Low Voltage Cable Sizing & Permissible Voltage Drop Calculations

1. Principles of Low-Voltage Cable Sizing

Cable sizing must satisfy at least three conditions simultaneously. First, ampacity: the cable must carry the continuous load current without the conductor exceeding its insulation rating — 70 degrees Celsius for PVC, 90 degrees Celsius for XLPE — with table values corrected for ambient temperature, installation method, and grouping of circuits. Second, the voltage drop at the far end must stay within limits. Third, the cable must withstand fault current for the full clearing time of the protective device.

In Thai practice, sizing follows the EIT electrical installation standard, which classifies wiring methods and tabulates ampacities for cable types conforming to the relevant TIS product standards. The engineer must fully specify cable type, insulation, installation method, and environment before opening the tables, because the same cable carries very different ratings in free air, in conduit, or directly buried.

2. Voltage Drop Calculation and Acceptable Limits

Voltage drop is the product of load current, cable length, and per-length impedance. The common three-phase working formula is root three times current times length times the sum of resistance times cosine and reactance times sine of the power factor angle. For small cables, resistance dominates; from roughly 95 square millimetres upward, reactance becomes significant and cannot be ignored. Resistance must also be taken at the conductor's actual operating temperature, not the 20-degree value.

The widely applied design criterion following the installation standard's guidance limits total drop from supply intake to point of use to 5 percent, apportioned roughly as 2 percent for feeders and 3 percent for final circuits. Some loads demand tighter limits: circuits with large DOL-started motors need an additional check of the transient dip during starting so contactors do not drop out and the motor can accelerate, while excessive drop on lighting circuits shows up as reduced output and shortened lamp life.

• Check three conditions together: ampacity, voltage drop, and fault withstand
• Always apply derating factors for ambient, wiring method, and grouping
• Total drop limit around 5% (feeders ~2%, final circuits ~3%)
• Include reactance for large cable sizes
• Verify the transient dip during starting for DOL motor loads

3. Verifying Cable Short-Circuit Withstand

The third and most neglected condition verifies the cable survives fault current for the protective device's clearing time, using the adiabatic relation: minimum cross-section equals the square root of I²t divided by the conductor-and-insulation constant k. Copper with XLPE has a higher k than with PVC because the insulation tolerates a higher short-circuit temperature. The I²t figure should be read from the breaker's actual let-through energy curves — especially for current-limiting breakers, which yield far lower values than naive time-based calculation.

The same check applies to the protective conductor: it must withstand the earth fault current for the clearing time, and the circuit length must not be so great that the far-end fault current falls below what the protective device needs to clear within the required time (the earth fault loop impedance condition). Long circuits can pass the voltage-drop check yet fail this one; remedies are a larger conductor, a lower protective device rating, or residual-current protection to clear low-magnitude earth faults.

4. Common Design and Installation Errors

Frequent errors include reading ampacity tables without applying derating factors — grouping many circuits in one tray cuts ratings substantially; using plan-view lengths instead of actual routed lengths with rises and detours, so real drop exceeds the calculation; provisioning future load at the breaker but not in the cable; and mid-run size changes without checking that the upstream device still protects the smaller section. Systems rich in triplen harmonics also need special attention to neutral sizing, since third-order harmonic currents add in the neutral rather than cancelling.

Prevention means a per-circuit calculation sheet showing all three conditions and the factors applied, so results can be cross-checked; measured route lengths; a post-installation voltage measurement at the far end under high load to confirm the design; and full recalculation of the whole path whenever load is added or boards are relocated — never judging by spare breaker capacity alone.

• Produce a per-circuit sizing sheet showing all three conditions and factors
• Use measured route lengths, not plan-view distances
• Measure the far-end voltage under high load after installation
• Size the neutral specially where triplen harmonics are significant