Elec Class

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Solved exercise · level 4 of 5

Sizing a line

Skills: Build, Calculate

Statement

Lamp L1 (40 W / 12 V) is supplied through a copper line of 20 m (there and back = 40 m, wire F1). With the current cross-section it gets too little voltage.

Choose for F1 the smallest cross-section among 0.5 · 0.75 · 1 · 1.5 · 2.5 · 4 · 6 · 10 mm² that gives L1 at least 11.4 V (95 % of U).

Parametric exercise: the values change with every draw; these are one example.

2 hints from Bastaya available in the lab
Solution

R_lamp = 3.6 Ω; to keep 95 % of U, R_line ≤ 189 mΩ; S ≥ 0.017 × 40 / 0.189 ≈ 3.6 mm², hence 4 mm². V1 then reads 11.5 V.

Course sheet of the chapter : Ohm's law & resistance

This chapter is covered by the paths: ISL-oriented exercises

Other exercises of the chapter

Simulated time00:00:00
Q = I·t-
W = P·t-

Circuit diagram CEI 60617

How does it work?

Each big hole is a circuit node: you push one leg of a component into it. The 4 small holes around it are connected to it: chips and leads plug in there.

  1. Place a component Drag it from the palette: it snaps onto the grid. R rotates it.
  2. Bridge neighbouring holes Pick the jumper chip (or C) and drag on the board: bridged holes become a single node.
  3. Draw a lead For long links and the power supply: from one big hole to another, or to a terminal.
  4. Close the loop The circuit is solved live (Ohm, Kirchhoff) and the diagram follows.

Shortcuts

Del
delete
R
rotate
C
chip brush
Esc
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drag background
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