millerjoe pop over to landscape design right next door.
They should have answers.
I ran 110 wiring underground for my last garden for my pond pumps and lights but that was a while ago and I doubt it was done right.:)

UF Type wire is a wire with a special jacket that can be buried directly into the ground . Kudzu is right, give it some time and you or someone else will eventually stick a shovel into it . For safety, PVC conduit is not a bad idea. I used direct burial wire with my landscape lights,put it on a GFI plug for safty and made a diagram of where I ran my wire .

I'm running UF wiring for my lamp post but that will be about 2 ft down. I'm wondering what size low voltage I should run for my pathway lights, etc. I know its dependent on the number of lights, but whats the calculation for that?

Thanks.

I'm not positive because I've never used low voltage wiring but your regular 110 wiring is not normally used for low voltage applications. I think there is one or two common sizes for low voltage wire which in most DIY stores comes pre-packaged off the shelf. You need to match the voltage rating of your fixtures to the rating of the low voltage wire. After that, you need to consider the length which you will run the wire. The label on the low volt wire package should give the details. Low voltage landscape lighting is more of a do-it-yourself job for the normal homeowner which the DIY stores have all kinds of packages and instructions on how to install these kinds of lights.

Depends on length of run and total watts
use 16 gage wire for runs up to 100' long (150 watts max)
use 14 gage wire for runs up to 150' long (200 watts max)
use 12 gage wire for runs up to 200' long (250 watts max)

this is right from Intermatics website (Malibu lighting)

Here is a link that might be useful: Link to Malibu lighting

Intermatic has a good selection of low voltage lights and an excellant customer service dept ( toll free) . My nieghbor installed Intermatic low volt lights and is very pleased with it.

millerjoe49-
If I'm understanding your latest question correctly, you don't really need a formula. Here's what you do: 1) pick out all the lights you want to use; 2) add up the wattage for each lamp (these are typically in the 18-35 watt range, with 20 watts very common); when you get your total wattage, buy a transformer that can handle the whole load, plus a little more. The general rule of thumb is to have a transformer be not to big and not too little. If it's smaller than the total wattage it won't work properly, or will burn out; if it's too big (the load from the bulbs is less than 50% of the transformer rating) then this can cause premature failure, too. For example, if I had 5 lights at 20 watts each, the load would be 100 watts; I'd probably get a transformer that was rated at least 125 watts (which would mean I could add another light in the future and not have to replace the transformer); and probably not more than about 150 watts. It's important to remember to replace burnt out bulbs promptly because too many burnt out bulbs can lower the load enough to cause problems. Lastly, if you have any fixtures that are going in a pond, make sure you get the special version of the transformer that handles both regular and submersible lights. You can either hook up all the lights to one run of wire, or, if it works better for you, you can have a second set of wires coming in from a different location. The main concern about wire gauge is that you get bigger gauge wire the longer the run, as listed above. If you want some helpful info, check out the Malibu site FAQ's:
Low voltage wiring FAQ's

Hi all. Hope I can get an answer to the following question.
I have 6 low voltage SNOC landscape lights and all hold 20W bayonet bulbs similar to those in most brake light bulbs. I wish to replace them with LED bulbs. WIll this damage the transformer? I have been told that it may.

Hi all. Hope I can get an answer to the following question.
I have 6 low voltage SNOC landscape lights and all hold 20W bayonet bulbs similar to those in most brake light bulbs. I wish to replace them with LED bulbs. WIll this damage the transformer? I have been told that it may.