Okay, this is a fun thought experiment! Let's brainstorm some potential uses for a 6/8 3W+G line in a 1979 house, considering the era and the wire characteristics you described

High-Power Appliances/Equipment (Keeping in mind 12kW is a lot for the time):
Electric Furnace/Baseboard Heating

While less common than gas in many areas, electric resistance heating was definitely an option in 1979. A supplemental or backup electric furnace, or a zone of electric baseboard heaters requiring a dedicated high-capacity circuit, could be a possibility.

Early Whole-House Air Conditioning (Heat Pump?)

Some of the very early whole-house A/C units, especially those using heat pump technology, might have required that much power. Though inefficient, they existed.

Commercial/Heavy-Duty Equipment (Less Likely, but possible depending on the owner's history)

Was the previous owner a hobbyist woodworker? A metalworker? A mechanic who worked on cars at home? Maybe it powered a large compressor, a welder, a kiln, or some other shop equipment.

Large Sauna/Steam Room

High-end amenity for the time, but a large electric sauna could require a significant power draw.

Solar Hot Water System (Electric Booster)

Some early solar hot water systems used electric heating elements as backup/boosters. A large system (e.g., for a family with several children) could have required a dedicated circuit.

Water Heater (Unlikely Primary, but Possible Booster)

While a standard electric water heater usually doesn't need that much, perhaps it was intended as a super-fast recovery booster, or for a very large tank.

Arc Furnace (Even less likely, but...)

Probably not, but if they were some kind of metallurgist they might have had a small arc furnace to experiment with.

Less Likely, But Still Potentially Possible



Amateur Radio Equipment

Dedicated power supply for a high-power amateur radio setup. While a 12kW capacity seems excessive, some enthusiasts went to extremes.

Early Computer Equipment

While personal computers were still relatively new in 1979, a serious hobbyist or someone working from home in a tech field might have had a large mainframe-type device which needed that amount of power.

Grow Lights

This is a modern consideration, but if the previous owner was growing something indoors on a large scale, they may have run power up to their "garden".

Hidden Safe Room

In the event of a break-in or some other sort of emergency, a panic room with dedicated equipment might have been installed by the homeowner.

Considering the Wire Characteristics



Stranded Wire

This is an interesting point. Stranded wire is generally more flexible and easier to work with, making it ideal for runs where movement or vibration is expected, or where tight bends are required. This could suggest a connection to something that moved or vibrated.

Shared Neutral with other 2F circuits

If it really is tied to a shared neutral, then that limits the likely use case to something that would be used at the same time as those second floor circuits.

Age/Style

The age and style being consistent with other original high-power lines (like the dryer) is key. It confirms it was likely intended for a major appliance or piece of equipment that was considered a normal part of a well-equipped home in that era.

Investigative Avenues



Talk to Neighbors (If Possible)

Someone who lived in the area in the late 70s/early 80s might have seen the previous owner using something connected to that line.

Check Old Building Permits: See if there are any records of electrical work done that mention the installation of a high-power circuit.Good luck with your investigation! It's a fascinating mystery. Flag for review