multiple ground rods
- Thread starter NIFE
- Start date
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tryinghard
Senior Member
- Location
- California
They should be 6' apart but have as many as you want, and they must be bonded together. See 250.53
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
NIFE said:
You need to clarify your question. Ground rods need only be driven at the service not for sub panels, etc. You would need an equipment ground not ground rods--- I think this is what you are getting at.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Dennis Alwon said:
What about subpanels within separate structures?
Let me explain it better.
this is pertaining to factory built buildings, built in factory, shipped on site by semi and craned onto a slab or over a craw space. each section is 12 ft wide 60 foot long. each having its own 125amp distribution panel on the back ends. the buildings are often used for schools. One common installation for lets say a 4 section wide building, would be: a existing distribution panel on a close building. to a transformer, then to a main disconnect panel say 400amp. then that panel feeding 2 200 amp switch panel. each 200 amp would then feed 2 of the 125 amp panels on the buildings.
distances were from trans to main 400amp disconnect 5 feet. the 400 and the 2-200's were mounted together. the 125 amp panels were 20 feet away from the transformer. the trans being in between the two.
In this perticular job, the eng plans called for no ground rod at trans. one rod at the 400 amp main, and 1 rod at each of the 4 125 amp building panels.
this is pertaining to factory built buildings, built in factory, shipped on site by semi and craned onto a slab or over a craw space. each section is 12 ft wide 60 foot long. each having its own 125amp distribution panel on the back ends. the buildings are often used for schools. One common installation for lets say a 4 section wide building, would be: a existing distribution panel on a close building. to a transformer, then to a main disconnect panel say 400amp. then that panel feeding 2 200 amp switch panel. each 200 amp would then feed 2 of the 125 amp panels on the buildings.
distances were from trans to main 400amp disconnect 5 feet. the 400 and the 2-200's were mounted together. the 125 amp panels were 20 feet away from the transformer. the trans being in between the two.
In this perticular job, the eng plans called for no ground rod at trans. one rod at the 400 amp main, and 1 rod at each of the 4 125 amp building panels.
let me give you more info.
This situation happened on a 4 section factory build building. each section being 12x60, and each having a 125amp main disconnect at the back feeding its own section.
The equipment on site was the 4 125amp panels, 2-200 amp switch panels mounted with a 400amp main disconnect. and a transformer.
the power was supplied around the corner of a near building, didn't see where it came from,(not my inspection)it wasn't a service though. to the transformer. from the transformer to the 400amp panel, the 400amp had 2-200amp breakers feeding the 2-200amp switch panels. then each of the 200amp feed 2 of the 125amp building panels.
the distances from the 400 / 2-200 to the closest 125 was 20 feet. in between 5 ft from the 5ft from the 400 and 15ft from the nearest 125 was the transformer.
The eng plans called for ground rod at the 400 amp main. and also a rod at every one of the 125 amp panels. and 1 at the transformer.
Wish I could draw the layout for ya
This situation happened on a 4 section factory build building. each section being 12x60, and each having a 125amp main disconnect at the back feeding its own section.
The equipment on site was the 4 125amp panels, 2-200 amp switch panels mounted with a 400amp main disconnect. and a transformer.
the power was supplied around the corner of a near building, didn't see where it came from,(not my inspection)it wasn't a service though. to the transformer. from the transformer to the 400amp panel, the 400amp had 2-200amp breakers feeding the 2-200amp switch panels. then each of the 200amp feed 2 of the 125amp building panels.
the distances from the 400 / 2-200 to the closest 125 was 20 feet. in between 5 ft from the 5ft from the 400 and 15ft from the nearest 125 was the transformer.
The eng plans called for ground rod at the 400 amp main. and also a rod at every one of the 125 amp panels. and 1 at the transformer.
Wish I could draw the layout for ya
steelersman
Senior Member
- Location
- Lake Ridge, VA
Dennis you were just telling me that you need ground rods at new sub-panels on the sub-panel thread? I'm confused now.Dennis Alwon said:
tryinghard
Senior Member
- Location
- California
NIFE said:
Grounding is required at each separate "structure"/building (250.32) and of course at the service disconnect (250.24).
The grounding at each structure does not include bonding jumpers to the neutral (250.24(5) & 250.142(B)), this only happens at the service disconnect (250.26 & 250.28). If there is a downstream transformer bonding the neutral will happen again if it is a separately derived system (SDS) (250.30(A)). If your application is qualified under NEC 2005 or older there might be a chance the neutral will bond downstream past the service disconnect if it meets ALL the criteria of 250.32(B), this is not likely with your applications and it is also eliminated in 2008 for new installations.
In summary each of the modular?s will have a ground earth connection that bonds only to equipment grounding at their respective sub panel and bonds to the neutral (grounded conductor) only at the service disconnect, SDS transformer or its first secondary disconnecting means.
The bonding jumper at the service disconnect and or SDS is the only item that allows a path for fault current to source without it overcurrent protection will not function. Downstream the effective ground fault path is achieved with equipment grounding (250 part VI and 250.118). If the neutral is incorrectly bonded downstream a dangerous parallel path will be formed to source.
tryinghard
Senior Member
- Location
- California
Some visuals may help:
tryinghard
Senior Member
- Location
- California
And:
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
steelersman said:
I am sorry I omitted the grounding electrode requirement for separate structures. Since the op didn't mention separate structures I ignored that situation. Of course you would need them there but not at sub panels within a structure.
I understand bonding and your right (not that you need me to tell you), each of the 125 sub panels have the neutral and the ground isolated. they use 4 conductors back to the transformer.
My main question still stands though, If you have say 2 sub panels 5 feet apart, with a ground rod at each, 5 ft. away from each other, can you get current between the 2. or more importantly can you get current through you, if you stand between the 2 rods.
And if effectively bonded, at what point in distance between the panels would you need if at all, another ground rod?
My main question still stands though, If you have say 2 sub panels 5 feet apart, with a ground rod at each, 5 ft. away from each other, can you get current between the 2. or more importantly can you get current through you, if you stand between the 2 rods.
And if effectively bonded, at what point in distance between the panels would you need if at all, another ground rod?
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
NIFE said:
I don't believe the 2002 changes anything. May I ask why you are driving ground rods for sub panels. As said before it is not necessary unless the panel goes to a separate structure. In your case of 2 sub panels 5 feet apart one would not use ground rods.
You will not get current thru you f you stand between the rods. Generally the rods for a service must be at least 6 feet apart but I am not sure why. There should be very little if any current normally travelling thru the ground rod. Ground rods are basically required for lightning protection only.
tryinghard
Senior Member
- Location
- California
NIFE said:
Additional grounding is only required if the sub panel is in a separate building/structure, sounds like in your case these are separate buildings. Once the additional grounding is installed at separate buildings these must connect to the service disconnect/or sources grounding causing it to be a larger ?grounding system? with multiple electrodes to earth. Grounding does not have current, its specific purposes are to:
[1]"limit the voltage imposed by lightning,
[2]line surges,
[3]or unintentional contact with higher-voltage lines?
NIFE said:
The grounding electrode conductor should be as short as practical, 20? or less, with as few bends as possible especially sharp ones. One electrode (as listed in 250.52) per structure ? not per sub panel - is all that is required. If it is installed in a conductive item like metallic conduit it must physically bond at each end of the conduit to disable a ?choking action? if used, this action could blow the conductor in half disabling it.
The rods (electrodes) serve no other purpose except to enable an earth connection (grounded). The grounding system is not needed at all to enable the circuitry to function; its not even needed to enable overcurrent protection to work for shorts. It never has current unless one of the items listed in 250.4(A)(1) is happening or a downstream bonding jumper is missing. The neutral connection to source ?XO? is what enables all returning current to source and the bonding jumper with equipment grounding enables the fault protection for shorts.
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