Connecting a Secondary Distribution Panel


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Old 01-19-04, 08:00 PM
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Connecting a Secondary Distribution Panel

Hi everyone. Our house has a 200 amp service located in the attached garage. Before the house was built, a separate garage was built with a 100 amp service (and it's own meter). To not have to pay for 2 hookups and have 2 electric bills each month, I did not have the electric utility restore service to the unattached garage. I figured I would just run power from the 200 amp service to it (which I was told no problem by the utility), and they removed the meter and capped off the lines.

To get power out there, I installed a 100 amp breaker in the house service, and ran wires buried in conduit to the main breaker on the 100 amp service. I ran 3 wires, had everything inspected, and all was OK. I'm now told there should have been 4 wires, and not 3. I'm a bit confused here - where would the fourth wire be connected? And does what I did have sound logic? This was done about 7-8 yrs ago.

Thanks for any help!

Jeff
 
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Old 01-20-04, 07:42 AM
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The fourth wire would be for the neutral conductor. The neutral can be bonded to ground at exactly one place in a service. This is usually done in the main panel or at the meter base. In order to rectify the situation, you should run a 4th wire to the garage, separate the grounds and neutrals in the subpanel. Unbond the neutral bar from ground (install a separtate ground bar for the bonded grounds). You should already have a ground rod driven at the garage from the previous service.
 
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Old 01-20-04, 09:03 AM
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If you have required conductors for a 3-wire 120/240 volt Feeder to a structure detached from the stucture where the Service is located, then the " 4th-Conductor" referred to seems to be an Equiptment Grounding Conductor.

There are Code Articles relevant to your situation, and if the "4th Conductor" is indeed the EGC, please inform us and we will submit the necessary Code requirements so that you can make Grounding connections that are Code-compliant.

Good Luck and Enjoy the Experience!!!!!!!!!
 
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Old 01-20-04, 11:06 AM
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The fourth wire should be a ground, not neutral.

Do you have any other lines (phone, cable, water, gas) running to the garage? If not you may not need a 4 wire feeder. If so you do need the 4th feeder, although I'm not sure how the code was when the original installation was done. Either way you'll need a ground rod at the garage, and probably separate neutral/ground bus bars.
 
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Old 01-20-04, 03:52 PM
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I'm kind of in unfamiliar territory here, so please forgive my questions! The detached garage has both water and gas going to it from the same supply for the house. The ground rod is present from the previous service.

Scott/Trinitro, I think what you are saying is to run the 4th wire from the main neutral/ground to the unattached garage ground rod, and separate the neutral bus there from the ground? I'm not seeing what this is going to do. What is the real intent of the 4th wire, if it is still attached to both the ground and neutral at the main box? It seems that if it is to isolate some type of errant voltage that gets to the neutral bus, isn't it still going to travel along all other neutral wires?

Pattbaa, kinda foggy about your question also. The garage is not used for any large machinery, just lights, outlets, a 110V air compressor, and a 110V wire welder. The previous owner had a 220V outlet that was wired wrong (using two 30A breakers) that I disconnected and have not hooked back into the box. I also got rid of those breakers just because.

Thanks for the help! And remember - you gotta be patient with us mechanical-type guys!
 
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Old 01-20-04, 04:24 PM
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Let's not get equipment grounding and panel grounding mixed up. So let's take one at a time.

For the Equipment Grounding Conductor (EGC):
  • You need a new EGC between the two buildings. This needs to be pulled through the same conduit as the three wires you already ran. It can be bare or green insulated, and it must be #8 copper or larger.
  • At both panels, the EGC connects to the grounding bar.
  • It is likely that you have no grounding bar in the detached garage panel, so you will probably need to buy and install one.
  • Then you need to remove the bonding screw (usually green) or strap that connects the detached garage's neutral bar to the panel case.
  • Then, in the detached garage panel, you need to move all branch circuit grounding wires from the neutral bar to the new grounding bar you installed.
For the panel grounding:
  • You need a Grounding Electrode Conductor (GEC) from the grounding bar of your detached garage panel grounding bar to the grounding rod. This should be #6 copper or larger.
  • This GEC also needs to run to the underground water line, and be connected to it within five feet of where it enters the building.
Let's make sure you're clear on what needs to be done first. We can explain the why later. The why is a very complicated thing, and it may take many words to cover it.
 
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Old 01-20-04, 06:43 PM
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Sorry, but I think I'm going to need step-by-step to understand this one. I did mis-state one item in my original post, so let me try to be clearer here:

Main Distribution Box on house (approx 10 years old)
1. 200 amp service
2. 100 amp 2-pole breaker to feed secondary box. 1 wire from each leg of the breaker going out hot, and 1 wire connected to neutral bus. There is a neutral/gounding bar on each side of the box, are these connected to each other? It has white insulated wires and bare ground wires attached to each. There is 1 wire connecting one of the bars to the ground rod.
3. Neutral/grounding bus connected to buried ground rod.
4. Bus is bonded to the panel case with bare wire.

Secondary Distribution Box on Garage (approx 20 years old)
1. 2 hot wires coming in to a 50 amp 2-pole breaker (main breaker)
2. Neutral wire coming in to neutral bus
3. Neutral bus connected to buried ground rod with #6 awg. I do not see any type of screw or strap attaching the neutral bus to the panel case.

What I need to do:
1. Connect a wire to the neutral bus at the main box, and run it through the conduit to the garage.
2. Install a ground bus in the garage box, and connect the wire to it.
3. Bond the new grounding bus to the panel case.
4. Remove all branch circuit grounding wires from neutral bus and attach to new ground bus. Leave branch circuit neutral wires attached to nuetral bus.
5. Remove wire going from neutral bar to the ground rod.
6. Attach new grounding bar to ground rod. Attach wire also to supply water line.

Now, correct me if I'm wrong, but I should now have in the garage:
1. 1 wire from main box neutral bus going to the garage neutral bus.
2. Neutral wires from branch circuits going to neutral bus.
3. No wire from neutral bus to ground rod.
4. All circuit neutral wires and ground wires going to the new ground bus.
5. Ground bus connected to ground rod and water line.

Once again, sorry for drawing this out, and thanks for your patience!

Jeff
 
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Old 01-21-04, 07:03 AM
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All you need to do in the main panel (house) is connect the newly ran #8 ground wire to the neutral/ground bar. In the subpanel (garage) you need to buy a ground bar (looks just like the neutral bar). Any ground connections presently connected to the neutral bar need to be moved over to the new ground bar (if you use metal conduit you probably won't have many grounds). the newly ran ground wire along with the present ground wire going to the rod need to be connected to the new ground bar. You most likely have a bonding screw/strap installed. It will basically connect the neutral bar to the box. SqureD boxes use a screw, Siemens boxes usually use a strap. I'm not sure of the other brands, I don't use them very often.

What you are trying to do is isolate the ground and neutral in the subpanel. The purpose of the 4th wire you need to run is to equalize the ground level between the main panel and the subpanel.
 
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Old 01-21-04, 08:50 AM
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This may be an issue of appying the Code retro-actively.Your installation may have been Code compliant at the time it was done. This qoute is culled from the NEC 2002 handbook-----

"In previous editions of the Code the Grounding Electrode Conductor was permitted to be connected to the Grounded Conductor ( Neutral, White wire) at a seperate structure.(But) this connection could provide a parallel path for the Neutral current along metallic piping" (Between the 2 structures).

You could use the Grounded Feeder Conductor for Equiptment Grounding if " (2), there are no continuous metallic paths bonded to the Grounding sytem in both structures." ( Art 250.32).

This is the complication----- as best I know, the Grounding Electrode alone is not suitable as an Equiptment Grounding Conductor for the seperate structure. If you bonded the Grounded Feeder Conductor (Neutral) to the Grounding Electrode Conductor and there was a "break" in the Neutral connection at any point between the 2 structures, the Neutral current would be conducted thru the Grounding Electrode and thru any metallic piping back to the "source".

I believe that you are justified in pointing out that your
installation did indeed confrom to the Code Grounding requirements that were in effect at the time.

What authority/person/ inspector is contending that you have a Code violation that must be corrected because it does not comply with the present Code?

Good Luck!!!!!!!!!! ( can't urge you "to enjoy the experience" in this circumstance)
 
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Old 01-21-04, 09:32 AM
charlie b
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Others have discussed what to do. I will add a bit of the "why." First, at both the main panel and the garage panel, the following will be true:

(1) Voltage from “left side hot” (call it “black”) to “right side hot” (call it “red” for now, even if the wire is also black) will be 220 volts.
(2) Voltage from black to “neutral” (call it “white”) is 110 volts.
(3) Voltage from red to white is 110 volts.
(4) In order to run a 110 volt load (e.g., receptacle outlets), you need one “hot” (could be red or black) and one neutral (white). The hot wire attaches to the breaker in the panel, and the neutral wire attaches to the neutral bus.

Here’s where the main panel and the garage panel are different:
(1) At the main panel, any “Equipment Grounding Conductors” (this is the formal, correct name, but it is also called “ground,” or “safety ground,” or “green,” or “bare wire”) can be connected to the same bus as are the white neutral wires. It's best to connect white wires to a neutral bus and green wires to a ground bus, but the two busses are tied together at the main panel anyway.
(2) At the garage panel, any green wires must be connected to a ground bus that is electrically isolated from the neutral bus.

BOTTOM LINE: AT THE GARAGE PANEL, DO NOT LET THE WHITE TOUCH THE GREEN!

Looking at your last post, under the “correct me if I am wrong,” your item #4 is wrong. You said “neutral wires and ground wires going to the new ground bus.” That must not happen at the garage panel. Neutral (white) wires go to one bus, and ground (green or bare) wires go to a different bus, and the two busses must not be connected to each other.

The simple answer to why the two panels are different comes from looking upstream to the next component in line. At the main panel, the next thing upstream is the serving utility. Current is trying to make its way back to the source, and once it reaches the main panel, it is nearly there. At the garage panel, the next thing upstream is the main panel. If the neutral and ground are tied together at the garage panel, then neutral current leaving the tool in your hand, and traveling back to the garage panel, can go backwards through the green wire, which is attached to the case of the tool, and then make its way through your hand, through your feet, into the concrete floor, through dirt, back to the ground rod at the main panel, and finally back to the source. Not good. So please re-read the sentence shown above in all caps.
 
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Old 01-21-04, 07:53 PM
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I understand now what needs to be done. As far as the "why" part, I think I'm going to have to read that a couple of times!

As far as someone questioning the way it is now, this topic came up from a co-worker wanting to do the same thing to a garage/workshop. I explained to him what I did, and we were overheard by another who said that it didn't sound like the way it should be done, and he started explaining the 4-wire approach. Up to now, I had not heard of single phase 4-wire. With the way I am paranoid around electricity, I figured I may as well ask the question here to see if I did it right. And if not, I've got no problem changing it. I figure if this is the code now, it's for a reason (which probably gave someone else a very bad day). This will not be a large expenditure in cost or my time to correct it, and if it's what I should do to protect me and mine, it's a moot point.

I guess I'm just having a hard time understanding what is actually wrong with the way it is right now, with 3 wires. I keep thinking that if anything happens at either box, they are both protected with a ground rod, and this is the path that any backward current will take? How is this different from having no sub-panel, just the main panel on the house, and a 3-wire electric stove or dryer? (Doing searches on the web, I've noticed that these are now all 4-wire cords, too!). Is it due to the fact that there is a separation distance between the 2 panels?

Thanks to ALL for the help here! Like I said, I'll probably read the posts about 10 times, then it'll hit me in the head and I'll say Oh, now I get it!
 
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Old 01-22-04, 05:33 AM
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3 wire vs 4 wire to subpanel

It may help for you to think of things from a slightly different perspective.

1) Instead of thinking of current flowing backward through the grounding system, think about how it flow normally in the wires.

2) Let go of the idea that the connection to earth ground is what protects you from electric shock. The connection to earth ground is for protection from transient overvoltages (everything from lightning to static electricity from walking across the carpet, to contact with high voltage supply lines). A properly designed system with _no_ ground rod will still have functioning breakers and still protect you from electric shock (think about it: circuit breakers and electric power work in _airplanes_, and I've never seen an airplane with a permanently installed ground rod

In normal operation of your electrical system, current should flow on a well defined set of conductors. Say for a single 120V circuit, current should flow on the 'hot' and the 'neutral'. Since AC is flipping polarity 120 times a second, as far as an isolated load is concerned (say a lightbulb just sitting in the circuit), 'hot' and 'neutral' are exactly the same. For an isolated load you could swap 'hot' and 'neutral' and everything would work the same. In normal operation, current flows in the 'hot' and the 'neutral' wires, passing through the load. The current is not supposed to flow anywhere else.

For reasons of transient voltage protection, the electrical system is electrically connected to earth ground. It is this connection that makes 'hot' and 'neutral' different beasts. (Note for the terminology buffs: the term 'neutral' comes from the connection to the transformer, _not_ from the connection to earth. There are good reasons to make your transformer neutral the earth reference, but this is not required, and in fact there are several electrical systems in which the transformer point connected to earth is _not_ the neutral. In fact, the terms used in the NEC are 'grounded conductor' and 'ungrounded conductor' to describe the wires which are supposed to be carrying current, and to distinguish between the current carrying conductor tied to ground and the current carrying conductor not tied to ground.) Because the 'neutral' is connected to earth ground, you find that the 'neutral' is always at very low voltage relative to things like water pipes and the like. This is useful, for example, with light fixtures, where you want the screw shell to be at low voltage relative to nearby metal objects.

However, while the 'neutral' is directly and explicitly tied to ground at the service panel, you want to make sure that they are separate everywhere else. The reason for this is that you want to make certain that the current flows in the 'neutral' conductors, and _not_ in the metal pipes or building frame or any other random bits of metal in your house. Thus the 'neutral' is supposed to be tied to ground at one point and one point only. Sometimes people say that electricity follows the path of least resistance. This is not true. Rather electricity follows _all_ paths in inverse proportion to their resistance. This means that if you connect a neutral wire to ground somewhere in the system, some current will continue to flow in your neutral wire, and some will flow in your grounding system. This current flow in the grounding system can cause damage (corrosion in pipes, for example), and can cause shock (say when a plumber cuts open a pipe).

Thus you tie your 'neutral' conductor to your grounding system at the main panel, but no where else. At a subpanel, you have to supply _as separate conductors_ your 'hots', your 'neutral' and your 'equipment grounding conductor' (blacks, white, green). In the subpanel, you have to have a separate bus for each, and you have to make certain that there is no connection between the 'neutral' (white) and the 'ground' (green).

Now, with your detached garage, you bump into an exception to the 'prevent parallel neutral paths' rules. The reason is that _as a separate detached structure_ you can sometimes treat the subpanel in that structure just like a main service panel coming into a house. In other words, you have a local grounding electrode system, and the neutral and ground are tied together. Under current code, the requirement for this sort of installation is that there be no metallic connection between the two structures; no metal conduit, no water piping, no fence, etc. If there were a parallel metallic path, then some fraction of the neutral current would flow in that parallel path, and could present a hazard. Under previous versions of the code, you might have been able to treat the subpanel in the detached structure as a service panel even if there were a parallel path.

Now, if you have been reading closely, you will say 'but wait, the earth itself is a conductor', and that even having the ground rod in the detached structure will mean that _some_ current will flow in that parallel path. You are correct! However such current flow through the earth is generally not considered a problem (though this is actually a contentious issue; current leakage is sometimes a problem on farms.)

Finally, you may ask which is better. When you wire a subpanel with a 4 wire feed and a separate ground, you are reducing the effectiveness of the 'neutral' to 'ground' connection for protection from transient overvoltage, but are increasing the safety with respect to parallel conductive paths. The exact trade-off will depend upon factors like distance, other metallic systems, etc. One can't really say which is better without significant engineering analysis...but IMHO for a detached garage close to a house I would go with a 4 wire system.

-Jon
 
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Old 01-22-04, 05:46 AM
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That's the best explanation I've seen so far... not even my teachers in college were able to come up with something like this..
 
 

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