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Where and How to Install a Green Ground Screw in a Panel


By Dave Rongey - Summary:

How to Bond the Ground Using the Green Grounding Screw of a Sub Panel: Grounding a Main Panel vs Grounding a Sub-Panel, Bonding the Ground System at the Main Electric Service Panel, When Two Terminal Strips are Connected Together, The Difference between Ground Terminals and Neutral Terminals


Guide to Grounding a Panel or Sub Panel

Electrical Question: I’m confused about adding a sub-panel. I bought a GE PowerMark Gold Load Center (125amps).

  • There are two neutral buses flanking the breakers and the buses are connected with a metal bar.
  • Since this is a sub-panel, I purchased an additional bus for the grounds, and connected it to the box (based on the pre-drilled holes, it fit only one place).
    • Question: The panel came with the notorious “green screw”.
    • It’s my belief I’m already “good to go” without the screw, however I’m not sure where I would put the screw even if I were supposed to use it?
    • The instructions point to a location for the screw on a diagram that does not match my box, or, I’m not understanding it correctly.
  • Basically, I believe I’m doing everything right, I just wish I understood better how that green screw makes any difference.
  • I see no holes in this box where I could put it where it would appear to serve any function.

Please a) confirm I’m doing this right, and b) shed some light on the green screw (in this box specifically, if possible). I appreciate your time, very much. Your book is priceless for a DIY’er, and I’ve already mentioned it to others.

I bought your Home Electrical Wiring Book and love it!
This electrical wiring question came from Pete Frank, in Erie, Pennsylvania.

Dave’s Reply:
Thanks for your electrical wiring question Pete, and Thanks for the Compliment about my Home Electrical Book!

How to Bond the Ground Using the Green Grounding Screw of a Sub Panel

home-electrical-codes-for-grounding

Preparation

  • Application: Adding a Sub Panel.
  • Skill Level: Intermediate to Advanced – Best performed by a Licensed Electrical Contractor.
  • Tools Required: Electricians pouch of hand tools and the various power tools necessary for installing the sub-panel.
  • Estimated Time: Depends on the type and size of the panel and available access to the project area.
  • Precaution: Any existing electrical wiring within the immediate area that may interfere with the installation of the sub panel should be identified and turned OFF and Tagged if necessary. Working in an electric panel is dangerous due to arc flash hazards and the possibility of electric shock.
  • Notice: Installing additional electrical wiring and a sub-panel should be done according to local and national electrical codes with a permit and be inspected.

Grounding a Main Panel vs Grounding a Sub-Panel

  • If the panel that is being installed is being used as the primary circuit breaker panel, and the panel is installed in the location that is at or near where the main electrical utility service is located, and the main ground wires are brought into this location, then the green ground screw is installed into the pre-drilled and pre-tapped hole of the neutral buss which will then bond the neutral buss to the grounded enclosure of the panel which is where the main ground wires are located and attached.

Bonding the Ground System of a Panel Located at the Main Electric Service

  • Keep in mind that the main panel is the only location where the ground system is bonded together with the neutral.
  • If the panel is being installed at the location of the main electrical service and the service panel components are not connected together using bonded electrical conduit and or approved fittings, then a ground wire must be brought into the panel and attached to the terminal strip which is shared with both the ground and the neutrals, the green ground screw must be installed which will then bond the panel enclosure to the ground system, and the other end of the ground wire must be bonded to the main ground system of the main panel.

Bonding the Ground of a Sub Panel

  • When a sub-panel is installed in another location then it is fed with a 4-wire feed that will have a separate ground wire and a separate insulated neutral wire, and each of those wires will be attached to a separate ground and separate neutral buss respectively, therefore the ground and neutral are not bonded together, but are instead kept separate within the sub panel.
  • Bonding the separate ground wire to the ground terminal strip will then provide the bond of the ground back to the main panel where the grounding system begins.
  • NOTE: When installing a remotely located sub-panel for an outbuilding for example, the sub panel may be fed with a 3-wire feeder, and a ground rod may be installed along with a connecting ground wire which is attached to a ground terminal strip that is directly attached to the panel enclosure.

When Two Terminal Strips are Connected Together

  • When there are two buses or terminal strips similar to what you have described, which are bonded together by a metal bar, in some cases the manufacturer of the panel will provide instructions whereby the metal bar between the two may be removed which then provides two separate terminal strips, one for the ground and one for the neutral, however one of the terminal strips will have the provided bonding hole for the green grounding screw, and that may be near the location where the metal bar was attached.

The Difference between Ground Terminals and Neutral Terminals

  • A ground terminal strip, which may be similar to the one you may have, is typically mounted directly to the metal enclosure of the panel, therefore there is a metal to metal contact.
  • However the neutral terminal strip is mounted using an insulated spacer which prevents a bond to the metal enclosure of the panel.
  • This is why the green grounding screw is not installed by the manufacturer of the panel, but is provided so that it may be installed when the application requires bonding of the ground.

IMPORTANT: The size of the ground wire will depend on the amperage of the panel.

More about Wiring Electric Panels

Home Electrical Sub-Panels
electrical-panel-wiring

  • Sub-Panel Electrical Wiring for the Home

  • Electrical sub-panel wiring considerations for the home complete with pictures. Careful planning for your Sub-Panel with immediate and future load considerations will help you understand how to size your Sub-Panel. This information will help as you consider a Sub-Panel and its size.

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