Here are 2 exam style cable calculation questions to practise on. Using mock exam / practise questions that have been written in the exact style you would find in your exam is one of the strongest revision techniques to employ.

The more exam style questions you try, the better you'll get and the more chance you will have of scoring the maximum mark available in this, the biggest question on all of the electrical exams

Practice question 1.

A new circuit is to be installed to supply a 3 kW 230 V single-phase motor having a power factor of 0.85. The circuit is to be installed using 70°C thermoplastic non-sheathed, single-core cables in surface metallic trunking for its 30 m length. The trunking contains four other similar circuits. The ambient temperature is 35°C and protection for the circuit is to be provided by a circuit breaker to BS EN 60898. The installation is a TN-C-S and has a Ze of 0.35Ω. Determine the most suitable and cost effective conductors, ensuring they are fully complaint.

Practice question 2.

A circuit supplying a 9 kW 400 V water heater is to be wired in SWA, 70°C thermoplastic cable clipped direct along its length of 43 m. The cable will pass through a thermally insulated wall of 100 mm the ambient temperature is 35°C. Protection is by a Type C circuit breaker to BS EN 60898 and Ze has been measured at 0.38 Ω. Determine a suitable CSA of live conductor for current carrying capacity and suitable CPC for thermal withstand.

SCROLL DOWN FOR THE ANSWERS. FULLY WORKED WITH REFERENCING

Answer to practice question 1.

Ib = 3000/(230 x 0.85) = 15.35 A
In = 16 A type C, table 7.1 (ii) OSG
Ca = 0.94, table F1 OSG
Cg = 0.60, table F3 OSG
Cf = 1
Iz = 16/(0.94 x 0.60 x 1) = 28.33 A
It = 32 A, reference method B, 4 mm, table F4 (i) OSG
mV/A/m = 11, table F4 (ii) OSG
volt drop = (11 x 15.35 x 30)/1000 = 5.07 V
Max volt drop allowed is 11.5 V, therefore this is permitted
Ib ≤ In ≤ It = 15.35 ≤ 16 ≤ 32, this is correct
mΩ/m for 4 mm and 4 mm is 9.22 table I1 OSG
R1+R2 = (9.22 x 1.2 x 30)/1000 = 0.33 Ω
Zs = Ze + (R1 + R2) = 0.35 + 0.33 = 0.68 Ω, max allowed is 1.37 Ω table 41.3 BS 7671
Ipf = 230/(0.68) = 338 A
t = 0.1 s, Fig 3A5 BS 7671 (instantaneous at 160 A, max allowed 0.4s Table 41.1)
k = 115, table 54.3 BS 7671
S = √(I^2 x t)/k = √(〖338〗^2 x 0.1)/115 = 0.9 mm
A suitable live would be 4 mm with a 1 mm cpc.

Answer to practice question 2.

Ib = 9000/(√3 x 400) = 13 A
In = 16 A MCB type C, Table 41.3
Ca = 0.94, table 4B1
Ci = 0.78, Appendix 4, 2.6, p423
Cf = 1
Iz = 16/(0.94 x 0.78 x 1) = 21.82 A
Reference method C
It = 25 A, 2.5 mm, table 4D4A BS 7671 p.454
mv/A/m = 15, table 4D4B BS 7671 p.555
Volt drop = (15 x 13 x 43)/1000 = 8.39 V, max volt drop allowed is 20 V, Table 4Ab p430
Ib ≤ In ≤ It = 13 ≤ 16 ≤ 25
mΩ/m for 2.5 mm and 2.5 mm is 14.82 table I1 OSG
R1+R2 = (14.82 x 1.2 x 43)/1000 = 0.76 Ω
Zs = Ze + (R1 + R2) = 0.38 + 0.76 = 1.14 Ω, max allowed is 1.37 Ω table 41.3 BS 7671
Ipf = 230/(1.14) = 201 A
t = 0.1 s, Fig 3A5 BS 7671 (instantaneous at 200 A, max allowed is 0.4s)
k = 115, table 54.3 BS 7671
S = √(I^2 x t)/k = √(〖201〗^2 x 0.1)/115 = 0.55 mm
A suitable live would be 2.5 mm with a 1 mm cpc.

Cable calculations 7 - 12

Cable calculations 13 - 18