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In California, I have a metal jbox and, in it, there is a junction for a 10/2 NM and a 14/2. These have different size ground conductors.

Does the code require a specific size ground conductor to pigtail to the metal box?

It stands to reason in my mind that I would use the largest circuit ground conductor size but I am looking any code specifics.

  • What is the size (Amperage) of the breaker feeding this circuit? Are you just asking about the size of a pigtail to the box? Is the feed coming on the 10 or the 14? i.e. not uncommon to have a larger wire size for a long outside run, where the feed would be 14 Ga. – Ecnerwal May 27 '19 at 17:43
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It needs to match the circuit grounding conductors in size, basically

The clearest indication of the minimum size a box grounding jumper needs to be comes from NEC 250.146 (emphasis mine):

250.146 Connecting Receptacle Grounding Terminal to Box. An equipment bonding jumper shall be used to connect the grounding terminal of a grounding-type receptacle to a grounded box unless grounded as in 250.146(A) through (D). The equipment bonding jumper shall be sized in accordance with Table 250.122 based on the rating of the overcurrent device protecting the circuit conductors.

This basically requires the box grounding jumper to be at least as large as the minimum size ground conductor permitted for a circuit of that amp rating. Further support for the OP's position, where the grounding jumper to the box must be the size of the largest ground wire coming into the box even if that ground wire is oversized as per NEC 250.122(B), comes from the precise text of NEC 250.148(C):

(C) Metal Boxes. A connection shall be made between the one or more equipment grounding conductors and a metal box by means of a grounding screw that shall be used for no other purpose, equipment listed for grounding, or a listed grounding device.

In particular, since it basically implies that the jumper between the grounding screw and the equipment grounding conductors coming into the box is an equipment grounding conductor, it means that this jumper must comply with all the equipment grounding conductor sizing rules in NEC 250.122 in order to conform with this section of the NEC.

  • Please explain further; you quoted the text of the code which basically says the jumper must only be sized to the circuit breaker rating, then you repeated that in your own words. It seems your position supporting the statement that "the grounding jumper to the box must be the size of the largest ground wire coming into the box even if that ground wire is oversized" contradicts both. – Jimmy Fix-it May 27 '19 at 22:13
  • @JimmyFix-it -- the intent of this was basically cumulative (this says that the ground wire can't be grossly undersized, while that part then brings the full 250.122 atop it). Do you have any suggestions as to how to reword this? – ThreePhaseEel May 27 '19 at 22:15
  • No sir, as always I defer to your electrical expertise, I just couldn't figure if the answer is Y or N. I guess sometimes, when the code is ambiguous, it is best to choose the most protective (and cautious) path... – Jimmy Fix-it May 28 '19 at 2:07

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