Electrical Conduits

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Timona

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Houston, Texas
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Graduate Electrical Design Engineer
  1. What is the permissible filling ratio for branch conduits?
  2. Does the code, if any, specify that conduits for light and power have to be run separately or can a single conduit carry wires for both power and light?
  3. When conduit enter any room space, can we stop the "solid conduit" run right at the spot where it enters that room and then just run a flexible conduit, to say the light / receptacle fixture or do we have to run the "solid conduit" all the way to where the light/ receptacle is to be located?
Please, if possible, provide NEC references so that I can also know where to fish next time.
 
1. Depends on number of conductors see Table 1 Chapter 9
2.Both is acceptable NEC 300.3
3. Converting to flexible conduit is acceptable. See applicable Section on wiring method.
 
Lastly, is there an interval for junction box placement? I have looked at the related junction box and etcetra article but could not find an answer.
 
Timona said:
Lastly, is there an interval for junction box placement? I have looked at the related junction box and etcetra article but could not find an answer.
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After 360° of bends. You can put them in more frequently if you wish.
 
infinity said:
After 360° of bends. You can put them in more frequently if you wish.
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360 degrees worth of bends is the only rule that the NEC strictly requires. Other factors come in to play, in terms of what you have to do to make it practical to install. Such as the factory length available of your cable, because splices have to happen in an enclosure. As well as the pulling tension and sidewall pressure. Southwire has a great spreadsheet for modeling the factors that affect the mechanics of your wire pull, and determining if your route is realistic to install.

Obviously, if I build a continuous mile of conduit, that would be asking for trouble. I've never seen a mile long wire reel, and the wire has a good chance it would get damaged after that much dragging and pulling. Yet the NEC has no rule against this. By contrast, I can pull wire with no difficulty at all through 5 consecutive 90 degree elbows, connected end-to-end, but this would not be allowed by the NEC.
 
Carultch said:
360 degrees worth of bends is the only rule that the NEC strictly requires. Other factors come in to play, in terms of what you have to do to make it practical to install. Such as the factory length available of your cable, because splices have to happen in an enclosure. As well as the pulling tension and sidewall pressure. Southwire has a great spreadsheet for modeling the factors that affect the mechanics of your wire pull, and determining if your route is realistic to install.

Obviously, if I build a continuous mile of conduit, that would be asking for trouble. I've never seen a mile long wire reel, and the wire has a good chance it would get damaged after that much dragging and pulling. Yet the NEC has no rule against this. By contrast, I can pull wire with no difficulty at all through 5 consecutive 90 degree elbows, connected end-to-end, but this would not be allowed by the NEC.
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You should watch utility crews. Mile long runs are uncommon but not impossible. In conduit would be rare though.
 
paulengr said:
You should watch utility crews. Mile long runs are uncommon but not impossible. In conduit would be rare though.
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Used to be in facilities at a state university system. Can't say we ever pulled a mile, but probably a good fraction of that.
 
Timona said:
  1. What is the permissible filling ratio for branch conduits
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The short answer is 40% by area, most of the time, which is for 3 or more wires. For a single wire, you can fill up to 53% by area, and for 2 wires you are limited to 31% by area. For short conduits 24" and less in length (i.e. nipples), you can fill them up to 60%

The reason for the various percentages is to imagine what would happen if you packed your wires wall-to-wall, and enclosed them in a virtual circle. Look up "circle packing in a circle" to see some examples. The fill percentages generally represent filling a virtual circle that is about 75% the inner diameter of the raceway with the wire qty desired. The "nipple rule" represents about 90% fill by diameter, which will "look full".
 
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