As I understand it, in a residential electrical system, the NEC requires that sub-panels have the neutral bus and the ground bus are separated. Separate conductors are provided for hots, neutral and ground, back to the main panel.
In the main panel, the branch circuits, including feeders for sub-panels, have all the neutrals and grounds tied together on the same bus. At least this is how the electrician wired my house (not a DIY).
If the ground and neutral are tied together in the main panel, what good is separating them “downstream” in the sub-panels?
Are outdoor subpanels treated any differently than indoor sub-panels (other than the weather resistence of the enclosure)?
Is there a requirement to add “local” grounding to sub-panels? And under what circumstances?
Replies
The basic principle is that within a structure that any metal that you come into contact with is at the same potential.
That is "ground".
The other basic principle is that if you have current normally flowing through a wire it will have a voltage drop.
So at the service disconnect the metalic water pipes are bonded to the panel. And the refigerator is also grounded to the same point via the equipment grounding conductor; ie the ground wire.
Thus if you touch the refigerator with one hand and the faucet with the other they are at the same potential.
However, if the refigerator is feed from a sub-panel and the grounds are connected to the neutral in the sub-panel then you would have voltage drop from the current in the neutral back to the main panel. So the refigerator and faucet won't be at the same potential.
Normally that difference would be too small to even notice. But if there becomes a high resistance connection on the neutral you can have enough difference to kill.
But if the in the sub-panel the neutral was isolated then there isn't any current in the ground from the sub to the main. Thus refigerator and faucet would have the same potential even if the neutral was fried.
Exterior panels for the same structure are treated the same.
"Is there a requirement to add "local" grounding to sub-panels? And under what circumstances?"
"Grounding" is not an exact term. Are you talking about a GROUND ELECTRODE SYSTEM?
A ground electrode system is not designed to affect the operation within a structure. It's purpose is for shunting surges from lightning.
There is no need for additional ground electrode systems within a structure.
A separate structure that has more than one circuit requires it's own ground electrode system.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Is it your opinion/belief/understanding that if the resistence is great enough,GFCI's will "nuisance" trip? Would the voltage drop cause enough difference in potential to be a "ground fault"?
So, aside from the theoretical question above, the answers are:
Sub-panels must have separate, isolated ground and neutral busses that remain separate back to the main distribution panel.
Outdoor sub-panels are treated as if they are within a single, continuous structure.
No extra ground electrode system(s) are required for sub-panels within the same structure.
"Is it your opinion/belief/understanding that if the resistence is great enough,GFCI's will "nuisance" trip? Would the voltage drop cause enough difference in potential to be a "ground fault"?"Has nothing to do with GFCI.The GFCI just compare current in the hot to current in the neutral for that circuit.A ground fault is where the current in the hot lead does not return via the neutral. To make up an example Lets say that you have a light bulb with 120 ohms resistance so that 1 amp flowed.Now the resistance of the neutral, either the neutral for the light bulb or neutral to the feeder for the sub-panel that the light bulb is feed from, goes up to 120 ohms.The current in the circuit is now 120/120+120) = .5 amps. But that is the same current in the hot or in the neutral.And if something was referenced to that neutral it would be 60 volts with respect to "ground"..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Bill,
Since we are on the topic... is it OK to tap into a subpanel to feed a subpanel in a detached building, assuming the building has its own independent grounding, and is it incorrect to connect the 1st subpanel ground in addition from the pass through sub to the second sub? or does every sub have to independently have to come off the main panel?
WSJ
Any sub-panel (in the same building or different) can come off anyohter panesl for residential applications.One problem that you get with sub-panels is that you no longer have demand factor. Residential calcualtions realizes that don't have all lgihts on all the time, all receptacels loaded to max, all burners and oven on hight all one, etc.But as you break it down, for example you have sub-panel for just a bathroom. They you need to size that panel and feed for the full 20 amps.Now with large industrail and commercial applications you get into the appropriate cooperation of overloads and their interrupting capacities. But that should not be a concern in residential..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Bill,
Thanks for the quick reply.
What I did awhile back was tapped into a 50 amp panel in my attached garage/workshop wired with 4 gauge cables from my main 200 amp panel to a ~25' 6 gauge run 40 amp panel in a detached building. Tapped off of and fed into 40 amp breakers at both ends plus two 8' grounding rods outside the building hooked to that panel with 6 gauge, plus the ground from the pass though sub also hooked up to the tapped panel.
A continuity check from the rods to my well/ground indicates they work. My question is the extra sub panel pass through (tapped) ground just redundant (safer?) or dangerous? ie. if the out building gets a lighting strike (I've had 3 to trees hit in 15 years...I live on a hill) am I better not to have that ground from the out building to the house connection, or do you think it's beneficial?
WSJ
Edited 7/15/2008 3:37 pm ET by WorkshopJon
As for as lightning you never know. Just too unpredicatable.In general, probably the safest for lightning, is farm distribution style.Then you haave the service on a pole. Then feeders to the different buildings radial style.But again you never know..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
If the ground and neutral are tied together in the main panel,
Bill gave a good explanation on the background, but I just wanted to add that WHERE the ground and neutral is tied together is decided by the local power utility. It may be the service panel, may be the meter socket, might even be somewhere else - but they are the ones who decree where it will be.
"Bill gave a good explanation on the background, but I just wanted to add that WHERE the ground and neutral is tied together is decided by the local power utility. It may be the service panel, may be the meter socket, might even be somewhere else - but they are the ones who decree where it will be."Not true, the NEC says the main bonding jumper SHALL be in the service disconnect enclosure. You may be thinking of where the ground electrode conductor lands. That can be anywhere from the service disconnect to the service point (usually the service head).
There will also be additional bonding in the meter base (the neutral lug is bolted to the can) and there are only 3 conductors upstream of that so everything is inherently bonded.
The since 2002 NEC ignores any neutral to EGC bonding on the load side of the service disconnect. Actually the only time it would matter anyway is if the nipple from the meter base to the service disconnect enclosure was metal. A lot of AHJs want to see a PVC nipple for that reason but since 2002 or so they would have a hard time citing a violation.
Not true, the NEC says the main bonding jumper SHALL be in the service disconnect enclosure.
That's actually consistent with what I said, and the practice of electric utilities. The question is 'what is the service disconnect' - could be the main breaker in the service panel, could be the meter socket. The meter socket is a service disconnect.
It is true that the utility decrees where the common bond point will occur - and if you violate their statement, you won't pass inspection and they won't connect you to the grid.
There will also be additional bonding in the meter base (the neutral lug is bolted to the can) and there are only 3 conductors upstream of that so everything is inherently bonded.
That's one arrangement, but not the only one. Local practice varies throughout the US - and again, the utility decrees how it is to be done, if you want to connect to the grid.
There are different types of meter sockets. The ones used in my area either isolate everything - so it's three wires in, three wires out - for utility A, or for utility B, which decrees that the meter socket is the common ground point, it's three wires in, four wires out.
Best advice is to check with your local utility regarding what THEY require - because they won't hook you up if you don't meet their requirements.
There might be a service disconnect in the meter socket enclosure (an all in one) but the meter socket is not the service disconnect.
There might be a service disconnect in the meter socket enclosure (an all in one) but the meter socket is not the service disconnect.
No, the meter socket can be a service disconnect. I've not seen a meter socket with a separate service disconnect, but they could be out there.
As specified in NEC, a service disconnect is simply a means to disconnect the service. It can be a fuse, a breaker, a meter socket - essentially anything that provides a method to disconnect service. In the case of the meter socket, the service is disconnected by removing the meter.
You will NEVER get an inspector to agree your meter socket is a disconnecting means, at least not a real inspector. Perhaps in a mobbed up place where the inspector works for bribes or some hick town where it is some mayor's idiot son in law you could.
I agree the Fire Department MIGHT pull a meter under a load in an emergency but they are pretty reluctant to do that these days, now that arc flash training is part of the program.
As I said in my top note, the PoCo usually bonds the neutral in the meter base but you have to bond it in the service disconnect enclosure by code. That is usually the main panel or a separate enclosure outside the house where the service entrance comes in. On the load side of the service disconnect neutrals and grounds are isolated. Up until 2008 there was an exception that made it legal to rebond the neutral if you went to a second building with a 3 wire feeder and no other metallic paths but that goes away as soon as your AHJ adopts the 2008
You will NEVER get an inspector to agree your meter socket is a disconnecting means, at least not a real inspector.
Not sure where you live or work, but I assure you that inspectors have no trouble with this at all. Granted, it's not all that common a situation, but in areas where the utilities do it that way, the inspectors have no trouble with it since it is NEC compliant.
NEC compliant???
How can you possibly say that is true?
Cite it.
Show me a listing for any meter base that says it is suitable as a disconnect.BTW the service disconnect is not the utilities area anyway., That belongs to the customer
Edited 7/15/2008 12:56 pm ET by gfretwell
"230.70 (B) Marking Each service disconnect shall be permanently marked to identify it as a service disconnect.""230.76 Manually or Power Operable
The service disconnecting means for ungrounded service conductors shall consist of one of the following:(1) A manually operable switch or circuit breaker equipped with a handle or other suitable operating means(2) A power-operated switch or circuit breaker, provided the switch or circuit breaker can be opened by hand in the event of a power supply failure"Having to break a seal and in some case remove a clamp is hardly Handle Like."230.79 Rating of Service Disconnecting Means
The service disconnecting means shall have a rating not less than the load to be carried, determined in accordance with Article 220. In no case shall the rating be lower than specified in 230.79(A), (B), (C), or (D)."Some use a meter socket that is rated for use as a disconnecting means..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
(1) A manually operable switch or circuit breaker equipped with a handle or other suitable operating means
Having to break a seal and in some case remove a clamp is hardly Handle Like.
I agree with you that it is not "handle like", but the utilities have been able to convince the regulating authority that it is "other suitable operation means".
What about a UL listing for one being used as a disconect?.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
What about a UL listing for one being used as a disconect?
Why would a service disconnect or a meter socket be UL listed? UL is a private company that offers voluntary testing of primarily consumer products. They test systems rather than components - so if they were to test the meter socket, they would test it with the meter, and might still not be willing to test it, since it is a component rather than a "system", by UL's parlance.
Used to be most consumer products were UL listed, but I have noticed in the last several years a lot of products that are not UL listed. Since it is not a requirement to have UL listing to sell a product, it appears companies are banking on the consumer misunderstandings about UL to save some money.
Regarding the meter sockets, they are standard Square D sockets. Used to be that the consumer bought them, but now the utility is supplying them. They require that they be stamped with the utility company approval (It's a label that says something like "this meter socket is approved by XYZ power and light for connection to their system") which implies they may be doing some further testing. I have a call into them to clarify what additional testing they do, but haven't gotten a response yet.
"Why would a service disconnect or a meter socket be UL listed? UL is a private company that offers voluntary testing of primarily consumer products. They test systems rather than components - so if they were to test the meter socket, they would test it with the meter, and might still not be willing to test it, since it is a component rather than a "system", by UL's parlance."COMPLETE ####.First I want to make it clear that I an using UL Listed in the generic sense. UL has several different approval programs that List, Certify, and other approval programs. So while I might have used the list the product might have been approved under a different system than. And the code does not reuire UL approvals. It can be approved by other organization, but UL does 99% of them.Now here is what the code requires."110.1 Scope
This article covers general requirements for the examination and approval, installation
and use, access to and spaces about electrical conductors and equipment; enclosures
intended for personnel entry; and tunnel installations.110.2 Approval
The conductors and equipment required or permitted by this Code shall be acceptable
only if approved.
FPN:See 90.7, Examination of Equipment for Safety, and 110.3, Examination, Identification,
Installation, and Use of Equipment. See definitions of Approved, Identified, Labeled, and Listed.110.3 Examination, Identification, Installation, and Use of Equipment
(A) Examination In judging equipment, considerations such as the following shall be
evaluated:
(1) Suitability for installation and use in conformity with the provisions of this Code
FPN: Suitability of equipment use may be identified by a description marked on or provided with
a product to identify the suitability of the product for a specific purpose, environment, or
application. Suitability of equipment may be evidenced by listing or labeling.
(2) Mechanical strength and durability, including, for parts designed to enclose and
protect other equipment, the adequacy of the protection thus provided
(3) Wire-bending and connection space
(4) Electrical insulation
(5) Heating effects under normal conditions of use and also under abnormal
conditions likely to arise in service
(6) Arcing effects
(7) Classification by type, size, voltage, current capacity, and specific use
(8) Other factors that contribute to the practical safeguarding of persons using or
likely to come in contact with the equipment(B) Installation and Use Listed or labeled equipment shall be installed and used in
accordance with any instructions included in the listing or labeling."Approved. Acceptable to the authority having jurisdiction.""Identified (as applied to equipment). Recognizable as suitable for the specific purpose,
function, use, environment, application, and so forth, where described in a particular
Code requirement.
FPN: Some examples of ways to determine suitability of equipment for a specific purpose,
environment, or application include investigations by a qualified testing laboratory (listing and
labeling), an inspection agency, or other organizations concerned with product evaluation.""Labeled. Equipment or materials to which has been attached a label, symbol, or other
identifying mark of an organization that is acceptable to the authority having jurisdiction
and concerned with product evaluation, that maintains periodic inspection of production
of labeled equipment or materials, and by whose labeling the manufacturer indicates
compliance with appropriate standards or performance in a specified manner.""Listed. Equipment, materials, or services included in a list published by an organization
that is acceptable to the authority having jurisdiction and concerned with evaluation of
products or services, that maintains periodic inspection of production of listed equipment
or materials or periodic evaluation of services, and whose listing states that the
equipment, material, or services either meets appropriate designated standards or has
been tested and found suitable for a specified purpose.
FPN: The means for identifying listed equipment may vary for each organization concerned with
product evaluation, some of which do not recognize equipment as listed unless it is also labeled.
Use of the system employed by the listing organization allows the authority having jurisdiction to
identify a listed product."Now I quickly found 2 POCO's requirements for meter sockets.From Consumers Energy."Important information & requirements:
òAll equipment must be UL listed and labeled"Niagra Mohawk pdf does not allow copy and paste, but it list 4 ANSI, UL, and NEMA standards and then stays that it must be approved and labeled by an AHJ accepted organization. How can they require this if the UL does not test COMPONENTS.And what about these COMPONENTS.I just went through my box of electrical parts and found breaker, raco box, Carlon plastic box, receptacles, GFCI, switches, and NM-B ALL WITH UL MARKIGNS.And what about thsi COMPONMENThttp://www.idealindustries.com/products/wire_termination/twist-on/twister.jsp"
Twister¯ Wire Connector
Twister
Features * Exceptional wire range capacity - only three sizes to stock
* Swept-wing design for comfort and greater leverage
* Live-action, square-wire spring
* Hexagonal shape allows connectors to be applied with standard nutdriver
* No pre-twisting required
* UL Listed and CSA Certified
* Reusable
* Shell rated for 105° C"And for NM cable connectors.http://www.arlcatalog.com/NM%20Cable/NM%20Cable%20Connectors%20Plated%20Steel.htm
'They have a graphic of the UL symbol..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
I think was unclear in my post. Let me try to clarify.
UL tests "systems", to use their terminology. So, if they test a breaker, for example, they test it in a particular service panel. They will then rate the breaker for use in that panel, but NOT for use in some other (untested) panel or configuration.
If I take a breaker to UL and ask them to test it, without providing a panel, they will say it is a component and they don't test components outside of a "system". If I supply a "system" or "subsystem" they will test it. So I can supply a service panel and breaker and they will test that as a subsystem, without requiring that I supply a house, a national electric grid, etc. :-)
My thought (without having called UL to confirm, but based on prior experience having products UL certified), was that they would decline to test a meter socket by itself, but would probably agree to test it if the meter were supplied as well, but only for use with that meter.
Hopefully that explanation clarified rather than confused :-) There is some subjectivity with what they consider a system and a component, and I have found them willing to listen to reasonable arguments about why a particular product should be considered one or the other.
It was interesting that you restricted it to a breaker which is part of a panel "system".But you ignored all of the examples that I gave of components.Such as a receptacle. Does it need to be tested with ever combination of a "system" that it can be part of with types of cables, different types of wire, different types and sizes of boxes, different type of coverplates, different types of cable/conduit connectors to said box.It quickly become 100,000 of combinations for one receptacle..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Such as a receptacle. Does it need to be tested with ever combination of a "system" that it can be part of with types of cables, different types of wire, different types and sizes of boxes, different type of coverplates, different types of cable/conduit connectors to said box.
I agree with you that UL's distinctions between "components" and "systems" seem arbitrary at times. I can only speculate about why they will test a receptable as a component, but a breaker as a system. My speculation would be simple practicality - it's not practical to test all the combinations of receptacles, boxes, wiring, etc. A breaker, however, can be readily tested for a small number of service panels.
We've never been able to get a definitive answer from UL on that question. When we submit something they say has to be tested as a system, sometimes we can convince them to test it as a component. It seems to me that their component/system distinctions are a vague and "gray", but that is just my opinion.
You are ignoring the orginal "problem".Meter sockets do have UL approvals..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Meter sockets do have UL approvals.
Do you have a particular meter socket in mind? I've not found meter sockets with UL approvals - the ones we use around here don't have UL approval.
Well as I SAID EARLIER and give you the names several POCO's have writen requirements that UL approved socket.And looking at the summary given by google there are many many more that do.But there are a few UL approved sockets.http://electrical.hardwarestore.com/14-51-meter-bases/general-duty-meter-socket-613650.aspxhttp://electrical.hardwarestore.com/14-51-meter-bases/individual-socket-meter-650958.aspxhttp://electrical.hardwarestore.com/14-51-meter-bases/meter-socket-horn-over-head-ug-654563.aspxhttp://kscdirect.com/item/MIL%2BU9701-RRL/MILBANK%2BMFG%2BCO_MILBANK%2B200AMP%2B3PH%2BUG%252FOH%2BMETER%2BSOCKET%2BLEVER%250Ahttp://www.doityourself.com/invt/1647809http://www.landisandgyr.com/NR/rdonlyres/6C11F9E6-F067-4A88-9359-0BE2884F742D/0/Pickett_NC_SC.pdf
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Well as I SAID EARLIER and give you the names several POCO's have writen requirements that UL approved socket.
What you posted earlier said UL listing was ONE option, not the ONLY option.
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From Consumers Energy."Important information & requirements:
òAll equipment must be UL listed and labeled""Is that is ONE option and the ONLY option..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Why is a utility even involved with the service disconnect in any way (beyond requiring one). The service disconnect belongs to the customer and that is where the NESC (the code utilities use) stops.
Edited 7/15/2008 10:57 pm ET by gfretwell