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Subpanel Questions

Posted by crimson_star (My Page) on
Tue, Nov 9, 10 at 12:22

I've got a shed that I'm wiring. I've used 12-2 to wire 15A outlets, 2 single switches each to a fixture, and three GFCI outlets to start each circuit. All of that goes back to an interior 100A subpanel with a 60A main lug and three 20A breakers. So far, does everything sound okay?

The big question for me is connecting that to my main panel which is about 45-50' away in direct line of sight on the exterior of my house. I'm thinking of going underground with conduit and connecting that up to a new 60A breaker on my main panel. If this sounds correct, what kind of cable is needed to connect the two panels (UF/TW) and what gauge (6-3, 4-3)? Any other things I may be overlooking will be greatly appreciated.


Follow-Up Postings:

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RE: Subpanel Questions

Not seeing a problem so far.

If you go underground with conduit (a good choice in my estimation):

1. Wire type: THWN. That is, when using conduit, pull individual conductors, not a cable. (BTW, if you're shopping at a big box store, you might want to ask for "THHN". Strictly speaking, it's THWN that's rated for wet locations such as underground runs but virtually all "THHN" is dual-rated as THWN too. Something to verify, however -- it's printed on the wire.)

2. At that distance (45-50'), voltage drop won't be a problem so should do fine with #6AWG protected by a 60A breaker in your main panel. You'll need three conductors: one red, one black and the other white. You should also pull an equipment ground which may be #10AWG.

3. The conduit run should be buried at least 18" to the top of the conduit.

4. As for conduit size, 1 1/4" PVC is a reasonably good option. You could also bump it to 2", which might make for a slightly easier pull. However, if the run is reasonably straight, as you imply, 1 1/4" should work fine.

****

Since you asked about UF cable, another possible option is to bury 6/3 UF directly (i.e., not in conduit). This would require trenching deeper to 24".


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RE: Subpanel Questions

Tom,

You should also pull an equipment ground which may be #10AWG.

Should pull an equipment ground? Isn't an equipment ground required?

I'm assuming the shed is a detached structure. When does he need ground rods? Or is that only when other metallic cables run like phone.


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RE: Subpanel Questions

"Should pull an equipment ground? Isn't an equipment ground required?

Point well taken. That's should as in MUST. :-)

"I'm assuming the shed is a detached structure. When does he need ground rods? Or is that only when other metallic cables run like phone."

S/he'll need ground rods in this case. The notable exception, which does not apply here, is when the detached structure is fed by a single 15- or 20-amp circuit.

[The "other metallic cables issue" had to do with (now) old-fashion three-wire feeders which are no longer legal for new installations with the adoption of the 2008 code.]


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RE: Subpanel Questions

Thanks to both of you for your help. As far as grounding, I've got an 8' grounding rod that I'll connect to my sub panel and drive into the ground/bury. Besides that and the three wires (2 hot, 1 neutral) do I also need to run a ground wire from the main panel to the sub or is the rod sufficient? If I do need to run a separate ground can it be in the same conduit as the other three wires? Now a couple of questions about sub panels. I wanted to be sure that I should unbond the neutral and ground bars to keep them separate. Also, I was told that whatever brand panel I buy that I should also buy the same brand breakers. Is this the case? Thanks again everyone.


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RE: Subpanel Questions

Depending on what version of the code your location is using you MIGHT get away with only two wires, *BUT* later versions of the code used in the majority of states require it unconditionally and frankly IT IS A VERY GOOD IDEA. Run the separate ground wire to this structure.

Yes, strictly you must use the breakers authorized by the panel manufacturer. Select a panel you can easily get breakers for wherever you shop. It varies from place to place. When I was in NJ, CutlerHammer was quite common but down in NC GE prevails. SquareD is usually available most places.


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RE: Subpanel Questions

"Besides that and the three wires (2 hot, 1 neutral) do I also need to run a ground wire from the main panel to the sub or is the rod sufficient? If I do need to run a separate ground can it be in the same conduit as the other three wires?"

The separate ground wire (a #10 conductor in this case) MUST be run in the same conduit. This grounding conductor -- also called an equipment ground -- is the "real path" that provides overcurrent protection (i.e., causes a breaker to trip in the event of a ground fault).

The ground rod (or rods) serves a different function. It is basically akin to a lightning rod for dispersing lightning strikes or high-voltage spikes that can occur in the power lines.

These days, you usually need two ground rods placed at least 6' apart and connected to you panel's ground bar with #6 AWG copper wire. One ground rod is sufficient only if your soil and moisture conditions (and distance to the POCO's transformer) provide for a relatively low resistance path. While the resistance can be tested with specialized equipment, it's easier and cheaper for a DIYer simply to install two ground rods.


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RE: Subpanel Questions

For clarification in my own mind: I'll have four wires run in my conduit between the two panels. Two hot (from the main panel's double pole breaker I add to lugs on sub), one neutral (from neutral bar on main to neutral bar on sub), and one ground (from ground bar on main to unbonded ground bar on sub.) I'll also add another grounding rod into the mix.

I think that answers all of my questions. Thanks very much to all of you for your help.


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RE: Subpanel Questions

Adding...I missed this "question" on the first pass:

"I wanted to be sure that I should unbond the neutral and ground bars to keep them separate."

Nods. The neutral and equipment grounds should be kept isolated. Whether you'll need to "unbond" or simply avoid bonding them at the subpanel depends on the manufacturer and the configuration of the panel.

Some panels -- typically those with a main breaker designed for use as a main panel -- come with a bonding strip or jumper already installed between neutral and ground bars. When these panels are used as sub-panels (nothing wrong with that, BTW), you need to remove the bonding connection.

Other panels -- and likely your panel, based on your description -- come with a small green connector that can be installed to bond the neutral to the box (and, thus, the grounding system). When used as a subpanel, you should NOT install this bonding lug.

BTW, if you have not already done so, you will very likely have to buy and install a ground bar in your panel. While some equipment comes with one or more ground bars already installed, most smaller panels do not. So that's probably another $4 - 5 to add to your budget. :-)

The general rule is this: The neutral (sometimes called the "grounDED conductor") is bonded with the equipment ground (a.k.a. the "grounDING conductor") at the first point in the service that has overcurrent protection (fuse or breaker). This is usually a house's main panel, although bonding sometimes occurs earlier -- for example, a disconnect outside at the meter pan.

Thereafter, the twain shall never meet. That is, the grounding system is an isolated "stand-by" path to handle faults, high-voltage spikes and to ensure that metallic things such as plumbing and appliances are at the same potential and not inadvertantly energized. Under normal circumstances, the grounding system should not carry current.

BTW, dunno if you have metallic plumbing running to that shed but, if so, that also needs to be bonded to the grounding system.


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Bingo.

Yup, you got it.

(Sorry about the cross-post. I had that window open for a long time while gabbing on the phone. :-) )


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RE: Subpanel Questions

"One ground rod is sufficient only if your soil and moisture conditions (and distance to the POCO's transformer) provide for a relatively low resistance path. While the resistance can be tested with specialized equipment, it's easier and cheaper for a DIYer simply to install two ground rods."

And the test generally requires driving another ground rod (and then removing it if not needed).

Since two rods eliminates the requirement to test, you might as well drive both and be done with it.


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RE: Subpanel Questions

On more question. My sub panel is inside my shed and I'm thinking I would like to have a way to turn off the power completely to the shed from outside right next to the shed. My central air unit has a master disconnect switch and I was wondering if it would be possible to connect something like that between the feed coming from my main panel to the sub panel. If so, would it need to match the breaker on the main (60a)? Browsing Home Depot's web site I see a 60A 240V disconnect for under $15 (See link).

Here is a link that might be useful: GE Disconnect


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RE: Subpanel Questions

"I would like to have a way to turn off the power completely to the shed from outside right next to the shed."

A disconnect rated at least as large as the breaker feeding the shed will be just fine.


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RE: Subpanel Questions

Dumb question: If the disconnect is listed as an AC disconnect for an air conditioner, can it still be used or is there something specific about it that makes it for use with an air conditioner?


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RE: Subpanel Questions

"If the disconnect is listed as an AC disconnect... "

Listed how?

"listed" has a very specific meaning in the NEC.
I have never seen a disconnect that was restricted by its listing to AC use only.

You do need to decide the purpose of the disconnect though.

Not all disconnects are listed to operate under load.
Some are designed to be a service disconnect, and are designed to break the connections when NO current is flowing.

If you use a panel with a main the only thing live after shutting down the main is the feed to the main itself.


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RE: Subpanel Questions

The disconnect in question was the one in the URL I posted a few posts up. Its description on Home Depot's site says "GE's 60 Amp AC Disconnect is used to cut off the power when servicing an Air Conditioning unit. Non-Automatic Disconnect." I've seen a few others in the $10-20 range that all mention air conditioners.


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RE: Subpanel Questions

"description on Home Depot's site"

That actually means nothing, and is just a description of a typical use.


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RE: Subpanel Questions

Thanks. I just wanted to be sure. So as long as the amperage matches my breakers (1x60A on the main = 3x20A on sub), it has the right number of connections for my wires (2x hot, 1x neutral, 1x ground), and supports the gauge wire I'm using (#6) any AC disconnect should work?


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RE: Subpanel Questions

You only need to disconnect the two hots since the extra hardware you are installing is not required.

The extra connections in the neutral and grounding line just introduce one more place for a bad connection to occur.


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