Bonding - ATS vs Main Panel

[cerbone0]

Currently have a 200A main panel that is also the main disconnect. Almost all the cables enter from the top. There are 36 spaces populated in a QO panel and all the neutrals and grounds are attached to the left and right neutral buses up near the main breaker. There are a few connected to ground bars further down along the sides. As expected, all the grounds for the earth rod, water line, and gas line all terminate at the main panel.

A 24KW Generac generator and 200A service rated ATS is being installed (neutral not switched). The ATS is going in the basement a few feet away from the main panel. The plan is to replace the 3-wire SEU cable from the meter to main panel with one going to the ATS. (Local authority has no issue with the distance from meter entry into the house.) Then a 4-wire SER cable will go from ATS to main panel.

I understand that the code wants neutral to ground bonding at the first service disconnect which would now become the ATS. The expectation is to remove the rod, water, and gas grounds from the main panel and terminate them in the ATS. Also, the neutrals and grounds are to be separated in the main panel and the bonding screw removed.

As you can imagine, the effort at the main panel, especially with all the short ground leads, is quite exhaustive.

I was analyzing the circuit paths and believe that if the main panel is left as-is, but the ATS bonding jumper is removed, since 4 wires connect ATS to main panel and generator, and since the main panel is the only bonding location, it appears that the electrical paths for ground and neutral currents are essentially the same either way.

Shouldn’t this be OK? What am I missing?


Sent from my iPhone using Tapatalk

 

[Little Bill]

If the ATS is service rated, then IMO, it is the 1st means of disconnect with OCP and is now the main. That makes the old main a subpanel and must be wired accordingly.

 

[cerbone0]

I understand this is the case according to code. Essentially, my question is more from a technical point of view. Why would the code dictate this when electrically, it’s accomplishing the same thing? The bonding is only in one place, the grounds and neutrals are separated everywhere else, wire sizes are appropriate, and the circuitry routes the faults to the same place.

I just don’t get the reasoning. The neutral and ground would be separated in the ATS and everywhere else except the main panel where it’s bonded. All fault currents, including from the ATS, would be routed th the main panel. The 200A SER cable between the main panel and ATS effectively extends the neutral and ground buses as if the two panels were acting as one.


Sent from my iPhone using Tapatalk

 

[cerbone0]

Wondering if anyone can shed some light on the reasoning as discussed?


Sent from my iPhone using Tapatalk