Cable calculations

Open Ended

Describe a situation where choosing the wrong cable size could lead to problems in an electrical installation.

Multiple Choice

Why is it important to select the correct size of electrical cable for an installation?

• To ensure safety and prevent overheating

• To reduce the cost of installation

• To make the installation process faster

• To use up leftover cable stock

Multiple Choice

Which of the following best describes the relationship needed to achieve coordination between the load, circuit conductor, and overload protective device?

• Ib ≤ In ≤ Iz

• Ib ≥ In ≥ Iz

• Ib = In = Iz

• Ib > In < Iz

Open Ended

List the steps involved in determining the correct cable size for an electrical installation.

Multiple Choice

What is the formula used to calculate the design current (Ib) of a circuit?

• Ib = $$\frac{P}{V}$$

• Ib = $$\frac{V}{P}$$

• Ib = $$P\times V$$

• Ib = $$\frac{V}{I}$$

Fill in the Blanks

If a single-phase load has a power of 1.7kW and operates at 230V, what is the design current (Ib)?

Multiple Choice

Which of the following combinations of power and voltage results in a design current (Ib) of 13.04A?

• 3000W, 230V

• 6500W, 230V

• 10kW, 230V

• 1700W, 230V

Fill in the Blanks

Calculate the design current (Ib) for a load of 6500W at 230V.

Multiple Choice

According to Stage 2, what must be true about the protective device rating (In) compared to the design current (Ib)?

• In must be less than Ib

• In must be equal to Ib

• In must be greater than or equal to Ib

• In must be double Ib

Multiple Choice

Using the information from Appendix B, what is the maximum measured earth fault loop impedance (in Ω) for a BS EN 60898 16A Type B circuit-breaker?

• 1.16 Ω

• 1.09 Ω

• 2.2 Ω

• 1.75 Ω

Fill in the Blanks

What are the correct protective device ratings (in amperes) for the following design currents: 13.04A, 28.26A, and 43.47A, as shown in the work examples?

Multiple Choice

Based on the work examples, match the following protective devices to their corresponding design currents: 60898 Type B Circuit Breaker, Semi-enclosed Fuse - BS 3036, BS 88 High Rupturing Capacity Fuse.

• 13.04A, 28.26A, 43.47A

• 16A, 30A, 50A

• 10A, 25A, 40A

• 13.04A, 30A, 50A

Open Ended

Explain how the rating factors (Ca, Ci, Cg, Cf) identified in Stage 3 can affect the selection of cable size in an electrical installation.

Multiple Choice

According to Table F1, what is the rating factor (Ca) for a cable with 70°C thermoplastic insulation at an ambient temperature of 35°C?

• 1.03

• 0.94

• 0.87

• 1.00

Multiple Choice

According to Table F2, what is the derating factor for a cable surrounded by 400mm of thermal insulation?

• 0.88

• 0.51

• 0.63

• 0.78

Multiple Choice

Which arrangement and number of cables in Table F3 results in the lowest rating factor for current-carrying capacity?

• Bunched in air, 12 cables

• Single layer on wall, 1 cable

• Single layer multicore on tray, 3 cables

• Single layer multicore on ladder, 5 cables

Multiple Select

Which of the following factors are considered in the formula for calculating the cable rating (Iz) as shown in Stage 4?

• Ambient temperature factor (Ca)

• Thermal insulation factor (Ci)

• Grouping factor (Cg)

• Cable length factor (Cl)

Fill in the Blanks

Calculate the minimum cable rating (Iz) required for a circuit with a nominal current (In) of 30A, using the rating factor 0.79 as shown in the work examples.

Multiple Choice

According to Table F4(i), what is the maximum current-carrying capacity (in amperes) for a 16 mm² conductor using Reference Method C (clipped direct)?

• 87 A

• 76 A

• 61 A

• 79 A

Multiple Choice

Which of the following is an important reason for referring to relevant codes of practice and standards when selecting cables?

• To ensure safety and compliance in electrical installations

• To reduce the cost of cables

• To increase the length of the cable run

• To avoid using any insulation

Open Ended

What are the key principles to consider when selecting the right size of cable for an electrical installation?