"ground" wire size for 150A main, USA

Discussion in 'General Electronics Chat' started by #12, Jun 20, 2012.

  1. #12

    Thread Starter Expert

    Nov 30, 2010
    16,345
    6,831
    I found my 3 month old thread about a bad ground on my breaker box and checked the box that says, "Do you really want to reply to this old thread?" but this site won't let me, so...here's a new thread.

    The bad connection resumed being bad long enough for me to find the illegal splice hidden behind all the smaller wires.

    What size wire for the NEUTRAL on a 150A main?
    USA, 240 VAC, center tapped, etc.
    I think 75 C is the right temperature and size "0" is the right diameter in copper for the energized wires. Probably THWN.

    Any "real" electricians here?
     
    Last edited: Jun 20, 2012
  2. BSomer

    Member

    Dec 28, 2011
    433
    106
    The ground wire that you are referring to is not sized according to the main breaker. It is sized according to the size of the incoming feeder wires. Look at the table 250.66 in the NEC.
     
  3. #12

    Thread Starter Expert

    Nov 30, 2010
    16,345
    6,831
    You have pointed out something I am confused about. That chart says, "grounding electrode conductor". I interpret that as the wire to the earth ground. The wire I need to replace is a current carrying common that carries all unbalanced current.

    Same thing?
     
  4. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
    304
     
  5. #12

    Thread Starter Expert

    Nov 30, 2010
    16,345
    6,831
    Doggone it! I always have trouble with "ground" and "common". (That's why I provided a drawing.) Oh...wait...wrong again. It's, "neutral". (So I edited the first post.)

    Anyway, is 75 C the "right" temperature for this?
    I mean, you can buy wires rated up to 200 C but that's rediculous.
    The temperature rating is a critical factor in sizing the wire.
    and I'm in Florida.
     
  6. BSomer

    Member

    Dec 28, 2011
    433
    106
    I guess I misunderstood your first post there. The chart I showed is for the grounding electrode conductor and the bonding conductor. The bonding conductor ties the neutral (grounded) conductor to the grounding electrode system, which is supposed to carry the fault current.

    The neutral, which you said is needed in post #3, should be sized to the same size as your phase conductors. According to my code book, table 310.15(b)(7), it should be a #1 copper or 1/0 aluminum at the specified 150A. Unless your local codes changed this NEC section, which does happen, you can get away with a smaller sized neutral if certain requirements are met.

    The temperature rating of the wire used will most likely be 90°C if you use THHN, THHW, THW-2, ... If sizing wires using the NEC table 310.15(B)(16) you have to use the 75°C column unless all the connctors, lugs and other terminating points are rated at 90°, this almost never applies though.

    If you are putting the wire in a conduit from you electrical panel to the meter you can use THHN. If the wire is going to be exposed to moisture, i.e. direct bury, use THW.

    I hope this helps a little. :D
     
    #12 likes this.
  7. #12

    Thread Starter Expert

    Nov 30, 2010
    16,345
    6,831
    Got it. Thanks.
     
  8. n1ist

    Active Member

    Mar 8, 2009
    171
    16
    In your first drawing, your temporary fix is neither legal nor safe. You can NOT use the water pipes as a current-carrying conductor. Also, if your bootleg neutral ever opens, you will likely destroy lots of your electronics due to overvoltage.

    Based on your comments, you should get a licensed electrician to fix things immediately. The risk of fire or electrocution is too great.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Using the water pipes for grounding is a bad thing, particularly in Florida. Many areas of FL have high sulfur content in groundwater, which will corrode copper; one of the reasons that there was a switch to PVC and CPVC pipe a number of years ago.

    Grounding to water pipes was OK up to at least the 1960's. Sometime later, the code was changed to require an 8-1/2' copper clad rod to be driven into the ground, and a solid copper wire run from the distribution panel ground bus to this rod. Later, the code was amended to require TWO of these rods driven a minimum of six feet apart, and the ground bus connected to both of these rods.

    I am not a licensed electrician. If you want to make certain that your electrical system is in compliance with the NEC as well as state and local codes, you will need to consult with a licensed electrician that is local to you.
     
  10. BSomer

    Member

    Dec 28, 2011
    433
    106
    I agree that this is not safe, however, it is legal and required. Current electrical codes require the grounding electrode system to be comprised of a connection to the metal water pipe with a jumper across the water meter, and a ground rod. New construction in houses require an additional connection to the rebar in the concrete floor / walls and some local codes require a connection to the metal gas piping.

    The grounding electrode system is to be bonded to the grounded (neutral) conductor at the point of entrance to the main electrical panel. So in essence his grounding system is paralleled with the neutral. The unballanced current is going through this wire into the ground and back to the source, which is probably on a plole outside his house. This is the reason he was getting a current reading on the water pipe in his original thread.
     
  11. DMahalko

    Senior Member

    Oct 5, 2008
    175
    14
    The neutral is for balancing voltage across the split-tap transformer, if you have more load on one leg of the transformer or the other.

    Technically the maximum load of the neutral will never be more than half the rated output of the power company's split-tap transformer.

    And if you have active loads on both the high and low side of the transformer, the neutral back to the transformer is not used or is minimally used. Power flows from transformer leg 1 (call it red) through the devices, to the neutral bar in panel, through the devices on on leg 2 (black), and back to the transformer.

    ,

    The primary purpose of the ground or earthing is to trip breakers and blow fuses. A short to ground is supposed to create enough current flow to trip whatever overload protection is available for the circuit.

    The ground is an unbroken path for current to flow. At one time neutral fusing was permitted, though if the fuse blows that means what looks like a safe neutral can actually be "hot" shock hazard.

    In recent times we've found another use for the ground wire, for ESD dissipation for protection of voltage-sensitive electronic devices and for surge dissipation with discharge tubes and MOVs, but that wasn't the intended original use.
     
  12. #12

    Thread Starter Expert

    Nov 30, 2010
    16,345
    6,831
    I see you guys had fun explaining the NEC to me but I just now got back to my computer. I was busy all day replacing the TEMPORARY fix that was keeping the illegal splice from destroying lots of expensive electronic devices.
     
    Last edited: Jun 21, 2012
Loading...