I do not know. I can only assume you have 14 gauge wire running 300 feet to a point where you want to power at least a 600 watt transformer on your 15 amp circuit. How many watts of lighting is out there already? How long is the run? Total planned wattage including timers and other devices on that circuit?

Thanks for response. There are no low voltage lights yet. I'm looking to hardwire a new transformer into the existing 120v line that currently powers my 120v driveway landscape lights. I'm looking for a transformer that can always be set to on when it receives power from that 120v line, which does have a timer setup in my garage to activate those driveway lights. Are there transformers that can be hard wired, and essentially have no timer so it'll just turn on and power my future low voltage lights, when the 120v line gets activated for the normal 120v lights I currently have.

Hope that is clearer. Thanks!

Yes. Understanding that the longer the run the more line loss you will have is important though, and add to that the current load on the circuit and you will have 3 parts of the answer. There are charts that describe how long a wire can be to carry a load at a given temperature and if you need to size up the wire past a certain point to keep it or the contacts cool. Safe loading is 80% of what the line can load to, to prevent heat and fire. A 15 amp circuit then would load to no more than 15x.8 follow? Or since Volts x Amps =Wattage then 15a x 110v = 1650w and safety dictates 1650w x .8 = 1320watts. Deduct light and transformer wattages (all devices) and length of run deduction from there. I should say my driveway is quite long so it influences my answer but your situation is different I would guess.