Where do I attach the Ground and Neutral inside a sub-panel?

Question

I'm wiring 100 amp subpanel to detached Garage with 2/2/2/4 copper. I'm hoping to replace the subpanel to my detached garage which was previously wired to run on a 40 or 60 amp subpanel with like 6 spaces powered by a tandem 15A/20A breaker with 10/2 cable. So far, I have already disconnected any and all breakers and sub panels that were going to my detached garage in an effort to streamline the new installation and eliminate any confusion as to what(if any) electrical wiring/circuits (is/are) going from the detached garage from the main panel.

Now that I removed the original wiring used for the detached garage, I am hoping to run a 125 amp subpanel on a 2-pole 100 amp breaker that will be placed inside of the garage where the conduit elbow comes up through the slab.

(((Just FYI If you're looking at my main panel and wondering why I have them there, there is a 2-pole 60 amp breaker on the top right and a 2-pole 30 amp breaker on the bottom left, I point those out because they are both disconnected. the 60 amp one was from my original garage subpanel. )))

The distance is approximately 94' plus the height going from the trenched conduit to the main panel and the subpanel.

I ran copper with 2/2/2/4 and now for due diligence, I am hoping to verify from a 3rd party where I connect the Neutral line and the ground lines on the main and subpanel before I attempt it.

My understanding is that on the subpanel, the two hot wires go to the left leg and right leg and on the other end, go into the breaker itself which then goes on the Main panel.

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(Here is Where I'm hoping to get some clarification)

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I'm guessing (But Not Sure) the Set Screw on the top center of the subpanel that is connected to both neutral bars is for the Neutral wire. Please let me know if I'm wrong.

There was an included plastic baggy that had a roughly 5" ground bar with a Set Screw on it and I'm not sure but I'm guessing but not sure that is where I connect the ground wire. Please let me know if I'm wrong.

On the Main panel, I see there is a flat Bar on the left side with (2) Set Screws, One of those on the bottom is being used to hold the Copper Ground Wire which is connected to the (2) posts that are buried and sticking out of the ground a couple of inches. I'm guessing But Not Sure, that the empty Set Screw is where I connect the #4 copper subpanel ground wire. Please let me know if I'm wrong.

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Feel Free to Roast me or Educate me. Thanks in advance.

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I've had maybe 3 or 4 legitimate electricians come out over the last 6 months and they all are booked up with work and I've ran out of time. I know a half dozen other "amateur" electricians but I don't have the confidence that they have so much as heard the term NEC or comprehend safety standards/measures. I feel more comfortable checking on sites like this one so I can try to get the process personally/firsthand.

Answer 1

Since you are connecting the feed from the sub-panel to the main panel, it really doesn't make any difference where you connect the neutral and ground, since they are bonded (connected) in the main panel. Still I like to separate the neutral from the ground, just in case the main panel becomes a sub-panel at some point (Like installation of a generator transfer switch between the meter and the panel). In your case, I see the neutrals and grounds are combined on a couple of bus bars, not a big deal, common practice.

You'll need some lug adapters to accommodate your wire sizes to connect the neutral and ground to the bus bars.

In your sub-panel, you'll need to "float the neutral" meaning isolate the neutral from the ground. Usually that just means not installing the bonding screw that comes with the panel. Also, because it's a separate building, you'll need ground rods connected to the grounding bus bar in the sub-panel. Yeah, it feels kinda redundant to have a ground wire to the main panel and still have to install ground rods at the out-building, but that's code.

Lastly, and this "opinion based". You might consider returning the new sub-panel for one that has PON (Plug on Neutral) capability. That way you don't need to get breakers with those curly pig tails. I recently installed a PON panel in my son's house and it made for a very clean install.

Answer 2

In your main panel there's a fat bus bar down the left side. It has a lug up high and another at the bottom (hidden from view behind some wires). Those lugs should accommodate your neutral and ground wires.

Unfortunately... you've kinda got the wrong panel for the detached garage. There's a requirement that all circuits in the building can be shut down with no more than 6 motions - by turning off no more than 6 circuit breakers, in other words.

Usually this is accomplished by using a main-breaker panel rather than the main-lug type panel you have there. You could add a QOM100VHCP or QOM125VHCP main breaker to that panel but it'll be more cost effective to return the panel and buy one that has a main breaker already installed.

Answer 3

Oh no! Send that panel back.

The dead giveaway is that they threw an accessory ground bar in the box for you. (and it's HOM, which is notoriously cheap, which means they wouldn't give you a ground screw let alone a ground bar unless UL was holding a gun to their head).

The need for an accessory ground bar is because it's a main lug panel which means it's intended as a subpanel. Subpanels require separate neutral and ground.

What's the matter with that, then? Subpanels in outbuildings require disconnect switches. The most cost-efficient way to get a disconnect switch, is to select a panel with a main breaker.

Either retrofit a main breaker, which many panels these days do permit... or send it back and get an appropriate panel. Speaking of that...

The deal with the copper wire

This conversation happens several times a week, "I got copper because I heard bad things about aluminum" and we find the user is intractable in that belief, and sincerely put their money where their mouth is, boy howdy! Which is ironic because the panel lugs are aluminum - for lugs it's the "universal donor" metal, plays well with both Al and Cu wire. Now, first on that, next time follow Harper's Law, "Buy the wire LAST".

Second, it seems fairly nonsensical to fear aluminum whilst buying a HOM panel. Whose buses are notoriously aluminum. Certain panels are made with copper buses. Square D "QO" of course but the price is prohibitive (unless you're using DC power, in which case it's the only game in town). I believe certain Siemens panels have Cu buses, but my favorite is the Eaton CH, a happy intersection of "industrial grade" and "sanely priced".

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The other thing I see is a picture of wire in incomplete PVC conduit. You must finish all the conduit, complete, before putting ANY wires into it! "But I won't be able to pull the wires in after it's done, the turns are too sharp/too many". Then you can't build it that way. It needs to be complete, and really, re-buried and tamped so wire pulling forces don't pull it apart. Now, if you're actually doing direct burial (with wire and depth legal for direct burial) and that's just the stub-up, then "nevermind".

Fun fact. Rigid Metal Conduit only needs 6" of cover.

Lugging neutral and ground

Neutral: Neutral MUST go to the neutral bar because normal service current travels on it, thus it must be rated for that thermal load. This appears to be a "Meter+main+panel" preassembled thing. I don't see any large lugs for neutral, since they wouldn't be necessary and again, HOM isn't going to just "throw one in".

So you will need to consult with your friendly neighborhood Square D dealer, and see what UL-Listed options exist for attaching a large lug to this. I believe it'll be something that double-screws into the neutral bar or something. Note that screw torques will be very critical on that - so if you have a way to correctly set inch-pounds of torque, you will need it.

Grounds are a bit easier because service current doesn't travel on ground, so we don't have thermal issues to worry about. As such, ground can be terminated more liberally - and it's also a smaller wire. Since this is a main panel, it can land either on the netural bar or the -- um, I don't see a ground bar. Well, that's fine in a main panel.

That bar looks like neutral, since it's coming from the meter and meters don't supply ground. However again in a main panel it matters not.

Worse comes to worst, use a Polaris connector to step ground down to its minimum size, #6, and land that on a neutral bar. Or lug it to the chassis with a bolt and nut.

"Why do I need #6 ground wire? The book says #8." The book also says #3 Cu for 100A feeder. When you oversize conductors, you must also oversize ground in proportion.

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Oh, speaking of "chassis of the panel", the grounding electrode wire needs to come in a cable clamp, and the empty knockouts must be plugged. They sell simple 10 cent plugs for that. They do make "oops" cable clamps meant to apply without removing the cable; you might get away with that for the grounding electrode.

The Romex traveling outdoors in conduit on its way into the panel is technically not legal, however if the inspector approved it like this, there you are.