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You gave the answer below to the question of the requirement for separate bus bars after the main disconnect box ... my question is ...

In what year did the NEC start to require this separation? What is the code number?

There are supposed to be two bus bars in every panel. One for neutrals, and one for grounds.

In the main panel ONLY, they are to be bonded together. This is what references your electrical system to earth voltage. It must only be done one place in a system.

Because of the required bond in the main panel, a great many electricians conclude that there is no difference between neutral and ground in the main panel since the N-G bond is right there. And they simply spam all neutrals and grounds onto the same bus. That logic is actually... reasonable... in the main panel.

However... If you want to be a precise and competent worker, then you separate them always, as if every panel were a subpanel, and let the neutral-ground bond do its job.

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    I completely agree with your last few paragraphs. I always separate neutral and ground, least to make converting a main panel to a sub a "snap". Of course I also work mainly in EMT, so there's no ground bar at all. .... Which explains why some panels don't come with them. – Harper Oct 28 '18 at 15:21
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You're after NEC 250.32(B)(1)

The applicable NEC section is 250.32(B)(1), which reads as follows ("equipment grounding conductor" = "ground" and "grounded conductor" = "neutral" for us, and exception 2 is irrelevant for this):

(1) Supplied by a Feeder or Branch Circuit. An equipment grounding conductor, as described in 250.118, shall be run with the supply conductors and be connected to the building or structure disconnecting means and to the grounding electrode(s). The equipment grounding conductor shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. The equipment grounding conductor shall be sized in accordance with 250.122. Any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrode(s).

Exception No. 1: For installations made in compliance with previous editions of this Code that permitted such connection, the grounded conductor run with the supply to the building or structure shall be permitted to serve as the ground-fault return path if all of the following requirements continue to be met:

(1) An equipment grounding conductor is not run with the supply to the building or structure.

(2) There are no continuous metallic paths bonded to the grounding system in each building or structure involved.

(3) Ground-fault protection of equipment has not been installed on the supply side of the feeder(s).

If the grounded conductor is used for grounding in accordance with the provision of this exception, the size of the grounded conductor shall not be smaller than the larger of either of the following:

(1) That required by 220.61

(2) That required by 250.122

The current state of this section was reached in the 2008 NEC, which forbade new construction from using the feeder neutral to provide ground for the fed system. (Your choices now are to provide a separate ground wire, which is what is normally done, or use a distribution transformer to create a separately derived system for the fed building, which has its own N-G bond.) Many of the installations that fall under the current exception were installed prior to the 1999 NEC, which is the first edition that forbade parallel current paths between the structures involved.

  • I agree with 1999 as the first required isolation of netural from ground. – Ed Beal Oct 28 '18 at 14:31
  • But that probably goes hand inhand with requiring grounds on feeders (ie interbuilding ones). I don't believe Code ever said "sure, tie neutral and ground together wherever". If they both exist, they would have always needed to be separated. – Harper Oct 28 '18 at 15:29
  • @Harper -- keep in mind that the "parallel current path" likely was not a feeder EGC (in fact, before '99, feeder EGCs were only allowed under an exception, if that). More likely, it was through metal plumbing or comms wiring.... – ThreePhaseEel Oct 28 '18 at 16:19

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