Cable sizing of sub-main electrical circuits (working examples)

Few words about Sub-main circuits

A sub-main electrical circuit can be defined as a circuit connected directly from the main LV switchboard to a sub-main distribution panel or a rising main for final connection of the minor current-using equipment.

The Code requires that the maximum copper loss in every sub-main circuit should not exceed 1.5% of the total active power transmitted along the circuit conductors at rated circuit current.

Similar approach could be followed for sizing conductor as for feeder circuit. However, assumption has to be made in the design for various characteristics of the sub-main circuit including design current, expected harmonic current (THD) in the circuit, degree of unbalance, etc.

Alternatively, an energy efficiency method introduced by the Code could also be used for preliminary cable sizing. This energy efficiency method for cable sizing requires the calculation of the maximum allowable conductor resistance based on the maximum copper loss requirement as stipulated in the code.

Percentage copper loss with respect to the total active power transmitted:

Therefore,

Table 4.2A and 4.2B in the Code provide a quick initial assessment of cable size required for the common cable types and installation methods used in Hong Kong for this example.

The tabulated current rating of the selected cable could then be corrected by applying the correction factors accordingly. The effective-current carrying capacity of the selected cable must be checked so that its value is larger than or equal to the nominal rating of the circuit protective device.

Calculate an appropriate cable size

A 3-phase sub-main circuit having a design fundamental current of 100A is to be wired with 4/C PVC/SWA/PVC cable on a dedicated cable tray. Assuming an ambient temperature of 30°C and a circuit length of 40m, calculate an appropriate cable size for the following conditions:

1. CASE 1 // Undistorted balanced condition using traditional method (cosθ = 0.85)
2. CASE 2 // Undistorted balanced condition with a max. copper loss of 1.5% (cosθ = 0.85)

Case #1

Undistorted balanced condition using conventional method:

• I_b = 100A
• I_n = 100A
• I_t(min) = 100A

Assume the correction factors C_a, C_p, C_g and C_i are all unity.

Refer to BS7671:2008, The Requirements for Electrical Installations,

Table 4D4A for 25mm² – 4/C PVC/SWA/PVC cable  – It = 110A
Table 4D4B for r = 1.5mV/A/m x = 0.145mV/A/m (negligible)

Case #2

Undistorted balanced condition with a maximum copper loss of 1.5%
(cosθ = 0.85)

Maximum copper loss method using Table 4.2A in the Code for initial assessment of an approximate conductor size required by calculating the max. conductor resistance at 1.5% power loss:

From Table 4.2A 35 mm² – 4/C PVC/SWA/PVC cable having a conductor resistance of 0.625mΩ/m is required. Refer to BS7671:2008, The Requirements for Electrical Installations:

Table 4D4A for 35mm² 4/C PVC/SWA/PVC cable It = 135A
Table 4D4B for r = 1.1mV/A/m x = 0.145mV/A/m

RELATED TABLES //

TABLE 4.2A

Multicore Armoured and Non-armoured Cables (Copper Conductor), Conductor Resistance at 50 Hz Single-phase or Three-phase a.c.

Go back to Cases ↑

TABLE 4.2B

Single-core PVC/XLPE Non-armoured Cables, with or without sheath (Copper Conductor), Conductor Resistance at 50 Hz Single-phase or Three-phase a.c.

Go back to Cases ↑

TABLE 4D4A

Multicore armoured 70C thermoplastic insulated cable

Go back to Cases ↑

TABLE 4D4B

Voltage drop (per ampere per-metre)

Go back to Cases ↑

Reference // Code of Practice for Energy Efficiency of Electrical Installations – Electrical and Mechanical Services Department – The Government of the Hong KongSpecial Administrative Region