Fire officials now have a cause for the fire burning in western Calaveras County.
CDF officials say the use of a grinder in dry grass sparked the blaze off Hunt Road at Salt Springs Valley Road north of Copperopolis at 5-30 Tuesday evening.
The fire is still at 400 acres and only 10 percent contained.
There is more than 500 firefighters assigned to the blaze, equipment includes 52 fire engines 21 crews, 11 dozers, 7 water tenders, 2 air tankers and 5 helicopters.
Officials are concerned about firefighter safety due to fatigue because of non-stop fire activity this past week.
Some roads are closed because of the fire, including Pool Station road at Circle XX, and Hunt Road at Highway 4. Motorists are advised to keep an eye out for fire equipment on the roads in the Highway 4 area.