NM-B in conduit?

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  #1  
Old 01-30-02, 12:39 PM
Sketchy
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NM-B in conduit?

I have some 4/3 NM-B (romex) emerging from under my house to the main service entrance panel. It needs to run about 1-2 feet up the side of the house to the panel. Is it OK to run the NM-B in PVC conduit? If so, what size? If not, can I strip the outer (black) insulation and run the individual wires in conduit (and what size)?

Thanks
-- Jim
 
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  #2  
Old 01-30-02, 06:23 PM
Wgoodrich
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Try 1" sch 80 PVC to sleeve that Romex for protection. Make sure the conduit enters the panel box using a terminal adapter and an LB and that the conduit terminates at the other end of the conduit inside the structure to ensure you have this cable protected also from sunlight and a wet location.

You will not be meeting the NEC if you install 3/4" sch 40 PVC unless you are above about 6' otherwise close to the earth would be normally be considered exposed to physical damage requiring the sch 80 PVC. Sch 80 PVC has a smaller inside diameter than sch 4o PVC is why I suggested the 1" sch 80 PVC.

Hope this helps

Wg
 
  #3  
Old 01-30-02, 06:54 PM
Sketchy
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Thanks Wg, that helps quite a bit.
-- Jim
 
  #4  
Old 01-31-02, 12:42 PM
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Is NM-B listed for use in a raceway outside of a house? Also, isn't 1" PVC conduit a tad small for the 4-3 NM-B? Didn't do a conduit fill calc so I don't know what size conduit should be used. Are you sure it's NM-B and not UF? I'm not so sure they make UF with #4-3 conductors though?
 
  #5  
Old 01-31-02, 01:57 PM
Sketchy
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It's NM-B, not UF. It needs to travel about 12 inches up the outside wall from the hole to the main panel. Once it disappears into the hole, which leads into the basement/crawlspace, it won't be running in conduit. Does the conduit fill calculation come into play with such a short length? Would it be better if I stripped the outer (black) insulation from the NM-B and just ran the wires up the pipe?
-- Jim
 
  #6  
Old 01-31-02, 03:05 PM
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Sketchy,

You can use the 1" pvc for protection. NEC only considers the conduit fill for complete conduit systems. If it was me, i'd use 1 1/4" pvc.
 
  #7  
Old 01-31-02, 05:31 PM
Sketchy
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Thanks thinmn. I assume you'd do the 1.25" pipe for the purpose of extra heat dissipation room? I'll look into that.
-- Jim
 
  #8  
Old 01-31-02, 05:45 PM
Jxofaltrds
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Thinman????

"You can use the 1" pvc for protection."

I agree. I am trying to find a NEC ref. for this. The best I can find is 342 1999 NEC.

It is my understanding conduit can "only" be used for protection.

342-7. Installation. Nonmetallic extensions shall be in-stalled
as specified in (a) and (b).
(a) Nonmetallic Surface Extensions.
(1) One or more extensions shall be permitted to be run
in any direction from an existing outlet, but not on the floor
or within 2 in. (50.8 mm) from the floor.
(2) Nonmetallic surface extensions shall be secured in
place by approved means at intervals not exceeding 8 in.
(203 mm), with an allowance for 12 in. (305 mm) to the
first fastening where the connection to the supplying outlet
is by means of an attachment plug. There shall be at least
one fastening between each two adjacent outlets supplied.
An extension shall be attached to only woodwork or plaster
finish, and shall not be in contact with any metal work or
other conductive material other than with metal plates on
receptacles.

An extension shall be attached to only woodwork or plaster
 
  #9  
Old 01-31-02, 07:09 PM
Wgoodrich
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I am sorry, I read 14/3wGrnd instead of 4/3wGrnd.

I don't have a piece of that cable to confirm what the diameter is accuratly of that cable.

He mentioned that he is protecting that cable 1 to 2'. 24" or less would be a nipple allowed to be filled 60% per chapter 9 table 4.

I looked at the approximate diameter of 1 - 4 awg thhn conductor in table 5 and found it to be .324". Multiply that times 3 conductors all being the same and you would have an approximate diameter of .97 then add to that the approximate diameter of a 10 awg bare grounding conductor equalling .102 in table 8 totalling a combined diameter of 1.07" diameter without adding for the thickness of the sheathing of the cable. Then back to table 8 finding a single bare conductor equalling that diameter I found the diameter of 900 mcm to be 1.09" then converting to area sqaure inch as per table 4 then 900 mcm would equal .94 square inch that would meet the requirement of being one conductor as requried for Romex in a conduit.

Back to table 4 telling us a limit of 60% fill on a nipple less that 24" long and you would have to have a required size of 1 1/2" Sch 80 PVC. 1 1/2" sch 80 PVC allows in 60% fill column of .94 square inch.

SPECIAL NOTE;

DID THAT MAKE SENSE? I think that is right but anybody that questions this calculation is highly welcome to speak up. I am not used to calculating it this way. Normally I measure the diameter of the cable and then go to table 8 and find a single cable that matches that diameter and go from there. In this case I didn't have a cable to measure and had to make a calculation that I can't remember the last time I did that calculation.

Hope this helps

Wg
 
  #10  
Old 01-31-02, 07:15 PM
Wgoodrich
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Jxofaltrds

Try looking up the following rule of chapter 9 of the NEC, that rule should hit what you were looking for.

copied section of the NEC that applies;
2002NEC

Chapter 9 Notes to tables rule 9;

(9) A multiconductor cable of two or more conductors shall be treated as a single conductor for calculating percentage conduit fill area. For cables that have elliptical cross sections, the cross-sectional area calculation shall be based on using the major diameter of the ellipse as a circle diameter.


Hope this helps

Wg
 
  #11  
Old 01-31-02, 07:38 PM
Sketchy
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I'm surprised this situation isn't more clear in the code, as it's a common situation (or maybe I'm misinterpreting your code discussion as uncertainty?). 90% of the houses in my neighborhood have the main panel on an outside wall, with the branch circuits and subpanel feeders exiting the panel, running down the wall a couple of feet in conduit, then disappearing into the crawlspace via a LB 90 degree turn. What is the standard practice for pro electricians? Do they run THHN in the conduit and join with NM once inside the house?

If it helps, I just measured the 4/3 NM cable. It averages around 3/4" in diameter, but since the individual wires inside it are twisted around each other, it makes spiral grooves running the length of the cable which can increase or decrease the diameter by 1/8" or so.

-- Jim
 
  #12  
Old 01-31-02, 08:21 PM
Wgoodrich
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Take a look and see if there is a -2 on the side of that cable just out of curiosity. If you find the -2 then your cable is a compressed cable reducing the diameter due to being compacted. This is common now in the building trade. That would answer as to why I came up with the 900 kcmill wire size for diameter of a single conductor using the other tables.

If we take you measurement of 3/4" and convert it then we would have a diameter of .75". In table 8 of chapter 9 a conductor equalling that diameter would be a 500 mcm insulated conductor. The conductors within your cable most likely is THHN and the area square inch of 500 mcm THHN in table 5 is .7073. We can then go directly to table 4 and find the conduit fill allowed for a nipple less that 24" long allowing a 60% fill of that conduit using sch 80 PVC for subject to physical damage we should find the required conduit minimum size to be back to the same size that Thinman said that he would install. The minimum conduit size should be 1 1/4" sch 80 PVC.

much simpler if you know the diameter of the cable as per the rules in the tables of chapter 9.


Sketchy, don't be surprise the round robin calculations we have to do to get an answer in the NEC. This seems to be the attraction to a lot of us concerning the NEC. Quite a challenge to say the least. Actually with the NEC changing every three years it can be a life long challenge.

Wg
 
  #13  
Old 01-31-02, 08:23 PM
resqcapt19
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Doesn't Note 2 to Table 1 in Chaper 9 tell us that the conduit fill rules do not apply to sections of conduit used for protection. I think that it does and any size that you can fit it in would be ok.

"2. 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."

I think thinman brought up the wet location issue. In my opinion the interior of any outside conduit is a wet location and NM cable is not permitted in wet locations.

Don(resqcapt19)
 
  #14  
Old 01-31-02, 09:15 PM
Wgoodrich
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resqcapt19, you are absolutely right. Worse is that thinman said it before but it didn't register. Guess you just said it bigger. Sorry thinman, I spaced rule 2 concerning sleaving only. Both of your are absolutely right if the cable will slide through it without damaging the cable then you are within code when just sleaving. Again my short comings shining through again!



I'm Not sure I can agree with your stance that it would be a wet location. See copy of NEC below.

225.22 Raceways on Exterior Surfaces of Buildings or Other Structures.
Raceways on exterior surfaces of buildings or other structures shall be raintight and arranged to drain.


If we follow the instructions of 225.22 then inside that conduit would not be exposed to weather. Then if we looked at definitions of Raintight as required in 225.22 and definition of a wet location. see below

Raintight. Constructed or protected so that exposure to a beating rain will not result in the entrance of water under specified test conditions.

Location, Dry. A location not normally subject to dampness or wetness. A location classified as dry may be temporarily subject to dampness or wetness, as in the case of a building under construction.
Location, Wet. Installations under ground 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.


I can see where your thoughts are coming from but if we read the minimums set in the NEC the Romex would be protected, inside a raintight conduit and allowed to become temporaryly wet. I am not sure as an AHJ you could come up with a Code section to support the belief this installation protected within a raintight protection would be considered as a wet location.

Not really against your stance on the subject just not sure it could be backed up by the NEC rules.

just my thoughts

Wg
 
  #15  
Old 01-31-02, 09:26 PM
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Now I'm really confused. The note that Don mentioned seems to render table 1 useless. After all, when is conduit used except for physical protection?
 
  #16  
Old 01-31-02, 10:31 PM
Wgoodrich
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John what Don is talking about are sections in the NEC that require special consideration due to a certain section of a circuit being installed in an area where exposed to physical damage. Example would be a wood structure where a Romex cable comes through the floor behind a washer that is liable to jump around and damage that cable. A piece of conduit would be required to protect that area that would be exposed to physical damage. While the wiring style is Romex it may be installed in a spot of concern. The majority of that circuit is approved and not subject to physical damage yet a section must be protected by sleaving it in conduit. Therefore this wiring design would not be a complete conduit installation and would land under that rule to of the Tables rules. An example of what we are talking about where there is rules requiring sections of a circuit to be protected by conduit can be found throughout the NEC. One example can be seen below.

334.15

(B) Protection from Physical Damage. The cable shall be protected from physical damage where necessary by conduit, electrical metallic tubing, Schedule 80 PVC rigid nonmetallic conduit, pipe, guard strips, listed surface metal or nonmetallic raceway, or other means. Where passing through a floor, the cable shall be enclosed in rigid metal conduit, intermediate metal conduit, electrical metallic tubing, Schedule 80 PVC rigid nonmetallic conduit, listed surface metal or nonmetallic raceway, or other metal pipe extending at least 150 mm (6 in.) above the floor.


Hope this helps

Wg
 
  #17  
Old 02-01-02, 10:55 AM
resqcapt19
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Wink

Warren,
Why does "raintight" conduit have to be arranged to drain????
Don(resqcapt19)
 
  #18  
Old 02-01-02, 11:47 AM
Sketchy
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wow, I'm not envying the job of my inspector right now.....or any inspector for that matter.
-- Jim

p.s. Wg, there is no "-2" on the cable
 
  #19  
Old 02-01-02, 11:53 AM
Wgoodrich
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225.22 Handbook commentary;
If raceways are exposed to weather or rain through weatherhead openings, condensation is likely to occur, causing moisture to accumulate within raceways at low points of the installation and in junction boxes. Therefore, raceways should be installed to permit drainage through drain holes at appropriate locations.

My opinion;
A conduit mounted outside should be arranged to drain in case if may become subject to temporary wetness. See the definition of a Dry Location below found in Article 100;

2002 NEC
Location, Dry. A location not normally subject to dampness or wetness. A location classified as dry may be temporarily subject to dampness or wetness, as in the case of a building under construction.

Considering the commmentary and the definition above I would say that inside a conduit that is weathertite should be considered as a dry location. The NEC declares conductors installed within a conduit installed underground to be a wet location. The NEC does not declare conductors in conduits mounted on exterior of buildings to be a wet location. If the Code declares one installation and is mute on the other installation concering declaring wet locations I would see it hard to find a content of the NEC supporting a ruling that inside that coduit mounted above ground on the exterior of a builging to be a wet location.

My thoughts for what they are worth;

Wg
 
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