Home Improvement Asked by IndyGopher on October 10, 2020
Thanks in advance for advice! I just finished framing my basement and am moving on to electrical. Hoping for advice on the subpanel. Main 200A Eaton panel is in the garage. I’ll need to run cable up into the attic, cross about half the house, then down into the utility room in the basement. I haven’t measured this yet, but I’m guessing about 80′ of cable. The basement is about 2200 sf with 2 bedrooms, 2 bathrooms, a wet bar, living room, workout room, utility closet, and large storage area. I haven’t finished my drawings yet, but I’m guessing about 15 circuits if I do separate lights and receptacles in most rooms. Mini-fridge, small countertop microwave, and a treadmill are the only notable loads. Everything else will be pretty standard: 2 bathroom GFCIs, bar GFCIs, and a bunch of lighting and receptacle circuits. No heat, range, freezer, AC, etc.
My research is telling me it’s much better to “go big” and overdo these things, so I was thinking of installing a 100 amp panel with 24 spaces (48 circuits) from Square D, using a 100 amp breaker at the main panel, and running 1-0/1-0/1-0/2 SER since I’ll be crossing insulation in the attic. I also understand the subpanel will need a ground bar installed. Is this overkill for my situation? It’s a large basement, but without a fridge, range, baseboard heaters, etc., am I overdoing it? I definitely don’t want to run out of space down the road, but I also don’t want to spend $2+ per foot on cable (plus wrestle that monster) if half that would do the job.
Anyone have opinions or advice for me? Also, do I sound like I know what I’m talking about enough to attempt this on my own and live to tell about it? 🙂 Thanks!
A 1/0 Al SER cable can handle 120A when landed on 75°C terminations, and we are allowed to round up to 125A by the 240.4(B) "round-up" rule as the next standard breaker size above 120A is 125A. As a result of this, and the fact that a BR2125 is only $10-$15 more than a BR2100, along with the fact that the HOM2448L125PGC is a 125A panel to begin with (there is no 100A version in main lug), there is really no reason to go with anything less, although if you wanted to upsize the panel further, the HOM3060L225PGC is only $15 more. (While you could run a 4/0 Al SER cable, you can't pull more than 125A out of your current panel anyway as the double frame breaker or subfeed lug required for a 200A feed takes up more space in your panel than you can make room for.)
While you're good to go on the one thing that often gets people in trouble with subpanels, namely grounding, as long as you keep your neutrals and grounds straight, thanks to the fact both panels mentioned above come from the factory with ground bars and a ground lug fitted already, you will have to remember that you'll need new Square-D HOMeline breakers for the circuits you're moving to your new panel, instead of being able to reuse the BR breakers you're pulling out of your existing panel.
There's one more thing you'll need to do in order to ensure a reliable installation. Your panel and breakers will have lug torque specifications labeled/marked on them (in inch pounds), and 110.14(D) in the 2017 NEC requires that you use a torque wrench or torque screwdriver to set the lug setscrews to that torque. Even if your jurisdiction has not adopted the 2017 NEC yet, it's still a good idea anyway, since you don't want your electrical system to lose you the race, now, do you?
Answered by ThreePhaseEel on October 10, 2020
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