# Chapter 14: Transmission lines question

#### Msilva_solarpro1

Joined Oct 26, 2007
2
Hi There,

I am looking for advice with regards to how to determine what is the best conductor layout on an underground trench; using direct burial conductors for multiple (10) ~ 500Vdc circuits.

I did read the Section Characteristic impedance, on Chapter 14 of the book; and found an interesting explanation of how capacitance and inductances depends on cable separation; but this theoretical exercise was done on an infinite tranmission line.

My case is real and has a limited lenght of 198 mts for the longest run, and about 20 mts for the shortest. How could I find the optimum conductor arrengment in order to minimize the losses (highest transmission efficiency possible)?

Thanks

Miguel

#### Ron H

Joined Apr 14, 2005
7,014
Transmission line effects are irrelevant for DC transmission. DC resistance is the only loss component.

#### beenthere

Joined Apr 20, 2004
15,819
Agree with Ron H. You might want to be careful about ampacity, though - the ability of the wire to carry current. When buried, it's smart to derate the capacity and use large conductors. Otherwise, heat induced resistance can create significant voltage drops.

#### Msilva_solarpro1

Joined Oct 26, 2007
2

To give to you a better idea of my case, at one side of my circuit I have a solar PV array with circuits with similar outputs (up to 20 circuits ~500 Vdc ~80A), at the other end I have a DC/AC inverter.

The conductors are sized properly (temp derrating, short circuit current, type, etc). Should I be worry about how the cables are layout inside the trench? (i.e. + - + -...N, distance between conductors on the same circuit, distance between conductors from different circuits, etc) or I should worry only about soil type, moisture content and heat disipation properies of the soil.

Ho could I determine what is the right trench width?

I will appreciate your feedback on this topic

Miguel

#### gerty

Joined Aug 30, 2007
1,302
Are you going to put the wire in conduit? If not you'll have to make sure to use direct burial cable (type uf)..

#### beenthere

Joined Apr 20, 2004
15,819
As long as the insulation holds up, layout shouldn't be a problem. As this sounds like a big effort, you might want to check into the expense of using plastic pipe to enclose the cabling. Keeping moisture away would probably help the lifetime of the insulation. In sandy soil, this might be of little concern.

#### thingmaker3

Joined May 16, 2005
5,083
You'll want AWG#4 at the least. #2 would be better with your length. UF is sufficient. Bury it at least 18" (46cm) under the grade to comply with NEC. Don't forget your warning tape buried above it.

40KW of solar power is quite impressive! You have 20 of those? I'd love to know more about this.