Updating electric in an attached shop. There are two sub panels, both fed 240 from the main panel in the house. The one has large aluminum conduit, with the conduit serving as ground. The smaller panel (screw in fuses!) just has a cable with red, black and white. I'd like to just run a separate ground for the smaller panel replacement to the conduit where they both enter the shop. It would be extremely difficult to get a new line to the main panel.
Shop is about 1000 sf.
Larger panel is on a 60A breaker. It might feed at most - a minisplit (MCA 15A) and a welder (under 20A) both legs, plus it might have 20a of misc 120 loads at one time. 6AWG in 2" conduit. The conduit is routed through the attic of the house, coming out in the ceiling of the shop.
Could get a line to the smaller from the larger, it might see 25A during vacuuming with a heater going etc though. Plug in vacuum and 120 heater intended for standard outlets, so 8-13.5A even if replaced with other models.