bonding grounding?? subpanel

bushwacker1December 14, 2011

My service goes from the meter on a pole outside to a main disconnect breaker in my detached garage. From there four wires go underground to the house where there is a main breaker and all the cicuits. In the house the grounds and neutrals are separated and there are 2 ground rods.

From the panel in the house, a 40 amp breaker feeds back to a subpanel in the garage mounted right next to the main disconnect breaker. The subpanel has a ground bar and there are 2 ground rods outside. The main disconnect has a bonding screw and a ground wire going to the ground bar in the subpanel.Is this setup right? It seems that now the neutrals and grounds are connected through the ground wire from the main disconnect to the ground bar in the subpanel.

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brickeyee

The ground rods should be out at the disconnect, not at a sub-panel in the same building.

Sun-panels require isolation of grounds and neutrals and no connection between the two.

They do not require ground rods if they are in the same building.

    Bookmark   December 14, 2011 at 1:42PM
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Ron Natalie

Brick is right. I suspect the main disconnect was moved from the house to the garage at some point, leaving the current hodgepodge.

    Bookmark   December 14, 2011 at 4:31PM
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bushwacker1

So let me get this correct. The ground rods for the garage should be connected to the ground in the disconnect in the garage and the subpanel fed from the house but located in the garage should not be connected to these ground rods??
Sorry this is confusing.

    Bookmark   December 15, 2011 at 7:19AM
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brickeyee

The only place there should be ground rods is at the main where the groundING (AKA 'safety ground') and the groundED (AKA 'neutral' from the POCO) are bonded together in a single building.

Sub panels are re2uired to be served by a 4-wire connection (2 hots, 1 neutral, 1 ground) and if they are in the same building do not require ground rods.

The rods are at a single place in a building, then main panel that has ground and neutral bonded.

    Bookmark   December 15, 2011 at 9:54AM
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bushwacker1

Ok, the fact that the sub panel in the detached garage is fed from the house doesn't make any difference? Thanks for all your help!

    Bookmark   December 15, 2011 at 12:16PM
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weedmeister

What is the distance from the detached garage main breaker panel to the house subpanel?

    Bookmark   December 15, 2011 at 3:47PM
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brickeyee

I did not notice you said detached garage in the first post.

If the garage is detached from the house then the first panel in the house is a sub-panel and requires ground rods.

Any panel fed from the first house panel requires a 4-wire feed but does not require ground rods.
Its ground is in the 4-wire feed.

Having additional ground rods on a panel that does not require them is probably not a good idea.

The earth is a poor conductor at 120 V.

The rods are for lightning and pole transformer leakage (the earth IS a good conductor at 7,200 V, the lowest normal distribution voltage).

If another panel is being fed from the main back in the garage it should be a sub to the house wit isolated neutral and ground and be bonded to the ground in the disconnect.
This is just to make sure it does not have any voltage on ground relative to the disconnect.

I would think very hard about adding an overcurrent device and feeding the box in the garage as a sub directly from the disconnect.

The whole 'exiting a separate building and then going back into it to supply power' just makes me uncomfortable.

I would be worried that a voltage could exist on the garage sub-panel relative to the disconnect.

    Bookmark   December 15, 2011 at 4:41PM
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bushwacker1

"Any panel fed from the first house panel requires a 4-wire feed but does not require ground rods.
Its ground is in the 4-wire feed."

The panel I'm asking about is in a detached garage. My original question was about this panel. I'll try to reword the ?. If the panel in the garage is on the same ground rods as the main disconnect in the garage isn't that mixing the bond and the ground? And is this permitted?

    Bookmark   December 16, 2011 at 1:31PM
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Ron Natalie

Each structure is required to have only one ground. You can fix this by bonding all the ground rods together (probably more preferable than ripping out the spurious ones).

It's pretty darned unambiguous in the code:
250.58 Common Grounding Electrode.

Where an ac system is connected to a grounding electrode in or at a building or structure, the same electrode shall be used to ground conductor enclosures and equipment in or on that building or structure. Where separate services, feeders, or branch circuits supply a building and are required to be connected to a grounding electrode(s), the same grounding electrode(s) shall be used.

Two or more grounding electrodes that are bonded together shall be considered as a single grounding electrode system in this sense.

    Bookmark   December 16, 2011 at 5:01PM
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mouselb

The theory behind neutral bonding is to maintain mirror current along ALL the neutral circuit runs in close proximity to corresponding hot circuit runs all the way back to ONE bonding at the first/main disconnect panel (two-leg, single-phase, wye, anything with neutral conductors--think zip wire). Detached-building entry panels require auxiliary grounding (rod(s), proper foundation reinforcement-bar, CONDUCTIVE cold water pipe. or etc.). Adjacent panels require cases/chassis wired/bonded (not neutrals) together to avoid possible deadly current between them in certain failure modes. Whether the main-disconnect-panel's ground wire attaches to some other SUB-panel on its way to ground is inconsequential to its grounding, save it fulfills the required gauge, total length, number of rods and etc.--and it commonizes both chassis. Keep ALL neutrals separate from ground runs to avoid current paths doing an end run-around to ground causing unwanted field radiation, RF interference, etc. (Do you have mobile phone, broadcast TV/radio and/or vibrating fillings problems? Do you run neutral WITH your three-way-switch runs?) Some meter/drops feed multiple main disconnect panels, each requires: neutral bonding, additional grounding (spaced rods); and if only one ground run is used, sufficient gauge to carry TOTAL of each panel's gauge (as if current carrying). Don't bond three-wire electric clothes dryer neutral, but wire chassis to ground--think trailer-home safety. Auxiliary grounding is a wonderful thing, just don't bond neutral anywhere but at MAIN disconnect panel--one must use or modify boxes for un-bonded neutrals.

This post was edited by mouselb on Fri, Jul 5, 13 at 2:52

    Bookmark   July 5, 2013 at 12:11AM
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Ron Natalie

Is there some reason why you followed up to a two year old thread to share that pearl of information?

    Bookmark   July 5, 2013 at 9:45AM
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