Subpanel box wiring

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I've got a small Square D breaker box I am wiring for service to a shed that I am having problems wiring.
The box will be supplied by 10/2 UF with ground coming from a 30 amp breaker in my home's breaker box. I have 10/2 NM wire with ground that is coming into the box. The box will power three circuits: one circuit with 5 recepticles, one circuit with two recepticles (for a small A/C and dorm fridge) and one circuit with some lights. Each circuit is run from a 20 amp breaker in the box using 12/2 with ground wire. The problem is that I'm not exactly sure how to wire-up the box. I have a diagram of the box here:
http://tinyurl.com/6fdbhs
Here's the way I think this should go: I have two connectors above each of the two breakers, so I assume I should pigtail the black wire coming from the feeder and connect those wires to the connectors over the breakers. Then connect each of the black wires going leaving the box for the three circuits to one of the connectors actually on the breakers themselves.
Then I would pigtail the white and ground for each circuit together and connect them to one of the four connectors on the center part of the bar to the right of the breakers. Then I would connect the white and ground wires coming from the feeder to each of the bigger posts on the top and bottom of the bar to the right of the breakers.
For the pigtail going to the circuit breakers I would use a red wire nut and 10 gauge black wire. For the pigtails going to the neutral and ground posts I would use a red wire nut and 12 gauge white wire.
Here's what I think it would look like:
http://tinyurl.com/5cb8f7
Am I way off base here?? There is no separate ground bar in this breaker box, so I assume all the grounds go to the neutral bar on the right. Since there are only four connectors on that bar I seem to have no choice but to connect the neutrals and grounds together.
I have a qualified electrician checking my work, but he isn't available for a few days and I don't want to make a mess. The circuit will not be energized until the electrician checks it out and I have passed county inspections.
Thanks!
John
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On Thu, 31 Jul 2008 10:05:44 -0700 (PDT), snipped-for-privacy@hotmail.com wrote:

First, you need 10/3 with ground to serve this. Even if you exploit the exception that lets you reground the neutral in a separate building the neutral still needs to be insulated if you use UF. The next thing is you need a ground rod. Since you are going to need 10/3, why not spend a few extra bucks and get the supplemental ground bus and be 2008 code compliant..
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On Jul 31, 2:13 pm, snipped-for-privacy@aol.com wrote:

Ah-- one hot wire goes to one breaker and one hot wire to the other?
But the problem is that I have already buried about 50' of 10/2 w/ ground wire and it has passed inspection. So I guess I would have to get a breaker box that can hold all three breakers on one hot feeder wire. And then I wouldn't need the ground rod, either...
Thanks,
John
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On Thu, 31 Jul 2008 11:34:39 -0700 (PDT), snipped-for-privacy@hotmail.com wrote:

If your inspector is OK with the uninsulated neutral, who am I to argue. Go with it. You still need the ground rod if you put breakers out there. (not a single branch circuit) With a 3 wire feeder you don't need the supplemental ground bus as long as you have enough holes in the one you have.
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Ah-- one hot wire goes to one breaker and one hot wire to the other?
But the problem is that I have already buried about 50' of 10/2 w/ ground wire and it has passed inspection. So I guess I would have to get a breaker box that can hold all three breakers on one hot feeder wire. And then I wouldn't need the ground rod, either...
Thanks,
John
You should have used 10/3 but I don't see any problem with the two wire. Install a jumper on the main lugs to the hot leg. Connect the neutral to the unbonded neutral buss, and install a separate ground detail and connect the ground to it. Then install a ground rod and connect a protected #8 conductor from the rod to the ground detail as well
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Yes, this was a plan that has been in my mind. I was looking at this site http://tinyurl.com/5o6gdq and it has a configuration similar to mine, except they only have one main lug and one breaker on it.
Thanks to both of you. I am going to stop by the inspector's office tomorrow and talk to them about my plan and see what they say. I'll post back when I have an answer, just for everyone's edification.
Thanks!
John
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On Thu, 31 Jul 2008 11:34:39 -0700 (PDT), snipped-for-privacy@hotmail.com wrote:

It's not clear from your information, but it sounds like you are only running 120 volts to the shed? If so, you can get away with the 10/2 w/g. On the feed side of the 10/2, does the black go to the 30 amp breaker and the white to the neutral bus? If so, it's 120. If both black and white go to terminals on a double breaker, then it's 220.
If you are running 220, then you need 10/3 w/g.
If it is 120, tie the black wire to both the top lugs, and the white wire to the neutral bus. Remove (or don't install) the long screw that ties the neutral bus to the metal box. Buy a separate ground bus (looks like the neutral bus), mount it in the box, and tie the ground from the feed to it, as well as a ground rod. The 12/2 w/g lines to your outlets get the black to a breaker terminal, white to the neutral bus, and ground to the ground bus.
It's a little unusual to use a 220 volt breaker box when you only have a 120 feed, but it will work and will be safe. Not sure if it meets code though.
HTH,
Paul F.
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*As others have mentioned already, you should have used a three wire cable with ground. You will only be able to pull 80% continuous load which gives you about 24 amps. You don't seem to have accounted for voltage drop.

*You can splice two #10 wires together and feed each terminal

*WRONG! The grounds and the neutral must remain separate in a subpanel. This has been discussed in this group many times already. Do a Google search of this group. You must purchase a separate ground bar for your subpanel and terminate all of your ground wires there. You also need to install at least one ground rod, though two is preferred and connect ar least a #8 wire to them and terminate that on the ground bar as well.

*The wires for the hots and the neutrals should be the same size. A #12 wire is good for 20 amps and goes on a 20 amp circuit breaker. #14 goes on a 15 amp circuit breaker.

*Buy a separate ground bar for your subpanel. The label inside of the panel will tell you the part number.

*How about posting some pictures of this installation?
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insinuating that he was OK with an uninsulated neutral. I was just lamenting that I've already gotten the buried cable that far along and that it would be a pain to replace now. I realize that it may have read differently and I apologize.
Paul Franklin: I am only planning to run the black from a single pole, 30 amp breaker from the main box on the house and the white from the neutral. I have been unable to find a box that didn't have two rows of breakers or I would have bought one. I'm considering running to the Square D distributor tomorrow to see if they have a box with a single row of breakers. I'm also planning to ask the inspectors if it meets code to only use one side of a two-column breaker box.
Thanks,
John
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insinuating that he was OK with an uninsulated neutral. I was just lamenting that I've already gotten the buried cable that far along and that it would be a pain to replace now. I realize that it may have read differently and I apologize.
Paul Franklin: I am only planning to run the black from a single pole, 30 amp breaker from the main box on the house and the white from the neutral. I have been unable to find a box that didn't have two rows of breakers or I would have bought one. I'm considering running to the Square D distributor tomorrow to see if they have a box with a single row of breakers. I'm also planning to ask the inspectors if it meets code to only use one side of a two-column breaker box.
*I don't recall ever seeing a circuit breaker panel that utilizes only one phase. You can use the 10/2, but you will only have 120 volts which may be okay for your needs. For all of the work that was necessary to put the wire in the ground, a simple upgrade to 10/3 would have been beneficial in the long run. Don't go digging again unless you feel that down the road you may need the added capacity. If you do dig the trench again you should consider installing PVC conduit for the power. Your material cost may be cheaper than UF cable. You may also want to consider installing a separate PVC conduit for a future phone or TV jack.
Post some pictures anyway so we can see the installation.
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snipped-for-privacy@hotmail.com wrote:

John, ask your electrical inspector if a "floating" neutral is required for your installation. Someone has already mentioned installing a ground bar kit in the sub-panel and not bonding the neutral and ground together. I don't believe a separate ground rod is called for or required. Common practice for sub-panel wiring is to float the neutral and use the ground at the main disconnect grounding point where it is connected to a ground rod and usually the cold water pipe also. The reason is the possibility of ground loop currents which can cause GFCI devices to trip and electrical and RF noise.
[8~{} Uncle Monster
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Will do, thanks for the info.
This diagram is a very good illustration of my original plan:
http://tinyurl.com/6y37sd
The only difference is that there are three breakers in my subpanel, instead of one, and my subpanel has two breaker columns.
John
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snipped-for-privacy@hotmail.com wrote:

That looks correct. Good electrical practice calls for one common grounding point known as a star ground. It's more important when dealing with low level signals in electronic equipment but I've seen some strange problems crop up in home, business and industrial electrical systems because of improper grounding.
[8~{} Uncle Monster
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Every structure requires a ground rod (electrode). So since this subpanel is in a separate structure, it gets its own ground rod, which is connected to the ground bar on the subpanel.
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Are you sure about this? It's my understanding that a separate structure may use a separate ground (but is not required to) if and only if there is no other conductive path between the buildings, such as a phone line. -- Doug
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On Fri, 01 Aug 2008 11:12:05 -0500, Douglas Johnson

250.32 Two or More Buildings or Structures Supplied from a Common Service. (A) Grounding Electrode. Where two or more buildings or structures are supplied from a common ac service by a feeder(s) or branch circuit(s), the grounding electrode(s) required in Part III of this article at each building or structure shall be connected in the manner specified in 250.32(B) or (C). Where there are no existing grounding electrodes, the grounding electrode(s) required in Part III of this article shall be installed.
The only exception is if the second building is served by a single branch circuit.
You are thinking about 250.32(B)(2) Grounded Conductor. Where (1) an equipment grounding conductor is not run with the supply to the building or structure, (2) there are no continuous metallic paths bonded to the grounding system in both buildings or structures involved, and (3) ground-fault protection of equipment has not been installed on the common ac service, the grounded circuit conductor run with the supply to the building or structure shall be connected to the building or structure disconnecting means and to the grounding electrode(s) and shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded
That goes away when your AHJ adopts the 2008. All buildings will be served with a 4 wire feeder (separate neutral/ground etc) after that.
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Under the National Electric Code (NEC), every building gets a ground rod (electrode) which is appropriately connected at the service/feeder entrance.
If this building is served by a utility with 240V/120V, they provide only 3 wires (two hots and a neutral), and the grounding electrode is connected to the service neutral at the service entrance. Thereafter, within that building, the ground and neutral are kept separate.
If the building is served by a feeder from another building, then a 240V/120V feeder should be 4 wires--two hots, a neutral, and a ground. The neutral and ground are kept separate at the feeder entrance. The building's grounding electrode is connected to the ground bar in this feeder entrance.
Under the 2005 NEC (and earlier), there is an exception to this requirement for a 4 wire feeder--if there is no other conductive path between the buildings (such as a phone line, water pipe, metallic gas pipe, etc.), then the ground wire and neutral in the feeder may be combined, allowing a 3 wire feeder. In this case, at the feeder entrance the grounding electrode is connected to the feeder neutral/ground. Thereafter, within the building, the ground and neutral are kept separate.
However, the 2008 NEC has done away with this provision, and all 240V/120V feeders will need to be 4 wires. 4 wire feeders are superior, so even if you are under the 2005 NEC a 4 wire feeder is recommended. In fact, the electrical distribution system would arguably be safer if the utility provided a 4 wire feeder instead of a 3 wire feeder. But that is another matter.
Cheers, Wayne
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Ok, so I spoke to the inspector today and we talked about jumping the phases and the grounding rod issue. He suggested that I put all my breakers on one phase. He said I could jumper them together, but that the connectors for the hot leads are not designed to hold two wires. He said that if I want to jumper them I would have to use a pigtail and wire cap. But he strongly suggested that I use only one phase. So I went and got a new box that has three breaker positions for each phase, which is perfect. It has a convenient screw spot in the neutral bar for bonding, so I will ask the inspector when he comes if I need to bond the neutral. I also bought the extra ground bar.
So on the ground rod-- he said I probably didn't need a ground rod, that it is debatable, but if it were his shed, he'd do the ground rod. The reason is that he would use one is because if there is ever a power surge that originates from the shed (IE lightning) that the #10 wire wouldn't be able to carry much. It has to carry the surge all the way back to the ground rod at the house. If there is a ground rod at the shed then it won't have to go all the way back to the house.
I pointed out to him that if the ground rod is there and is causing RF problems or tripping my GFCIs that all I have to do is disconnect it.
I didn't get a chance to ask about the floating neutral, sorry. He was in a hurry, so I got cut short.
I'll post some pics tonight.
Oh, also, about the loads I'm going to have on this: I have one circuit that will be running some occasional woodworking tools, like bandsaw, chop saw, drill press, etc. Occasionally I might run a router on it, but I can do that up at the garage if it trips breakers. It would be rare, at any rate. I have another circuit that will run some lights-- two flourescent lights over my work bench, a small ceiling fan with light and one light outside over the door. The final circuit only has two recepticles-- one for a small, window air conditioner and maybe a dorm fridge on the other.
Thanks for all the great advice on this!!!
John
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You do not bond the neutral, since you are providing an equipment grounding conductor in your feeder. The neutral and EGC should be kept separate like a regular subpanel.

A ground rod is required, there's no debate. :-) See gfretwell's post for the NEC citation.

Ground rods don't cause RF problems and don't cause GFCIs to trip. Whenever a GFCI trips, it is due to a sufficiently large (>5 ma) current imbalance in its current sensing transformer. Almost always this is due to a problem with the equipment plugged in. Very rarely the imbalance is a phantom caused by RF interference from something like a hand-held walkie-talkie. In that case a better GFCI should be found that is more resilient to electrical noise.

Seems like if your air conditioner and fridge come on at the same time, and you are running your table saw, you may exceed 30 amps and trip your 120V feeder breaker. You should check the nameplate ratings for those items and add them up.
Cheers, Wayne
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Noted.
Ok. But I was doing one, anyway, so you'll get no debate from me on it! I would rather be over-prepared for whatever the inspector wants, so if that means ground rod, then a ground rod it will get.

Noted.
Will do. If it turns out I am right at the edge of the system's capacity then I can do without some things like the A/C or the fridge and still be perfectly happy with what I have.
Thanks!
John
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