Voltage Drop Calculation

<h2>Why voltage drop matters</h2>
<p>As current flows through a conductor, the resistance of the conductor causes a voltage drop along its length. Excessive voltage drop causes equipment to underperform — motors run slower, lamps are dim, heating elements produce less heat.</p>

<h2>BS 7671 limits (Appendix 12)</h2>
<ul>
<li><strong>Lighting circuits:</strong> 3% of supply voltage = <strong>6.9V</strong> on a 230V system</li>
<li><strong>Other circuits (sockets, heating, motor circuits):</strong> 5% = <strong>11.5V</strong></li>
</ul>

<h2>Voltage drop formula</h2>
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Voltage drop (V) = (mV/A/m × Ib × L) ÷ 1000
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<ul>
<li><strong>mV/A/m</strong> — voltage drop per ampere per metre from BS 7671 tables (depends on cable type, CSA and installation method)</li>
<li><strong>Ib</strong> — design current in amperes</li>
<li><strong>L</strong> — length of cable run in metres</li>
</ul>

<h2>Worked example</h2>
<p>2.5mm² twin and earth cable, clipped direct, mV/A/m = 18 (from Table 4D2B)</p>
<p>Design current Ib = 16A, cable length = 18m</p>
<p>Voltage drop = (18 × 16 × 18) / 1000 = 5184 / 1000 = <strong>5.18V</strong></p>
<p>Limit for socket circuit = 11.5V. <strong>PASS</strong> ✓</p>

<h2>Key exam points</h2>
<ul>
<li>Lighting limit: 3% (6.9V). Other: 5% (11.5V)</li>
<li>Formula: (mV/A/m × Ib × L) / 1000</li>
<li>mV/A/m values come from BS 7671 appendix tables</li>
<li>If voltage drop exceeds limit: increase cable CSA or reduce run length</li>
<li>Voltage drop adds to any volt drop in the supply — check from origin</li>
</ul>