Subpanel Requirements

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  #1  
Old 10-13-05, 04:53 PM
tngoodguy
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Subpanel Requirements

The main service panel in my house is in an interior room, about 20 feet from the wall where the meter base and exterior disconnect are mounted. I read in a "Do it yourself" book that this panel should be treated as a subpanel because it is not located directly at the service entrance.

When I take the cover off the panel, I see that all the common (white) wires are connected to the bus on the right, and all the ground (bare copper) wires are connected to the bus on the left. The black and red wires are all connected to breaker switches. From what I read, there is a problem if this is a subpanel, because both the common and ground buses are joined together by a metal strip at the bottom.

Is this in fact a problem, and if so, why is this a problem? What could happen as a result? And most importantly, what do I do about it?
 
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  #2  
Old 10-14-05, 06:49 AM
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Originally Posted by tngoodguy
The main service panel in my house is in an interior room, about 20 feet from the wall where the meter base and exterior disconnect are mounted. I read in a "Do it yourself" book that this panel should be treated as a subpanel because it is not located directly at the service entrance.

When I take the cover off the panel, I see that all the common (white) wires are connected to the bus on the right, and all the ground (bare copper) wires are connected to the bus on the left. The black and red wires are all connected to breaker switches. From what I read, there is a problem if this is a subpanel, because both the common and ground buses are joined together by a metal strip at the bottom.

Is this in fact a problem, and if so, why is this a problem? What could happen as a result? And most importantly, what do I do about it?
Any idea as to what year the electrical service, etc., was installed? How many wires were installed from the main disconnect to the other panel?
Are those wires installed in metallic conduit?
 
  #3  
Old 10-14-05, 08:46 AM
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Originally Posted by tngoodguy
The main service panel in my house is in an interior room, about 20 feet from the wall where the meter base and exterior disconnect are mounted. I read in a "Do it yourself" book that this panel should be treated as a subpanel because it is not located directly at the service entrance.
Does this panel house the main breaker for your service or is there a main breaker outdoors or otherwise near the meter? This is the distinguishing characteristic between a main and a subpanel. A main panel houses the main disconnect, and there can be only one per structure.

Today, a panel 20 feet inside the building would probably be installed as a subpanel by putting the main breaker outside near the meter, however things were done differently in the past so you could have a main panel some distance into the structure.
 
  #4  
Old 10-16-05, 07:09 AM
tngoodguy
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Thanks to both of you for responding. To answer your questions, I think this was done somewhere in the 1975-76 range, but not sure. That is when this house was added on to and a significant amount of remodeling done.

There is in fact a 200 amp breaker switch in a metal box immediately below the meter base on the outside wall, with metal conduit between the two and then conduit below the disconnect going into the crawl space under the house. The single cable (looks like 4/0 AL SE) under the house is not in conduit. It just goes through the floor joists and up through the flooring to the breaker panel. The breaker panel also has a 200 amp main breaker, so there are two main disconnects, which from your response sounds like a problem.

My question really is this -- is it a problem that the ground and common buses are connected, and what could this cause? Should I just remove the connector between the two?

Thanks again.
 
  #5  
Old 10-16-05, 07:42 AM
ollie
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tngoodguy
The 200A breaker in the disconnect switch, under the meter. are there to protect the feeder to the panel. The 200A breakers in the panel are not necessary but are not a problem. As for the rest ???
Ollie
 
  #6  
Old 10-16-05, 09:36 AM
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My first NEC code book was 1975 and....

I threw it away!!

I'm going out on limb here but I don't think a grounded conductor can be used as the equipment grounding conductor from the service disconnect to the subpanel per the 1975 NEC (the time the house was remodeled).

The general rule is the grounded circuit conductor cannot be used as an equipment grounding conductor on the load side of the service disconnect (2002 NEC Article 250.142 Use of Grounded Circuit Conductor for Grounding Equipment). There are exceptions, of course, but in your case they don't apply.

Is there an electrical inspection sticker located in the service disconnect or the subpanel?
 
  #7  
Old 10-16-05, 01:56 PM
ollie
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After reading you post again I don't think this is a sub panel. The main disconnect is out side it is feeding the main panel inside. There's nothing wrong with this setup. In the main panel, and only in the main panel, the bus bars are connected together to provide a low impedance path for clearing ground faults. If the bus bars were not connected together there would be a high impedance path for ground faults to clear, a very dangerous condition.
Artical 250.142 does not apply in this case. I only prohibits using a grounded (white) wire as an equipment ground (bare or green wire) down stream of the main panel. At one time this was allowed, no longer.
Ollie
 
  #8  
Old 10-16-05, 02:52 PM
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What am I missing?

Originally Posted by tngoodguy
There is in fact a 200 amp breaker switch in a metal box immediately below the meter base on the outside wall, with metal conduit between the two and then conduit below the disconnect going into the crawl space under the house.
This is your service disconnect.


Originally Posted by tngoodguy
The single cable (looks like 4/0 AL SE) under the house is not in conduit. It just goes through the floor joists and up through the flooring to the breaker panel. The breaker panel also has a 200 amp main breaker, so there are two main disconnects
This the panel inside your house, correct??

How many wires are there in the single cable (looks like 4/0 AL SE)?
 
  #9  
Old 10-17-05, 05:23 AM
tngoodguy
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Thinman:

Yes, the cable goes up through the floor into the panel inside the house.

Well, you asked the right question about number of wires. I didn't recall how many wires were in the cable, so I took the panel cover off to see. Lo there are three wires, two black and one neutral (bare). All three are multi-strand, but the neutral strands seem to be woven around the two conductors inside the sheathing, which at the panel end are pulled and twisted together to be attached to the lug. Anyway, this neutral goes into a lug which is connected to both buses on either side of the breaker array, which is the metal strip I referred to earlier. Had I really looked the first time, I would have seen there is no way to remove this. So, all of this means to me that the buses MUST be connected. This is one of those things that I just didn't comprehend when I first looked at the panel.

Thanks to everybody for your help, and please respond if my understanding is incorrect or I have missed something else.

P.S. - I am still curious. Forgetting about my situation, on a subpanel fed from a main panel, what is the reason the ground and common buses should not be connected? What could happen if they were?

Thanks again.
 
  #10  
Old 10-17-05, 05:27 AM
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When you have a sub panel, there is current flowing between the main panel and the sub panel on both hot wires and on the neutral (the neutral current being the difference between the current amounts on each of the other two wires). If you have the neutral and the ground wire connected at each end then you will have current flowing on the ground wire, which is something you do not want.
 
  #11  
Old 10-17-05, 07:59 AM
ollie
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In addition to what recraft said,
The ground system is a safety system in case of a ground fault, that is electricity going where it's no supposed to, like the frame of the washer. For this system to work it must have a low resistance so the fault will generate enough current to clear the fault (electric speak for turning off the circuit) at the main panel. To accomplish this the grounding system must not only take the shortest most direct route back to the main panel, it should also have the lowest resistance as possible so it can generate current high enough to trip the breaker. Connecting ground to neutral in sub-panels would increase resistance and create multi paths in the grounding system, defeating the purpose of the system.
If you want more info on grounding and wiring "Wiring Simplified" is a good book to start with.
Ollie
 

Last edited by ollie; 10-17-05 at 09:51 AM.
  #12  
Old 10-17-05, 10:28 AM
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Originally Posted by tngoodguy
Thinman:

Anyway, this neutral goes into a lug which is connected to both buses on either side of the breaker array, which is the metal strip I referred to earlier. Had I really looked the first time, I would have seen there is no way to remove this. So, all of this means to me that the buses MUST be connected.
I disagree. I'll bet dollars to donuts that the rule prohibiting use of the grounded circuit conductor (neutral) for grounding equipment, etc., after the service disconnect (load side) WAS in effect in 1975.

You're sure that the metal strip cannot be removed? Who made the sub panel? Is there a Cat No. inside it somewhere?
 
  #13  
Old 10-18-05, 09:18 AM
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In this case, the cable needs to be replaced with a 3 wire +bare cable, and the bus link removed, to comply with modern codes.
 
  #14  
Old 10-19-05, 01:55 AM
tngoodguy
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ollie and racraft, thank you -- this is understandable. This was really part of my question.

thinman -- there is a sticker inside the panel that says "ITE Circuit Breaker Company", then "Catalog Number EQ40MBB", "1 Phase 3-Wire 200amp". I can assure you that the bus connector cannot be removed without destroying the innards of the panel. It may be worth noting that the part of my house where this panel is located was originally built in 1966, with very significant addition and remodeling in the 1975-76 range.

classicat -- I think you are sharing the bottom-line answer, which I would do in a hearbeat if my existing set-up poses danger. It looks to me that to replace the cable with 3-conductor plus ground would require a new sub-panel, along with assuring proper grounding at the meterbase/external disconnect.

So, is this a dangerous situation?

Thanks again to all.
 
  #15  
Old 10-19-05, 04:07 AM
ollie
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tngoodguy
I don't think you have a problem. But for peace of mind spend a hundred bucks and get a electrican in to check the panel. Not the guy down the street who knows electricty but a real sparkey.
Ollie
 
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