Wiring outbuilding

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Recently I had a breaker box installed in an outbuilding (Homeline, 70 amps max, with 2 20 amp breakers). Power is supplied via an underground cable from the main fusebox in the basement of my house. The distance is around 100 ft and the cable is 12-2 w/ground. I wired the buidling myself (four circuits, 2 per breaker). The circuits supply 1 light and 1 receptacle in one room and 1 light and 1 receptacle in the other. However, as there was no ground bar, I tied all the grounds with the neutrals on the neutral bar. A bonding screw was included but not used. My questions:
1. Does the NEC permit this? Is it safe? (I know that tieing neutral and ground together on a subpanel in the same structure is a no-no.)
2. Should the building have its own ground rod? There are no connections between house and building except for the underground feed.
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B. Adams wrote:

You need to add a "grounding electrode." (probably 2 ground rods if you really want to get picky.) If I understand what you said correctly, you should add the bonding screw in order to ground the metal panel box.
What kind of breaker do you have in the house to protect this circuit?
Best regards, Bob
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Obviously what you have done is wrong. I am not sure if the grounding rod is an acceptable fix or not.
However, a 100' 12/2 line is pretty long. As long as you just have 100w lights and clocks on it you should be okay, but a significant load will exceed your voltage drop allowance real fast.
If you are not familiar with it, the link below should be helpful. http://www.electrician.com/vd_calculator.html
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I think 12/2 is a bit small for a 100' subpanel feed to an outbuilding. I would have suggested 10/3 as the cable of choice for a light load such as yours.
It is a code violation and a potential safety hazard to have the grounds and the neutrals connected together in a subpanel. Buy an appropriate ground bar for your panel and install in the subpanel and relocate all of your ground wires to it.
You do need to install at least one ground rod for your outbuilding and connect it using at least a #6 wire to the ground bar in your sub panel.
John Grabowski http://www.mrelectrician.tv

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B. Adams wrote:

No the US NEC does not permit that. Since there is an Equipment Grounding (bonding) Conductor (EGC) in the cable you must use it to ground all non current carrying parts of the electrical installation back to the Service Equipment in the house. You need to install an add on EGC buss bar in the Garage's panel. These are available wherever SquareD equipment is sold. You then put all the EGCs on the add on buss bar and keep all of the neutrals on the built in buss bar. The reason for not running the neutrals and EGCs on the same buss bar is that in the event of an open neutral between the garage and the utility's transformer the voltage on all exposed metallic parts of the garage's electric system will go dangerously high if the EGCs are not separate from the neutrals.
The separate building does need it's own grounding electrode system. The minimum is a driven rod, eight feet or more in length, which measures less than 25 ohms resistance to earth after installation. The code requires that a second rod be added unless the first measures < than 25 ohms. Once you install a second rod the code is satisfied so most electricians install two rods to be done with it. The two rods must be at least six feet apart but more is better. Best practice would be to have driven the two rods through the bottom of the wire trench at ten and twenty feet from the building respectively and connect them back to the panel with bare number 2 AWG run in the bottom of the trench. Since it sounds like the trench is long back filled it is too late for that. Just drive the two rods at least six feet apart and connect them back to the garage panel with bare number four copper wire. Using an acorn clamp on each rod run the wire from the furthest rod back to the nearest rod through it's acorn clamp and back to the add on buss bar in the garage panel. That bare copper wire is called the Grounding Electrode Conductor (GEC). It must be protected by conduit if it is exposed to severe physical damage such as from lawn mowers or power trimmers. -- Tom Horne
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As I wrote in my original post, a bonding screw came with the breaker box but was not used. The instructions read: "When it is required to bond box to neutral plate, use long screw enclosed. Insert through hole in neutral plate and thread into hole in box." Does the NEC require it, in addition to adding a ground buss?
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B. Adams wrote:

You *don't* need to install another buss. (I'm assuming you are running a 240V feeder to the garage, and you are using the white wire for one of the hot legs.) In this application, your little panel is considered "service equipment", and it can have a common ground and neutral buss. But the metal box has to be grounded, and that's what the bonding screw accomplishes. If you are running 120V to the panel in the garage, and the white wire is neutral, then you should install a ground buss in the panel and leave out the bonding screw -- but I don't know why your would go to the expense of using a breaker panel in the outbuilding if you just ran a 12/2 120V circuit.
I don't know that the equipment grounding conductor in a 12/2wg cable is sufficient for a grounded neutral wire. You really should have used bigger cable. But maybe it's OK. Your whole installation doesn't make a lot of sense to me.
Best regards, Bob
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Thanks for your input. It's 120V, not 220, and it's to a small utility buidling. Also, the load is generally very light, just a couple of 100w bulbs. Occasionally a space heater, which pulls 11 amps.
Are you suggesting that the breaker box was not necessary? The guy who did the work was not a licensed electrician, btw, just a neighborhood handyman who had done work for me before. I hired him to replace the old cable after the building lost power. It turned out the old underground cable had been spliced with pvc tape and had burned in two! There was already an old rusty fuse box in the building and he replaced that with the new breaker box. Who did the original work I do not know, but it sure was a hell of a mess.
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B. Adams wrote:

Do you know what kind of cable he used? It should have been UF cable (underground feeder). It looks like regular NM-B cable, but it has a tough PVC jacket that is weather-proof and sunlight resistant. UF costs a lot more than NM, and it's very difficult to work with.
I think all you *really* needed was a deep double-gang switch box for your light switches. You also need to use GFCI outlets. A ground rod woulda still been a good idea but I don't think necessary.
You have a 120V service box with multiple circuits; you need to install a ground rod.
Bob
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B. Adams wrote:

I'm not going to suggest it I'm going to state it as fact. By installing that circuit as a feeder that supplies overcurrent protective devices he has made work for you with no benefit. If he had simply installed a multi wire branch circuit to the out building he would have doubled the available power without triggering the requirement for a grounding electrode system. Let me suggest that you install a two pole switch assembly in place of the breakers now in use and add a ground bar. This will turn the underground wire into a branch circuit and relieve you of the requirement to install a grounding electrode system. -- Tom Horne
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Let me suggest that you install a two pole

Had the CB box been properly installed as a branch circuit with neutral and ground separate would the additional ground rods be necessary?
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John Gilmer wrote:

NO, at least not if the circuit from the house was not suppling Over Current Protective Devices. The exception that allows you to avoid installing a grounding electrode system in a separate building applies to buildings supplied by a single branch circuit with an Equipment Grounding (Bonding) Conductor run with the circuit conductors. The US NEC specifically says that a multiwire branch circuit is a single branch circuit for purposes of that exception. The definition of branch circuit is "the circuit conductors between the final overcurrent device protecting the circuit and the outlet(s)." By using the twelve gage underground conductors to supply over current protective devices the local handyman has turned it into a feeder vis.. "Feeder. All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device." The quoted materials is copyright 2002 National Fire Protection Association. -- Tom Horne
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IIRC, the minimum size for feeders is 30A, which means that the #12 cable is undersized, and the installation cannot meet code with the subpanel in place.
You need to either remove the subpanel, and turn this into a multiwire branch circuit, or replace the feeder cable with a minimum of 10/3 UF. Given the distance involved, I would use at least #8 to compensate for voltage drop, if not #6.
--
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Bob Vaughan | techie @ tantivy.net |
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Bob Vaughan wrote:

You did say you were working from memory but I would be interested to know what national code language sets a minimum size for a feeder. If it is a local amendment or code I'm not interested. -- Tom Horne
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HorneTD wrote:

215-2(a)
Feeders smaller than 30A might not make any sense, but I can't think of anything inherently unsafe about them, so this might be one of those cases where the NEC has a solution in search of a problem. I wouldn't dig up a buried cable just to satisfy this part of the code unless an inspector told me I had to. (I might dig it up because I realized too late that 20A wasn't big enough, or because the voltage drop is unacceptable, but not just to satisfy this point in the code.)
Bob
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zxcvbob wrote:

You're right! There it is in 215-2(a)(2) in plain black and white. I really appreciate your taking the time to find that reference. I was obviously unfamiliar with it. Now there is absolutely no good reason that the OP would want to keep that panel in the out building. If he combines both circuits in the out building into one branch circuit he could then supply it with the twelve gage two wire feeder but what would be the point of that. His best course of action is to remove the panel from the out building and substitute a large J box with a switch mounted in a raised cover to function as the building disconnecting means. That switch should be rated for the entire ampacity of the twenty ampere circuit. At least then he will not be obliged to install a grounding electrode system. -- Tom Horne
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Inserting that screw is the same as connecting your grounds and neutrals together. NO. Do not insert the green bonding screw in the neutral bar and screw it into the threaded hole for your subpanel. This is only done in your main panel. I'm beginning to wonder if you should be doing this type of work. From what you have posted so far it seems as though you have shortcomings in your knowledge concerning basic wiring.
John Grabowski http://www.mrelectrician.tv
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B. Adams wrote:

No the US NEC Does not require or permit the installation of the bonding screw in this case. To install it would be a violation of 250.24 (A) (5) vis.. 250.24 Grounding Service-Supplied Alternating-Current Systems. (A) System Grounding Connections. A premises wiring system supplied by a grounded ac service shall have a grounding electrode conductor connected to the grounded service conductor, at each service, in accordance with 250.24(A)(1) through (A)(5). (5) Load-Side Grounding Connections. A grounding connection shall not be made to any grounded circuit conductor on the load side of the service disconnecting means except as otherwise permitted in this article. -- Tom Horne
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UF was indeed used and GFCI receptacles.
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NO. In a subpanel, the NEC requires that the neutral NOT be bonded. Bonding is *only* for service entrances.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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