Hello,

I have a old (1980) portable generator with a 30A, 240V twistlock receptacle (3-prong) with 2 hots and gnd. It also has 120V, 20A receptacle as well on the panel of the generator.

Do I have to run a seperate neutral from panel to the generator and terminate the neutral to the neutral at the generator? Or do I connect the gnd from the generator receptacle to the neutral at the panel?

Thanks in advance

Are you talking about connecting to the house panel?

Finding the wiring diagram would be helpful. Do you have the instruction manual?

You need to figure out of that 3d prong is the neutral (it probably is), then figure out if it is bonded to the generator frame. That is just a few minutes with your meter.
Use the neutral and the ground on the 120v outlets as references.

If this is the neutral and isolated from the frame you would connect it as a non-SDS supply. The transfer switch will transfer the ungrounded connection and the neutral is a direct connection along with an EGC to the generator frame that you will need to add.

If it is for a panel that has utility power, it goes to a transfer switch, not to the panel. If the gen don't have neutral, you can't power 120 loads with 240 side

Just a note of caution!!

YOU must prevent any 'backfeed' to your utility (POCO) wiring. As you profile describes you as an EE, I will suggest you locate a qualified electrical contractor within your local area to connect your generator.

You also must be mindful of the carbon monoxide exhaust fumes from the gen! Three EMS trips to the local hospital today for people who had gens in open door garages.

Be safe