Load Calculation
Step 1 is to do a NEC Load Calculation to determine how much extra power you can send to the subpanel. Assuming you have 200A service and just the single 200A panel right now, you start with a Load Calculation for your entire house. This includes:
- The size of your house (square feet)
- Certain required circuits (kitchen, bathroom, laundry)
- Fixed appliances such as water heater
- HVAC - highest of H or C
- Cooking equipment - which has a bunch of special rules
and a bunch of other details. It is not "what I think I use" (that's too low!) and it is not "add up the breaker handle numbers" (that's too high!).
Once you know how much your existing Load Calculation is, you have a few choices:
- Install a subpanel with a feed up to the difference between your total available (200A most likely) and your Load Calculation.
- Upgrade your electrical service because you are already too close to the limit for what you want to add.
- Install equipment that manages your loads so that, for example, the water heater shuts off if you turn on the dryer and would otherwise be over the 200A limit. If you want to do this then we'll need more details and it will be a new question.
Of course moving existing things will affect this. If you have a 30A dryer (50A is not normal for US residential dryers) and move it from the main panel to the subpanel then instead of (for example) 120A in use and 80A available for the subpanel you will have 90A in use and 110A in the subpanel - but that 110A will have to include the 30A dryer.
Feed Breaker, Wire, Panel Size
The feed breaker determines the maximum amount of power you can send to the subpanel. It can be any size you want, but the larger it is, the larger the wires. Typical sizes are anywhere from 50A to 200A, though 200A feed from a 200A panel doesn't make much sense.
Wire size depends on feed breaker size, wire type and voltage drop. At 75', voltage drop is not a real concern, so just the feed breaker. The three wire types generally involved are:
- Copper cables
- Copper wires
- Aluminum wires (and some cables)
See an ampacity chart for details. But generally aluminum carries less current at a given size than copper, so it has to be a larger size for any given feed breaker size. Copper cable is generally using the 60 C column which is a larger size than the 75 C column for individual wires in conduit. But the bottom line is that copper is so much more expensive than aluminum that for almost any subpanel feed it makes sense to use aluminum. Some typical sizes (ground wires are smaller):
- 60A = Copper cable 4 AWG, copper wire 6 AWG, aluminum wire 4 AWG
- 70A = Copper cable 4 AWG, copper wire 4 AWG, aluminum wire 3 AWG
- 80A = Copper cable 3 AWG, copper wire 4 AWG, aluminum wire 2 AWG
- 90A = Copper cable 2 AWG, copper wire 3 AWG, aluminum wire 2 AWG
- 100A = Copper wire 3 AWG, aluminum wire 1 AWG
- 120A = Copper wire 1 AWG, aluminum wire 1/0 AWG
You gain nothing but higher wire costs by using copper. You gain nothing by using larger wire than you currently need except if you plan on a heavy-up that would allow sending more power to the subpanel.
90A with 2 AWG aluminum is a popular combination, and should be enough for your needs.
Circuits
- Dryer - nearly all standard US clothes dryers use a 30A circuit. (Exception: heat pump dryers that use less!) So if you really need 50A then this is something special.
- EV charging - nearly everyone will do fine with 20A or 30A. 50A is overkill for most people. Exception are people who drive a lot (Uber, Lyft, etc.) or who have more than one EV.
- Lights - 15A is plenty
- Outlets - if you have any plans to use larger power tools then 20A circuits make sense. Otherwise it is generally up to you. The only real extra cost is larger wire (12 AWG instead of 14 AWG).
- Refrigerators - if these are standard residential refrigerators, 15A is plenty. I've got a 15A circuit that runs a refrigerator/freezer, freezer, computer and a bunch of other stuff. Finally got it split to two circuits to cut down on some interference when compressors kick in, but otherwise no big deal.
Your subpanel load really comes down to ~ 20A (more or less, based on the typical outlets + lights + refrigerators, keeping in mind that these things don't use a lot even though they are on 15A or 20A circuits) + dryer (likely 30A not 50A) + EV (30A is plenty). So a proper Load Calculation for the subpanel is in order but I think 90A is more than enough and 70A is probably enough, though the devil is in the details.
