main lug hot wire connector
 

A main lug has 2 spaces for 4 circuits. There are two places to connect the two phases of hot wire to. If I only have one hot wire available can I create a connector between the two locations where you are supposed to connect hot wires in the main lug? Is that safe or is that a big no-no?

The one hot wire available will never have more than 30A and the main lug is rated at 60A so don't worry about overloading anything.

Is this for a subpanel?

You would have to splice them and not use the lug for 2 wires.

exactamundo

by splice you mean outside the box?

what is the difference between doing that and connecting them inside the box?

Splice them inside the box.
The lugs are rated for 1 wire only.

Since your subpanel will be on only one leg, be sure to balance your load across both legs.