1/C VS 3/C MV Cable in Conduit

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philly

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What is typically industry standard, best practice, or preference for installation of 1/C vs 3C MV cable (up to 35kV class) in PVC conduit. For example 500MCM type MV-105 cable in 6" conduit.

I've always understood 1/C was best due to ease of installation but curious to hear experiences from others.
 
A conduit for a cable must be sized as if the cable was a single conductor the same diameter.
 
I absolutely hate 3C in one conduit and make it a point to avoid it unless it’s a really old installation that needs to be reconducted in existing conduit.
it makes the pull difficult, make any maintenance difficult, and should something happen to a riser termination, if you want to replace just one cable it’s next to impossible.

your already digging the ditch or boring. Pull in or install 3 separate conduits and make it 1C per conduit.

Those that suggest 3 per conduit are looking at conduit savings only. Not installation costs IMO
 
Hv&Lv said:
I absolutely hate 3C in one conduit and make it a point to avoid it unless it’s a really old installation that needs to be reconducted in existing conduit.
it makes the pull difficult, make any maintenance difficult, and should something happen to a riser termination, if you want to replace just one cable it’s next to impossible.

your already digging the ditch or boring. Pull in or install 3 separate conduits and make it 1C per conduit.

Those that suggest 3 per conduit are looking at conduit savings only. Not installation costs IMO
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To clarify I was referring to (3) phases of the same circuit. You can install 3 single conductor 1/C cables or a 3/C cable with all (3) individual conductors in an overall cable jacket. In both cases these would all need to be in the same conduit. NEC does not all the separation of individual phase conductors (at least not that I'm aware of) in separate conduits. In either case I have not seen any installations where these are separated into different conduits.
 
philly said:
To clarify I was referring to (3) phases of the same circuit. You can install 3 single conductor 1/C cables or a 3/C cable with all (3) individual conductors in an overall cable jacket. In both cases these would all need to be in the same conduit. NEC does not all the separation of individual phase conductors (at least not that I'm aware of) in separate conduits. In either case I have not seen any installations where these are separated into different conduits.
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Sure it does. Look in 300
 
I'm sticking with my original response, and adding that I believe that a single conductor (in this case, a cable of equal diameter) requires a larger conduit than three individual conductors.
 
LarryFine said:
I'm sticking with my original response, and adding that I believe that a single conductor (in this case, a cable of equal diameter) requires a larger conduit than three individual conductors.
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Even considering that the single cable has a 53% fill and three cables have a 40% fill?
 
LarryFine said:
I'm sticking with my original response, and adding that I believe that a single conductor (in this case, a cable of equal diameter) requires a larger conduit than three individual conductors.
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I don't know anything about MV cable, but it's easy to do the comparison with Aluminum SER cable.

On the large side of things, 4/0-4/0-4/0-2/0 SER from Southwire has an OD of 1.496". If we use Chapter 9 Table 5A, 4/0 XHHW compact Al has a diameter of 0.590", and 2/0 has a diameter of 0.490" (we could use a bare 2/0 at 0.376", but I think that would be unusual).

So for minimum conduit internal area, we need to compare 1.4962/0.53 = 4.22 to (3 * 0.592 + 0.492)/0.4 = 3.21 The single cable is worse, by a factor of 1.3. [The above aren't areas, but if you multiply them by pi/4, they are the minimum conduit areas in each case.]

Just to check the smaller side of things, 6-6-6-6 Al SER has an OD of 0.717", and #6 XHHW compact Al has a diameter of 0.260". Now we are comparing 0.7172/0.53 = 0.97 to 4 * 0.262/0.4 = 0.68. Here the single cable is worse by a factor of 1.4.

For a simple approximation of a 3 conductor cable, the diameter of a circle enclosing 3 packed circles is approximately 2.154 times the packed diameter. So the comparison is 2.1542/0.53 = 8.75 vs 3/0.4 = 7.5. For a ratio of 1.17; the actual case is worse as the cable has an outer sheath.

Circle packing in a circle - Wikipedia

en.wikipedia.org en.wikipedia.org

Cheers, Wayne
 
In my opinion in underground pvc conduit -or duct- is preferable for both 3 single core cables or 3 core-cable. The close instrument and control cables- parallel running- will be shielded or will run on steel conduits.

If there are parallel cables a certain unbalance -it depends on distances-it could be in single core cables case. However, the ampacity it is better in single core cables case and the voltage drop is less for three core cable. If there are single core cables in different conduits -pvc- the voltage drop is high but the ampacity is high too.

With respect the easy installation, up to 500 mcm [240 mm^2] it is -more or less-the same. But in case of conductor higher cross section area single core is preferable.
 
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