second one in lieu of an official test if there is a reasonable possibility that it will help in any way. At this point, I can't find any information that would point me to specific issues that might be solved.

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    If the resistance is too high then the easiest path to ground is through you.
    – ratchet freak
    Commented May 10, 2016 at 8:28
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    @ratchetfreak current flows through every available path, so why does the "easiest" path matter?
    – Tester101
    Commented May 10, 2016 at 9:08
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    @ratchetfreak, just what current is taking this path?
    – Speedy Petey
    Commented May 10, 2016 at 11:35
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    @ratchetfreak 'Tis true that as resistance goes up, current goes down. However, if the ground rod is protecting you from electrocution, how are those GFCI guys still in business?
    – Tester101
    Commented May 10, 2016 at 11:37
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    I think the 25 ohms might just be an arbitrary value that was chosen at some point. Maybe the person that proposed it had a reason for it, but I'm not sure that reason was ever published outside the proposal. Maybe somebody could dig through the NEC archive to find the original proposal, but short of that I'm not sure we'll ever know. In reality, you want the value to be as close to 0 as possible.
    – Tester101
    Commented May 10, 2016 at 13:33

3 Answers 3

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Well it is the National Electrical Code that requires 25 Ohms or less. Notice the exception after #5.

Here:

250.53(A)(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.

(3) Supplemental Electrode. If multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall not be less than 1.8 m (6 ft) apart.

Informational Note: The paralleling efficiency of rods is increased by spacing them twice the length of the longest rod.

(B) Electrode Spacing. Where more than one of the elec-trodes of the type specified in 250.52(A)(5) or (A)(7) are used, each electrode of one grounding system (including that used for strike termination devices) shall not be less than 1.83 m (6 ft) from any other electrode of another grounding system. Two or more grounding electrodes that are bonded together shall be considered a single grounding electrode system.

The reason contractors elect to install a second ground rod is that it is cheaper than paying for the ground resistance testing.

The advantage to a low resistance is during a lightning strike or large ground fault even a small resistance creates a large voltage differential. This could be dangerous for persons in contact with grounded parts of the system at that moment. Hence the reason for the requirement.

Otherwise, during normal operation there isn't a noticeable advantage to consumers.

For an older home you may already have a grounded metal water pipe exiting the building so you already have a supplemental electrode. If not adding an extra rod may help protect you and your valuable electronic equipment during a lightning strike or large voltage transient on the line.

Good luck!

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    You can pound all the ground rods you want, without a proper lighting protection system, you don't stand a chance against a direct lightning strike. Even with a lightning protection system in place, there's still a high probability that your stuff will cook..
    – Tester101
    Commented May 10, 2016 at 11:42
  • You re-stated my question, but basically said that ground rods only help with lightning strikes (nearby? direct? induced current?)
    – hazzey
    Commented May 10, 2016 at 12:45
  • @hazzey When it comes to lighting, grounding only helps with induced energy caused by nearby lightning strikes. The grounding system is not designed as a lighting protection system, and will not protect against a direct lightning strike.
    – Tester101
    Commented May 10, 2016 at 13:28
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I'm gonna go out on a limb and say in a typical setting there is no difference. The 25 ohm thing is meaningless, especially considering that the NEC seems to agree in that if the 25 ohm requirement is not met you can simply sink another rod and not worry one bit about the resistance. So why bother with the 25 ohm requirement in the first place.

Ground rod and other electrodes have pretty much nothing to do with the day to day functionality of the electrical service in a home. So one rod or two you will see absolutely NO difference in "performance".

Please do not let anyone tell you circuit current is "drained" to a ground rod, or that you can "ground" a receptacle or circuit by installing or connecting to a ground rod, or some other silly explanation of why they are required.

The main purpose of electrodes is explained just right in Archon's post (+1) above:

For an older home you may already have a grounded metal water pipe exiting the building so you already have a supplemental electrode. If not adding an extra rod may help protect you and your valuable electronic equipment during a lightning strike or large voltage transient on the line.

Even then, lightning is a fickle mistress and will do what she wants.

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Have a higher resistance ground would increase the amount of time for a breaker to trip.

Reference: https://m.youtube.com/watch?v=3vvvv5QVZoA

(After 16:55)

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    This answer needs expanding. Links tend to die, please give a bit of information from the linked resource so that this answer will last. In particular, Why do we care about breaker trip time?
    – Chris Cudmore
    Commented Oct 2, 2017 at 15:10
  • A ground rod has nothing to do with clearing faults.
    – Tester101
    Commented Oct 2, 2017 at 15:34

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