This is the main panel, the green neutral bonding screw is there. How to differentiate the neutral and ground bars? In this case, does it matter which one I use as ground or neutral for the interlock kit installation?
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In a main panel with bond, there is no (functional) difference.
Many workers will still separate the functions, and "you tell" based on "what's in this bar? - White or Green/bare?" Also, Neutrals are one wire per hole, while grounds in most bars are allowed to be multiple wires per hole (check the label on the box for details and torques - and always use a torque driver.)
In a bonded main, mixing them is allowed, as they are "the same" due to the bond.
You can use that lug for the neutral from the generator. I would not use it for ground. Go to a neutral or ground bar for that.
It appears all the bars in this panel are configured as neutral bars. There are ~56 neutral lugs, and 40 spaces, so barely enough neutrals. If you needed to run this panel as a sub, you would need accessory ground bars.
On a service panel, the metal chassis is grounded. It's important to be able to separate neutral and ground.
The key feature of a neutral bar is the insulation: it is insulated from the panel chassis. You only need to remove a ground screw or strap, and the bar is isolated from chassis.
Neutral bars have a heavy, high-current path between the bar and neutral lug, which is itself isolated from the chassis It is obvious that the neutral lug-to-bar connection is heavy, and designed to flow a lot of current all the time.
Ground bars are, by design, in direct contact with the panel chassis. This can't be removed; there'd be no way to isolate them even if you wanted to.
Ground bars may not have a dedicated electrical connection to neutral. They may rely on the case itself for their current flow to the other ground bars and any neutral-ground bonds.
In a main panel, there is nothing wrong with using neutral bars to attach grounds.
Do not put neutrals on ground bars. Their current paths are not designed to flow high current continuously.
If you see "an awful lot of neutrals", consider the worst-case for the panel, of every space in the breaker filled with a tandem/duplex/twin breaker driving a single circuit; thus you need twice as many neutral screws as you have spaces. If you have a gross excess of that number, then most likely the manufacturer intended some bars to be ground bars.
"No, the neutral and ground should never be wired together. This is wrong, and potentially dangerous. When you plug in something in the outlet, the neutral will be live, as it closes the circuit. ... However, if there is something wrong and the neutral is disconnected, it will make the appliance dangerous."