Ground Rod at a construction trailer Y or NO

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eljefetaco

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Fanwood, NJ
I ran a 480v 3 phase feeder to a transformer for a 120/208 3 phase service. I am feeding two trailers and a guard booth. There is a panel board located 10' behind the trailers with a main breaker panel. I then fed the tralers with 4 wire feeders 100amp ser 2hots, neutral and equipment grounding conductor. Since I have a egc I don't think I need to put a ground rod from the steal trailer frame to earth. If anything I should bond my egc to the steal frame. Any input would help. I'm still looking at article 250.
 
You need grounding electrodes at the transformer, 250.30(A) the guard shack and the trailers. 250.32(A).

The trailer frames should be bonded to the EGC if not already done.
 
Since I am feeding the trailers with a 2 pole breaker doesn't it make the trailers a branch circuit and thus I can apply 250.32 A exception? What exactly is the purpose of the rod? If I was running a feed to a shed with a copper water feed would I need to bond that to my egc as well as a ground rod or could i use the water pipe instead. unless rod is for lighting protection i don't see why it is needed in the event of a fault current. i'm not worried about the cost i am just trying to educate myself and understand what is the right way.
 
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eljefetaco said:
Since I am feeding the trailers with a 2 pole breaker doesn't it make the trailers a branch circuit and thus I can apply 250.32 A exception? What exactly is the purpose of the rod? If I was running a feed to a shed with a copper water feed would I need to bond that to my egc as well as a ground rod or could i use the water pipe instead. unless rod is for lighting protection i don't see why it is needed in the event of a fault current.
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The wording of the exception is quite poor.

It generally has been interpreted to mean there is only a single circuit in the structure, not that a single circuit feeds the structure.

Otherwise you would almost never need a GES at a seperate structure since generally they are only fed by a single circuit.

Its not about fault current in any case. The grounding system does nothing for that.
 
eljefetaco said:
Since I am feeding the trailers with a 2 pole breaker doesn't it make the trailers a branch circuit and thus I can apply 250.32 A exception?
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The number of poles is not what matters, a feeder is any circuit that has breakers down line. (See the definition of Feeder and Branch circuit in Article 100) Your suppling panels in the trailers so they are feeders.


What exactly is the purpose of the rod?
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Great question, one that I really can't give you great answer to. 250..4(A)(1) tries to answer that question.



If I was running a feed to a shed with a copper water feed would I need to bond that to my egc as well as a ground rod?
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Yes, any time you supply a separate building or structure you must treat it like a service except you do not bond neutral to ground you bond the EGC to ground.
 
When they install the plumming to the trailer I could use the the copper line if it meets the gec requirements. Correct?So if by chance the exterior shell becomes energized a ground rod wouldn't help to solve that problem. Only if the shell was attached to the egc. (fpn The earth is not a good ground fault path) Am I right with this logic? And thanks for your continued input.
 
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eljefetaco said:
When they install the plumming to the trailer I could use the the copper line if it meets the gec requirements. Correct.
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If they run a metal water line to the trailer that meets the requirements of an electrode you have to use it. (250.50)

And then you will have to add another electrode as well. 250.53(D)(2).
 
eljefetaco said:
Since I am feeding the trailers with a 2 pole breaker doesn't it make the trailers a branch circuit and thus I can apply 250.32 A exception?
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250.32 (A) exception only applies to branch circuits not feeders. If, for instance you fed the trailer with a MWBC that would be considered one circuit. Your feeder is supplying other loads thus an electrode must be used as stated.
 
okay, put what about the shell? I know it isn't required to be connected to the gec. Am i right to assume the gec do not help if the trailer because energized for some reason
 
eljefetaco said:
okay, put what about the shell? I know it isn't required to be connected to the gec. Am i right to assume the gec do not help if the trailer because energized for some reason
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If the trailer frame became energized then the gec would help because it is tied back at the panel ground. :-?
 
eljefetaco said:
okay, put what about the shell? I know it isn't required to be connected to the gec.
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The trailer shell / frame etc are required to be bonded to the grounding conductor.

The electrodes are required to be bonded to the grounding conductor.



Am i right to assume the gec do not help if the trailer because energized for some reason
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In my opinion you are 100% correct.

I can't give you a good reason they NEC requires the grounding electrodes all I can tell you is they are required.

When I do construction trailers I install two rods at each trailer to comply with 250.56 which we have not even mentioned yet.
 
090527-1847 EST

I would suggest the reason you may want ground rods, and even better a long conductive water pipe connected to the trailer structure and the EGC is to make the EGC, trailer structure, and earth all at about the same potential.

This would minimize the likelihood of a shock when entering the trailer.

The likelihood may be low, but the possibility of stray currents in the earth are possible.

.
 
gar said:
I would suggest the reason you may want ground rods, and even better a long conductive water pipe connected to the trailer structure and the EGC is to make the EGC, trailer structure, and earth all at about the same potential.

This would minimize the likelihood of a shock when entering the trailer.
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This is a common thought but is it really possible unless your literally standing on the rod?

I don't think so.
 
250.32 Buildings or Structures Supplied by a Feeder(s)
or Branch Circuit(s).
(A) Grounding Electrode. Building(s) or structure(s) supplied
by feeder(s) or branch circuit(s) shall have a grounding
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.
Exception: A grounding electrode shall not be required
where only a single branch circuit, including a multiwire
branch circuit, supplies the building or structure and the
branch circuit includes an equipment grounding conductor
for grounding the normally non?current-carrying metal
parts of equipment.
(B) Grounded Systems. For a grounded system at the
separate building or structure, an equipment grounding conductor
as described in 250.118 shall be run with the supply
conductors and be connected to the building or structure
disconnecting means and to the grounding electrode(s). The
equipment grounding conductor shall be used for grounding
or bonding of equipment, structures, or frames required
to be grounded or bonded. The equipment grounding
conductor shall be sized in accordance with 250.122.
Any installed grounded conductor shall not be connected
to the equipment grounding conductor or to the grounding
electrode(s).

there you go no rod necessary. the egc is bonded to the frame and the panel. having unneccessary ground rods can cause other problems. so, the 'just throw money at it' plan is not always the best.
 
iwire said:
The number of poles is not what matters, a feeder is any circuit that has breakers down line. (See the definition of Feeder and Branch circuit in Article 100) Your suppling panels in the trailers so they are feeders.




Great question, one that I really can't give you great answer to. 250..4(A)(1) tries to answer that question.





Yes, any time you supply a separate building or structure you must treat it like a service except you do not bond neutral to ground you bond the EGC to ground.
Click to expand...
This is soooooooooooooo misunderstood .
 
090527-2118 EST

iwire:

It is true that if you inject current into a vertical monopole ground rod that you will find the voltage drops very quickly as you move radially from the rod.

If you go to a long horizontal rod, such as a water line, then the drop won't be as rapid. Also the voltage in the earth relative to the long horizontal rod will be very much dependent upon the source of the ground current.

Suppose the water pipe is 20 ft away from the trailer and parallel to the trailer. Now put the current injector further away, like 40 ft from the trailer, thus 20 ft from the water line, and centered on the middle of the trailer. Make the current injector a horizontal rod 20 ft long. Virtually all of the current will flow between the water line and the injector. Very little current from the injector will be in the earth close to the trailer.

.
 
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