Electrical Wiring: Under the pad or over head?

[bmxdad]

I'm thinking of adding some 3/4" schedule 40 under the pad, which has not been poured yet, that starts under the sub-panel and goes to each wall and some 240v boxes. I want some 110v 20amp outlets and some 240v 30/40amp circuits. Is there a wire limit on what can be in each conduit?

I want to make sure this is worth doing before going through all the work, only to find out I can't do it ... either because of code or something else. Pierce County, Washington has adapted the 2008 NEC, and I have a copy of it, but haven't found anything to help ... yet. It's a big book!

Thoughts?

 

[SunsetsAndFriends]

I like the idea of conduit under slab. I just have concern about leaking under slab conduit which I saw on a job recently.


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[TheEquineFencer]

I'm thinking of adding some 3/4" schedule 40 under the pad, which has not been poured yet, that starts under the sub-panel and goes to each wall and some 240v boxes. I want some 110v 20amp outlets and some 240v 30/40amp circuits. Is there a wire limit on what can be in each conduit?

I want to make sure this is worth doing before going through all the work, only to find out I can't do it ... either because of code or something else. Pierce County, Washington has adapted the 2008 NEC, and I have a copy of it, but haven't found anything to help ... yet. It's a big book!

Thoughts?

http://www.electrician2.com/calculators/rf_calculator.html

You can run 3/4 to each, but I'd run a little larger than what you think you might need to allow for future circuits. I'd even run some to each of the four walls even if you don't use them right now, you'll know where they are for future use. As far as water inside the conduits, if the proper conduit and wire is used and installed properly water is a non issue even if some does get into the conduit. I assume you are going to run something like THHN and not Romex inside the conduit? Romex in a conduit is a no no now.

 

[Thumper68]

I ran quite a bit of stuff under my slab before I poured,

1 2" conduit for the table saw, jointer and router table

1 3/4" conduit for the post mounted vise feeds a outlet in the .post, nice having power right there for grinders, sawzall etc..
1 1" conduit for the future jib crane, again will service outlets in the post and be avalible for a elec hoist if needed.

1 4" pvc with ground wire for the dust collector for the table saw, jointer and router table.

I did run a 3/4" galv pipe for air but somehow it got lost in the pour, my guess is that one of the helpers stepped on it and pushed it down.

1 4" pvc that feeds the outside use that for the water line, power for the sewage transfer pump heat lines for the well house.

Wish I had added a few more for different things and I will if I ever build a new shop or add on to this one.

 

[bmxdad]

... I assume you are going to run something like THHN and not Romex inside the conduit? Romex in a conduit is a no no now.

Thanks... nice to know.

So I guess you spliced Romex at a j-box, where the conduit came up?

 

[Stuart in MN]

If it's under the floor you need to use THWN wire. However, most of it is dual labeled THHN-THWN these days, you just need to check for sure when you buy it.

There's usually no good reason for installing Romex in a conduit, but it's perfectly okay as long as 1) the conduit is sized correctly and 2) it's not below grade or in wet areas.

 

[bmxdad]

... There's usually no good reason for installing Romex in a conduit, but it's perfectly okay as long as 1) the conduit is sized correctly and 2) it's not below grade or in wet areas.

Why? What else should be used? Where in NEC 2008 is this information?

Isn't under the pad below grade? Or is below grade under the top level of the ground, that the pad is on?

 

[Stuart in MN]

Romex (actually, NM-B or nonmetallic cable - Romex is a brand name) is not rated for use in wet areas, and a buried conduit is considered a wet area. You can get nonmetallic cable rated for underground use, it's called UF-B. It has thicker insulation than NM-B so it's even stiffer and more difficult to put in a conduit.

There's nothing in the NEC that prohibits installation of NM-B or UF-B in a conduit, other than the conduit has to be sized properly. For a flat type cable like NM-B you base the conduit size on the largest dimension of the cable, which is its width, and then you go find a conduit sizing calculator and figure out what size conduit is required.

However, installing NM-B in a conduit is a pain in the neck since it's thick and stiff and hard to pull. If you're going to run wire in conduit, the best choice is to use individual wires instead of a bundled cable like NM-B or UF-B. It fits better in a conduit and is easier to pull. As mentioned earlier, use THHN/THWN rated wire, as it's rated for use in underground conduits.

 

[bmxdad]

Ahh ... makes sense now. That explains alot. Thanks.

 

[Rookie2]

conduit is cheap ! I over sized mine and you always think of other circuits after you pour. I also ran air lines across to the other walls under the pad .

 

[Charles (in GA)]

Ahh ... makes sense now. That explains alot. Thanks.

I highly suggest that you obtain a copy of "Illustrated Guide to the National Electric Code" by Charles R Miller. The 4th edition covers the 2008 code (what version of the code is used in your area?)

You can find the book on Amazon. It will be hugely beneficial to understanding the code, as it gives illustrations, examples and the code references. It pulls references from various places in the code and puts them together to explain a particular thing, such as wet locations. I consider the book invaluable.

Charles

Example page from the book.

 

[bmxdad]

Tacoma Public Utilities (TPU), according to their website :

The current Electrical Code of the City of Tacoma adopted the 2008 National Electrical Code (NEC) in its entirety.

 

[TheEquineFencer]

Why? What else should be used? Where in NEC 2008 is this information?

Isn't under the pad below grade? Or is below grade under the top level of the ground, that the pad is on?

I'm not going to get in a ******* match, but I think Romex is not rated for use inside Conduit, 2008 is old code, check latest code requirements. I know around here you get failed for doing it now. Before the last code change you could.

Yes you can splice at a box where it comes out of the concrete.

I'd run a couple of "pull strings" in the conduit, one for now and one for later.

 

[Stuart in MN]

The NEC 2011 and NEC 2014 don't have any prohibitions against using it in conduit either, but it's entirely possible there are local regulations against it.

 

[bmxdad]

Tacoma has adopted NEC 2008, so any changes in NEC 2011 or NEC 2014 don't apply ... right?

 

[Charles (in GA)]

Why? What else should be used? Where in NEC 2008 is this information?

Isn't under the pad below grade? Or is below grade under the top level of the ground, that the pad is on?

ARTICLE 100—DEFINITIONS

Location, Wet. Installations underground or in concrete
slabs or masonry in direct contact with the earth; in locations
subject to saturation with water or other liquids, such
as vehicle washing areas; and in unprotected locations exposed
to weather

The code is difficult to "read" because you have to put things together from several different places in the book sometimes, for it to make sense. People overlook the Definitions, but there is a lot of information in that section.

Charles

 

[Highbeam]

Tacoma has adopted NEC 2008, so any changes in NEC 2011 or NEC 2014 don't apply ... right?

Are you in the City of Tacoma or in TPU's service area? Pierce county doesn't issue electrical permits, the Washington State department of Labor and Industries does. If you're in Pierce county and not in TPU land then you'll be getting a state permit and they make us follow the 2012(or was it 2011?) nec last time I got a permit.

 

[bmxdad]

TPU's service area ... so TPU service, but not there reg's ... Nice

Your right ... they have this:

NOTE: The 2014 National Electrical Code is adopted effective July 1, 2014.
Permits purchased after June 30, 2014 will be inspected using the 2014 NEC.

Oh well ... at least I'm learning now, before later.

Thanks again GJ

 

[sberry]

The main advantage of under slab is going around doors and the need for hangers. Overhead is slow, every step up and down ladders.

 

[brewchief]

We ran a bunch of 1/2" and a couple 3/4" to various spots when we did my friends 50x100. I wouldn't bother with the 1/2" again and would run all 3/4" with a couple 1".

Yes there are fill limits for each type of conduit and there is also derating required if over so many currant carrying conductors.

We didn't bother with a pull string, I just sucked a pull line through with a shop vacuum when we were ready to pull wire.

 

[Highbeam]

Ive vacuumed pull lines through a few long conduit runs. It works very very well.

 

[TheEquineFencer]

Ive vacuumed pull lines through a few long conduit runs. It works very very well.

I have too, but if you already have wires in the conduit and later on want to add some more, a pull string works pretty good.

 

[bmxdad]

As I pull wire through I'll be adding a pull line just in case. I have one run already that is maxed out that I wont though. It has 4 hot, 4 neutral and one ground for 4x 20 amp circuits. Wire will be #12 THWN/THHN.

 

[TheEquineFencer]

FWIW....
Chapter 9 of the NEC titled "Tables" says in "Notes to Tables", note (9), A multiconductor cable or flexible cord of two or more conductors shall be treated as a single conductor for calculating percentage conduit fill area. For cables that have ellipticall cross sections, the cross-sectional area calculation shall be based on using the MAJOR diameter of the ellipse as a CIRCLE DIAMETER

What this is saying, is that, when you install a multi-conductor cable such as romex in conduit, a single romex pull would be a single conductor and have a 53% fill limit, while two conductors (two separate THHN wires or two Romex pulls) would have a 31% fill limit and more than two conductors would have a 40% fill limit...... the limits are specified in Table 1 of Chapter 9........... AND you would take the fattest width of the romex and calculate the area using the simple Pi times the radius squared formula. Thus a typical 12/2 w/grd Romex that measures about .375 the wide way, is .375 divided by 2 equals .1875 and that squared is .03516 times Pi (3.1416) is .1104 square inches of area PER ROMEX. Table 4 of Chapter 9 tells us that for a 1/2 conduit the 53% (one conductor) fill is .182 and the 31% (two wire) fill is .094 and the 40% (three wire) fill is .122

Thus a single 12/2 Romex is all that will fit in a 1/2 EMT conduit.

Looking at 3/4 EMT, the two wire 31% fill is .165 so two Romex pulls will not go in 3/4 either. You have to go all the way to 1" EMT to get a two wire fill large enough, .268 to accommodate two pulls of Romex.

HOWEVER, all of this being said............ Note (2) tells us that the conduit fill limits of Table 1 applies only to complete conduit or tubing systems and is not intended to apply to sections of conduit or tubing used to protect exposed wiring from physical damage. While I thought there was a length limit to use of EMT as physical protection and not as a complete system, I cannot find it in the code right now.

That being said, common sense tells us that when you have a 1/2 conduit whose 100% fill is .304 sq in, and a ID of .622" that two 12/2 Romex cables will not fit in it very well if at all.
However, it is not compliant according to code: NEC 310.120
All conductor and cables shall be marked...

 

[bmxdad]

No Romex type wires are going in the conduit, just individual THWN/THHN wires. They way I figure it, using tables below, is that I can get 9 #12 wires for each conduit.

Table 4: Articles 352 and 353 - Rigid PVC Conduit (PVC), Schedule 40, and HDPE Conduit (HDPE)

Table 5: Dimensions of Insulated Conductors and Fixture Wires

Edit: Annex C, Table C.10a shows 9 too.

 

[revamped]

I am really glad you posted this. Planning my electrical now; footer poured in Silverdale and 8ft stem walls being poured this week. I really need to get in gear on the electrical plan. I was going to go through walls, and ceiling, but reading this I think I might just put in a 1" conduit to each 20ft section of wall and a second for each 220 drop going in. The math is driving me nuts because I haven't planned out the circuits yet but I know I want a 4plex every 12 feet on concrete walls and each interior framed wall, and one 220 on each rear wall and in the loft ready to go high on the lift. I guess maybe I shouldn't have planned to take on the electrical myself... lol. time to buy some books.

 

[Milton Shaw]

I ran part of my dust collection system under the slab also. If you know where tablesaw/jointer/plainer are going to be then run ductwork under slab it makes a lot neater shop that running it all overhead later.

 

[bmxdad]

Make sure you plan out how and where they come up out of the floor. There is one that I wish I would have moved a couple inches over ... no big deal, but something to be aware of.

 

[homebuilt burner]

A different comment, but I saw one wood working shop where they had poured a 12"x12" chase in the concrete floor for running electrical and dust collection to the various machines. Then had a wooden cover flush with the floor. Maybe not a great idea for a wet location but otherwise it offers a lot of flexibility.

 

[Travv]

No Romex type wires are going in the conduit, just individual THWN/THHN wires. They way I figure it, using tables below, is that I can get 9 #12 wires for each conduit.

Table 4: Articles 352 and 353 - Rigid PVC Conduit (PVC), Schedule 40, and HDPE Conduit (HDPE)

Table 5: Dimensions of Insulated Conductors and Fixture Wires

Edit: Annex C, Table C.10a shows 9 too.

Don't forget to derate the ampacity for the number of current carrying conductors in your raceway.

 

[sberry]

As I pull wire through I'll be adding a pull line just in case. I have one run already that is maxed out that I wont though. It has 4 hot, 4 neutral and one ground for 4x 20 amp circuits. Wire will be #12 THWN/THHN.

There is a super temptation to want a circuit for everything. It also could be done with less conductors and may even work better on long runs if a couple circuits were to be heavily loaded.

 

[Kaizen]

great thread. What is the preferred method of where all this pipe goes? under the dirt? I can see a 1 inch going attached to metal in middle of the slab but a 4 inch for vacuum? is that totally under the dirt?

 

[simpler=better]

Sounds like you've got your head on straight