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We are remodeling our 2,750 sqft 4-bedroom home to add two basement studio units. The home is 1910s and I'm rewiring the 200-amp panel as I go along, and trying to plan circuits wisely to leave enough spare capacity and circuit breakers for the new units. But my initial back of the envelope calculation shows I'm already out of room based on current code requirements (I'm even leaving lighting circuits off for simplicity):

Upstairs (100 amps)

  • 20 amps AFCI for Bedroom 1 and Bedroom 2 (<10 receptacles total)
  • 20 amps AFCI for Bedroom 3 and Bedroom 4 (<10 receptacles total)
  • 20 amps GFCI for Bathroom 1 upstairs
  • 20 amps GFCI for Bathroom 2 upstairs
  • 20 amps for Washer / gas Dryer

Downstairs (40 amps)

  • 20 amps GFCI for hallway powder room
  • 20 amps AFCI for Living/Dining (<10 receptacles total)

Kitchen (110 amps)

  • 20 amps GFCI Kitchen countertops 1
  • 20 amps GFCI Kitchen countertops 2
  • 20 amps AFCI Fridge (2014 NEC)
  • 20 amps shared Dishwasher/Disposal
  • 20 amps Microwave
  • 10 amps (maybe) for gas range/oven, if it needs its own circuit

Basement Appliances (30 amps)

  • 15 amps for gas furnace
  • 15 amps for gas tankless water heater

...and that's 280 amps already, and I have even thought about lighting!

New requirements

  • 2nd laundry room (approx. 200sqft)
  • Central AC unit(s) (don't have currently but will add for tenants)
  • Receptacles/Lighting/Kitchen for two studios (approx. 500sqft each)

Wondering if I'm doing something wrong here, I considered backing up to doing a load calculation but I think the rules above for dedicated circuits are straight NEC and the guidance on receptacle count (assume 1.5amp per receptacle) is pretty common from what I've seen in my searching.

[EDITS]

Thanks for all questions/comments. Multiple commenters identified my biggest misunderstanding, which was equating capacity with usage, and assuming once the circuit breakers added to 200 amp I had to stop adding circuits. It appears I have to look into subscription/oversubscription, and more importantly plan for branch circuits/subpanels to identify my larger issue, which is a lack of space on the panel for all these separate circuits.

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    The NEC rules for load calculation are a totally different beast -- see Article 220 for details. We have enough information for a good start here, though, but I need to know a bit more -- how many amps does your air conditioner pull, if you have one that is? Also, there will be a single laundry for the main house and both studios, I take it? – ThreePhaseEel Jun 27 '18 at 11:44
  • Wait, that's it? Only 260? You're not even starting to load up that panel. 200% oversubscription is normal, and this panel is only 70% subscribed. And you're having pajel space problems too, is there an opportunity to replace this panel with a m.u.c.h larger one? – Harper Jun 27 '18 at 13:43
  • Since you have gas water heater and furnace and range my guess that your average draw will be less than 50 amps with peak values well below 100a. I base this after many home upgrades of panels that just needed more branch circuits. In truth I have not found many homes with gas that needed more than 60 amps and with energy efficient lighting that number is coming down, the big energy hog is usually a AC unit requiring close to 1/2 of the total load then an electric dryer as the next largest load. – Ed Beal Jun 27 '18 at 13:49
  • @ThreePhaseEel is correct, you are trying to predict a panel size by using the maximum overcurrent protection for a circuit, which is not the connected load. For example an electric water heater is 4500W which is 18.75 amps but it is protected by a 30A/2P breaker. An electrical designer will use NEC Article 220 to calculate the ELECTRICAL DEMAND which is even less than the connected load. Best example your AC and electric heat should not run at the same time (dissimilar loads). – Retired Master Electrician Jun 27 '18 at 16:25
  • NEC Article 220 is not an easy Article to understand and is usually saved in electrical training until the last year of an apprenticeship course. So needless to say, I really don't think it should be done by a DIYer. You should use a capable trained designer to do a true calculated load. Regardless you OCPD, Connected and Demand amperages all apply to the same system. Each one serves a different purpose. – Retired Master Electrician Jun 27 '18 at 16:31
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Not even close

Look at how service panels work. There are actually two Poles, L1 and L2. Since you are dealing in 120V loads, you can pull 200A off each of them.

A 240V load like a house air conditioner pulls off both at once. You have none, however.

Aside from the 120V issue, you are supposed to radically oversubscribe panels. A family will use the power a family wants to use, how the circuits are subdivided will not affect demand.

The purpose of breakers is to protect wiring. If you subdivide a circuit, say, into 3 circuits, each set of wiring needs its own circuit protector (breaker). That's all that is. And Codes have required a lot of circuit division of late, which is why each bathroom and bedroom requires its own circuit and you are putting 6 circuits in a kitchen.

Tom rewired his kitchen from three 15A circuits to twenty 20A circuits, even provided 2 separate circuits on each countertop receptacle. From the panel it looks like a leap from 45A to 400A, but Tom's wife doesn't know that. She might raise her usage from 30A to 40A as she learns not to be skittish about breaker trips, but that's all.

A lot of your general-use circuits are dedicated circuits with almost no load on them. Fridge, gas range, gas furnace, gas water heater, lighting circuits... These might draw an amp or two yet you are figuring 15-20A for each. This is just the nature of satisfying modern codes, which do a lot of subdividing circuits that would've been combined before. A lot of European homes have 6 breakers.

One thing we do not oversubscribe is big 240V loads that are likely to be on at the same time (i.e. Not electric heat and A/C).

Another type we do not oversubscribe is loads we plan to run 24x7, e.g. A Bitcoin miner will buy a house and (if he listens to me) will spread the servers around the house so the existing 30A A/C can cool it. If he is putting twenty 10A servers around the house, he has to make sure they are balanced 10 on each pole, and provision a full 100A for them, plus the 30A A/C.

In other words we think about the loads that are likely to be on at once, worst case, and fudge in a safety factor.

Make it legal

This studio subdivision, are these mancave/art studio, or do you even have the sparkle in your eye of renting these units out? If the latter, get to the city permit agency and have a lengthy talk about subdivision - if it is legally possible and what utility divisions are required to do it. For instance you will need separate panels for the studios, because the tenant must be able to reset his own breakers without having to call you. And they may need to be (or be easily rearranged into) main panels with their own meter. That's code saying that, not my opinion.

My opinion is, existing units often have these ganky arrangements where a tenant has to call maintenance to reset a breaker, or get into tense fights with landlords over excessive use. Those are terrible which is why they've been written out of most codes today. Just last week we had a landlord in here who evicted her tenant due to excessive electricity use, then admitted the electricity use did not drop after the tenant left, and said "surely it can't be my ________" which it most certainly was. The landlord was a complete ____ obviously, but the landlording business turns people into that. Good meters make good tenants.

And now is the time to be fitting that stuff, it will be much harder later. Aside from the obvious, once the units become rental units, the work must be done by a licensed professional. The moment of it becoming a rental unit is an important detail that defines how much you can DIY.

  • thanks! I updated the question and will read through the links you sent. The subdivision is all legal and permitted (what a hassle that was). I will talk with an electrician about planning a separate subpanel for the rentals (everything is outside so no access concerns) and to reality-check my plan. Right now I'm just replacing all the existing K&T branches in the main house, and getting everything updated back to the panel (or close by) so it can all be tied together properly during the unit additions. – elihu Jun 28 '18 at 19:07
  • @elihu -- also, with the second laundry room (and perhaps other things as well, such as exterior lighting/receptacles), you'll want to set things up so they can be easily converted to be on a separate house loads service in order to comport with NEC 210.25(B) (provisions for a 4th meter would not be amiss either) – ThreePhaseEel Jun 29 '18 at 1:58
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Almost every installation has way more total amperage of circuit breaker capacity than the main panel breaker capacity. Remember that the circuit breakers are there to protect the circuit wiring from shorts and overloads. The code also provides recommendations for the circuit wiring sizes (i.e. current carrying capacity) for various types of room applications so that there do not get to be unsafe conditions of overloading certain circuits when typical combinations of loads are deployed.

The only real way to evaluate whether a 200A mains service is adequate for a dwelling is to carefully evaluate the load rating of all the actual devices that are likely to be in use at the same time and add up those loads and see if you are over subscribed or not. Many times you will find that this analysis will place focus on the biggest load devices because they will possibly consume a lions share of the main panel total capacity. For example air conditioner + electric water heater + electric car charging socket.

One thing to consider is that you are just not going to be going around and plugging in 15A loads into the outlets in every room all at the same time.

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You have plenty of power at your disposal

Running through the simplified method (NEC 220.82) for each unit in turn and then the commons (house) loads under the following assumptions:

  • Each studio is 500ft2 and the commons are 250ft2 (200ft2 laundry room + 50ft2 for any hallways, entryways, etc), leaving 1500ft2 for the main house
  • The main house has four kitchen receptacle circuits (treating the dishwasher/disposal and microwave circuits as SABCs, which is often a conservative assumption) + a laundry circuit at 1500VA each
  • Each studio has two kitchen receptacle circuits at 1500VA each
  • The commons has a single laundry circuit at 1500VA
  • The HVAC loads are commons loads, using a 4 ton/48kBTU, 13SEER A/C (Trane XR13) as an exemplar unit drawing 28A@240VAC to provide a "realistic worst case" for a modern air conditioner

gives us:

  • 10800VA (4500VA lights + 7500VA small appliances, 10kVA at 100% demand factor and 2kVA at 40% demand factor) or 45A@240VAC for the main house
  • 4500VA (1500VA lights + 3000VA small applainces, all at 100%) or 18.75A@240VAC for each studio
  • 8970VA (750VA lights + 1500VA small appliances, all at 100%, + 6720VA (28A * 240V) for HVAC at 100% demand factor) or 37.375A@240VAC for the commons loads

which adds up to 120A for the whole shebang, well under the 200A of power the utility is giving you.

However, you gotta keep 'em separated!

However, your single-panel plan is a non-starter, primarily because NEC 210.25 forbids commingling loads across unit subdivisions, and this will become an issue if you try to sell this place, especially to another landlord. As a result, you'll need to provision for subdivided metering and power, and this will start all the way back at the meter socket, or sockets as the case will be, and continue through the entire electrical architecture of the building.

While there are several different ways of implementing multi-tenant metering (individual sockets or meter-mains on a utility sealed trough, ganged meter sockets, and modular multi-tenant metering solutions), I would recommend a meter-main stack, also known as a meter-pack, for this application as it's a single item (less work for the electrician to install) and makes downstream rearrangement easy as the main breakers are in the meter-pack, making everything downstream all subpanels. In particular, the Eaton 1MP4124R stands out for this application's needs and its versatility to meet local utility requirements:

  • It can be field retrofitted for ring-type or ringless meter mounting
  • It can be used for underground or overhead (with a DSxxxH2 hub) service
  • It can be fitted to meet a variety of local utility specifications, including CECHA and EUSERC
  • A copper bus version is available (1MP4124RC) if your AHJ has an aluminum allergy
  • The bottom meter socket can be blanked out with a 1MMBC125B, allowing it to provide 3 meters now while having expansion room for a separate commons meter later
  • And, it uses readily obtainable type BR breakers for the tenant mains

As to what goes inside this meter-pack, your electrician should be familiar with what the local utility wants for metering, but we can specify the tenant mains now: a BRH2125 for the house and a BRH2100 per studio (2 total). The fourth tenant main is left unpopulated to provide future provision for separating the common area loads onto their own meter, or you can specify a BRH260 there if you want the common area loads on their own meter from jump. (The H in these stands for a higher fault current interrupting rating -- BRHxxx breakers can break a 22,000A short, while ordinary BRxxx breakers can only open a 10,000A short. I'm specifying the BRH here to provide an appropriate safety margin against utility upgrades that increase the fault current on your service -- modern main breaker residential panels specify a 22,000A interrupting rating on their main breakers as a bare minimum.)

From there, we move onto the feeders that go off to the individual dwelling unit panels, as well as the grounding electrode conductor, which routes through one of the small knockouts on the bottom center into the bonding block located there. Instead of running separate cables or conduits to each panel, though, we use conduit nipples to run the feeders into a 6" by 6" metal trough (gutter, wireway) with field-made knockouts (factory KOs on a wireway this size are too small for our usecase) mounted below the meter-pack and extending out to the right (so it doesn't get in the way of where an underground service goes into the meter-pack, or where the grounding electrode conductor goes for that matter). Furthermore, these feeder conductors are smaller than expected, as the rules in NEC 310.15(B)(7)(2) apply, which allows them to be downsized to 83% of what they would otherwise be, just like service conductors, as they carry the entire load of a dwelling unit each.

As a result, the main house feeder consists of 3 1/0AWG XHHW-2 aluminum wires, each studio feeder consists of 3 2AWG XHHW-2 aluminum wires, and the common-loads feeder consists of 3 6AWG THWN copper wires as it can't be downsized. This means that a pair of 2" nipples can be used, with one carrying the main house feeder with room left over for the common loads feeder, and the other carrying the two studio feeders.

Attached to the trough using more of the 2" nipples from above are the panels for the various dwelling units involved, which are all main lug panels configured as subpanels (i.e. neutral and ground bars separate) in NEMA 3R (outdoor) enclosures:

  • The main house should use at a minimum a 42 space panel -- this puts you into 200A or 225A bussing, but that's what you need to get the kind of space you're after, and doesn't pose any issues electrically speaking. A 54 or 60 space panel would not be amiss here whatsoever, though!
  • The studios don't need so many circuits, so a 30 space, 125A panel can be used for each studio, although if you wished to put larger panels in for them, feel free to be my guest.
  • Last but not least, the subpanel for the commons loads can be another 30 space, 125A unit (or larger, depending on what was used for the studios). If you didn't spring for the fourth meter+main initially, then this panel is fed as a subpanel off of a 60A, 2-pole breaker in the main house's panel, thus making it so that the commons loads are on the main house (this may be what is expected for accessory dwelling units in your area, check with your AHJ for details). If you do wire it this way, make sure to route the feeder for this subpanel back into the trough from the main house panel via the same nipple the feeder for the main panel came in from instead of a separate conduit or cable.

As to the make and model of panels to get, Siemens PL stands out as an obvious first choice (copper bussing, factory ground bars, 54 space options, and all this for a reasonable price), with Eaton BR as a backup plan in case you don't have a Siemens supplier in your area or your AHJ needs the series combination rating with the BR tenant mains in the meter-pack, as series ratings like that don't work across breaker manufacturers. You may be able to repurpose your existing panel as a secondary feedthrough off the house main using a set of subfeed lug blocks and some more of that 1/0 Al in conduit as a feeder, though, so don't have the electrician trash it!

If your AHJ has other ideas...

It may be the case that your AHJ requires accessory dwelling units (like your studios) to be on the same meter as the main dwelling. In that case, I'd still use the four-panel setup as before, but instead of feeding them from a meter-pack with individual meters, I'd instead use one of two configurations:

  • A rule-of-six meter-main such as the Eaton MB1212L200BTS, main breakered as above, with a separate main for the common loads panel. This has the advantage that individual tenants (or the common loads) can be turned off without disturbing other tenants, but has the disadvantage that there's no single shutoff means (some AHJs or utilities require a single-handle main disconnect), and it also makes feeding 200A to the main house more difficult (especially in EUSERC areas, as the MBE1212L200BTS doesn't support BJ branch breakers for feeders >125A). It also only comes in aluminum bussing, which is a problem for some rather skittish AHJs, and has few bypass options, which is troublesome for some utilities.
  • A single-disconnect, 8/16 space, feed-through lug equipped meter-main such as the MB(E)816B200BTS, again configured with the same feeder breakers as before including a separate common-loads main. This has the advantage of providing a single shutoff means as well as a clean path to providing 200A to the main house, but has the downside that providing 200A to the main house means that shutting off the main house shuts off everything else as well (as you need to use the feed-through lugs to do that). It also has the upshots of being available in copper bussing and a wider variety of bypass options, as well as being easier to make EUSERC compliant.

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