Question 1

What electrical calculations do UK electricians need to perform?

Accepted Answer

UK electricians regularly perform several key calculations to ensure BS 7671 compliance: voltage drop (Regulation 525), cable sizing for current-carrying capacity (Appendix 4), earth fault loop impedance (Zs) for disconnection times, prospective fault current (Ipf), and maximum demand with diversity factors. These calculations verify that installations are safe, efficient, and will pass EICR inspections.

Question 2

How do I calculate voltage drop for a circuit?

Accepted Answer

Use the BS 7671 formula: Voltage Drop = (mV/A/m × Ib × L) ÷ 1000. The mV/A/m value comes from Table 4D1B (or equivalent) in Appendix 4 and already accounts for both conductors. Ib is your design current in amps, and L is the one-way cable length in metres. The result must not exceed 3% for lighting (6.9V) or 5% for power circuits (11.5V) from a 230V supply.

Question 3

What cable size do I need for a 32A circuit?

Accepted Answer

It depends on installation method, cable length, and grouping. For short runs (<15m) using Reference Method C (clipped direct), 4mm² T&E handles 32A with margin. For longer runs or where voltage drop is a concern, 6mm² provides better compliance margin. Always verify both current-carrying capacity from the appropriate table AND voltage drop before finalising cable selection.

Question 4

What is mV/A/m and how do I use it?

Accepted Answer

mV/A/m stands for millivolts per amp per metre - it's the voltage drop rate for a specific cable type. Find the value in BS 7671 Appendix 4 tables (e.g., Table 4D5 for flat T&E cables). The value already accounts for the go and return conductors. Multiply by your design current and cable length, then divide by 1000 to get voltage drop in volts.

Question 5

Are these calculators compliant with BS 7671:2018+A2:2022?

Accepted Answer

Yes, all our electrical calculators use values and methods from BS 7671:2018+A2:2022 (18th Edition with Amendment 2). This includes mV/A/m values from Table 4D1B, current-carrying capacities from Section 4, and voltage drop limits from Regulation 525. We update calculators when new amendments are published.

Question 6

What's the difference between cable sizing and voltage drop calculations?

Accepted Answer

Cable sizing determines the minimum cable size to safely carry the required current without overheating (thermal limits from Appendix 4 tables). Voltage drop checks whether that cable size delivers adequate voltage to the load over the circuit length. A cable might pass current capacity but fail voltage drop on long runs - both criteria must be satisfied for a compliant installation.

Question 7

What is earth fault loop impedance (Zs)?

Accepted Answer

Zs is the total impedance of the earth fault current path - from the transformer, through supply cables, your installation's line conductor, and back through the CPC and earth. It determines how much fault current flows during an earth fault, which affects whether protective devices disconnect within required times (0.4s for socket circuits, 5s for distribution circuits).

Question 8

How do I calculate Zs for a circuit?

Accepted Answer

Zs = Ze + (R1 + R2), where Ze is the external earth fault loop impedance (from the DNO or measured at the origin), R1 is the line conductor resistance, and R2 is the CPC resistance. The measured or calculated Zs must be less than the maximum Zs for your protective device from BS 7671 tables to ensure disconnection within required times.

Question 9

Do I need to calculate voltage drop for every circuit?

Accepted Answer

Best practice is yes, especially for circuits over 10-15m or with significant loads. BS 7671 requires compliance but doesn't mandate documentation for every circuit. However, failing an EICR due to excessive voltage drop after installation is costly to remedy. Our calculators make pre-installation verification quick and simple.

Question 10

What are the voltage drop limits in BS 7671?

Accepted Answer

For installations supplied from the public network: 3% for lighting circuits (6.9V from 230V) and 5% for other uses including socket outlets (11.5V from 230V). For private supplies, limits increase to 6% and 8% respectively. Runs over 100m can add 0.005% per additional metre, up to 0.5% extra maximum.

Question 11

Can I use these calculators for commercial and industrial installations?

Accepted Answer

Yes, BS 7671 applies equally to domestic, commercial, and industrial installations. Our calculators cover single-phase and three-phase circuits, with specific tools for commercial lighting, warehouses, server rooms, industrial motors, and construction sites. The same regulations and calculation methods apply regardless of installation type.

Question 12

What is the adiabatic equation and when do I need it?

Accepted Answer

The adiabatic equation (S = √(I²t)/k) calculates minimum CPC size to withstand fault current energy without damage. It's required when standard CPC sizes from tables might not be adequate - typically for high fault currents, long disconnection times, or when using reduced CPCs. The equation ensures protective conductors survive fault conditions.

Question 13

How accurate are online electrical calculators?

Accepted Answer

Our calculators use official BS 7671 table values and established formulas. Results match manual calculations using the same data. For standard installations, accuracy is excellent. Complex scenarios involving multiple grouping factors, high ambient temperatures, or thermal insulation may require additional verification using the full design calculation method in Appendix 4.

Question 14

What's the difference between single-phase and three-phase voltage drop?

Accepted Answer

Single-phase voltage drop uses the standard formula: VD = (mV/A/m × I × L) ÷ 1000. Three-phase balanced loads use: VD = (mV/A/m × I × L × 0.866) ÷ 1000, where 0.866 is √3/2. The three-phase mV/A/m values in BS 7671 tables are already adjusted for three-phase use, so check which table you're using.

Question 15

Why do I need to consider installation method when sizing cables?

Accepted Answer

Installation method affects cable cooling and therefore current-carrying capacity. A cable clipped direct to a surface (Method C) dissipates heat better than one in conduit (Method B) or thermal insulation (Method 100). BS 7671 Appendix 4 provides different current ratings for each method. Using the wrong method can result in undersized or oversized cables.

Circuit Type	% Limit	Max Volts (230V)
Lighting circuits	3%	6.9V
Other uses (sockets, fixed)	5%	11.5V
Private supply - lighting	6%	13.8V
Private supply - other	8%	18.4V

Electrical Calculators UK

What Are Electrical Calculations and Why Do They Matter?

What is BS 7671 and Why Does It Matter?

Key BS 7671 Requirements for Calculations

Which Calculator Do I Need?

"Is my cable run too long?"

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"EV charger installation"

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Core Electrical Calculators

Voltage Drop Calculator

Cable Sizing Calculator

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Submain to Outbuilding

SWA Armoured Cable

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Solar PV

Home Battery Storage

Heat Pump

Commercial & Industrial

Three-Phase Motor

Commercial Lighting

Warehouse & Industrial

Server Room / Data Centre

Agricultural & Farm

Construction Site

Special Locations (BS 7671 Part 7)

Swimming Pool

Hot Tub & Spa

Marina Shore Power

Caravan & Camping

12V DC Systems

How Voltage Drop Is Calculated

The Basic Formula

Example Calculation

BS 7671 Voltage Drop Limits

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