5 ms·

It depends on the impedance of the soil. If impedance is too high, you need additional and/or longer rods, spaced accordingly. The code says MINIMUM of 8ft in

It depends on the impedance of the soil. If impedance is too high, you need additional and/or longer rods, spaced accordingly. The code says MINIMUM of 8ft in contact with soil. MAXIMUM earth ground impedance of 5 Ohm. So you need to keep driving in rods (or use an alternate grounding system) until you are below 5 Ohm - hence your multiple rods.

sokoloff 7y ago
I don’t recall the NEC saying that. My recollection is that code is two ground rods, spaced 6+ feet apart, is sufficient for code without measuring resistance at all. Alternatively, if you can prove that a single ground rod has less than 25 ohms to earth, then a single rod suffices. If I have time later, I’ll try to look up the actual reference, but I remember reading and thinking “that makes no engineering sense!” so it stuck in my head.
- supermatt 7y ago
  I think you may be right: Its actually IEEE and NFPA that recommend a maximum of 5 Ohms for the grounding system. NEC state less than 5 Ohms for sensitive equipment, 25 Ohms otherwise. (NEC 250.56) - but as you mentioned, the measurement is only taken from first rod and a second can be inserted without testing (so it could be over 25 Ohms still, but based on the code it doesnt even matter....) As an aside, its 200 Ohm in the UK (but it HAS to be measured - cant just blindly drive 2 electrodes and call it a day).
  - sokoloff 7y ago
    This discussion triggered in me the question, "Obviously, I can't just stick a meter probe in the ground [as I'd be measuring the dominant resistance of that probe rather than the grounding rod in series]. So, how would I reasonably measure the resistance of an earth connection?" I came up with the (mildly hazardous) solution of putting a 120VAC line from the power company supply down the ground rod and measuring the current flow back to the PoCo (grounded) neutral conductor and then went googling. There are a few other (safer) ways that might interest HN readers, so sharing this video: https://www.youtube.com/watch?v=Yg6G5VUSsWA https://www.youtube.com/watch?v=Yg6G5VUSsWA
    - mcguire 7y ago
      They have several hundred dollars of equipment there and have a stripped end of a piece of bell wire plugged into the 120VAC extension cord. I love electricians (and electrical engineers).
madaxe_again 7y ago
Yeah - 5 Ohm is the bit that matters. I’m in the process of installing an off-grid solar energy system at my home, which the previous owner had wired up, ungrounded, with a generator. I am not exactly following code, as my earth comprises a 5 meter RSJ left over from construction buried 30cm down under a dirt road - we have very little soil over bedrock, but the soil is full of iron. It’s only half buried but I’m at 2.2 Ohm, so it should do the job - have soldered a nice fat cable to it to be my common ground.
robomartin 7y ago
> It depends on the impedance of the soil. If impedance is too high, you need additional and/or longer rods, spaced accordingly. I did the calculations. One rod would have done it. Two if you wanted redundancy. The plan checker wanted four. Want to know why? He wanted one next to each post. As simple as that.
- opwieurposiu 7y ago
  My county also has a bureaucratic nightmare for a building permits department. I avoid interaction with them however I can. Sheds less than 120sqft do not require solar permits, so I am trying to cram 1.7kw of panels on to my garden shed. Also looking at getting another few kw mounted on a wheeled trailer which the building dpt does not have dominion over. They want $1000 for a permit which would increase my payback time by at least 3 years. I closely follow the relevant codes that improve my children's safety, but about half of the codes in my county are more about bureaucracy then safety.
  - robomartin 7y ago
    That's what happens when bureaucracies need to justify their existence. Sad. Not saying safety isn't important. Quite to the contrary. However, there's safety and then there's making life impossible with things that don't really add to or enhance safety.

sokoloff 7y ago

I don’t recall the NEC saying that. My recollection is that code is two ground rods, spaced 6+ feet apart, is sufficient for code without measuring resistance at all. Alternatively, if you can prove that a single ground rod has less than 25 ohms to earth, then a single rod suffices. If I have time later, I’ll try to look up the actual reference, but I remember reading and thinking “that makes no engineering sense!” so it stuck in my head.

supermatt 7y ago
I think you may be right: Its actually IEEE and NFPA that recommend a maximum of 5 Ohms for the grounding system. NEC state less than 5 Ohms for sensitive equipment, 25 Ohms otherwise. (NEC 250.56) - but as you mentioned, the measurement is only taken from first rod and a second can be inserted without testing (so it could be over 25 Ohms still, but based on the code it doesnt even matter....) As an aside, its 200 Ohm in the UK (but it HAS to be measured - cant just blindly drive 2 electrodes and call it a day).
- sokoloff 7y ago
  This discussion triggered in me the question, "Obviously, I can't just stick a meter probe in the ground [as I'd be measuring the dominant resistance of that probe rather than the grounding rod in series]. So, how would I reasonably measure the resistance of an earth connection?" I came up with the (mildly hazardous) solution of putting a 120VAC line from the power company supply down the ground rod and measuring the current flow back to the PoCo (grounded) neutral conductor and then went googling. There are a few other (safer) ways that might interest HN readers, so sharing this video: https://www.youtube.com/watch?v=Yg6G5VUSsWA https://www.youtube.com/watch?v=Yg6G5VUSsWA
  - mcguire 7y ago
    They have several hundred dollars of equipment there and have a stripped end of a piece of bell wire plugged into the 120VAC extension cord. I love electricians (and electrical engineers).

madaxe_again 7y ago

Yeah - 5 Ohm is the bit that matters. I’m in the process of installing an off-grid solar energy system at my home, which the previous owner had wired up, ungrounded, with a generator. I am not exactly following code, as my earth comprises a 5 meter RSJ left over from construction buried 30cm down under a dirt road - we have very little soil over bedrock, but the soil is full of iron. It’s only half buried but I’m at 2.2 Ohm, so it should do the job - have soldered a nice fat cable to it to be my common ground.

robomartin 7y ago

> It depends on the impedance of the soil. If impedance is too high, you need additional and/or longer rods, spaced accordingly. I did the calculations. One rod would have done it. Two if you wanted redundancy. The plan checker wanted four. Want to know why? He wanted one next to each post. As simple as that.

opwieurposiu 7y ago
My county also has a bureaucratic nightmare for a building permits department. I avoid interaction with them however I can. Sheds less than 120sqft do not require solar permits, so I am trying to cram 1.7kw of panels on to my garden shed. Also looking at getting another few kw mounted on a wheeled trailer which the building dpt does not have dominion over. They want $1000 for a permit which would increase my payback time by at least 3 years. I closely follow the relevant codes that improve my children's safety, but about half of the codes in my county are more about bureaucracy then safety.
- robomartin 7y ago
  That's what happens when bureaucracies need to justify their existence. Sad. Not saying safety isn't important. Quite to the contrary. However, there's safety and then there's making life impossible with things that don't really add to or enhance safety.