In medium voltage networks of 6-35 kV, at least the following methods of laying single-core cables in the ground are known:
1️⃣ Directly in the ground;
2️⃣ In the ground in a common pipe;
3️⃣ In three different pipes arranged in a bundle.

In 110-500 kV high voltage networks, option 2️⃣ is missing, and only options 1️⃣ and 3️⃣ are possible.

There are many advantages of laying cables in pipes:
✅ mechanical protection of cables;
✅ protection of cables from aggressive environment;
✅ protection of cables in case of intersections with communications and other cables;
✅ reducing the requirements for the quality of trench bottom preparation;
✅ the possibility of building a line using horizontal directional drilling (HDD);
✅ the possibility of phased construction (first, the pipe route is completed and backfilled, and then, as the cable is ready or funded, the cables are pulled into the pipes);
✅ the possibility of prompt repair and replacement of the line;
✅ reduce the temperature of the soil and the risks of its drying out;
✅ others.

Of the disadvantages of laying cables in pipes, perhaps one thing is known – it is believed that laying cables in pipes reduces the ampacity of the cable line. Today let us draw attention to the fact that the situation with pipes is much more complicated.

The table shows the results of calculating the ampacity of 10 kV cable line for cases 1️⃣-2️⃣-3️⃣, provided that there are no currents in the screens (for example, this is achieved by grounding the screens on one side, which is acceptable in 6-35 kV networks for lines up to 2-3 km long or even more). It can be seen from the table that:
👉 laying cables in a common pipe REDUCES the ampacity (ratio I2/I1 ranges from 0.83 to 0.97, and 0.9 in average).
👉 laying cables in three separate pipes INCREASES the ampacity (ratio I3/I1 ranges from 1.00 to 1.14, that means increasing by up to 14%).

Additionally, it can be noted that three pipes give the best effect (up to 14%) under the following conditions:
⭐ with increased thermal resistivity of the soil;
⭐ with increased pipe diameter.

So, the idea that laying in pipes reduces the ampacity is, in general, incorrect. Instead of reduction by 3-17% (10% in average), we can see increasing by 0-14%. The reasons of such an interesting behaviour were described in the book and in a number of posts in the public LinkedIn group “High Voltage Cable Lines“.

Option 2️⃣ is typical for medium voltage cables. Option 3️⃣ is typical for high voltage cables. Thus, the following conclusion can be drawn:
➡️ In 6-35 kV networks, pipes reduce ampacity;
➡️ in 110-500 kV networks, pipes increase ampacity.

The post is based on the example of single-end grounding of the screens, that is, the absence of losses in the screens. This is not always the case in 6-35 kV networks, however, even with both-ends grounding of the screens (so-called “solid bonding”), the situation in the 6-35 kV networks will remain the same – pipes reduce ampacity there by 3-17%.