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Can anyone help me spec a sub-panel for a basement renovation? The plan is to run a 6/3 AWG feed from a 60A breaker in the main panel to a sub-panel which will be flush mounted in an interior stud wall in a closet. Here are the circuits I need:

  • Two 20A outlet circuits for a small kitchen
  • One 20A for a garbage disposal
  • One 20A for a dishwasher (I'd prefer to share this with one of the two kitchen outlet circuits -- if that's allowed)
  • Two 15A circuits shared across bedroom/bathroom/living room outlets
  • One 15A circuit for lighting (3 rooms, all LED)
  • One 20A (Amperage TBC) circuit for bathroom under-floor heating
  • 20A circuits get 12/3 wire and 15A circuits get 14/3
  • GFCI outlets in kitchen and bathroom, of course.
  • Furnace stays on it's existing circuit from the main panel
  • I will run all this by the city inspector after I pull permits

Questions:

  1. Is this a reasonable plan?
  2. Is 60A sub-panel enough or should I be going to 100A?
  3. What kind of breaker do I need for the main panel? 60A single pole?
  4. Can the sum of the sub-panel breakers be greater than the breaker in the main panel from which it's fed? (I think yes, as is the case in main panels)

A recommendation on reasonably-priced panel and breaker choice would be much appreciated. There are so many choices out there.

  • What square footage are you dealing with here? Also, what are the nameplate ratings on the disposal, dishwasher, and bathroom radiant heating system? – ThreePhaseEel Nov 8 '17 at 4:26
  • Also, that bathroom will need a 20A circuit for its receptacles... – ThreePhaseEel Nov 8 '17 at 4:38
  • @ThreePhaseEel Around 1100 sq ft. Mini-kitchen, bathroom, living room, one bedroom, and the rest (40%) unfinished. nameplate ratings unavailable because I have not purchased them yet. Why does the bathroom need it's own 20A when it will never have anything greater than a hairdryer in use? I don't think my upstairs bathrooms have dedicated circuits. – Timbo Nov 8 '17 at 4:45
  • bathrooms are a bit funky in the NEC. You can have several bathrooms using 1 20A circuit for all the receptacles in those bathrooms and then the bathroom lights off on some other circuit, or you can have a 20A circuit that feeds the receptacles+lights in a single bathroom, but bathroom receptacles need to be on a 20A circuit that has no non-bathroom outlets on it in any case. – ThreePhaseEel Nov 8 '17 at 4:59
  • Do you plan to have gas service available at the kitchenette in this unit? – ThreePhaseEel Nov 10 '17 at 1:52
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What kind of closet will this be? Can't put a panel in a clothing closet.

A bathroom needs a 20A because a hair dryer is 13A all by itself nevermind the curling iron or hair straightener often set out to warm up while it's in use. Also whatever else, and lights, fan, heatlamp if that's on the circuit too.

I get your idea that it's YOUR house and YOU can cheap out on the electrical system and use your smarts not to overload it. However a) don't overestimate your sense of load size. b) I certainly hope for your resale value that you are not the last human who uses this house. And c) I fail to see the value of cheaping out on electrical. Given the astonishingly low cost of the parts, you should be optimizing for performance not economy. Case in point:

The ampacity of the subpanel

First, separate the supply breaker from the subpanel proper. You can have a 30A breaker in the main feed a 225A panel (the 30A certainly won't melt the 225A panel!) The reverse is not true, obviously. You would go with a higher amp subpanel mainly so you have extra breaker spaces for future expansion, because running out of spaces is a nightmare that is so easily averted right now, when you buy the subpanel, and extra spaces are cheap. So in your case I'd go for a 30 space for instance. Finishing with 60% of your spaces unused is a good place to be. It's so easy and cheap to just add circuits if you have the panel space, so things like under-sink water heaters, electric car, welder, become easy decisions.

Beware of double-stuff breakers. Many panels advertise "24-circuit (12-space)" - the 24 circuits relies on double-stuff breakers, which are more expensive. Those are bad news because increasingly you are required to use exotic breakers like GFCI, AFCI, 2-pole and the like, and those cannot be had in double-stuff. So you are stuck with the lesser capacity e.g. 12 circuits. It's cheaper to get the number of spaces you want, so you can use the simplest breakers allowed.

Which panel? The breakers must match the panel. Eaton CH panel, Eaton CH breakers. Most manufacturers make several incompatible lines (CH/BR, Homeline/QO, etc.) The ruling factor is the cost of exotic breakers such as GFCI, AFCI, 2-pole GFCI and the like. So you can cross off several models. Those exotics are being forced down our throat, so get used to them. I am also not a fan of tenant-grade cheapies (Homeline, BR). Siemens has a lot of promise: quality panel with sanely priced exotics.

The panel should be fed with a 2-pole breaker in the main panel, unless you are doing something intentionally weird, but then why would you use /3 feeder? You should know this, by the way.

I'm a little concerned with a knowledge gap here, and encourage you to hit a library for a book on wiring houses, and read it cover to cover. You don't want a knowledge gap. You especially don't want one after the drywall is up but before inspection. If your skill is lacking, do the works in EMT conduit so it's easy to fix later!

The circuits

Now, you say you want to run /3 cable. Either you don't know what you're saying, or you intend to double the circuit so you get 2 circuits out of each cable (a multi-wire branch circuit). That's awesome. (MWBCs not unfamiliarity).

I was a little worried that you were underpowering things like the bathroom and kitchen, but if you intend to supply them with a /3 and MWBC, that means double the circuits, and the cook will thank you. However MWBCs absolutely require a 2-pole breaker, which means you will need 2 breaker spaces for each one. Like I say, you go through spaces faster than you expect!

Honestly MWBCs are falling out of style because so many circuits require GFCI or AFCI. And that requires either the uber-exotic 2-pole GFCI/AFCI breakers, or correct branching at the destination and then GFCI or AFCI deadfaces or receptacles. And the savings is pretty small compared to running 2 separate /2 cable runs and avoiding MWBCs altogether.

The bathroom(s) require following one of two strategies:

  • Each bathroom gets a dedicated 20A circuit for ONLY loads in that bathroom, including wired loads like lights, fan, etc. I am not a fan of this one, because BOOM you are now in pitch black, holding a red hot hair straightener 1/4 inch from your ear, and can't even set it down because you can't see to not set it in the sink full of water. (however if the hardwired loads together exceed 50% of circuit capacity, looking at you fan-heater, that circuit cannot serve any receptacles at all, and this method doesn't work.)

  • A dedicated 20A circuit serves only receptacle loads in any number of bathrooms. The hardwired loads are served potentially from other circuits that serve other rooms.

There is one circuit that you really will want to be 100% dedicated, and that is the refrigerator. Code requires a GFCI since it's in the kitchen, even though that's not really the use-case GFCIs were intended for. And you really don't want any other appliance tripping the fridge and ruining your food. For this reason GFCIs on fridges are a terrible idea. I say install a dedicated circuit with a single (not the usual duplex) receptacle, so only the fridge can possibly be plugged in. Use a GFCI breaker, and I would certainly never tell you to switch it to a plain breaker 5 minutes after the inspector walks out the door.

  • In dwelling unit kitchens, the fridge receptacle is still exempt from GFCI protection since it isn't serving a countertop surface, at least as of NEC 2014. – ThreePhaseEel Nov 8 '17 at 12:39
  • @Harper many thanks. I don't want to go cheap so I will use a panel with extra spaces. Regarding the /3 wire you are right I don't know what I'm saying. I thought /3 wire meant hot/neutral/ground for a 120V single-phase 100A subpanel. Now I realize that 6/3 means hot/hot/neutral for a two-phase 240V installation and that ground is not counted in the numbering, right? I will run a 20A for bathroom outlet, and tie bathroom light into the other light circuit. – Timbo Nov 8 '17 at 14:07
  • @harper - FYI, I went to school for electronics and have plenty of experience running extra branch circuits and adding onto existing systems, but have never replaced a main panel or subpanel so I'm not up on terminology or NEC code. I plan to hire someone to handle the supply line from main to subpanel, and I'll just run the branch circuits. That will save me money while insuring the critical elements are looked at by someone who knows what they're doing. Also will hit up city inspector for knowledge. – Timbo Nov 8 '17 at 14:11
  • Now I'm wondering if I ought to run 240 to the sub-panel. I may want to make this a stand-alone "apartment" someday in which case I'll need 240V for a dryer outlet. Is that much harder? Just means bigger wire, bigger panel, right? I can see I need an electrician... – Timbo Nov 8 '17 at 14:27
  • @Timbo It's one more wire. It's almost crazy not to do it that way. Every subpanel will be designed with the expectation that you'll deliver 120/240 split phase. And don't be bashful about pulling the main-sub wire (or better, fat conduit). 95% of electrical work is actually light carpentry work. – Harper Nov 8 '17 at 15:35
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Panels and clothes don't mix

First off -- putting a panel (or any overcurrent device) in a clothes closet, or any other place where easily ignitable materials are stored, is forbidden by the NEC in 240.24(D):

(D) Not in Vicinity of Easily Ignitible Material. Overcurrent devices shall not be located in the vicinity of easily ignitible material, such as in clothes closets.

Size matters (in more ways than one)

First, we'll address whether your feeder is the correct size. We start with the 3300 VA lighting and general receptacle load -- this is the 3VA/ft2 rule for dwelling unit general loads (stated in NEC table 220.14 and restated in NEC 220.82/83) applied to your 1100 ft2 space. Atop that, we need to add 1500VA each for the two kitchen receptacle circuits, and another 1500VA to serve as an allowance for the dishwasher/disposal circuit. Finally, we add a 2000VA allowance for the bathroom radiant heat. This sums up to 9800VA; no demand factors can be applied since we aren't to 8kVA of non-HVAC loads and we're adding new HVAC (the bathroom radiant heat), forcing us to use the procedure from 220.83(B). Dividing this by 240V gives us just under 41A of load, so your 55A (6/3 NM-B) feeder would be adequate at the moment.


However, if you plan to turn this into a dwelling unit (apartment), it will need to have a minimum of a 83A (#4 Cu or #2 Al THHN) feeder due to 310.15(B)(7) points 2 and 3 applying an 83% demand factor:

2) For a feeder rated 100 through 400 A, the feeder conductors supplying the entire load associated with a one-family dwelling, or the feeder conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the feeder rating.

(3) In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.15(B)(7)(1) or (2).

to the 100A minimum feeder rating for a single dwelling unit feeder.

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