8 & 9. Cable Selection

Question 1

Explain the circuit design process

Answer 1

1. Determine the design current (Ib).
2. Select the rating for the protection device (In).
3. Calculate the cable rating (Iz).
4. Select a cable size from BS 7671 which will carry the current rating (It).
   - Make sure to pick the correct installation method.
5. Calculate Voltage Drop (Vd)
6. Check for Shock Risk constraints.
7. Check for Thermal Constraints

Question 2

What is the allowable volt drop?

Answer 2

3% for lighting circuits

5% for LV power circuits

Question 3

When checking for thermal constraints, what is K?

Answer 3

70 Degrees: 115
90 Degrees: 143

Question 4

What is the Maximum Ze for:

A TN-S Earthing System
A TN-C-S Earthing System

Answer 4

TN-S: 0.8 Ohms
TN-C-S: 0.35 Ohms