Help wiring new house

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jeffesonm

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Looking for some help wiring a house I've designed, and am in the process of building (you may have seen my recent plumbing post too) I've done a bunch of wiring in the past up to and including running a subpanel, but first time doing a whole house and first time with a slab. Austin, TX, 2020 NEC.

House is a 2 bed, 2 bath, 1150 sq ft house, slab on grade, one story. Would be pretty straightforward except for no real attic or basement, and not much ceiling for half the place. So I'm trying to think hard about where/how I will route wires, especially so before this slab gets poured.

Below is rendering of the interior of the house, taken from the S end in between the living and dining areas and looking back towards the N. Imagine a regular house, but you raised the roof up 2.5' and put a row of 2.5' tall windows all the way around in the space created. Exposed roof trusses and roof deck. There will be ceilings over top of the bathrooms and bedrooms, but then there's clear space above those ceilings and below the roof assembly. Hoping to minimize and wires/mechanicals up in that space.

interior.png


Here is the electrical plan I drew up:
electrical_plan.jpg


What circuits will I need? That would help plan what if any other conduit runs I might want. So far I've got:

Kitchen
  • Fridge - 15a, 14g
  • Microwave - 15a, 14g
  • Countertop/small appliance - (2) 20a, 12g, gfci
  • Dishwasher - 15a, 14g
  • Single wall oven - 30a, 220v, 10g
  • Cooktop (gas) and range hood - 15a, 14g
Bathrooms
  • Receptacles - 20a, 12g, gcfi
  • Lights/fan - 15a, 14g
Laundry
  • Washer/gas dryer - 20a, 12g, gfci
General
  • Receptacles - 20a, 12g, one circuit ea bedroom, living room, dining room
  • Lighting - 15a, 14g, probably three total, however makes sense
  • Outdoors - 20a, 12g, gfci - maybe two or three total
The service/meter will be mounted on a pole about 11' from the house. 2" conduit run down from the meter to the panel inside the house on what is the West wall there. Coming from the panel it will be easy enough to get wires to the NW corner bedroom, and the bathroom/kitchen along the W wall.

It will be more difficult to reach the East side of the house since the center hallway is open to the ceiling, so I thought it'd be good run conduit over there. At first I was thinking one fatty 1" with a bunch of circuits, but reading more now it sounds like 3 or possible 4 circuits per conduit is the max due to derating/wire fill. So maybe two separate 3/4" conduits to the "other side"? THWN in conduit with a PVC box on either end and then I can switch to NM-B?
 

wwhitney

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My initial questions/comments:

- The 2020 NEC requires an exterior "emergency disconnect" for the electrical service. Do you have that, or has Austin amended that section (230.85)?

- How are you handling the overhead lights in the area with open roof trusses? Type of fixture, how you're suspending it, and whether you can run the wiring concealed.

- I suggest running wiring for a future electric dryer and electric range now, leaving it disconnected in a blanked off box.

- What's the HVAC? I don't see that on your list.

- Your master closet looks big enough a couple receptacles would be a good idea.

- Master bedroom you need a receptacle between the two doors. Kitchen you need a receptacle on the island, 2020 NEC might require two, bears checking.

- Are you subject to any kind of energy code that requires certain lighting controls?

- Garbage disposal? Depending on the amp ratings, it and the dishwasher could be on a single 20A circuit.

- Bathroom lights don't need to be on a circuit of their own--the lights in each bathroom can be on the bathroom's receptacle circuit, or they could be on a general lighting circuit.

- As for conduits under the slab, wiring within them would have to be other than normal Romex (type NM). You could use type UF cable and then the conduit could be just a sleeve for the under floor portion with stub ups into the stud cavities. Or you could run conduit complete from box to box, and pull individual conductors (THWN-2). Either way, for #12 and #14, you are limited to 9 current carrying conductors in a conduit before you have to upsize the conductors (or downsize the breaker). For 120V L-N circuits, that would be (4) 2-wire circuits.

Cheers, Wayne
 

WorthFlorida

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An ambitious project for sure. For what my two cents is worth, having owned five different homes in four states for the last 45 years, you learn what works and doesn't work in a home design.

A project like this is great and a lot of fun, but for electrical work do hire an electrician to do some of the work. Your local power company may only accept a licensed electrician to pull a permit. They will not connect power until most of the electrical work pass inspections. It is wise for the electrician to do the mast if needed, the meter and breaker panel connections. The electrical will know what the inspectors want to see especially for grounding and bonding. Do call the power company to get what it takes to get a connection. Most lightly you will need to provide the cable to the pole and an electrician would know exactly what type and size. Any delay or rework required without an electrician may get expensive.

Whitney covered most of the items for electrical. A few things I see is where are the exterior doors? It looks like it will mostly be sliding doors due to the character of the home. Building codes may dictate at least two exterior doors. If you have a slider door or regular door in the rear of the home, a wall switch (three way) may be needed. All exterior doors must have a light fixture on the exterior. It looks like the kitchen table area is all sliding doors. It's possible that two exterior light fixtures may be needed at each sliding door (this is where an electrician would know). The kitchen table area may need a three way switch configuration. One by a door if there will be one and as you enter the area from the home. Present figure shows to turn on the kitchen table light you have to walk to the other side of the room. If I'm not mistaking three exterior outlets may be required on the home. As WWHITNEY asked, where will the AC be located. It must have a disconnect device and an GFCI outlet for servicing. Bathroom fans are not required if a window can be opened but always place fans in the bathroom. Texas is the same as Florida, humid as it can be.

Design wise, the sliding door to the bathroom is a PITA. Difficult to use all the time to grab the door to close. You can have a regular door swing into the bedroom area. I had that on one house because of a small bathroom. The bedroom door needs to swing the other way into the bedroom. Getting into the room to get to the closet or bathroom use, it will need to swing very wide to walk around it.

Bathrooms and closest won't work as one room. It does seems to give more usable space and convenience but in use it will be problematic. For one, no privacy if someone is squatting. Showering moisture will work into the closet and may cause mildew conditions. I would put a closet door to the bedroom and a small pocket door to bathroom so it can be closed most of the time and used if needed. This is where a pocket door is useful with occasional use. If you have the space make a throne room around the toilet. That would mean a full wall for the shower. As long as the there is 18" on each side of the center of the toilet it meets code. Placing a door keeps it private. We have it in our present home and it is one of the nicest features of the home.

I hope my suggests gives some insight.

house.jpg IMG_4925.jpeg
 
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wwhitney

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what the inspectors want to see especially for grounding and bonding.
Good point. To the OP, does your slab have any footings (or turned down edges) in direct contact with the soil? If so you need to install a concrete encased electrode (Ufer). Then you won't need any ground rods. If you don't have a footing in direct contact with the soil, you won't be able to install a Ufer, and you'll need 2 ground rods (8' long, fully buried, 6' apart at least). If I recall your water supply is non-metallic, so that's a non-issue.

If you have a slider door or regular door in the rear of the home, a wall switch (three way) may be needed.
Three ways are never required by the NEC, although they are the only practical solution for the the requirements on stair lighting. For a typical room, 2020 NEC 210.70(A)(1) just says (in part) "The wall-mounted control device shall be located near an entrance to the room on a wall." And 210.70(A)(2)(2) says you just need one exterior light at each "entrance or exit with grade-level access". Local amendments could change those requirements.

Exterior receptacle requirements are found in 210.52(E). Basically one at the "front and back" accessible from grade; one at any porch, deck or balcony, accessible from the porch, deck, or balcony level. Plus the requirement WorthFlorida mentioned about a service receptacle sufficiently close to any outdoor HVAC equipment. All of the above could conceivably be handled by two receptacles, but more may be better. I like just putting them on the receptacle circuit for the room on the other side of the wall. I also like the Arlington In-Box http://www.aifittings.com/catalog/inbox/

With a free account, you can read the 2020 NEC online at http://nfpa.org/70

Here are the Austin, TX local amendments to the 2020 NEC, you should review them:

https://library.municode.com/tx/aus...nt_code?nodeId=TIT25LADE_CH25-12TECO_ART4ELCO

Cheers, Wayne
 

WorthFlorida

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Jeffisonm, a little heads up. Not sure how keen you are on home building requirements.

When you submit any kind of plans to the local jurisdiction for permits and the permits are issued, it does not guarantee that the drawing and plans were without omissions. Though some may catch items, no guarantees. On an inspection something may come up that is needed and were not in the plans. The omission must be addressed for a final CO. Also, some local jurisdictions may require an architect's stamp on the drawings. A designed on a computer may not hold up for a submission. You'll need to talk with the your building department for the exact requirement, otherwise you can lose a lot of time.
 

jeffesonm

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Thanks both for the detailed feedback and comments.

Austin lets a homeowner do their own electric (with a permit and inspections) however the service, mast, meter and panel need to be installed by a master electrician. I met with mine yesterday an we went through the details and it sounds like it will be much easier to do the mast/panel on the house, vs on a separate pole near the house, then conduit to panel, etc. Revised location is on floor plan below. This one also better shows the locations of the doors:

asdfasdf.jpg


- The 2020 NEC requires an exterior "emergency disconnect" for the electrical service. Do you have that, or has Austin amended that section (230.85)?
Austin requires a fancy meter box $$ with a built in disconnect.
- How are you handling the overhead lights in the area with open roof trusses? Type of fixture, how you're suspending it, and whether you can run the wiring concealed.
Not sure. So far I've picked up a few dome type fixtures for over the living room, but that's it.
- What's the HVAC? I don't see that on your list.
Good catch. Ducted mini split unit will be mounted among the trusses somewhere, probably above the bedroom in the NW corner. Probably one big run of exposed spiral duct to feed most of the open area and then one branch each to get the bedrooms. 220v 20 amp
Your master closet looks big enough a couple receptacles would be a good idea.
Agree, one on each wall would do the trick I think.
Master bedroom you need a receptacle between the two doors. Kitchen you need a receptacle on the island, 2020 NEC might require two, bears checking.
"Island" will be more like a freestanding work table, so no islands. A floor outlet would be ideal but I figure that's a no-go in a kitchen.
Are you subject to any kind of energy code that requires certain lighting controls?
Austin follows 2015 IECC with amendments, and in general is a big sticker for energy codes. So yes I'm sure but have not researched it much, yet. I have considered it might be worth investing in those remote switches (I've used Lutron's product in the past) to simplify wiring to the exposed ceiling lights among the trusses.
Garbage disposal? Depending on the amp ratings, it and the dishwasher could be on a single 20A circuit.
Not sure on the disposal... never had one, but they sure do seem popular. But both on a 20a sounds good to me, worse case I figure one more to the same area.
- Bathroom lights don't need to be on a circuit of their own--the lights in each bathroom can be on the bathroom's receptacle circuit, or they could be on a general lighting circuit.
Good to know. Maybe what I had read was the receptacles need to be on their own circuit, separate from the lights.
As for conduits under the slab, wiring within them would have to be other than normal Romex (type NM). You could use type UF cable and then the conduit could be just a sleeve for the under floor portion with stub ups into the stud cavities. Or you could run conduit complete from box to box, and pull individual conductors (THWN-2). Either way, for #12 and #14, you are limited to 9 current carrying conductors in a conduit before you have to upsize the conductors (or downsize the breaker). For 120V L-N circuits, that would be (4) 2-wire circuits.
I guess either would work, but from the little research I've done it seems like THWN is the way to go. But I will plan a few extra conduit runs.
Good point. To the OP, does your slab have any footings (or turned down edges) in direct contact with the soil? If so you need to install a concrete encased electrode (Ufer). Then you won't need any ground rods. If you don't have a footing in direct contact with the soil, you won't be able to install a Ufer, and you'll need 2 ground rods (8' long, fully buried, 6' apart at least). If I recall your water supply is non-metallic, so that's a non-issue.
Yes ufer is required, I need to add that once they get the rebar set. And unfortunately electrician said they also want two ground rods (in addition to the ufer.)
Exterior receptacle requirements are found in 210.52(E). Basically one at the "front and back" accessible from grade; one at any porch, deck or balcony, accessible from the porch, deck, or balcony level. Plus the requirement WorthFlorida mentioned about a service receptacle sufficiently close to any outdoor HVAC equipment. All of the above could conceivably be handled by two receptacles, but more may be better. I like just putting them on the receptacle circuit for the room on the other side of the wall. I also like the Arlington In-Box http://www.aifittings.com/catalog/inbox/
Good call on those, I will add one at each patio and I guess one by the "front"/side door too. Slick looking flush mount box, thanks for the heads up on that one.
 

wwhitney

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A floor outlet would be ideal but I figure that's a no-go in a kitchen.
Floor outlets require a special box listed for the purpose, but I see no reason why you couldn't put one in the kitchen. As a plus, it would provide underground feed to the area in case you ever want a fixed island.

Good to know. Maybe what I had read was the receptacles need to be on their own circuit, separate from the lights.
Each bathroom needs one receptacle by the sink, and that receptacle has to be on a circuit that either (a) only serves loads in that bathroom or (b) only serves receptacles in bathrooms. So the only restriction on the bathroom lights is that if you feed two bathroom receptacles with one circuit, then you can't put the bathroom lights on that circuit.

Yes ufer is required, I need to add that once they get the rebar set. And unfortunately electrician said they also want two ground rods (in addition to the ufer.)
Check the Austin amendments to see if the ground rods are actually required in addition to the Ufer (which would be stupid). A contractor saying "they also want X" may be shorthand for "one time, one uninformed inspector asked for X and I didn't feel like checking that or contesting."

Cheers, Wayne
 

jeffesonm

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When you submit any kind of plans to the local jurisdiction for permits and the permits are issued, it does not guarantee that the drawing and plans were without omissions. Though some may catch items, no guarantees. On an inspection something may come up that is needed and were not in the plans. The omission must be addressed for a final CO. Also, some local jurisdictions may require an architect's stamp on the drawings. A designed on a computer may not hold up for a submission. You'll need to talk with the your building department for the exact requirement, otherwise you can lose a lot of time.
Austin requires an architect stamp for >20' or more than 1 story. I do have an engineer who worked on the slab, spans, shear, etc. Your concern is valid though. While planning/building did a limited technical review of my plans during the permit process, but it was mostly for stuff like space in front of fixtures, hallway sizes, tempered glass requirements, etc. I'm sure there will be a few random things I don't catch until the inspector does, and hopefully they won't be expensive ones!
A project like this is great and a lot of fun, but for electrical work do hire an electrician to do some of the work.
Agreed. As per above, electrician will be doing the service and placing the panel, and then I'll be doing all the runs, lights, switches, etc.
Whitney covered most of the items for electrical. A few things I see is where are the exterior doors? It looks like it will mostly be sliding doors due to the character of the home. Building codes may dictate at least two exterior doors. If you have a slider door or regular door in the rear of the home, a wall switch (three way) may be needed. All exterior doors must have a light fixture on the exterior. It looks like the kitchen table area is all sliding doors. It's possible that two exterior light fixtures may be needed at each sliding door (this is where an electrical would know). The kitchen table area may need a three way switch configuration. One by a door if there will be one and as you enter the area from the home. Present figure shows to turn on the kitchen table light you have to walk to the other side of the room.
I'll have to look into this a bit more. The sliding patio doors on either ends of the house go out to patios, but both are fully fenced in. I may (or may not) add another fence gate somewhere near the kitchen side patio but the general idea is entry/exit will be through the side door. This house is on a corner lot and I want it to "face" the side street and otherwise have the "front" yard be a more enclosed patio.
If I'm not mistaking three exterior outlets may be required on the home. As WWHITNEY asked, where will the AC be located. It must have a disconnect device and an GFCI outlet for servicing. Bathroom fans are not required if a window can be opened but always place fans in the bathroom. Texas is the same as Florida, humid as it can be.
My interpretation is the same, one at each patio and another by front door. Hard to say if the 4x4 landing by the front door counts as a porch, but I like outlets anyway.
Design wise, the sliding door to the bathroom is a PITA. Difficult to use all the time to grab the door to close. You can have a regular door swing into the bedroom area. I had that on one house because of a small bathroom. The bedroom door needs to swing the other way into the bedroom. Getting into the room to get to the closet or bathroom use, it will need to swing very wide to walk around it.

Bathrooms and closest won't work as one room. It does seems to give more usable space and convenience but in use it will be problematic. For one, no privacy if someone is squatting. Showering moisture will work into the closet and may cause mildew conditions. I would put a closet door to the bedroom and a small pocket door to bathroom so it can be closed most of the time and used if needed. This is where a pocket door is useful with occasional use. If you have the space make a throne room around the toilet. That would mean a full wall for the shower. As long as the there is 18" on each side of the center of the toilet it meets code. Placing a door keeps it private. We have it in our present home and it is one of the nicest features of the home.
I genuinely appreciate the thoughts here. The Ms. and I gave lots of consideration to the issues you discuss, and I spent a lot of time researching the same. This is a small 2/2 in a hip part of the city, close to downtown. Our imagined occupants (like us) are a couple or maybe one single person, possibly with one kid, but more likely a dog. I say all this to set the stage for why I'm okay making some design decisions that might not appeal to the widest of audiences, but will hopefully be very appealing to some.

The throne room (or "poop room" as we called it) was the subject of much debate. The tradeoff as you noted is privacy vs space. Having only glass on the two sides of the shower will make both the shower, toilet and surrounding space feel bigger (at the expense of privacy.). It's not a huge bathroom (or house, really) and we figured the other bathroom was ~10 ft away if you wanted privacy.

Likewise much debate around the bedroom/bath/closet arrangement. My ideal setup has the closet and bath are separated from the bedroom by a corridor or entry, that leads to the rest of the house. This lets one person get up and get ready in the morning while leaving the other sleeping in peace. The arrangement depicted gets you close, although you do have to walk back through the bedroom after getting ready.

I like your pocket door idea, I will give that some more thought. It provides the best of both worlds, although it would sacrifice some usable closet space by having doors on two sides of the walk in. Pocket door either way is probably a good idea.

ceilings.jpg

This drawing shows which interior rooms will have interior ceilings. You may notice the closet does not... the idea being you'll be able to see up and out those high windows wrapping along the NE corner. It will also let lots of natural light in, some of which should filter into the bathroom. Downside again is privacy, as now your bathroom (and kind of bedroom?) are open to the rest of the house. Another plus for pocket doors, when you want them (company is over, for example)

The only real master bedroom windows are the 6' wide sliders out to the back patio. I've considered some type of operable "skylight" or other type of transparent/translucent panels in the ceiling, but I'd want some way to close them off if you're the type who wants it real real dark when you sleep.

Likewise no "ceiling" for the shower, although to your point I want to keep moisture in. I'm currently thinking a skylight of some sort, perhaps one of those polycarbonate bubble types. But then I'll need a wall mount exhaust fan. Many details to sort out, but right now under slab requirements are most immediately pressing.
 

WorthFlorida

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:) Good planning.

About the island, a working table is not quite the same. A fixed island appeals to everyone and kitchen storage is always desirable under the island. It may not be the look you're looking for but always try to make it as big as the space can handle. Our island with the kitchen sink is 4'x8', closer to 5x9. It took one granite slab to cover. During parties and diners, everyone gravitates to the island. Good for a buffet layout too.
 

jeffesonm

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Thanks! If you have any other suggestions please don't be shy, I really do appreciate hearing other perspectives and ideas.

The island/work table will be ~8' long by 36"-42" wide. I plan to buy/build/repurpose some sort of industrial piece so it will depend in part on what I come up with, but something like the images below. Storage one one end and then open space on the other where you can tuck 2-3 barstools underneath. Exact size will also depend on how the space feels once the walls are up and cabinets are in. I'm a big fan of building scale models so will probably mock up a plywood worktable in 96x42 and see how it feels.

island1.jpg island2.jpg


Floor outlets require a special box listed for the purpose, but I see no reason why you couldn't put one in the kitchen. As a plus, it would provide underground feed to the area in case you ever want a fixed island.
I wasn't sure if a kitchen is considered a wet/damp area or if there are otherwise any code provisions that would prohibit it, but this would be ideal so I'll investigate further. I could aim for the island corner closest to the living room area which would be a good 10' from sink, dishwasher, etc.
 

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And unfortunately electrician said they also want two ground rods (in addition to the ufer.)
Someone pointed out to me that the power company sometimes has rules on top of the NEC. Rules like "every service must have a ground rod." (Perhaps on the theory that a ground rod is one type of grounding electrode that the power company can easily verify is present.) So check your power company requirements (here they publish a technical manual called the PG&E Green Book).

If the power company requires 1 ground rod, and the NEC requires 0, then one should be enough, you shouldn't need two ground rods. The NEC requirement for two ground rods instead of one only applies when the only electrodes are the water pipe and/or ground rods (or pipes or plates).

Cheers, Wayne
 

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Thing about the PVC transitions on either end (where it leaves the slab)....

If I go to the conduit as raceway route and use UF cable, I can just run that in the walls, correct? Perhaps a bushing on the end for abrasion resistance?

Alternatively, using THWN I’ll need to continue the conduit in the wall to the first box, or right to the panel on the other end?
 

wwhitney

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Thing about the PVC transitions on either end (where it leaves the slab)....

If I go to the conduit as raceway route and use UF cable, I can just run that in the walls, correct? Perhaps a bushing on the end for abrasion resistance?
That would be the "conduit as sleeve" route. Raceway is a synonym for conduit (more or less, you could say EMT is a raceway that's not conduit). Bushings on the ends would be a good practice (not sure if they are required).

Alternatively, using THWN I’ll need to continue the conduit in the wall to the first box, or right to the panel on the other end?
THWN requires a complete conduit system from box to box, so that's "and right to the panel." Also, no more than 360 degrees of bend between pull points, which have to be accessible after the building finish is installed.

A sleeve is subject to none of those rules, but pulling through more than 360 degrees of sleeve would be asking for trouble.

Cheers, Wayne
 

wwhitney

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Well, EMT is Electrical Metallic Tubing, not Electrical Metallic Conduit. While RMC is Rigid Metal Conduit. I believe there are places where the terminology "conduit" excludes EMT. But they are definitely both raceways.

Cheers, Wayne
 

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Slab was poured and house is dried in. Pleased to report that all under slab plumbing and electrical landed where I was hoping. Final layout is below. All conduit runs are 3/4". Two runs from the panel to the interior of the house (2 x 3/4" each) and then one run across the open living area so I can easily do a 3 way switch. Next up is rough in.

D7108592-BAF5-4CA5-8D82-FE7C7B6C5439.jpeg


This is my first time using plastic conduit. I plan to run the circuits in THWN through the conduit. One end will terminate at the panel and then the other end will terminate within a junction or switch box (which will need to remain accessible) and then switch to romex and run like usual through the studs.

IMG_3741.jpg

^^example of conduit coming up through my newly poured floor

How do I actually transition the conduit to a box? Get a metal box, knock out some of the 3/4 knock outs and then one of these male fittings? Regular metal locknut to secure it? Do I need a bushing covering the threads?

Screen Shot 2021-01-17 at 10.56.02 PM.png


I saw these at the store.. what are they used for?

Screen Shot 2021-01-17 at 10.56.30 PM.png


Also I'd like to terminate both 3/4" runs at location A in one larger box, then have a bunch of different circuits coming out of there. I've been reading about how many current carrying conductors I can run in conduit, derating, and also about MWBC/shared neutral, box fill etc. Much reading. Anyhow even the biggest metal box (4 11/16 sq) doesn't seem to have enough cu in for 6-8 circuits.

Can I just get one of those 6x6x4 PVC outdoor boxes and then drill a bunch of holes in it for the PVC conduit coming in, and romex connectors going out? And then mount it so the cover is flush with the drywall? The would be in a laundry room so not super concerned with it being a little ugly.
 

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The connector is how you terminate pvc conduit to a metallic box or a pvc box if you have to drill holes. The nuts are sold separately, the same sizing as metal. The threads are a little longer than metallic connectors since pvc boxes are thicker walled. If you use a metal box to transition to NMC be sure the box is grounded with to the green wire.

-------------------------------

3/4" to 1/2" adapter. This is used for pvc box with 3/4" insert when you need 1/2".
upload_2021-1-18_7-42-1.png
 

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wwhitney

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Get a metal box, knock out some of the 3/4 knock outs and then one of these male fittings? Regular metal locknut to secure it? Do I need a bushing covering the threads?
Yes, yes, not required for small conductors but some people still like to use them. Or you can use a PVC box, there are some with KOs, or you could carefully drill the correct size hole in a box.

I saw these at the store.. what are they used for?
That's an alternative way to terminate PVC conduit to a KO. You slide it through the KO from the inside, and then glue a coupling onto it, trapping the box wall between the box adapter and the coupling.

I'll respond to the other questions shortly.

Cheers, Wayne
 

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Also I'd like to terminate both 3/4" runs at location A in one larger box, then have a bunch of different circuits coming out of there.
What's the upside? Seems like separate boxes would be neater, less chance of spaghetti, and you could use smaller boxes.

On the box side of things, a 1-1/2" deep 4-11/16" square metal box (e.g. Raco 247) and a 1-1/2" extension ring (Raco 250) are each 29.5 in^3; if you add a 1" deep 2 device mudring (Raco 819), that's another 15 in^3. So that gets you up to 74 in^3 pretty easily in a 2x4 wall with 1/2" drywall.

On the conduit side, a 3/4" Schedule 40 (?) PVC conduit can hold 21 #14 or 15 #12. Derating is 70% for 7-9 CCCs and 50% for 10-20 CCCs. Base ampacity for derating is 25A for 90C #14 and 30A for 90C #12.

So at 70% derating, #14 is still good for 15A, and #12 is still good for 20A. At 50% derating, #14 is only good for 12.5A; so it can't be used for receptacles, but it could still be used for a lighting circuit, say, and protected at 15A if the calculated load is 12.5A or less. At 50% derating, #12 is good for 15A, so it can be used for a 15A circuit including receptacles. [#10s are a possibility, as they have a 40A 90C ampacity, so after 50% derating they are still good for 20A, but 3/4" PVC can only hold 9 #10s, so that doesn't come into play at this conduit size.]

MWBCs can be trouble with AFCIs, you either need double pole AFCIs, or to use a panel/breaker manufacturer whose AFCIs don't have a GF component (GE and I think one or two others, maybe BR?)

So for 15A circuits, best case with #14 is 4 MWBCs, 8 CCCs, 13 wires including EGC. Upsizing to #12s to allowing 50% derating only allows you to fit in one more 2-wire circuit, since 3/4" conduit can only hold 15 #12 wires. Hardly seems worth it, but it is an option. Or without MWBCs, you could use #12s, 7 2-wire circuits with 14 CCCs, 15 wires totals, only one 2-wire equivalent worse than using MWBCs, at the expense of a large wire size and a full conduit.

For 20A circuits, best case with #12 is 4 MWBCs, 8 CCCs, 13 wires including EGC. Without MWBCs, you are limited to 4 2-wire circuits, 9 wires total.

Jumping back to the box size question, worst case for one conduit with #14s is 4 MWBCs coming in, 8 two wire circuits going out. So that's 12 coming in, 16 going out, 1 allowance for EGC = 29 wires @ 2 in^3 = 58 in^3 required. No problem with the (1-1/2" + 1-1/2") deep 4-11/16" box with a blank cover. With 2 devices in a mudring, you're up to 66 in^3, still fine.

With everything the same but changing to #12s, each allowance is now 2.25 in^3. So that would be 65.25 in^3 for a blank cover, now you need the mudring volume. And with 2 devices in the mudring you'd be slightly over at 74.25 in^3, but one device in the mudring would be fine. [A 3-1/4" deep 4-11/16" box (Raco 260) is 66.7 in^3, and a 3/4" deep 2 device mudring (Raco 840) is 8.8 in^3, so that would make 75.5 in^3, which would work for 2 devices.]

Cheers, Wayne
 
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