2365 203 #20 - Volt Drop

What is voltage drop?

• A temporary power cut

• The rise in voltage due to heat

• The reduction in voltage as current travels through a cable

• The increase in resistance at high load

What causes voltage drop in an electrical circuit?

• Poorly earthed circuits

• Cable joints

• Resistance in conductors and cable length

• The age of the RCD

What is the BS7671 recommended maximum voltage drop for final circuits supplying lighting?

• 3%

• 5%

• 10%

• 1%

What is the maximum voltage drop allowed for socket outlet circuits under BS7671?

• 1%

• 3%

• 5%

• 7.5%

What unit is voltage drop measured in?

• Watts

• Volts per amp per metre (mV/A/m)

• Ohms

• Amps per metre

What values are needed to calculate voltage drop?

• Zs, PFC, and power factor

• Cable length, current, and mV/A/m rating

• Device rating and fuse size

• CPC size and test button results

What happens if voltage drop is too high?

• The circuit becomes more efficient

• The lighting gets brighter

• Appliances may malfunction or underperform

• The RCD won’t trip

What formula is used for voltage drop calculation?

• Zs = (V × I) ÷ 1000

• Vd = mV/A/m × I × L ÷ 1000

• Ib = In – Iz

• VD = Ohms ÷ Resistance

In the formula Vd = mV/A/m × I × L ÷ 1000, what does L represent?

• Load

• Loop impedance

• Cable run length

• Lighting rating

In a single-phase circuit, L = total length of cable run. In a three-phase circuit, what does L represent?

• Half the cable length

• The distance to the load

• The one-way run only

• The total length between source and load (no doubling)