Wires vs Cables in 1inch PVC conduit

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Old 06-03-18, 12:34 PM
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Wires vs Cables in 1inch PVC conduit

I am running Romex cable 12/2 inside the wall and then it goes out in the garage...so I will need to run it inside a PVC conduit for about 12 feet from almost the bottom...about 6 inches off the ground to the ceiling of the garage
I know normally - we'd only run wires inside a conduit like THHN and there are tables and calculations to calculate the total number of wires allowed in particular size conduit....so I have two questions:
1. Should I actually strip the PVC jacket as I enter the conduit ?
2. If I don't have to strip the jacket but can run the Romex cable through the conduit - will that then change the maximum count of wires - as volume will surely be different

thanks!
 
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Old 06-03-18, 12:41 PM
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1. Should I actually strip the PVC jacket as I enter the conduit ?
No. The wires are not marked or rated for outside the sheath.

2. If I don't have to strip the jacket but can run the Romex cable through the conduit - will that then change the maximum count of wires - as volume will surely be different
When using cable it isn't wire count it is volume. Cable shouldn't exceed 45% of the conduit volume.

I will need to run it inside a PVC conduit for about 12 feet from almost the bottom...about 6 inches off the ground to the ceiling of the garage
If any portion is outside you can not use NM-b. You can use UF.
 
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Old 06-03-18, 01:02 PM
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Based on your description it's not totally clear if the conduit is on the outside of the structure or inside when stating 6 inches above ground; did you mean floor?.

For a cable such as NM in conduit the cable area is calculated based on the widest dimension of the cable and is treated as a single conductor.
 
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Old 06-03-18, 05:47 PM
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When you're using conduit as protection, the NM-B can just be run inside it.

It's not recommended for long runs as it's a pain to pull through, but for your application, it seems perfect. Just run it through the conduit, then into a standard cable clamp when it enters the box. Clamp the conduit every few feet, and you're all set.
 
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Old 06-03-18, 06:36 PM
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pattenb - you are right...I meant garage floor...so the run is on the inside of the garage...

Ray; when looking up what's allowed inside 1inch PVC conduit - I keep finding tables that specify number of wires.... and all of them assume you are running individual wires...not cables

Another edit...to rephrase my questions 2:
SO if I am allowed 23 of AWG12 inside 1inch conduit.... that obviously speaks of wires.... so how many actual 12/2 cables could I run inside the 1inch conduit ??
 
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Old 06-03-18, 06:53 PM
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If just using the conduit as a sleeve for protection you can use 3/4 inch for 12/2. The conduit just needs to be large enough not to damage the wire when pulling it through. Sleeves are usually looked at is being 10 ft or less and being open on each end, not terminated at a box. I think you will be okay with 12 feet.
 
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Old 06-03-18, 06:58 PM
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You have to do the fill calculations manually to figure out how many 12/2 NM cables will fit in 1 inch conduit. The fill requirements for conduit only apply to complete conduit systems. You have to calculate the area of the wire based on its cross section. For NM as I said previously you use the widest dimension of a cable to figure area.
 
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Old 06-03-18, 07:02 PM
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You need to calculate it out using the tables in chapter 9 of the NEC. See notes 5 and 9

Cables are calculated as though they are a single conductor using the largest dimension of the cable. IF there are more then 2 cables in the raceway you may only fill that raceway to 40% (2 cables only 31% is allowed)
 
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Old 06-03-18, 07:40 PM
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I think I will just run the romex cable to a junction box, then switch over to THHN to run inside the conduit and then switch back to romex at the other end....
The project I am doing is basically running a new cable/ circuit from the panel - which is inside the garage but on the opposite side from the wall that will then go to the basement....
Cables will run in the attic and basement / walls ...except for the 12 feet in the garage - this is the part I am trying to solve here with my questions....
Now I only need single run 12/2 but want to make sure that in the future I can add more runs...
Based on the tables 1 inch conduit can handle like 24 no.12 wires... ..when I did a "dry" run - 3 romex cables 12/2 inside 1 inch conduit were already tight

I think this will make more sense to just splice the romex with thhn
 
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Old 06-03-18, 08:32 PM
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Even though 1 inch conduit can handle a lot of #12 wires there is amperage derating to take into account when there are more than three current carrying conductors within the conduit.
 
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Old 06-03-18, 08:50 PM
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While technically you do need to derate when there are over three wires, it really does not effect anything until you get over 9 current carrying conductors in a raceway when using 90 degree wire.
 
 

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