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Thread: Grounding subpanel?

  1. #1
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    Default Grounding subpanel?

    Now have run 3 wire and ground to a detached garage (PVC) and wondering what the correct method is for bonding the panel to earth (ground rod or water pipes)? Is there a standard fitting for exiting the service panel/exterior wall with the ground wire to the ground rod? Instead of using a ground rod can I bond to the water pipes? Is this also to be done outside the dwelling? Water pipes are a fair distance from the house where the wiring to the garage originates.

  2. #2
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    Since you ran 4 wires (two hots, a neutral, and a grounding wire), you do the following:

    DO NOT install the green bonding screw to the neutral bar -- it must remain electrically isolated from the panel. Install the white wire in your feeder here.

    Buy an accessory ground bar for your panel if it didn't come with one and screw it to the panel chassis. The green or bare grounding wire in your feeder goes to this bar.

    Install at least one ground rod, and maybe two. You have to have these even if you have a metal water pipe ground. The ground electrode conductor also goes to the ground bar in the subpanel. You can just drill a hole in the wall (angling downward) and run the ground wire through it, or do it nicer by installing a 1/2" PVC LB fitting and a length of PVC conduit down to the dirt.

    I'm confused with your water pipes. If there are any water pipes IN the detached structure where the new panel is located, they must be bonded to the ground bar in the subpanel. If these pipes are metal in the ground before entering that structure, then you must ground them within 5' of entering the detached building. If these pipes are just in the yard somewhere or at the house, you don't bond this panel to them.

    You didnt' say what size wires are feeding the detached building. The size of these wires determines the minimum size wire to use for ground rods and water pipes. If you use #6 to the ground rods, that will be compliant with whatever size you've run. If the grounding wire in your feeder is at least #8, you can probably only drive one rod at the detached building and say that the feeder EGC is also the bonding wire to a second electrode at the house (assuming you have at least one rod there).
    Mark
    Kent, WA

  3. #3
    Unregistered Guest

    Default The water pipes...

    Quote Originally Posted by suemarkp
    Since you ran 4 wires (two hots, a neutral, and a grounding wire), you do the following:

    DO NOT install the green bonding screw to the neutral bar -- it must remain electrically isolated from the panel. Install the white wire in your feeder here.

    Buy an accessory ground bar for your panel if it didn't come with one and screw it to the panel chassis. The green or bare grounding wire in your feeder goes to this bar.

    Install at least one ground rod, and maybe two. You have to have these even if you have a metal water pipe ground. The ground electrode conductor also goes to the ground bar in the subpanel. You can just drill a hole in the wall (angling downward) and run the ground wire through it, or do it nicer by installing a 1/2" PVC LB fitting and a length of PVC conduit down to the dirt.

    I'm confused with your water pipes. If there are any water pipes IN the detached structure where the new panel is located, they must be bonded to the ground bar in the subpanel. If these pipes are metal in the ground before entering that structure, then you must ground them within 5' of entering the detached building. If these pipes are just in the yard somewhere or at the house, you don't bond this panel to them.

    You didnt' say what size wires are feeding the detached building. The size of these wires determines the minimum size wire to use for ground rods and water pipes. If you use #6 to the ground rods, that will be compliant with whatever size you've run. If the grounding wire in your feeder is at least #8, you can probably only drive one rod at the detached building and say that the feeder EGC is also the bonding wire to a second electrode at the house (assuming you have at least one rod there).
    ...will be those that are now within about 10' of the structure. This one will be tapped into and taken into the garage. I used #8 stranded THHN for all 4 wires so it sounds as if a single ground rod is acceptable and use #8 or better solid copper to join the panel to the rod and the rod to the water pipe?
    Thanks, Mark!

  4. #4
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    Default The Bonding Screw?

    I have run feeder cable [two balck, one white, and one bare copper ground] from a main 100amp house panel to a main 100amp panel in my garage. The information on the garage panel states that in some applications the bonding screw should be removed and in some cases it should remain. The electricity in the garage will be used for general tools only. eg. welder, lathe, drill press, lights, etc. In laymans terms [someone who does not even know what all the abrievations mean], could someone tell me the reason why the white [common wire] is sometimes grounded and why sometimes it is not.

  5. #5
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    The white wire is always grounded once at the main service. Normally, you are prohibited from regrounding it again except for detached structures with no grounded conductive paths back to the main panel (e.g. via CATV wires, metal pipes, etc). The reason for this is the neutral carries current, and it therefore has some voltage drop. The longer the run and the higher the current, the worse the voltage drop is.

    If you have a detached building with only a 3-wire feeder (no separate grounding wire but a regrounded neutral), there will be a small current flowing between the detached building ground rod and the house ground rod. The current will be proportional to the voltage drop. Yes, this drop is only a few volts, so the current is minimal. But this current can bother animals that feed from grounded metal troughs, have a large distance from their front to back feet, and can sometimes cause corrosion. If there is a metallic path between the buildings, you could burn up a phone or CATV wire because it is effectively paralleling the neutral. Its also not good for noise/interference reasons.

    If the panels are in the same building and the neutral was regrounded, the grounding wires of different circuits could have voltages between them, and again this voltage would be the same as the voltage drop on the neutral in the subpanel feeder. This can cause sparks when two grounded items touch each other and current flows from that few volts of voltage difference. This won't shock you, but causes arcs and can degrade the grounding conductors because they have a better integrity when they don't carry current.

    If you run a 4th grounding wire to the detached structure and float the neutral, that grounding wire normally never carries current so it has no voltage drop. This keeps the earth at the same potential at each building so no current is flowing through the earth. This is the better setup and should be done unless cost prohibitive.
    Mark
    Kent, WA

  6. #6

    Default

    Mark,

    >If you have a detached building with only a 3-wire feeder (no separate grounding wire but a regrounded neutral), there will be a small current flowing between the detached building ground rod and the house ground rod. <

    Is this still within code tp have a detached building serviced with 3-wire or do new installions all have to be 4-wire?

    Bob

  7. #7
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    NEC still allows a 3 wire feed to a detached structure as long as there are no grounded conductive paths between the structurees. However, some local codes mandate 4 wire feeds in all cases once past the main disconnect (e.g. Washington State).
    Mark
    Kent, WA

  8. #8
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    Default Fourth wire?

    So is it safe to say that if a fourth wire [bare copper] is used to continue the grounding from the house panel to the panel in the detached building, that the white common wire should not be grounded. Also since the panel in the detached building is grounded directly from the main panel in the house, is a grounding rod for the second panel no longer necessary?

  9. #9
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    If you run a 4th grounding wire, you will always be code compliant. In this case, you DO NOT connect the neutral to the panel chassis or grounding bar. Only the 4th grounding wire connects to the panel chassis and ground bar and ground electrode conductor.

    Whether you run 3 or 4 wires, a detached structure supplied by a feeder still must be regrounded to a ground electrode system. The ground electrode system at the main panel won't be of too much use for a detached structure because it is too far away -- too much impedance in the wire run.
    Mark
    Kent, WA

  10. #10

    Default

    >too much impedance in the wire run.<

    I did read somewhere, that if you run #8 for ground to the detached structure, you can get away with only one Ground Rod since the ground rods at the main pannel will count. Is that correct or is there some maximum distance restriction?

    What is the best way to connect the ground wire at the subpannel? Can you use a terminal connection that is bolted to the pannel can?

    Thanks Bob
    Last edited by bobm; July 14th, 2005 at 01:46 PM.

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