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Home / Technical Articles / 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.

Cable Sizing Of Sub-Main Electrical Circuits (Working Examples)
Cable Sizing Of Sub-Main Electrical Circuits (Working Examples)

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.

For a 3-phase 4-wire circuit (assuming balanced, linear or non-linear):

Active power transmitted via the circuit conductors //

P = √3 · UL · I· cosθ

Total copper losses in conductors //

Pcopper =  (3 · Ib2 + IN2) · r · L

where:

  • UL – Line to line voltage, 380V
  • Ib – Design current of the circuit in ampere
  • I1 – Fundamental current of the circuit in ampere
  • IN – Neutral current of the circuit in ampere
  • cosθ – Displacement power factor of the circuit
  • r – a.c. resistance / conductor/ metre at the conductor operating temperature
  • L – Length of the cable in metre

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

Percentage copper loss

Therefore,

Max. a.c. resistance

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:

  • Ib = 100A
  • In = 100A
  • It(min) = 100A

Assume the correction factors Ca, Cp, Cg and Ci are all unity.

Itmin

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)

Conductor operating temperaturet= 30 + 1002 / 1102 · (70 – 30) = 63°C
Ratio of conductor resistance at 63°C to 70°Cr = (230 + 63) / (230 + 70) = 0.98
Voltage dropu = 1.5mV/A/m · 0.85 · 0.98 · 100A · 40m = 5V (1.3%)
Active power transferred (P)P = √3 · 380V · 100A · 0.85 = 56kW
Total copper losses in conductors (Pcu)= 3 · 1002 A2 · 0.0015Ω/m / √3 · 0.98 · 40m
= 1.02kW (1.82%)

(Cable size selected is not acceptable if the maximum allowable copper loss is 1.5%)

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:

Maximum copper loss method

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

Conductor operating temperaturet= 30 + 1002 / 1352 · (70 – 30) = 52°C
Ratio of conductor resistance at 52°C to 70°Cr = (230 + 52) / (230 + 70) = 0.94
Voltage dropu = 1.1mV/A/m · 0.85 · 0.94 · 100A · 40m = 3.5V (0.92%)
Active power transferred (P)P = √3 · 380V · 100A · 0.85 = 56kW
Total copper losses in conductors (Pcu)= 3 · 1002 A2 · 0.0011Ω/m / √3 · 0.94 · 40m
= 716kW (1.28%)

(Cable size selected is acceptable, i.e. power loss < 1.5%, under undistorted and balanced conditions)

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.

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

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.
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

TABLE 4D4A - Multicore armoured 70C thermoplastic insulated cable
TABLE 4D4A – Multicore armoured 70C thermoplastic insulated cable

Go back to Cases ↑


TABLE 4D4B

Voltage drop (per ampere per-metre)

TABLE 4D4B - Voltage drop (per ampere per-metre)
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

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

21 Comments


  1. Abdirahman
    Aug 15, 2020

    Hello well calculation, how about distribution transformed cable in 100Kva in secondery side and distance?

    Thank you


  2. Md Fazlul Haque
    Jun 29, 2020

    Dear Sir,
    How can I upload data/article at this page.

    Best Regards

    Md Fazlul Haque
    Bangladesh


  3. Md Fazlul Haque
    Jun 29, 2020

    how do u get the vale of 230 in case of ratio of resistance calculation, please. overall nice article..

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