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In a tool shed on my property, I've removed an old fusebox, and installed a breaker box to provide power to a couple lights and outlets.

Obviously I don't want anyone to get shocked, and am a little confused by the grounding here.

Here is the setup:

  1. On the electrical pole, there is an electrical meter with a double 100 amp breaker (i.e. the "shutoff").

  2. That directly connects to my box, with nothing else inbetween.

  3. The cables coming from the pole into the shed brings in two wires - one red, one black, and is wrapped in many small copper wires that collectively act as the ground (or common?)

enter image description here

I have two questions:

  1. Do I attach all my Common and Grounds to the copper wire from the pole?

  2. The box itself isn't connected to the ground (multimeter showed no continuity between ground and the box). Should I ground the box to the common?

Basically, I think the breaker box I'm (re-)using is meant for a subpanel, but I guess I'm using it as the main panel, because it's just a tool shed.

Yes, this shed is the only thing serviced from that pole and electrical meter (this shed used to be a tiny old house, the new house is on a different pole and meter entirely).

How should I go about grounding it?

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    Can you get us photos of the labels on the inside left and right of the box? Commented Dec 12, 2022 at 1:26
  • What country is this in?
    – Armand
    Commented Dec 13, 2022 at 22:26

3 Answers 3

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All metal boxes get grounded first

So yeah. If you're sticking an outlet in a metal box and you have your Romex coming in, you don't take the ground to the outlet like you do in a plastic box. You take it to the metal box (one hole is tapped #10-32 for ground screws). And the outlet or switch picks up ground via the box metal, OR you pigtail a ground wire to it.

In a panel like this, the ground needs to go to the metal of the subpanel.

Since it's an outbuilding you also need local grounding rods. Hopefully your guy provided an Ufer ground when pouring your concrete pad. Otherwise you'll need to drive a $10 ground rod and then, another $10 ground rod some distance away. You can do it with one rod if it passes a $500 impedance test. I know which way I'd go.

You have a legacy, grandfathered ungrounded feeder

This is illegal to install today. It was banned in 2008 which is no surprise given the trouble it causes. In the UK it's positively a menace - all this applies to you, including the bit where GFCI won't save you. That's why "PEN" was banned for subpanels in 2008.

The key difference on an American system is we take local ground rods very seriously. Brits don't have them much. The better the ground rods, the better the protection. What we do really works, although John says it won't work in the UK for reasons I disagree with. The whole point of ground rods is assure that the dirt under the feet is near the voltage of the grounded thing you are touching. It doesn't matter if some other dirt hundreds of feet away is a different voltage. You're not touching that other dirt.

However assuming it was legal when installed, it is grandfathered and you can continue to use it assuming you install it legally. Did you use a torque wrench to set the torques for the lugs? Not doing that is just the kind of thing that makes you lose the neutral wire, and then you have the problem John Ward discusses above.

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    @JaminGrey -- I suspect the 'hole" where a screw should be is where the bonding screw should go, but I'd need to see the labeling on the inside left and right of the breaker box to be sure exactly what's going on here (and what part you should order, for that matter) Commented Dec 12, 2022 at 5:26
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    The torque wrench is very important. Not tight enough and it comes loose, which is obvious, but too tight and you can rip the lugs right out. If you shear off just some of it, then it creates increased resistance on the ground and can cause a fire when you get a short. Get a torque wrench for electrical work!
    – Nelson
    Commented Dec 12, 2022 at 7:46
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    @Jamin neutral not common. That's very important. Each circuit must have its own dedicated neutral. Bonding screws are nice when you have them, but they're not generic - they are a custom fit to each panel. There are a couple sites there for an accessory ground bar. Suitable models are listed on the panel label. I would install one then run a bare #8Cu wire between neutral and ground bar. That will be necessary later when you upgrade to 4-wire feeder. Commented Dec 12, 2022 at 19:24
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    @JaminGrey I'm thinking more in the future if the opportunity should arise to upgrade that feeder or add a ground wire. If the grounds are already separated in the panel it's easy mode. As the video described, not doing so is a risk. NEC requires two ground rods 8' long at least 6' apart (cattycorner on the building is better). Make full use of any existing ground rods. Commented Dec 12, 2022 at 19:50
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    @JaminGrey Well given that the correct accessory ground bar for your panel is like $6 and will bolt up in 5 seconds, and you already have #8 copper....I think that'll be less overall effort than a snipe hunt for the correct grounding screw. As far as ground rod we give code compliant answers here and IME Code has good reasons for everything (not much bureaucratic stupidity in it). Two rods are required unless one passes a costly test, but your AHJ (permit issuing authority) is the one to ask about variances. I would count the existing ground rod as 1 of the 2. Commented Dec 12, 2022 at 20:41
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As a main panel, not fed from another panel, it needs it's own grounding.

Usually done by using two ground rods driven into the ground and connected with a single ground wire. Your local power inspector will tell you if you need one or two rods.

As a main panel, you can connect the ground wire to the neutral (the one on top) bus. There should also be a place for a screw on the neutral bar to bond it to the panel case.

Will also need to add a cable inlet to the plain hole the wires come though to protect the wire insulation. It gets nasty if that hole cuts the insulation.

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    Good catch about the bonding screw.
    – crip659
    Commented Dec 11, 2022 at 23:53
  • I'm not standing infront of the box right now, but judging from the photo, I don't see a spot for the grounding screw. Can I use a regular green grounding screw and copper wire to manually just tie the panel to the common bar? I think I see several spots where a grounding screw might be a proper fit.
    – Jamin Grey
    Commented Dec 12, 2022 at 2:46
  • My brother is saying, he remembers a grounding spike being there, so I'll see if we can find it. If not, we'll put in a new one. ¯_(ツ)_/¯
    – Jamin Grey
    Commented Dec 12, 2022 at 2:52
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Important Edit:

@Walker notes below in their comment that in UK red is hot, black is neutral and copper is ground from pole. My comments below all assumed this was a US installation, with red and black opposite hot phases of 120V and copper neutral; OP indicated they measured about 110V between each of red and black and "ground", so this seems likely to be US usage.

Some clarification just in case:

The cables coming from the pole into the shed brings in two wires - one red, one black, and is wrapped in many small copper wires that collectively act as the ground (or common?)

The many small copper wires are almost certainly the common/neutral, NOT ground. They provide the electrical return path for the incoming "hot" red and black wires. Voltage measured red to neutral or black to neutral in the US should be 120V. The red and black are different "phases" of hot and the voltage measured between them should be 240V.

For "grounded outlet" safety, the "equipment ground" wire (e.g. the third prong in a 3 prong plug) is wired back to eventually connect to the ground bar in the main electrical panel. In that main/first electrical panel ONLY, the ground bar is bonded (electrically connected) to the neutral bar.

This is so if e.g. an appliance metal frame accidentally comes in contact with a "hot" wire, the current immediately flows through the equipment grounding wire back to the main ground bar and then neutral as a short circuit, and the breaker trips, shutting off the power and thus making it safe to touch. This normally does not involve the "earth ground" (e.g. grounding electrodes 8 feet into the ground or equivalent).

Should I ground the box to the common?

All electrical panels/boxes should have both the metal enclosure and the grounding bar (if present) connected to the equipment grounding conductor (not the neutral/common).

In the main (first) panel/enclosure ONLY you should "bond" (electrically connect) the ground bar to the neutral bar.

In that main panel/enclosure you should also bond an earth ground (known as Grounding Electrode Conductor, usually attached to e.g. grounding electrodes in the ground) to the grounding bar. The GEC may also need to be bonded to the metal enclosure where it enters that enclosure.

Edit1: Adding a grounding bar to your box It looks like your box may be Square D Homeline brand. They make add-on grounding bars like the one below: enter image description here

I believe a bar like the one above would be mounted to the two pre-existing holes near the bottom right of your photo, one above the other, on the small "bumps". This size bar would give you six lugs to attach wires; I believe longer bars are available if you need more. You would attach your Grounding Electrode Conductor to one lug, and each circuit's Equipment Grounding Conductors to other lugs. The two screws supplied with the grounding bar would electrically bond it to the metal enclosure.

I believe that according to recent code you need something like a Kenny Clamp: enter image description here to bond your Grounding Electrode Conductor (which should be 6 ga copper or larger IIRC for 100A service) to the box as it enters from the outside. The Grounding Electrode Conductor needs to be unbroken (not spliced) from its connection to the first/main grounding electrode all the way to its termination at the ground bar. The Kenny Clamp allows an electrical connection at the box entrance without interrupting the GEC.

IIRC, the grounding electrodes need to be at least 8 feet long in the ground and at least 5/8" diameter, with at least 2 spaced 6 feet apart. They are easily available in home centers/hardware stores as pointed copper plated steel grounding rods. Don't use the short skinny 4 foot ones as those are not Code compliant.

Finally, to bond the neutral/common bar to the box and thus to ground, make sure the connection is a big enough size to handle the maximum expected current. I am not sure of Code on this, but suspect you will need larger than 12 ga; Square D should have an appropriately beefy bonding clamp/solution available.

Edit2 Regarding the neutral bus bonding screw to the metal enclosure, I found this set (use one): Square D part number PKNBSCP enter image description here Instructions: enter image description here The empty hole among the lugs on the neutral bar is for the bonding screw. Such a hole will have a hole in the panel skin underneath, allowing for the self-tapping bonding screw to make a good connection, using substantial turning force on the screwdriver.

Edit3:

Found the panel model, I think: enter image description here This seems to be an older model, labeled "Square D - HOM612L100SCP Homeline Load Center with Cover, 100-Amp Fixed Main Lugs, 1-Phase, 6-Space, 12-Circuit, Indoor"

Wiring instructions are said to be on the label barely visible on the left inside wall of the enclosure, as ThreePhaseEel suggested.

The add-on grounding bar kit (up to 4 can be mounted using the 4 pairs of mounting holes at the corners of the back panel): enter image description here enter image description here

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  • The voltages are as you suggest, ~110v between either phase and ground. I neglected to check between both phases, but since that's coming straight off the service pole, I suspect the electric company got it right.
    – Jamin Grey
    Commented Dec 12, 2022 at 2:39
  • It sounds like what I need to do is: (A) use a grounding screw in the breaker panel to ground the panel to the common bar, (B) attach grounds and common to the common bar (since there is no grounding bar, and only one or two breakers anyway), and (C) make sure there is a ground wire tied to the common bar that grounds to a 8ft copper spike in the ground. Am I comprehending that right?
    – Jamin Grey
    Commented Dec 12, 2022 at 2:39
  • @JaminGrey Yes and no - I edited answer to clarify details
    – Armand
    Commented Dec 12, 2022 at 3:34
  • In the UK the outside copper wire are grounded the red is live the black neutral.
    – Walker
    Commented Dec 13, 2022 at 13:55
  • @Armand can you provide a Square D part number for that bonding screw kit? Commented Dec 14, 2022 at 4:52

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