Subpanel grounding

[bnd]

I have a house that was feed over head from a remote pole service to the old type service entrance 3 wire cable which ran into the main panel. This panel was grounded with a rod. Later an addition was added at the point where the over head drop attached to the house so the drop was relocated to the far end of the new addition and a new main panel was added at this point. It to was grounded with a rod. This now made the original panel a subpanel and they feed it from the new panel with 3 wire ( 2 hots/1 n ).My ? is,#1 does code require a ground wire bonding between rods and panels.#2 If so is the separation of the grounding bus & the neutral bus required in the subpanel.#3 Can the subpanel be treated the same as a subpanel noted in 250.32/A of the NEC which allows either a grounding rod or a pulled bonding wire.
My inspector wants home owner to pull in a bonding wire and I fell they are safe with the rods at each panel.

 

[hornetd]

Re: Subpanel grounding

In order to guard against the possibility of a high impedance condition on the neutral conductor between two panels located in the same building the separate Equipment Grounding Conductor (EGC) is required. With a separate EGC and the neutral of the feeder kept aloof from ground at all points beyond the service disconnect you can avoid having neutral currents flowing on interior metal piping and other metallic systems and structural components that are not designed to carry current. The code permits the use of the second ground rod at the feeder supplied panel but it may only be connected to the EGC at that point and not to the grounded conductor.

250.54 Supplementary Grounding Electrodes.
Supplementary grounding electrodes shall be permitted to be connected to the equipment grounding conductors specified in 250.118 and shall not be required to comply with the electrode bonding requirements of 250.50 or 250.53(C) or the resistance requirements of 250.56, but the earth shall not be used as the sole equipment grounding conductor.

#1 does code require a ground wire bonding between rods and panels.

The Code requires an EGC between the two panels that is run with the feeder conductors in the same raceway or cable.

#2 If so is the separation of the grounding bus & the neutral bus required in the sub panel.

Yes, you must keep them separate.

250.24 Grounding Service-Supplied Alternating-Current Systems.
(A) System Grounding Connections. A premises wiring system supplied by a grounded ac service shall have a grounding electrode conductor connected to the grounded service conductor, at each service, in accordance with 250.24(A)(1) through (A)(5).
(5) Load-Side Grounding Connections. A grounding connection shall not be made to any grounded circuit conductor on the load side of the service disconnecting means except as otherwise permitted in this article.

#3 Can the sub panel be treated the same as a sub panel noted in 250.32/A of the NEC which allows either a grounding rod or a pulled bonding wire.

The answer to this question is contained in the title of the article you are asking about. It reads "250.32 Two or More Buildings or Structures Supplied from a Common Service." You are not talking about two buildings but rather two sections of the same building. Even if it were two buildings it would need to be free of any other metallic pathways between the two structures in order to be acceptable without an EGC that is run with the feeder in the same raceway cable or trench.

It is also worth noting that, in the case of separate buildings, if there is more than a single branch circuit to the second building it is required to have a grounding electrode system regardless of whether or not an EGC was run with the feeder. So it is not a choice between a "either a grounding rod or a pulled bonding wire" but rather a choice between whether or not to run an EGC to a separate building that is free of any metallic pathways between it and the building in which the feeder supplying it originates.
--
Tom

[ May 03, 2003, 03:58 PM: Message edited by: hornetd ]

 

[bnd]

Re: Subpanel grounding

Tom
Thanks,
The problem I have is that house is a flattop with a slab floor so getting a EGC between panels well be costly.The owner has little money and is very old so I'm tring to make it as safe as posible for her and still stay some what in the scope of the code. We sometimes must balance the 3 C's- Code/Cost&Commom Sense.
Bob

 

[tom baker]

Re: Subpanel grounding

Remember the grounded neutral conductor is a current carrying conductor. If you reground the neutral at a subpanel, some of the neutral current will flow on any conductive metal. Conduit connectors are not designed to carry current, they will heat and cause the wood to dry out and start a fire.
Mike Holt has a video "open neutrals" that shows how this happens in houses with the power shut off, several duplexes had fires from low voltages of 10-15 volts for long periods of time from pyrophoric carbonization. You can order the video from his office its about $20.

 

[bnd]

Re: Subpanel grounding

Tom
If I treat the second panel as a subpanel as required and separate the ground and neutral bus, this well put the EGC on the subpanel ground rod and put the neutral on the main service grounding system (the water line is also bonded at that point). Since there are no metallic ties between the main part of the house and the addition (the house is wired in romex), Does not that give me the required fault to ground protection needed at the subpanel and is not that the purpose of the prescribed forth wire (EGC). I feel that the ground rod at the subpanel would better service the purpose of fault dispersal then a long EGC conducter. Where am I wrong?
Thanks for the heads up on video-I well get one.
Bob

 

[david]

Re: Subpanel grounding

250.4 General Requirements for Grounding and Bonding.
(A) Grounded Systems.
(5) Effective Ground-Fault Current Path.


The earth shall not be used as the sole equipment grounding conductor or effective ground-fault current path.

David

 

[don_resqcapt19]

Re: Subpanel grounding

Bob,

I feel that the ground rod at the subpanel would better service the purpose of fault dispersal then a long EGC conducter.

A ground rod will almost never be able to clear a fault. The inpedance of the earth will limit the current to a level too low to open the overcurrent protective device. When I teach a grounding class, sometimes I have them install a 10' rod in the yard. We then connect to the hot wire from a 15 or 20 amp circuit to the rod and put a amp-clamp on the wire. It has never tripped the breaker and the most current I've ever read on the meter was 7 amps and that was after 3 days of heavy rain. The grounding electrode system is only intended to help provide protection from faults outside of the system, like lightning or a high voltage line contacting the 120/240 volt lines.
Don

 

[hornetd]

Re: Subpanel grounding

If you read the NEC section I have quoted below very carefully you will see that the purpose of grounding is not to clear faults within the grounded system but rather to limit the voltage to ground during a fault, do to lightning, or during accidental contact with distribution lines. Items three and four accurately describe the circuits used to clear faults within the system as bonding. This is the reason that Don Ganiere, who goes by the nom de plume don_resqcapt19 on this forum, wrote a proposal that is now under consideration by several code making panels to change the term "Equipment Grounding Conductor" to "Equipment Bonding Conductor." This would be a tremendous step forward in changing the industry's understanding of the purpose of grounding.

"250.4 General Requirements for Grounding and Bonding.
The following general requirements identify what grounding and bonding of electrical systems are required to accomplish. The prescriptive methods contained in Article 250 shall be followed to comply with the performance requirements of this section.
(A) Grounded Systems.
(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.
(2) Grounding of Electrical Equipment. Non?current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials.
3) Bonding of Electrical Equipment. Non?current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
(4) Bonding of Electrically Conductive Materials and Other Equipment. Electrically conductive materials that are likely to become energized shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
(5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be used as the sole equipment grounding conductor or effective ground-fault current path." ?2002 National Fire Protection Association.
--
Tom

[ May 08, 2003, 04:14 PM: Message edited by: hornetd ]

 

[bnd]

Re: Subpanel grounding

Tom
The bulb is becoming brighter but not to full glow yet .
I understand what you are saying and it goes against my understanding of the perpose of the ground. Can't beleive I went 30 years not knowing this.
Now that you have destroyed all my theories, tell me where does the fault dissipate to.
Bob

 

[don_resqcapt19]

Re: Subpanel grounding

Bob,
A fault is no different than any other circuit. The current must return to the source to have a circuit. In a grounded system with a ground fault, the current must have a path back to XO at the transformer.
Don

 

[dereckbc]

Re: Subpanel grounding

Bob, any fault is seeking the source. In the case of the AC service, a fault is looking for the grounded conductor or neutral conductor at the transformer. Don't be fooled, the neutral does not need to have a earth connection to clear a fault. A line to neutral fault completes the path. A line to ground fault is completed by the MBJ back to the neutral to complete the path.

 

[russellroberts]

Re: Subpanel grounding

Don't feel like the Lone Ranger Bob. I've been in it for almost 20 yrs. with very little formal training.

Since I've been visiting this and other forums,and learning volumes, I actually am understanding why to go with knowing how.

Russell

 

[bnd]

Re: Subpanel grounding

Thank guys for the input,
Now could someone tell me the difference between a subpanel in the same building and a subpanel in a outbuilding.
250.32/B1 as I read it, says if a EGC is ran with the feeders a separation of the ground bus & the neutral bus is required.
250.32/B2 says if a EGC is not ran with the feeders then the neutral is required to be bonded to the ground bus & the ground rod provided at the outbuilding.
Now how does 250.32/B2 deffer from the setup I have, where the subpanel is feed without benefit of a EGC and has a grounding rod?
Now Tom points to the possibility of a high impedance condition on the neutral between panels, is this not also a concern with 250.32/B2?
I know its hard to enlighten those of us flounder in the dark ;-).
Bob

 

[physis]

Re: Subpanel grounding

My understanding of the ground rod has always been that without it essentially the voltage coming off of the utility transformer is floating with respect to the earth. That means that although the output of the transformer is 240 volts with reference to itself, when referenced to the earth, if not tied to it somehow, it could be literally thousands of volts. So if your shoes are wet and you touch 110, it's not 110.

 

[bennie]

Re: Subpanel grounding

physis: An ungrounded transformer will have zero volts to earth. The only coupling is capacitance which at 240/120 volts is very low.

480 volts will indicate 277 volts on a high impedance voltmeter. Impedance will be very high, unlikely that any voltage will be measured with a wiggins type tester.

[ May 09, 2003, 07:53 PM: Message edited by: bennie ]

 

[physis]

Re: Subpanel grounding

Bennie,

That's only if there is never a connection to earth somewhwere. And the earth is electrical in nature itself, hence lightnig. I can't actually defend the assertion (thats why I posted it as my understanding). It's something I was told decades ago and it makes sense to me. I don't have a problem considering that the earth could be an unknown potential in series with a transformer.

[ May 10, 2003, 01:45 PM: Message edited by: physis ]

 

[bennie]

Re: Subpanel grounding

I,m sorry you've lost me.

 

[physis]

Re: Subpanel grounding

It has nothing to do with the original post either. I'll see what I can find on it and start a new topic.

 

[david]

Re: Subpanel grounding

?Now how does 250.32/B2 deffer from the setup I have, where the subpanel is feed without benefit of a EGC and has a grounding rod??

bnd,

It has to do with the path that is provided back to the source. In your sub-panel set up you ran a three- wire feed to a sub-panel, isolated the neutral from the equipment ground. Tied the equipment ground to a rod and the only return path that you provided between the equipment ground and the source was earth. Fault current would travel through earth until it found a less resistive way to the transformer.

Even though the code allows a three wire feed to a secondary building there is a required bond between the equipment ground and the grounded wire (neutral), fault current is being directed to the grounded wire as the return path back to the source.

Remember the code only allows a three-wire feed to an out building if there is no other metallic connection between the two buildings. You are attempting to direct current to one path back to the source


David

[ May 10, 2003, 01:05 AM: Message edited by: david ]

 

[david]

Re: Subpanel grounding

?Now how does 250.32/B2 deffer from the setup I have, where the subpanel is feed without benefit of a EGC and has a grounding rod??

bnd,

If you are referring to your original post, it has to do with the path that operating current is being directed back to the source.
You do not what to provide current a path to the bare equipment ground you are attempting to direct current to the insulated grounded conductor as the return path back to the source.You do not want current finding multiple paths back to the source. That is the reason you only bond the equipment ground to the grounded conductor at the service.

David