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I was just told by the electrical inspector that I need grounding clamps on the cold water and gas line in addition to two 8' grounding rods. As far as I'm aware the NEC only says you need to have a maximum resistance of 25 ohms, and I know the burden of proof is on me, but this seems excessive.

I've seen many new construction properties, all which passed inspection with less.

I know I have to do it. I guess I just want to know if it really takes this much grounding, to stay under 25ohms?

  • Yeah, it turns out dirt is a rather poor conductor of electricity. That does explain why they go to all the trouble to mine copper, instead just wrapping insulation around dirt. – Harper - Reinstate Monica Jul 14 at 18:49
  • Per my answer you may still be above 25 ohms but as long as you have 2 electrodes that is all code requires 1 can be a water pipe 250.53.5.D but a supplemental is required 250.53.2 for the code references not in my answer. But all the conductive plumbing even duct work is required to be bonded. – Ed Beal Jul 14 at 18:51
  • What about the gas line? He said it had to be both. I don’t even have gas appliances. Just the furnace. – mreff555 Jul 14 at 22:56
  • @mreff555, yes, the gas line must be bonded too -- I believe that's about avoiding static electricity sparks near gas. And your furnace does indeed count as a gas appliance. Note though, if your furnace has an electrical connection for its electronics, that may include a ground wire that also bonds to the gas pipes, in which case you would be all set. – Nate S. Jul 15 at 17:39
  • Also, the new construction you've seen that passed inspection with less probably had a "ufer" ground, aka a concrete encased electrode in the form of a connection to the foundation rebar, and that actually provides a better ground connection than traditional ground rods. – Nate S. Jul 15 at 17:42
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Yes the additional water and gas lines need to be connected but it is not necessarily for grounding resistance.

The gas meter is grounded not to be part of the grounding electrode system but because the internal pipes may become energized. Code actually requires this.

The water pipes, if not part of the grounding electrode system, may also become energized thus need to be connected to the system.

You may find out the water pipes and your 2 electrodes do not get you down to 25 ohms. When I first started taking measurements, I expected very low values with water pipes and ground rods, but found quite often the values were above 25 ohms. In fact, many times I saw values up to 4x or close to 100 ohms, unless I connected the well casing or an Ufer ground.

So the more grounding you have the better:

  • 25 ohms =4.8 amps to ground
  • 15 amps to ground 8 ohms
  • 20 amps to ground 6 ohms

The max voltage to ground in residential with split-phase 240v service is 120v to ground so you can see to clear a 20 amp breaker your grounding resistance for a fault would need to be below 6 ohms.

So it is code if metallic water pipes and gas lines for them to be bonded to your electrode system.
The best grounds turn out to be Ufer or concrete encased and water pipe (well casings are great but only require bonding if there is a pump).

Some older homes have disconnected from their wells for city water hook ups. Your 2 ea 8’ rods separated by at least 6’ are the minimum code allowed.

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    Note: you can still use a well casing as a grounding electrode, even if your water supply no longer comes from that well. (Just make sure the grounding electrode conductor is actually tied to the casing at the wellhead instead of relying on water plumbing elsewhere in the house for grounding continuity.) – ThreePhaseEel Jul 14 at 20:14
  • I agree but on the homes I worked on the city required the pumps to be pulled and in those cases the ground to the well was also taken out. – Ed Beal Jul 14 at 20:34
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You're mixing up grounding and bonding.

Grounding is primarily provided by the ground rods, and is used to dissipate natural sources of electricity (lightning, static electricity) to the earth. This can potentially be done by a single ground rod, but how well it works depends on the local soil conditions -- if you want to use a single rod, you need to test and make sure that single rod provides a less than 25 ohm connection to the soil. If you add a second rod, you're pretty much guaranteed to get under 25 ohms in most soil conditions (or maybe not, per Ed Beal's comment), so the NEC does not require you to test in that case. Since the test (either buying the tester or hiring someone who has one to do it) is quite a lot more expensive than just putting a second ground rod in, most people opt for two rods.

In addition to that, your other plumbing must be bonded to your electrical system's ground. This is so that your pipes and the housings of your grounded appliances are always at the same potential, and there's no chance you'll be shocked when you touch your sink and fridge at the same time.

There used to be a provision in code for using your plumbing systems instead of one of the ground rods, but that dates back from the time when water pipes were always metal. Now that plastic water lines are becoming more common, using them instead of a ground rod on a new installation is no longer allowed in most areas, but they must still be bonded.

So in short, two ground rods, plus bonding to both the water and gas systems, is the easiest-to-meet requirement. You can possibly get away with one ground rod + bonding to water and gas, but to do that you'll have to do an expensive test, and depending on your soil, you might have to install the second one anyway.

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  • Nate I have seen 2 rod systems close to 100 Ohms. This value is much higher in the summer than the winter when the ground is wet. Areas that have very deep water tables some times have the highest readings but you are correct it depends on the soil. – Ed Beal Jul 14 at 18:19
  • @EdBeal, woah interesting. Do you know what soil type those ground rods were going into in that install? – Nate S. Jul 14 at 18:20
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    Not just 1 but many locations. clay in Sonoma county ca summer months , in the foot hills of forestville and Sebastopol ca well over 100 with ground rings over 200’ in length, in Oregon it depends on the area foot hills I have seen as low as 10 and as high as 75 (75 with 30’ of driven rods 10’ apart). I have a multi point earth meter ,borrow one from work 1 point every now and then to check mine out. Most have never tested because it is so cheap but one of my early commercial jobs had a requirement for 3 ohms with a bonus for less than 1ohm if I would not have made 1 if not for a well casing – Ed Beal Jul 14 at 18:30

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