What size sub-panel and wire do I need for my three-season room?

Question

I have a 15x22 room that is roughly 50' from the main panel, that I am converting from a 3 season room, into a conditioned space (heating and cooling, enclosed with windows and doors, etc.).

Currently there is a single 110 outlet in this room. This will not be sufficient as I want this to be a fully functional room that I can use most of the year. That said, I want to put in one of those split wall heating/ac units, maybe a ceiling fan, lights and enough outlets to fill the room while still meeting code.

I live in upstate New York. I plan to put a back-up fridge (which I know is supposed to be on its own circuit for starting amps), maybe a plug in lamp, tv, and a few other small items that won't necessarily all be on at the same time.

I looked at Home Depot and found a 6awg 3 wire cable with ground that says the max amperes are 55. So my question is, basically, on the proper amperes for this room, a proper sized sub panel and the correct breakers, so that I don't overload the sub panel and catch the wire on fire from overheating.

As I looked around HD's website, I began feeling like I would need a 100 amp sub panel at a minimum as the available slots for what I need don't seem to come on a panel that is rated for the 6awg wire specs. Do I need a larger (4awg) wire and a minimum of a 4 slot panel? I figured I could put lights one 1 slot, outlets one 1 and a double pole breaker for the heating/ac unit on one. Any help mapping this out would be pretty great.

Thanks, Bob

p.s: The main panel is pretty full, but I had installed an electric tankless water heater that took 6 spots for 3 double pole breakers for each heating element that I can steal the space from since the wife doesn't like that the 27,000kW makes the lights in the house flicker, so we went back to a standard tank water heater.

Panel is a Square-D Homeline HOMC30UC Series S01.

Answer 1

You don't need that big of a feeder, but if you're going to put in a subpanel, you might as well take the chance to free up space in your main panel

Doing an Article 220 calculation for your feeder alone, with 990VA for the lighting, a 1500VA extra allowance for receptacle loads, and 2875VA for a 10A@230V mini-split (12kBTU/20SEER) at 125% (as the largest motor on the feeder), gives us a 22A feeder, far short of even the 60A that 6/3 NM is limited to. However, I would run either a 1/0-1/0-1/0-2 Al SER cable (good for a 125A feeder) or a 1.5" EMT (with stud shoes, but also good for 125A tops with 1/0 Al XHHW-2s in it) between the main panel and the subpanel.

This allows us to put in a 125A, 24 or 30 space, main lug panel for the subpanel (a HOM2448L125PGC provides a nice, easy package solution if you wish to stick with Homeline), with a HOM2125 for the feeder breaker. Even with two 15A AFCIs (HOM115CAFI in Homeline) for receptacles, and a 15A double pole breaker (HOM215) for the air conditioner, this still leaves ample room for future expansion, which is a good thing considering that your main panel is basically full: you can get 2 spare spots by pulling out the other 40A breaker from the tankless experiment, but that's it, and you'll need those spaces to retrofit AFCI protection as there are no double-stuff AFCIs.

The HOM2125 replaces one of the unused 40A breakers that originally went to the tankless heater, by the way. Also note that you will need to torque all lugs to specification with an inch-pound torque wrench or torque screwdriver; this requirement was enshrined in Code in 2017 with 110.14(D), in addition to being a good idea lest your electrical system pull a Greg Biffle on you.