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I've got a circuit in the basement carrying a handful of outlets. This circuit has no ground wire. I want to add a couple of outlets and want them to be grounded. I may even want gfci outlets on them in the future. I have no reasonable way of getting a ground wire to the panel but I do have easy access to another circuit that does have a ground wire. Is it acceptable to borrow or share a ground wire between circuits? If not, why not?

Both circuits are 20A with 12 AWG wire.

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Here is a good article written by Mike Holt.Replacing 2-Wire Ungrounded Receptacles. –  Tester101 Dec 12 '11 at 13:46
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4 Answers 4

up vote 15 down vote accepted

No, you cannot extend the equipment grounding conductor of one branch circuit to another branch circuit.

Here is what NEC 2011 has to say about replacing receptacles with no equipment ground.

National Electrical Code 2014

Article 250 - Grounding and Bonding

VII. Methods of Equipment Grounding

250.130 Equipment Grounding Conductor Connections. Equipment grounding conductor connections at the source of separately derived systems shall be made in accordance with 250.30(A)(1). Equipment grounding conductor connections at service equipment shall be made as indicated in 250.130(A) or (B). For replacement of non–grounding-type receptacles with grounding-type receptacles and for branch-circuit extensions only in existing installations that do not have an equipment grounding conductor in the branch circuit, connections shall be permitted as indicated in 250.130(C).

(C) Nongrounding Receptacle Replacement or Branch Circuit Extensions. The equipment grounding conductor of a grounding-type receptacle or a branch-circuit extension shall be permitted to be connected to any of the following:
(1) Any accessible point on the grounding electrode system as described in 250.50
(2) Any accessible point on the grounding electrode conductor
(3) The equipment grounding terminal bar within the enclosure where the branch circuit for the receptacle or branch circuit originates
(4) For grounded systems, the grounded service conductor within the service equipment enclosure.
(5) For ungrounded systems, the grounding terminal bar within the service equipment enclosure

Which means you can add a grounding conductor, but only at or before the service panel.

NOTE:
The grounding electrode system (of which the grounding electrode is a part), dose NOT include branch circuit equipment grounding conductors. The grounding electrode system is a collection of conductors used to connect the service equipment to earth, and is typically located at or near the service entrance only.

More from the NEC

National Electrical Code 2014

Article 406 - Receptacles, Cord Connectors, and Attachment Plugs (Caps)

406.4 General Installation Requirements. Receptacle outlets shall be located in branch circuits in accordance with Part III of Article 210. General installation requirements shall be in accordance with 406.4(A) through (F).

(C) Methods of Grounding. The equipment grounding conductor contacts of receptacles and cord connectors shall be grounded by connection to the equipment grounding conductor of the circuit supplying the receptacle or cord connector.

(D) Replacements. Replacement of receptacles shall comply with 406.4(D)(1) through (D)(6) as applicable. Arc-fault circuit-interrupter and ground-fault circuit-interrupter type receptacles shall be installed in a readily accessible location.

(1) Grounding-Type Receptacles. Where a grounding means exists in the receptacle enclosure or an equipment grounding conductor is installed in accordance with 250.130(C), grounding-type receptacles shall be used and shall be connected to the equipment grounding conductor in accordance with 406.4(C) or 250.130(C). \

(2) Non–Grounding-Type Receptacles. Where attachment to an equipment grounding conductor does not exist in the receptacle enclosure, the installation shall comply with (D)(2)(a), (D)(2)(b), or (D)(2)(c).

(a) A non–grounding-type receptacle(s) shall be permitted to be replaced with another non–grounding-type receptacle( s).

(b) A non–grounding-type receptacle(s) shall be permitted to be replaced with a ground-fault circuit interrupter type of receptacle(s). These receptacles shall be marked “No Equipment Ground.” An equipment grounding conductor shall not be connected from the ground-fault circuit interrupter type receptacle to any outlet supplied from the ground-fault circuit-interrupter receptacle.

(c) A non–grounding-type receptacle(s) shall be permitted to be replaced with a grounding-type receptacle(s) where supplied through a ground-fault circuit interrupter. Grounding-type receptacles supplied through the groundfault circuit interrupter shall be marked “GFCI Protected” and “No Equipment Ground.” An equipment grounding conductor shall not be connected between the grounding type receptacles.

(3) Ground-Fault Circuit Interrupters. Ground-fault circuit-interrupter protected receptacles shall be provided where replacements are made at receptacle outlets that are required to be so protected elsewhere in this Code.

(4) Arc-Fault Circuit-Interrupter Protection. Where a receptacle outlet is supplied by a branch circuit that requires arc-fault circuit interrupter protection as specified elsewhere in this Code, a replacement receptacle at this outlet shall be one of the following:
(1) A listed outlet branch circuit type arc-fault circuit interrupter receptacle.
(2) A receptacle protected by a listed outlet branch circuit type arc-fault circuit interrupter type receptacle.
(3) A receptacle protected by a listed combination type arc-fault circuit interrupter type circuit breaker

(5) Tamper-Resistant Receptacles. Listed tamper resistant receptacles shall be provided where replacements are made at receptacle outlets that are required to be tamper-resistant elsewhere in this Code.

(6) Weather-Resistant Receptacles. Weather-resistant receptacles shall be provided where replacements are made at receptacle outlets that are required to be so protected elsewhere in this Code.

There's a good YouTube video, where Mike Holt explains the options for Replacing two-wire receptacles

Consult your local government and/or a licensed electrician, to determine which version of code is used in your area. Codes do change, and you'll always have to follow the code adopted in your area.

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+1 Thanks for the reference and the good article. –  BMitch Dec 12 '11 at 15:52
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Wow. Thank you very much for digging up the code on this. This answers half my question. I guess I can speculate on the "why" half of my question: 1) It's confusing to anyone doing work on the electrical in the future. 2) A ground fault on one circuit may present a hazard to anyone assuming the other circuit is not energized. 3) In the exceptionally rare case there is a ground fault on both circuits simultaneously, the ground wire load capability may be exceeded. -- Am I missing anything? –  Paul Dec 12 '11 at 16:46
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@Paul: one reason I thought of was in the event of disconnecting an old circuit. Someone would not know that the grounding for another circuit is provided by the line they disconnected. –  BMitch Dec 12 '11 at 18:31
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@Tester101 Unfortunately, the "The NEC is right because they are the authority and they are the authority because they are right" produces a dangerous logical short-circuit. :) –  Paul Dec 12 '11 at 19:05
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Thanks again everyone. I'm clearly better off with ungrounded, properly-labeled outlets in my new rec room. Which is unfortunate since it is a great place for kids to make a mess. –  Paul Dec 12 '11 at 19:12
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To answer your question you can pigback onto existing Ground Wires with no problems. That is what normally happens in a home.

Whether the building codes in your country permit it is another story.

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There is no ground circuit. A circuit is a loop of wire that typically has a load (appliance) on it and a difference in potential (voltage) between the two wires involved.

A ground system is designed to provide a single return point to earth with very low resistance so that it will absorb any electrical power that comes in contact with it. That prevents a human from absorbing that energy, and also allows quick failure (tripping of the circuit) when something goes wrong instead of your appliance sitting there waiting for you to touch it and get a very nasty shock.

Every ground wire is connected to a bus bar and from there to a rod (very often a cold water pipe) that is driven into the ground. An important thing is to never create a loop with the ground wire -- only allow one path to grounding point / bus bar (otherwise you may create a circuit and encounter a floating ground). Other than that, you should be able to tie into any single ground wire you can get your hands on unless you have a complex distribution system.

Understanding multiple / "independent" grounds

General Wikipedia article

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Just because it can be done, does not always mean it should be done. –  Tester101 Dec 12 '11 at 15:59
    
@Tester101 Yet, in this case, I can't think of a reason why it shouldn't be done. In radio tower setups or server rooms, grounding paths are completely independent from circuit paths -- many circuits will ground to the same equipment rack and through fat copper straps to the ground point. Electrically speaking, they are also independent inside a home (the bus bar), but for convenience come in 3-wire bundles that follow the same path. (For fun, research lightning grounds, or how to keep your equipment running during a direct strike). –  Jeff Ferland Dec 12 '11 at 16:06
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You are getting things confused here. First there are differences in home wiring, as opposed to industrial wiring. Second when thinking lightning strike, you're thinking Ground Electrode Conductor (GEC). When thinking ground fault, you're thinking Equipment Ground Conductor (EGC). –  Tester101 Dec 12 '11 at 16:46
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Grounds can be joined between circuits, but as Tester points out in his answer, your situation isn't something NEC would approve of.

GFCI won't know the difference since it's looking for a leakage of current out the hot that doesn't return via the neutral, so how you get grounded (be it the ground wire or the bath tub) doesn't mater.

See also this post neutral/ground connections.

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@Tester101, what does NEC have to say on this? –  BMitch Dec 12 '11 at 14:28
    
found it 406.3(c). –  Tester101 Dec 12 '11 at 16:47
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