EP500 PRO connection to Transfer Switch

I have a EP500 Pro and have recently purchased a Reliance PRO/Tran 2 transfer switch. Hooking up the transfer switch to the main circuit breaker panel looks straightforward. My question is how best to hook up the EP500 Pro to the transfer switch which is about 40 feet away. Obviously I need to use the L14-30 socket. Should I use a generator cable. use heavy duty wire, or something else?

I purchased a 50 ft Reliance 30a L14-30 cable from Lowes which was the cheapest price I could find at the time. This was Several years ago and was around $150. The cable is more flexible than if you use the orange 10 gauge southwire romex and adding your own plugs.

Thanks for the info.The cable being more flexible will be better for my installation.
I found a 40ft Reliance 30a L14-30 cable on Amazon for $110 so I ordered it. Not sure yet but I may cut off one plug and hardwire it to the transfer switch as I may put the cable into a conduit to make a neater installation

You may want to check out this thread on the AC300 with Transfer Switch.

Assuming the EP500 is similar, if you want to keep the transfer switch connected at all times to use pass-through UPS functionality, yo will have to use the EP500 in “online” UPS mode. Offline will not work with that switch because the neutral is not switched.