Required number of ground rods

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Jpflex

Jpflex

Electrician big leagues
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Victorville
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Electrician commercial and residential
Quick question and probably easy for most.

At my companies main panel there is a ground rod bonded to neutral and equipment grounding bonding conductor.

Boss wants to place another (1) ground rod (not 2 at 8 feet apart) in the sub panel feeding shipping containers converted to housing.

Grounds and neutrals will be separated at sub panel as normal but are we required to have a ground rod at both main and sub panel? Is it allowed, permissible, unnecessary or not allowed? Anyone know specific code? Thanks
 
If these are separate structures you should have to put a grounding electrode at each one of them, and if rod type electrodes you either need to prove one is 25 ohms or less or use at least two.
 
ptonsparky said:
OP. That includes your converted shipping containers. Not just the sub in your post.
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The main Panel is away outside from shipping containers Converted to rooms and a kitchen with its sub panel Inside
 
kwired said:
If these are separate structures you should have to put a grounding electrode at each one of them, and if rod type electrodes you either need to prove one is 25 ohms or less or use at least two.
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Would you happen to know specific code?
 
Jpflex said:
Would you happen to know specific code?
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Same thing that requires grounding electrodes at all buildings/structures 250.50.

If these containers are utilized as a fixed structure, they get no different treatment on this than if they were more conventional construction methods.

If you connected multiple these shipping containers to effectively make one building then you only have one building and only need one supply and one GES.
 
kwired said:
Same thing that requires grounding electrodes at all buildings/structures 250.50.

If these containers are utilized as a fixed structure, they get no different treatment on this than if they were more conventional construction methods.

If you connected multiple these shipping containers to effectively make one building then you only have one building and only need one supply and one GES.
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So all buildings get their own ground rod plus main breaker away from building gets its own even though buildings are already bonded to ground rod at main breaker away from buildings?
 
Jpflex said:
So all buildings get their own ground rod plus main breaker away from building gets its own even though buildings are already bonded to ground rod at main breaker away from buildings?
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Ok I should have backed up a little further, 250.50 tells us what electrodes to use. If none of the first few mentioned (CEE, Water pipe, structural metal) are present then you must add one the other electrodes, simplest and most common choice is usually ground rod(s).

250.24 is about grounding service supplied AC systems

250.32 is about grounding buildings or structures supplied by a feeder or branch circuit. Feeder supplied buildings almost always need a grounding electrode system, and requirements are nearly the same as for service supplied buildings.

Buildings or structures supplied by a single branch circuit are about the only time you are not required to have a GES.
 
Jpflex said:
So all buildings get their own ground rod plus main breaker away from building gets its own even though buildings are already bonded to ground rod at main breaker away from buildings?
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The purpose for a grounding system at separate buildings is for lightning protection.
 
tom baker said:
The purpose for a grounding system at separate buildings is for lightning protection.
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Careful how you say that. These grounding electrodes are not part of lightning protection systems. They do help divert energy to earth when there is lightning caused transients on the system though.
 
rlundsrud said:
I think he means it helps prevent voltage gradients when lighting strikes nearby.
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It gives that transient energy a path that is hopefully lesser impedance than the path it otherwise might take and causes a majority of that energy to take the designated path. Current will still take all paths available, but lesser impedance paths will carry more of it.

Lightning is high enough energy that the more directly you are in/near the path the more gradient you will see in/near proximity to that path.

Lightning strikes a pole a mile away from you, there often will be electrodes at every pole and any other structures between you and the pole that took the strike. Those will divert more current to earth the closer you get to the main strike location. You might see little or no rise in voltage at your point.

Lighting strikes a pole just outside your house, chances are a fair amount of current finds a way into the house and finds other paths to earth whether intentional paths or not. Voltage gradients are possibly all over the place as well, but only for the duration of the event. If you are lucky you won't become a path between any of those gradient points. Well bonded/grounded metal structure might just turn into a Faraday cage around you in that sort of situation.
 
@kwired gave you the code section but yes if you feed a separate structure then, in most cases 2 rods are required at least 6' apart however it appears your boss is either knows he can get 25 ohms or he knows he wil not be being required to have 2 rods.
 
kwired said:
Careful how you say that. These grounding electrodes are not part of lightning protection systems. They do help divert energy to earth when there is lightning caused transients on the system though.
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I will post the code section later. I have the pdf code on my phone and it’s hard to find.
 
tom baker said:
The purpose for a grounding system at separate buildings is for lightning protection.
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Yes that’s why I said by each building already being bonded to main breaker panel and it’s corresponding grounding bar they are already grounded for lightning flash hazard protection. But I guess being redundant is ok
 
You can debate the purpose and need but the bottom line is if you want the installation to be Code compliant then 250.32:
(A) Grounding Electrode. Building(s) or structure(s) supplied by feeder(s) or branch circuit(s) shall have a ground ing electrode or grounding electrode system installed in accordance with Part III of Article 250.
 
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