A sub-panel has a cable of three 12 gauge wires for HVAC coming into the box through a big knock out hole. The HVAC people did not put a connector on the cable. Is there a connector designed to be added after the fact which would snap around the wires and get pushed into the hole from the inside of the panel? The panel is in a wall with no good way to get to the outside of the box at this point.
Yes, there are connectors that can be slipped onto the cable and popped into place in the panel knockout. They do not snap over the cable though, you must disconnect the wires and slide the connector over them.