Question 1

What is the maximum cable length for a 32A ring final circuit?

Accepted Answer

It depends on the cable size and acceptable voltage drop. For a 2.5mm² T&E ring (mV/A/m of 18), the effective impedance is halved because current flows both ways. At 32A design current and a 5% limit (11.5V), the total ring length can be approximately 40m before the drop becomes excessive. Always calculate using the actual load and route length rather than relying on rules of thumb.

Question 2

Why does my shower feel weak after a long cable run?

Accepted Answer

Shower heating elements are resistive loads, so power output drops with the square of voltage: P is proportional to V squared. A 10V drop on a 230V supply reduces the actual voltage to 220V, cutting heating power by roughly 8%. On long runs with undersized cable, this effect is very noticeable as reduced water temperature at higher flow rates.

Question 3

What is the difference between 3% and 5% voltage drop limits?

Accepted Answer

BS 7671 permits a maximum 5% drop for power circuits (11.5V at 230V) and 3% for lighting circuits (6.9V at 230V). Lighting uses the tighter limit because even small voltage reductions cause noticeable dimming and can shorten lamp life. These limits apply to the total drop from the origin of the installation to the furthest point of utilisation.

Question 4

Do I use design current or MCB rating for the voltage drop calculation?

Accepted Answer

Use the design current Ib, not the MCB rating In. The design current represents the actual expected load on the circuit. Using the MCB rating would overestimate the drop and could lead to unnecessarily oversized cables. However, for circuits where the load may grow to the full MCB rating (e.g. socket outlets), it is prudent to check at a higher current.

Question 5

Can I exceed the 5% voltage drop limit for motor starting?

Accepted Answer

BS 7671 Regulation 525.1 Note 2 permits voltage drop limits to be exceeded for transient conditions such as motor starting, provided the supply voltage remains within the equipment's rated tolerance. In practice, motor starting currents of 6-8 times full load current cause a brief dip that is acceptable as long as it does not affect other connected equipment.

Question 6

How do I calculate voltage drop step by step?

Accepted Answer

Use the formula Vd = (mV/A/m × Ib × L) ÷ 1000. Look up the mV/A/m value from Table 4D5 for your cable type and installation method. Multiply by the design current in amps and the cable length in metres, then divide by 1000. Compare against 11.5V for power (5% of 230V) or 6.9V for lighting (3% of 230V) per Regulation 525.1.

Question 7

What is the maximum cable run for 2.5mm² cable at 32A?

Accepted Answer

For 2.5mm² T&E with mV/A/m of 18, at 32A and a 5% limit (11.5V): maximum length = 11500 ÷ (18 × 32) = 20m. Ring circuits effectively halve the impedance so can extend further. Always calculate rather than relying on rules of thumb.

Question 8

Does voltage drop affect my appliance performance?

Accepted Answer

Yes, particularly for resistive loads. Power output is proportional to V², so a 10V drop (4.3%) on a 10.5kW shower reduces output by approximately 8.5%. LED lighting may flicker or dim. Motors may fail to start. BS 7671 limits exist specifically to prevent these effects.

Question 9

What mV/A/m values do I use for SWA cable?

Accepted Answer

SWA cable values are from Table 4D4A (thermoplastic) and Table 4E4A (thermosetting). Values differ from T&E. For example, 6mm² 2-core SWA has approximately 6.4 mV/A/m compared to 7.3 for T&E. Always use the correct table for the cable type being installed.

Cable Size (mm²)	mV/A/m	Max Length at 32A / 5% (m)
1.5	29	12
2.5	18	20
4	11	33
6	7.3	49
10	4.4	82
16	2.8	128

Voltage Drop Calculator

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Quick Reference — mV/A/m Values (T&E, Method C)

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