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According to NEC 2014 250.130 :

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) To an equipment grounding conductor that is part of another branch circuit that originates from the enclosure where the branch circuit for the receptacle or branch circuit originates
  • (5) For grounded systems, the grounded service conductor within the service equipment enclosure.
  • (6) For ungrounded systems, the grounding terminal bar within the service equipment enclosure

This allows for a non grounded branch circuit to borrow a ground from another branch circuit. (related stackexchange question Is it OK to borrow a ground wire from a different circuit?)

My question would be what would be the best way to bond it? Run THHN inside a conduit? Is a conduit necessary? Is bare copper bad? Should I use a bonding lug on the outside of a box? Should you wrap the conduit in green tape? Can you use green THHN without a conduit? Is it bad to use romex and just cut off the non-grounding wires?

I'm trying to think of the best way to do it without confusing future electricians too much.

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    It depends: are you trying to retrofit ground for an existing branch circuit (which was installed in compliance with code at the time, prior to grounding requirements), or are you looking to install a brand new branch circuit today in 2021? You're not allowed to "retrofit" on new work. – nobody May 11 at 1:22
  • Can you post photos of the inside of the box you're trying to bring grounding to? – ThreePhaseEel May 11 at 23:07
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You can barrow a ground from another circuit as long as they come from the same panel or grounding electrode conductor.

The size of the grounding conductor depends on the branch circuit. See table 250.122.

the maximum required size is 6awg when connecting to the existing GEC. (Even if tying into a #4 ufer ground or concrete encased electrode)

The minimum size is 14 awg if you are grounding a 15 amp circuit and since you won’t find anything smaller that makes 15 amp circuits Easy.

When you have a larger circuit let’s say a 60 amp you have to use a #10 ground So you have to ground off of a #10 ground or larger (most electricians will pull a (#10 and use that for multiple branch circuits) if I am doing an entire home I will pull a #10 unless actually grounding a sub then I go #6 (I don’t carry #8 normally) I run the length of the home in the crawl space or attic and drop a j box every 20’ or so. (Yes boxes are needed and the box it self requires a ground to the box if there is a splice in the box.)

If tapping the ground from a higher amperage circuit inside that is ok.

I do not run grounds in conduit unless outside and smaller than #6.

So just run 6 to a outside location.

Thhn/thwn dual rated green # 10 is my normal wire but it can be bare. If in conduit, No you don’t color the conduit green.

I think I hit all of your points.

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  • Is the reason you do not run grounds in conduit because of personal preference or a code specification? You mention you run a #10 through the attic with junction boxes for splices - but later imply that you don't use conduit. Are you running bare thhn into and out of junction boxes? – Vigrond May 10 at 23:42
  • Conduit is not required (possibly it is in Chicago) but the grounding conductor is not a current carrying conductor the only time or will see current is in a fault condition or the use of a lighting controll that uses the ground and those are limited to .5ma 1/10 of what it takes to trip a GFCI. It is cheaper and faster to run #6 than 10 in conduit. I would miss a lot of jobs adding that much unneeded cost. The wires still require a bushing in metal boxes. I run bare #6 but green#10 thhn (bare wire only has a type copper/aluminum and size). Note aluminum can’t be used within 18” of earth. – Ed Beal May 11 at 0:00
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You said conduit

You don't get to rely on legacy rules when you have conduit between your boxes. It is presumed to be easy to pull out all the wires in the conduit and change them to the correct wires.

So do that.

It's always been true that multiple circuits in conduit can share 1 ground. That's not even in that rule.

Don't forget, metal conduit IS the ground path

A lot of novices come upon older conduit work, see no ground WIRES, and conclude that the conduit is too old to have been built with ground wires, and that's a deficiency they must correct.

Not true at all. Older conduit work (which predates PVC) is certainly metal conduit.

All non-flex metal conduit (even brand new work!) allows the metal shell of the conduit to carry the grounding. Thus, finding no green/bare wires in metal conduit is perfectly normal.

Certain flex conduits also provide a ground path.

Metal boxes are grounded in any case

What also surprises novices is that devices can pick up grounds off the metal box. After all, they're used to plastic boxes which are not grounded obviously, and so they are used to running ground wires to the device only and not the box.

Not with metal boxes. Ground must come to the box FIRST - that's Code. If it comes in via wires, it must go to a ground screw on the box (they give you a 10-32 screw hole for that purpose).

Then, the devices can pick up ground off the box via one of several ways:

  • Switches ground through the mounting screw heads.
  • Receptacles with a "self-grounding" feature ground via a contact wiper on the mounting screw threads.
  • Receptacles with clean flush bare metal contact with the box (i.e. not floating above it via drywall ears) ground that way.
  • The remainder use a ground wire to the metal box, to that grounding screw, ground clip, etc.

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