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I am adding a sub panel in an unattached building. My sub panel has the connection for the 2 hot wires coming from the source panel (a 40 amp double breaker) In addition it has one neutral/ground bar which has a green screw head for the ground and a row of places to connect neutrals. I am seeing advice on sites like yours to not connect the ground wire on the same bar as the neutral. So with only one bar I am not sure what to do with my ground wires. Do I need to install a second ground bar? I have the ground wire coming from the house that is well grounded through the house 200 amp service as well as a buried ground rods outside of the outbuilding. Do I use both and if so do I connect them both to the same ground bar?

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Jay L's answer sounds good. The green head screw usually comes separately and is provided to bond the neutral bar to the panel and should not be used in a subpanel. –  user24125 Sep 21 '14 at 5:50

3 Answers 3

The neutral and ground MUST NOT be bonded at a sub-panel. They should only be bonded at the main service panel. If you bond them anywhere other than the main service, the neutral return current now has multiple paths, including though your ground wire.

You should be able to buy a second bar for the sub-panel if it really is meant to be used as a sub-panel. The neutral bar will need to be isolated (it should have plastic insulator separating it from the case). The ground bar should be bonded to the case.

The ground wire from the house must be connected to the sub-panel ground. Since it is a detached building the ground rods are also required by code and should be tied to the sub-panel ground. The ground to the service is the most important because it will provide the low resistance path back to the service in case of a fault.

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required by code!? What code? Can you site the code section? Can you quote the code? –  Tester101 Jun 11 '13 at 16:34
2008 NEC 250.32 –  JayL Jun 12 '13 at 4:22
“Grounding Electrode. Building(s) or structure(s) supplied by feeder(s) or branch circuits(s) shall have a grounding electrode or grounding electrode system installed in accordance with Part III of Article 250.” –  JayL Jun 12 '13 at 5:33

Changes to the Electrical Code now REQUIRE all unattached buildings with sub panels, to have the Neutral Bar Grounded with #6 wire to the panel box then to a grounding rod in the earth. (Rule 10-208)

You CANNOT use the ground included in say #10/3 wire as your sub panel ground anymore.

You are to treat the sub panel just as a service that is coming in from the meter, the only conductors that should be entering your sub panel now are your 2 Hot Feeds, and your Neutral. (no ground from the main panel.)

Now the confusing part, Rule 10-208 (b) states that you can ground any metal non current carrying components (outlet boxes) through a separate conductor back to the main panel if the building does not house livestock. However, since you must now have a bonded neutral in sub panels, this is more of a pain to attempt since you have a separate grounding circut right there already.

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What rule is this you refer to? And just how do you know it applies to the OP? We don't know where either of you are from. –  Speedy Petey Sep 20 '14 at 23:02

You guys are all nuts you dont make sense. I have the same box im putting in my shed. I have a problem with my 200 amp main breaker. Its real old it needs to be changed. Im not paying the electric company 450 dollars to turn it off and on i go in the box live. Just email me back a 200 amp 25 year old box the grounds and neutrals Are on both sides of the box the long bars and yes it takes murry breakers. My garage has the same box he has all it really is a one 220 breaker kill switch it has 4 ten going to it thn copper at 145 feet run in pvc are you a electrician. Well my ground and nuturals in the house box go on seperate screws on bar and i run a two double 30amp breaker should go smaller with two 20amps well the bar is not has plastic from grounding out. So my netural and ground to bar the main is like that and no ground wire to metal box and i got two single breakers in garage im using for 110 use. 15Amps for all the lighting and a twenty for outlets. I use a extra wire for ground on oulet to bar i ran ten thin to the outlets over board i run a compressor. You are not makeing english so if he was to ground the box it woul need its own ground rod by the shed.to the house he would get shocked. Me i use my tools out side in the rain am i dead i will turn the breaker off when i leave the garage soaking wet somtimes no shock. So if you ran two heavy duty extension cords to your hose with a groun its the same thing. I touch the white wires and ground in box nothing hit a hot i can die. So make sense to me 15 years i ran the garage like that. My stell saw we ill trip the breaker if i work it to hard 1500 watt electric heater works well.

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Your experience does not exempt the rest of the world from following code. –  longneck Jul 6 at 14:50

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