Verified answer

It does sound like you may have a 3 phase supply, it would be odd in a residential, but normal in commercial. 150 amp fusing for 2/0 AL may have been legal when installed. It sounds like a type of commercial "high leg delta" installation was meant to feed two panels. They ran larger conductors for the legs that will feed a single phase panel that feeds 120v loads. Those legs would typically be protected by 200 amp fuses. The smaller wire was meant to be part of the feed for a 3 phase panel that used all three wires and fed motor or hvac loads. Not that uncommon in "historic" parts of town, but still a bit odd for a residential service.

If it is a "high leg delta" it should be orange tape, but it could be a wye style and the blue tape is missing from the "C" leg.

If you have a meter these are the typical readings:

High leg, All legs measured phase to phase should be 240v, A phase and C phase to ground should be 120v, and B leg to ground should be around 206v. This type should have B leg marked orange.

Wye, All legs measured phase to phase should read 208v, All phases measured to ground should read 120v. Typical markings for this are one leg unmarked with the others marked red and blue.

The shutoff outside, it sounds like by how you are describing it, is required due to the location of the panel relative to the meter box (outside)...I believe it (the breaker panel) cannot be much more than 5 feet from the meter box (I could be inaccurate by a couple of feet here) or a shutoff is required. That additional 2/0 wire is the separate Neutral wire required on a main shutoff outside. It is not a 3rd hot (3-phase) if only because of the smaller size of the cable and also because it is unusual (not at all impossible mind...just unusual) for there to be a 3-phase set up in a single family residential building.

I am curious. How can you have an additional hot on a single phase system? Could it be that in the past someone had 3 phase coming from the service pole? Look again. Maybe one of the phases were tapped to make it look like third phase.

Warning: If you are using aluminum wire, be sure to periodically check the connections in the panel to make sure they are tight. Aluminum will cold flow (move away from the point of pressure). If connections get too loose arcing in the panel can ocur. Something I learned 39 years ago as an eletrical apprentice.

You have a 240 volt three phase delta high leg service. Two of your conductors are 120 volts each and together they are 240 volts. The third wire you have with red tape is the high leg or wild leg. It has 200 to 208 volts on it. Sometime in the past there was need for a 3 phase service and is no longer needed.

He is absolutely speakme about the place the wires enter the provider panel from the outside and connect to primary breaker. These wires are in most cases aluminum and can be measurement 00 or higher. There's also a neutral conductor that connects to the neutral bar. Codes state that anti-oxidants are required where distinctive metals come collectively.

Sounds like a 3 phase service entrance comes into your property.

Must have been a three-phase connection there at one time or another.