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I want to make sure I have this right, as it will be inspected and I'd rather not have to redo anything.

Current service: 150A, panel is in an interior hallway and the main disconnect breaker is outside. Currently ~1150sf, 3BR, 2BA, gas appliances. Adding an in-law suite - ~768sf with an additional bath and kitchenette (gas appliances there, too).

Rather than running the new circuits all the way back to the current panel (an additional 40' of cable for each run), I'd like to install a subpanel in the new laundry room adjacent to the addition. It would service 4-6 20A breakers.

This is what I'm thinking:

  • New double pole 60A (type QP) from the house panel to something like the attached in the new laundry room

  • 6/3 stranded copper Romex (<50')

  • Remove tie bar between ground and neutral in the new subpanel

Does that sound right? As mentioned, I'm pulling permits for all the work, so if there's something anyone thinks I should be aware of, feel free to let me know.

BTW: The existing panel is a Siemens, fwiw.

Thanks in advance!

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  • Does your laundry area have a toilet and a sink? Some do. You can't put a panel in a bathroom or bathroom area.
    – ArchonOSX
    Apr 23, 2018 at 20:26
  • Otherwise it looks like you have a plan.
    – ArchonOSX
    Apr 23, 2018 at 20:48
  • Thanks - No toilet or sink in the laundry room. It's a big-ish laundry area (8x15) and the proposed subpanel will go on the wall opposite the dryer. There will be ~5' of clearance in front of the new panel. Apr 23, 2018 at 22:12
  • How many kitchen receptacle circuits are there in the existing kitchen, how many kitchen receptacle circuits will the new kitchenette have, and will the new suite be fully provisioned as a "dwelling unit"? Also, are there any other large electric loads in the house that we'll have to take into account? Apr 23, 2018 at 22:35
  • Speaking of large electric loads -- does the house have A/C, and if so, what's the nameplate rating on that air conditioner? Apr 24, 2018 at 3:35

1 Answer 1

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Go for it -- just don't starve new the panel for slots!

You have plenty of headroom on your existing service to add the new feeder (the 220.82 calculation for your existing house comes out to a mere 45A, assuming your kitchen receptacles are on 2 circuits and your A/C is reasonably modern/efficient as I used a representative 13 SEER for calculation purposes), and a 60A feeder is more than enough for your guest suite, as NEC 220.82 pegs it at a mere 29A (given that it doesn't have its own laundry facilities -- even if it did, it'd be only 35A).

The wire's fine, too. Even though 6/3 NM is only rated for 55A, you can't get 55A breakers, so the 60A breaker is fine as per 240.4(B), and the Article 230 provisions for service sizing don't apply to feeders.

One suggestion I will make here, though, is not to starve the new subpanel for slots -- something like a Siemens P3040L1125CU would be ideal here, even if it seems like massive overkill at first. That way, there's ample space to arrange your load across circuits how you see fit. Pulling the tie bar on the new subpanel here is precisely the correct move, as well -- neutral and ground never meet again, after all.

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  • Thank you so much - I really appreciate it. I did a load calc based on square footage, et al (www.ask-the-electrician.com/residential-electrical-load-calculation.html#beginAdv) and came up with a current load of 65 amps, but that's still light and gives me plenty of room for the paltry requirements for the addition. Stack and its users are great - Than you again! Apr 26, 2018 at 10:21
  • @user3340470 -- we thank users around here by upvoting and/or accepting their answers :) Apr 26, 2018 at 11:32

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