Need to add ground to subpanel
Thank you in advance for any help!
I have a ground problem, and would like some advice to fix it. This is a 1912 Victorian that had electrical upgrades in the 70s or 80s.
There is a recently repurposed, large (40A) dryer run into a subpanel to provide 2 new 20A circuits for a remodel. One circuit for a microwave, and one circuit for a gas furnace. Apparently the ground is insufficient, as the furnace wouldn't work correctly until I provided it with a good, known, dedicated ground from a different circuit for testing purposes. The furnace would throw strange codes that didn't make sense. Everything else works fine on either circuit, including the microwave.
I'll describe the re-purposed circuit, and would like suggestions on ways to fix this problem. There will be an inspector coming to look at the furnace and 2 new circuits, and since this will be an "after the fact" permit, I think he may pass not only something to code, but something that is otherwise safe. He seems to have a fairly easy going attitude.
At the front of the house, there is the main 100A panel. There is a double 40A breaker that feeds steel conduit that has 3 black conductors, I believe 6 or 8 gauge (can't remember now, but was completed as part of a HUD upgrade in the 70s or 80s at the time of the 100A main panel replacement). There is no ground in the conduit. The conduit was run in the 4 ft crawlspace about 25ft to the middle of the house. A new subpanel is installed, and each hot leg of the original large wire was wire nutted to feed the hot leg of the two new circuits, which are 12ga Romex. The neutrals are wire nutted together. The grounds of each 12ga circuit are screwed to the inside of the subpanel. The steel conduit uses the set screw type of connector to attach it to the panel. I do not believe that the steel conduit has its own dedicated ground at the main panel side. Obviously, the older portion of the circuit is fairly weathered, though the crawl space does stay dry. California :)
Is the steel conduit designed to provide proper ground, and if so, what can I do to try and restore its functioning?
If there needs to be a dedicated ground wire, is there any alternative to trying to push a new conductor through all the existing conduit? I'm just not experienced enough to know if there's enough room to add a conductor.