so im installing a sub panel to detached guest house approximately 50' from the main panel. running all electric appliances electric range,electric tankless water heater, and a/c unit. all three of these appliances require 60 amp breakers #6/3.and also want to add 20 amp breaker for 5 recessed lights and 5 receptacles. question is what size panel do i need and what size wire will be used . main panel is fed with two 100amp breakers .
Just to give you a rough idea of what a load calculation would look like, here's a quick rundown.
This one is easy. You just take the square footage of the building (to the outside of the wall), and multiply by 3 volt-amperes (VA).
Branch Circuits Required
Next, for each required branch circuit, add 1,500 VA. So if there's a kitchen, that's 2 small appliance branch circuits (3000 VA). Have a bathroom? That's another required circuit, and another 1,500 VA. How about a laundry area? Another circuit, another 1,500 VA.
Appliances Fastened in Place
This gets a bit more complicated, as you'll have to check the nameplate of the appliances. You'll also have to note if it's a 120 volt, or 240 volt appliance.
For this, you'll take the nameplate rating of whichever equipment is larger. So get the values for both the heating and cooling equipment, then use the larger for the calculation.
Cooking equipment (ranges, ovens, stoves, and cooktops) has it's own complicated calculations, so it's difficult to estimate without knowing what equipment is installed.
Getting the Size
To convert from volt-amperes to amperes, you'll have to divide the value by the voltage. So for the general lighting and branch circuits required, you'll likely divide by 120 volts. For the other appliances, you'll probably use 240 volts.
Once you've converted everything to amperes, you can simply add the values. The total is the size of the service you'll need.
After you know that, you can start figuring out what size breakers, panels, and conductors you'll need.
While you said the buildings are 50' apart. Keep in mind that it's not the distance between the buildings that matters, it's the length of the wire between panels. If the conductors are too long, you'll have to account for voltage dropped across the wires. This could mean you'll have to use larger conductors.
After you do the maths, you may find that it makes more sense to install a separate service to the building. Instead of trying to feed the building through your existing service.
While this may not provide all the information you need, it should at least help you understand a bit about what's needed to figure out what size service you need.