how many ground rods for a detached building ?

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Requirement is the same as the residence.
(Dependent on other electrodes present, resistance, etc). Normally Two
 
Yup, there's the prevalent installation that Larry mentioned and then there is the NEC. If the rods are an electrode in the GES then two are required unless you can prove 25Ω or less on the single rod.
 
Or as we say "Drive two and go home" its 2 due to diminishing returns
2 rods decreases resistance by 60%
2 rods decreases resistance by 40%
This is from AEMC
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But is the 2 rod requirement also on the seperate building sub panel that has the 4 conductors (120/240)? Thought the requirements was only for a supplemental ground rod and not a seperate GES, thus only requiring one rod. Code reference please?
 
Fred B said:
But is the 2 rod requirement also on the seperate building sub panel that has the 4 conductors (120/240)? Thought the requirements was only for a supplemental ground rod and not a seperate GES, thus only requiring one rod. Code reference please?
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250.32 Buildings or Structures Supplied by a Feeder(s)
or Branch Circuit(s).


(A) Grounding Electrode.
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Buildings or Structures Supplied by a Feeder(s) or a Branch Circuit(s) by feeder(s) or branch circuit(s) shall have a grounding
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electrode or grounding electrode system installed in
accordance with Part III of Article 250. The grounding
electrode conductor(s) shall be connected in accordance
with 250.32(B) or (C). Where there is no existing grounding
electrode, the grounding electrode(s) required in 250.50
shall be installed.

III. Grounding Electrode System and Grounding
Electrode Conductor

250.53 Grounding Electrode System Installation.

(A) Rod, Pipe, and Plate Electrodes. Rod, pipe, and plate
electrodes shall meet the requirements of 250.53(A)(1)
through (A)(3).
(1) Below Permanent Moisture Level. If practicable, rod,
pipe, and plate electrodes shall be embedded below permanent
moisture level. Rod, pipe, and plate electrodes shall be
free from nonconductive coatings such as paint or enamel.
(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.
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Roger
 
Fred B said:
But is the 2 rod requirement also on the seperate building sub panel that has the 4 conductors (120/240)? Thought the requirements was only for a supplemental ground rod and not a seperate GES, thus only requiring one rod. Code reference please?
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In theory if you have a ground wire going from the main disconnect, where you have your two ground rods driven in, you would not need to drive two more rods, because it all part of the same system. Your panel in the separate building is not bonded, thus you have a 4 wire feed and you building is grounded via your ground wire to the sub-panel


“ shoot low boys their riding shetland ponies”
 
McLintock said:
In theory if you have a ground wire going from the main disconnect, where you have your two ground rods driven in, you would not need to drive two more rods, because it all part of the same system. Your panel in the separate building is not bonded, thus you have a 4 wire feed and you building is grounded via your ground wire to the sub-panel


“ shoot low boys their riding shetland ponies”
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A separate structure requires a GES when fed by a feeder even when there is an EGC with the feeder conductors.
 
I've always driven only one when the building is fed with a 4 wire 240v sub-feed. Most of the separate buildings I see are metal, so I grind the purlin and install a #4 mechanical lug and take a small piece of #4 bare cu. and bond the metal frame of the building. Doing this in conjunction with driving a single rod has always been sufficient for my inspections.
 
Rick 0920 said:
I've always driven only one when the building is fed with a 4 wire 240v sub-feed. Most of the separate buildings I see are metal, so I grind the purlin and install a #4 mechanical lug and take a small piece of #4 bare cu. and bond the metal frame of the building. Doing this in conjunction with driving a single rod has always been sufficient for my inspections.
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Even if inspectors let it go it is not code compliant unless of course the metal framing is per 250.52(A)(2). IMO one rod is all that is ever needed but my opinion is not code compliant either.

Roger
 
if the requirement is the second ground rod be a minimum of 6 feet from the first one, it could be argued the ground rods at the service is more than 6' away :unsure:.
 
acrwc10 said:
if the requirement is the second ground rod be a minimum of 6 feet from the first one, it could be argued the ground rods at the service is more than 6' away :unsure:.
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I was at a building once where someone had drilled into a sidewalk in front of the building and pounded in two ground rods. There was no more than a foot between them. Presumably it passed inspection.
 
acrwc10 said:
if the requirement is the second ground rod be a minimum of 6 feet from the first one, it could be argued the ground rods at the service is more than 6' away :unsure:.
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But the conductor between would be a bonding jumper, min size would be 8 awg
 
roger said:
Even if inspectors let it go it is not code compliant unless of course the metal framing is per 250.52(A)(2). IMO one rod is all that is ever needed but my opinion is not code compliant either.

Roger
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I was not implying that the metal building was one of my electrodes. I'm in agreement with you about only needing one electrode. I was stating that the metal structure needed to be bonded. I apologize for the confusion.
 
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