I came across a panel the other day that was wired like a sub-panel with neutrals and grounds separated, and had no main breaker.
The disconnect was located outside.
No other panel located in the house.
Is it required to have the main panel wired as a sub-panel when the main disconnect is elsewhere?
The panel inside is not a “main panel”. It is a remote distribution panel. The panel outside is the “main panel”
All panels (with few exceptions) downstream of the service panel, need to have the neutrals and grounds separated.
The “main panel” is the panel with the service disconnect.
In other words, the inside panel was a sub panel, and sounds like it was wired correctly.
Most of the times I see this, in my area, it is because the service drop/lateral is not an a good spot. The service panel needs to be as close as practical to the meter, and when the meter is on the exterior wall of a master bedroom for example, they will make the service panel (and disconnect) outside, so the remote distribution panel can be installed anywhere inside they want.