0

I have an outbuilding (200 ft from house main panel) I have run a 4 wire to the subpanel but wanted to put a ground rod in after reading some things. Is it ok to have both or do I need to disconnect the 4th ground wire from the panel?

1
  • "Allowed? It's MANDATORY!" - Suze Orman, answering a self-employed person asking if there's a way they can pay taxes quarterly instead of all at once in April. Apr 29, 2022 at 20:08

3 Answers 3

2

You need BOTH. They do different jobs.

Current flows in loops. No loop, no current.

You may have heard some poppycock about "current wants to get back to ground" no it doesn't. Current wants to get back to source. It doesn't want to go to ground unless ground is the source! (which would be the case for natural electricity such as lightning or ESD).

Human-generated electricity coming from a transformer wants to get back to the transformer.

Now inside the main disconnect (first location past the electric meter), we have an intentional bond among neutral and the ground wires. Also, tying the ground wires to actual earth is always a good thing so there are ground rods there and certain other places. Because of these, people often get fuzzy-brained about the difference between

  • the neutral wire
  • the ground wire
  • actual dirt

Neutral is the normal and intended path for working current. Ground is the "fault catcher" so that fault current takes the ground wire back to the neutral-ground bond, instead of going through a human body or starting a fire. For a bolted fault aka dead short between hot and ground, this will flow LOTS of current and trip the circuit breaker.

So we need a nice fat ground wire back to the panel for that reason. We need a ground rod to arrest ESD. (not likely to get lightning on interior wiring).

Why not combine neutral and ground? Because when you do, ground isn't ground anymore. It's neutral. So if the neutral wire has a problem, it floats the neutral to 120V (that will happen, that's why we insulate it). You don't want that tied to ground or it will nail anyone who touches anything grounded.

0
3

This is a common question, search for "how to wire a sub-panel" and you'll get tons of info. In a nutshell, detached structures require local installed (2 ground rods) at the structure. Glad you ran a 4 wire run. In the sub-panel, you connect the 2 hots as typically done. Connect the neutral to the neutral bus bar. And connect the ground wire from the main panel to the grounding bus bar as well as the connection from the locally installed ground rods. DO NOT BOND (CONNECT) THE NEUTRAL TO THE GROUND IN THE SUB PANEL. While that's required in main panels, it's not code in sub-panels. Most sub-panels come with a green grounding (bonding) screw that's not installed. Don't install it! If it is already installed, remove it. The concept is generally referred to as "floating the neutral" or more technically as isolating the neutral from the ground in a sub-panel. There are good technical reasons for this, but too much to get into in a short answer.

3

NEC 250.32(A) Specifically requires a grounding electrode system for a detached building, which ground rods are normally the primary method for an outbuilding. You will likely need 2 to comply with NEC 250.53(A)(2). The ground wire still needs to be attached, NEC 250.104(A)(3).

If pouring a new slab for an outbuilding your local AHJ (inspector) may require an Ufer (concrete encased) electrode instead of rods.

The 4th wire and the neutral should remain isolated from each other in the detached building.

3
  • 2
    Thanks for the contribution. Just for grins, I kept trying to figure out what Ufer stood for. After some research, I found that Ufer is the name of an Army consultant during WWII who invented the concrete incased grounding system for extremely dry climates (not that I have to worry about that in Western Washington!). Anyway, Ufer is not an acronym, it's the name of the inventor. Hope you enjoyed this comment. Apr 29, 2022 at 15:07
  • @GeorgeAnderson WA is one of those jurisdictions that require Ufer for new slab construction. Even though maybe not a superiority performance issue my experience is ground electrode conductor and connection is better protected with a ufer ground. WA does have pretty dry soil east of you. Apr 29, 2022 at 15:31
  • yeah, the other side of the "hill" (otherwise known as the Cascade Mountain range, mid-state) is completely different weather than the west side. A LOT drier. The joke about us on the west side is we don't tan, we rust! Apr 30, 2022 at 1:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.