My mom just bought an old house (1917). The Main Breaker has 3 wires two hot from a 30amp 220 breaker and one ground going from the main into a sub-panel, there is no room to feed a 4th neutral wire. So there is not a neutral wire. Can I add to the sub-panel a grounding rod from the ground to a grounding pole into the dirt? I then want to use the sub panel to only hook up the one 3 wire 220-volt plug. The ground to the ground bus in the sub-panel would be connected to the ground in the main and the grounding rod at the sub-panel. The 2 hots to a 30 amp 220-volt breaker in the sub-panel. The sub-panel would never have 110 volts or anything using a neutral and would not have a neutral bus. As long as only the one 220 volts was connected this way would this be safe?