sub panel neutrals and grounds

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  #1  
Old 12-30-06, 10:57 AM
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sub panel neutrals and grounds

Just curious, whats the reason the grounds and neutrals have to be seperated in a sub panel? Wouldn't those wires come from the same ground/neutral bar in the main panel? I have an existing 30A 230v 3-wire circuit going to a detached garage for my air compressor. It is currently hooked to an old fused type safety switch and I would like to maybe install a sub panel with breaker. I understand if there is no metallic connection between the house and garage a 3 wire might work, is this true? Do I need a ground rod? The cable is 10 gauge underground feeder.

Thanks for the info
 
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  #2  
Old 12-30-06, 11:04 AM
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A sub panel in the same building as the main panel needs to have the ground and neutral bares separate because the neutral wires carry current. As such, the neutral bar is not at zero volts, but rather is slightly above zero volts. Using a proper four wire circuit would have the return current flowing on both the neutral and ground wirs if they are connected at the sub panel.

If you have an outbuilding where there are no metallic paths between the buildings then you are allowed a three wire feed to the out building. However, I do not recommend this. It;s too easy ij the future for you or someone else to ignore the rules (or be ignorant of them and the dangers by not following them) and add a water line, or telephone line, or whatever between the buildings.

A ground rod is required for an outbuilding with a sub panel regardless of whether it is a three or four wire feed.
 
  #3  
Old 12-31-06, 03:10 PM
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Would that be a 8ft ground rod with 10ga bare wire for a 30a circuit?
 
  #4  
Old 12-31-06, 03:13 PM
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That would be an 8 foot ground rod, but the cable needs to be #6 copper, if my memory serves me correctly.
 
  #5  
Old 12-31-06, 03:18 PM
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Thanks for all the info. HAPPY NEW YEAR!!
 
  #6  
Old 12-31-06, 06:51 PM
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If it is a detached building, you can use a 3 wire cable for 120/240 as stated above, plus the 3rd needs to be insulated, not bare.

If it is bare (with the exception of an overhead triplex), you cannot use it as a neutral.
 
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Old 01-01-07, 02:46 PM
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classicsat, you are saying that the wire from the ground rod to the panel needs to be insulated is that correct?
 
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Old 01-01-07, 03:00 PM
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NO, the neutral in the feeder cable must be insulated. I assume you have 10/2UF which is NOT legal or safe to use as a 120/240v feeder. You need THREE insulated conductors.
The 3-wire feeder exception is that you can omit the grounding conductor. A neutral is always required. With 10/2 you do not have enough insulated conductors.
 
  #9  
Old 01-01-07, 03:19 PM
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could you explain why that 10/2 w/ground would be a safety issue? That circuit has been powering my compressor for 14+ yrs. with no problems. Just trying to learn.
Thanks
 
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Old 01-01-07, 03:34 PM
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Because the 240v compressor is very different from a 120/240v feeder.

240v is two hots and a ground. At 240v your bare wire is a ground as it should be. It should never any carry circuit current. It should only carry fault current, which in the real world is almost never.

At 120/240 you need two hots, a neutral and ground. With a 10/2 you only have two insulated conductors, so either they are two hots or one hot and one neutral. If they are two hots all you are left with is a bare wire. The bare wire CANNOT carry regular circuit current.
 
  #11  
Old 01-01-07, 05:23 PM
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I understand what you are saying but what I really want to know is why the bare wire can't carry any current. The bare wire has some insulation around it and would go to the same neutral/ground bar in the main panel as an insulated neutral wire would. Why is this such a no-no, what can happen?
 
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Old 01-01-07, 07:10 PM
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The bare wire has no insulation around it. It has the jacket around it, but no insulation. There is a difference.

The other reason it is not allowed is because code does not allow it.
 
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Old 01-02-07, 03:24 AM
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Ok, you have convinced me. If I have to change the wire I might as well use a 4-wire cable and future proof it. This cable runs underground for about 25ft between the house and garage. What are the chances I could use the old wire to pull the new one without having to dig up my yard?
 
  #14  
Old 01-02-07, 05:43 AM
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Get out the shovel and start digging. Perhaps put in conduit. Better yet, put in TWO runs of conduit.
 
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Old 01-02-07, 04:05 PM
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Let me ask you one more question on this subject and then i'll leave you alone.The issue here is that I need one more insulated conductor between the main panel and the new sub panel.I do actually have conduit running between the house and garage. It is the gray electrical stuff-either 1 1/4 or 1 1/2 I forget. Inside that conduit is a rubber air hose that supplies air to my workshop and one RG-6U coax cable from a garage mounted TV antenna to a basement distribution block. This would be way to easy so I'm sure there is some code against it, but could I run that one insulated wire I need between the main panel and the sub panel through that conduit? Thanks for all your help.
 
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Old 01-02-07, 04:11 PM
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No you cannot. Nor would you want to even if it were allowed.
 
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Old 01-02-07, 04:12 PM
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You guessed it...NO...Sorry You cannot run Low voltage and high voltage in the same conduit. You also need to realize how big (exactly) the conduit is your are now going to run b/w house and garage for the size wire you are going to pull. BTW...I have accidentaly pulled one 12/3/uf in underground by using the 12 that was already there but it was not fun however funny it was for the onlookers, but i had about 4' of conduit feeding both ends underground...good luck.
 
  #18  
Old 01-03-07, 03:33 AM
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Racraft, your answers to my and other questions remind me of living at home as a kid. You ask your parents a question, they say no, you ask why, they say because I said so!! (NO EXPLANATION) As in the above question why can't a neutral wire and coax be in the same conduit? Will my TV blow up when my compressor kicks on? Will my panel start broadcasting the nightly news? I'm joking of coarse but the "whys" are important to me. Without the whys I don't learn anything which is why I joined this forum to begin with. Maybe you don't know the answers or don't have the time to explain but it sure would be helpful.
 
  #19  
Old 01-03-07, 04:54 AM
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When running electrical wires it is necessary for the current going each direction through a raceway or cable to be equal. The reason for this is because of electrical fields and electromagnetic radiation produced.

If you put the neutral wire in a separate raceway then you will have unequal current through the the original cable. You will also have current near low voltage wires which is a safety issue.
 
  #20  
Old 01-03-07, 03:28 PM
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Thank you
Steve
 
  #21  
Old 01-03-07, 08:56 PM
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Wink

!!! radiation produced!!!

Thats it!! I'm outa this field! I'm gonna find something safe, that I CAN CONTROL.

You engineers scare me! If I may, I would like to give you all a big thanks!
Science is way cool. But I can't find anyone to sponsor my experiments.
They just don't get it.

Oh Well.
 
  #22  
Old 01-05-07, 07:31 PM
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The new sub panel is in (4 circuit CH)
30A breaker in main panel
10Ga 4-wire UF from house to garage
8ft ground rod w/6ga stranded wire to added ground bar in panel
neutral to isolated neutral bar in panel (bonding screw removed)
two hots to lugs in panel
Neutrals and grounds to correct bars in panel for out going circuits
Thank God for 50 degree weather in Wisconsin in Jan. (easy digging)
Did I miss anything?
Thanks everyone for your help
 
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