6.4 Voltage Drop Calculations and Circuit Segregation Rules

6.4 Voltage Drop Calculations and Circuit Segregation Rules

Quick Summary: Satisfying thermal current-carrying capacity ($I_z \ge I_n$) alone does not guarantee a safe or functional electrical installation. Under SS 638 Clause 525, electrical conductors must be sized so that the Voltage Drop ($V_d$) between the point of supply intake and any final utilization point (luminaires, socket outlets, motors) does not exceed 4% of the nominal supply voltage. Furthermore, to maintain electromagnetic compatibility (EMC) and prevent lethal flashover accidents, SS 638 mandates strict physical Circuit Segregation between Band I (Extra-Low Voltage / Data / Telecom) and Band II (Low-Voltage Power 230V/400V) circuits.

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1. Statutory Voltage Drop Limits under SS 638

Excessive voltage drop in distribution cables reduces motor torque, causes light flicker, overheats electronic power supplies, and leads to contactor chattering. SS 638 sets maximum allowable voltage drop limits measured under full design load conditions ($I_b$):

$\text{Maximum Permissible Voltage Drop Percentage } V_{d,max\%} = 4\%$

Absolute Voltage Limits for Singapore Supplies:

1. Single-Phase 230V Supply: $V_{d,max} = 4\% \times 230\text{ V} = 0.04 \times 230 = 9.20\text{ V}$

   • Minimum permissible utilization voltage at socket/appliance terminals = $230\text{V} - 9.2\text{V} = 220.80\text{ V}$.

2. Three-Phase 400V Supply: $V_{d,max} = 4\% \times 400\text{ V} = 0.04 \times 400 = 16.00\text{ V}$

   • Minimum permissible line voltage at motor intake terminals = $400\text{V} - 16.0\text{V} = 384.00\text{ V}$.

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2. Voltage Drop Calculation Equations

Voltage drop across a copper or aluminum conductor route is calculated using tabulated millivolt drop figures published in SS 638 Appendix 4:

Standard Voltage Drop Formula

$V_d = \frac{(mV/A/m) \times I_b \times L}{1000}$

Where:

• $V_d$: Total calculated circuit voltage drop in Volts ($ ext{V}$).
• $(mV/A/m)$: Tabulated voltage drop per ampere per metre route length from SS 638 tables.
• $I_b$: Circuit design current in Amperes ($ ext{A}$).
• $L$: Single-way circuit route length in metres ($ ext{m}$).

Temperature Adjustment Factor for Conductor Operating Temperature

Tabulated $(mV/A/m)$ values in SS 638 are published for maximum conductor operating temperature ($70^\circ\text{C}$ for PVC, $90^\circ\text{C}$ for XLPE). When conductors operate below full rated thermal capacity, a temperature correction factor may be applied to avoid over-sizing long cable runs:

$(mV/A/m)_{actual} = (mV/A/m)_{tab} \times \left[ 1 - \frac{\text{Factor}}{100} \right]$

However, for exam calculations and conservative engineering design, the published tabulated $(mV/A/m)$ value is applied directly.

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3. Step-by-Step Worked Calculation: Cable Upsizing for Voltage Drop

Example Scenario:

A $230\text{V}$ single-phase sub-main feeds a remote sub-distribution board located at a route distance of $L = 45\text{ metres}$ from the main intake switchboard.

• Circuit Load: Design current $I_b = 40\text{ A}$.
• Protection Device: $40\text{ A}$ MCB ($I_n = 40\text{ A}$).
• Cable Type: Multi-core PVC insulated armored copper cable ($70^\circ\text{C}$), clipped direct to wall (Method C).
• Ambient Conditions: Standard $30^\circ\text{C}$ ambient air ($C_a = 1.0$, $C_g = 1.0$).

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Step-by-Step Solution:

Step 1: Initial Selection Based on Thermal Current Capacity ($I_z \ge I_n$)

Consulting SS 638 Table 4D2A (Multi-core $70^\circ\text{C}$ PVC Armored Copper Cable, Method C):

• $4\text{ mm}^2$ PVC Cable: Tabulated capacity $I_t = 38\text{ A}$ (Non-Compliant: $38\text{A} < 40\text{A}$).
• $6\text{ mm}^2$ PVC Cable: Tabulated capacity $I_t = 47\text{ A}$ (Compliant: $47\text{A} \ge 40\text{A}$).

Thus, based strictly on thermal current rating, a $6\text{ mm}^2$ copper cable is selected.

Step 2: Perform Voltage Drop Verification on $6\text{ mm}^2$ Cable

Consulting SS 638 Table 4D2B for $6\text{ mm}^2$ 2-core PVC cable:

• Tabulated voltage drop $= 7.3\text{ mV/A/m}$.

Calculate actual voltage drop over $45\text{ m}$ route length: $V_d = \frac{(mV/A/m) \times I_b \times L}{1000} = \frac{7.3 \times 40\text{ A} \times 45\text{ m}}{1000} = \frac{13,140}{1000} = 13.14\text{ V}$

Verification Check vs Statutory Limit:

• Maximum permissible voltage drop for $230\text{V} = 9.20\text{ V}$.
• Calculated $V_d = 13.14\text{ V} > 9.20\text{ V}$.
• RESULT: REJECT $6\text{ mm}^2$ CABLE! (Fails voltage drop compliance despite passing thermal current capacity).

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Step 3: Upsize Cable to $10\text{ mm}^2$ and Re-Calculate

Consulting SS 638 Table 4D2B for $10\text{ mm}^2$ 2-core PVC cable:

• Tabulated voltage drop $= 4.4\text{ mV/A/m}$.

Re-calculate voltage drop: $V_d = \frac{4.4 \times 40\text{ A} \times 45\text{ m}}{1000} = \frac{7,920}{1000} = 7.92\text{ V}$

Final Verification Check:

• Calculated $V_d = 7.92\text{ V} \le 9.20\text{ V}$ ($3.44%$ voltage drop).
• TECHNICAL CONCLUSION: The sub-main cable MUST be up-sized to $2\text{C } 10\text{ mm}^2$ PVC/SWA/PVC Copper Cable to satisfy SS 638 statutory voltage drop limits.

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4. Circuit Segregation Rules under SS 638

Circuit segregation involves physically isolating wiring systems operating at different voltage levels or serving different system functions to eliminate physical damage, electrical flashover, and electromagnetic interference (EMI).

Classification of Voltage Bands per SS 638:

+-------------------------------------------------------------------------+
| BAND I: EXTRA-LOW VOLTAGE (ELV)                                         |
| - Voltages <= 50V AC r.m.s. or <= 120V ripple-free DC                   |
| - Applications: Fire Alarm Systems, Telecom, Data LAN, Building         |
|   Management Systems (BMS), Access Control, Security Telephony          |
+-------------------------------------------------------------------------+
                                    VS
+-------------------------------------------------------------------------+
| BAND II: LOW VOLTAGE (LV) POWER & LIGHTING                              |
| - Voltages > 50V AC up to 1000V AC (or 120V DC up to 1500V DC)         |
| - Applications: 230V Single-Phase Power, 400V 3-Phase Motors, Lighting  |
+-------------------------------------------------------------------------+

Mandatory Rules for Segregating Band I and Band II Circuits:

1. Prohibition of Shared Enclosures: Cables of Band I (ELV) and Band II (LV) must NOT be drawn into the same conduit, duct, or compartment of trunking unless effective physical separation is implemented.
2. Methods of Physical Separation:
   • Separate Containment: Installing completely independent steel or PVC conduits, or separate cable trays, for ELV data and LV power.
   • Compartmentalized Trunking: Utilizing multi-compartment surface trunking fitted with continuous metallic or rigid PVC physical dividing partitions/baffles.
3. Insulation Exception Rule (SS 638 Clause 528.1): Band I and Band II cables may occupy the same containment unit without a physical barrier ONLY IF every cable in the duct is insulated to the highest voltage present (e.g., all ELV signal wires insulated for $600/1000\text{V}$ AC).
   • Note: Even if insulated to $1000\text{V}$, physical separation is still strongly recommended by EMA to prevent electromagnetic noise induction into sensitive data circuits.
4. Fire Alarm & Emergency System Segregation: Wiring for emergency lighting, fire alarm bell circuits, and smoke extraction fans must be wired in fire-resistant cables (e.g., FP200, MICC) and completely segregated from general lighting and power wiring to preserve circuit integrity during building fire evacuations.
5. Prohibition of Common Neutral Conductor: Neutral conductors must never be shared across different sub-circuits or between Band I and Band II systems.