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Para um condutor de aterramento do equipamento de retroajuste, seu gabinete está no ponto 2 da NEC 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) 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
Então, o seu plano com o parafuso dividido é bom. Você pode usar a THHN verde sem um conduíte para isso, desde que esteja protegido contra danos mecânicos e roteado dentro da estrutura do prédio, a propósito, conforme 250.120 (C):
(C) Equipment Grounding Conductors Smaller Than 6 AWG. Where not routed with circuit conductors as permitted in 250.130(C) and 250.134(B) Exception No.2, equipment grounding conductors smaller than 6 AWG shall be protected from physical damage by an identified raceway or cable armor unless installed within hollow spaces of the framing members of buildings or structures and where not subject to physical damage.
Se você precisar fazer uma corrida exposta, no entanto, um cabo de aterramento blindado vazio será mais barato e mais fácil de manusear do que ter que passar o conduíte apenas por um fio terra.