I have a 200 amp main service panel. I am installing a second dwelling unit 150 feet away on my property. What size wire will I need to achieve 100 amp to my new load center in the second dwelling unit? It will be in conduit underground.
There are a few different issues involved:
- Minimum Wire Size
For that, you go to an ampacity table. That tells you that for 75 C wire in conduit (the wire may be rated 90 C, for various reasons 75 C is the one you usually need to work with for this), the choices are 3 AWG copper or 1 AWG aluminum or larger. Feeders such as this are almost always done using aluminum due to cost.
- Voltage Drop
You asked for 100A. Generally that means 80A continuous with possibly occasional peak demand up to 100A, with most of the load at 240V (oven, EV charging, water heater, clothes dryer, etc.). So we go to a voltage drop calculator and plug in 80A and 240V and get 2.42% for 1 AWG aluminum. That is perfectly fine, so no need to upsize. Even at 120V, the 80A voltage drop is < 5%. So don't worry about it.
- Load Calculation
But before you get started, you need to make sure this will actually be allowed. That requires a load calculation. This is a complicated formula that takes into account large loads (e.g., HVAC, water heater, clothes dryer, oven, etc.), square footage (to take care of random plug-in stuff) and other factors. You actually need two load calculations: the existing building (to see how much capacity can be spared out of 200A service) and the new building (to see how much capacity it needs). If the numbers work out OK, great. But if, for example, the existing building has a load calculation of 125A then you can only use 75A in the new building (the feed could be larger, but the new building's load calculation would have to show that it all works out OK). There are a number of ways of adjusting things if needed, but you need to do this before you get started with any work, as it may affect whether you need a service increase (heavy-up) or choose different appliances to lower the load.
Appliance choices can make a huge difference in load. Electric resistance heat (cheap to install, expensive to run) + electric water heater + electric cooktop and oven + traditional electric clothes dryer will have a very different load calculation than the same building with high-efficiency heat pump + heat pump water heater + electric cooktop and oven + heat pump dryer, which will in turn be very different from high-efficiency heat pump (or high-SEER air conditioner + gas heat) + gas water heater + gas cooktop and oven + gas dryer. No absolute right or wrong - each choice is a tradeoff between available utilities (do you even have gas available?), equipment and installation costs, running costs, etc. But you need to figure it all out before you blindly assume "100A feed, meaning also only 100A for the original building".
You need to start with a Load Calculation for your ADU.
A lot of people just pick a number off the wall, like they see a 100A panel whose price they like. Not how it works. You must make a plan for what all the major built-in appliances will be, then do a Load Calculation according to NEC Article 220 (recommended: alternate method in 220.82) or whatever worksheet your municipality gives you, which may differ.
This will then dictate to you the minimum size of feeder you need. You are allowed to choose a feeder larger than that, but you can't "round down" (e.g. if your Load Calculation is 93A you can't use 90A wire, but 120A is fine).
The feeder then dictates the minimum amps of the panel. Larger is fine. If you need 93A then a 100A, 125A or 200A panel is all fine; since all those numbers are larger than 93. It's like tire speed ratings on your car. The size of the main breaker in those panels doesn't matter because it's only being used as a disconnect switch. (due to economies of scale, a panel with a main breaker is cheaper than one with a disconnect switch that is not a breaker).
We recommend in the strongest possible term that you go for PLENTY of breaker spaces. Nobody ever came back here and said "Help, my panel has plenty of extra breaker spaces and I want to add something. What do I do?" Spaces are CHEAP.* So don't be bashful about taking a 125A or 200A breaker panel if it means more spaces at a sane price. Many panels also come bundled with breakers, which can be handy.
How to look up the wire size
Look for Table 310.15(B)(16) aka Table 310.16. This is a big and messy table with six columns of numbers. You want the "Aluminum, 75C" column. Pick the next one that is larger than your Load Calculation requirements or desires.
Note that the "Round Up" rule applies here. So if you decide on 1/0 wire which is 120A, they don't make a 120A breaker, so you "round up" to 125A. You can't plan to use the extra amps! So if your Load Calc says 123A then 120A wire is not good enough.
If you find a really simple table with 2 columns of amp numbers and you're like "This table is SO much easier! Why didn't you send me here?" Because it's the wrong table. If it (or anyone) is telling you #4 copper and #2 aluminum is 100A, they're looking in the wrong table. That table does not apply to you.
What about the distance, though?
I don't even bother to do the calculations until 170 feet, so I'd say don't worry about it. If it feels right, or is advantageous, feel free to bump up 1 numeric wire size, but I wouldn't bother to do more.