BS 7671 Cable Sizing Calculations Explained
- Chris Downing
- Apr 16
- 4 min read
BS 7671 cable sizing calculations are one of the most important parts of electrical design.
Get them right and the installation is safe, compliant, and fit for purpose. Get them wrong and you can end up with overheating cables, excessive voltage drop, nuisance tripping, failed inspections, or a system that simply does not perform as intended.
In this guide, we explain cable sizing calculations in practical terms, what factors affect cable size, and why proper design matters before installation begins.
What Are Cable Sizing Calculations?
Cable sizing calculations are used to determine the correct conductor size for a circuit based on the electrical load and installation conditions.
The aim is to ensure the cable can:
carry the required current safely
remain within permitted voltage drop limits
withstand fault conditions
work correctly with the selected protective device
Cable sizing is not just about picking a cable that “looks about right”. It is a design calculation based on the actual circuit duty and installation method.
Why Cable Sizing Matters
Correct cable sizing is essential for both safety and compliance.
If a cable is undersized, it may:
overheat under normal operation
suffer excessive voltage drop
fail under fault conditions
not coordinate correctly with protective devices
If a cable is oversized, it may still work, but it can increase project cost unnecessarily and make installation harder than it needs to be.
Good cable sizing finds the correct balance between safety, performance, compliance, and practicality.
What Factors Affect Cable Size?
Several factors influence the final cable size selected.
1. Design Current
The starting point is the design current of the circuit.
This is based on the load connected to the circuit and is usually expressed as current in amps. Higher load means higher current, and higher current generally means a larger cable may be required.
2. Protective Device Rating
The cable must be suitable for the protective device selected.
This means the cable must have sufficient current-carrying capacity to work properly with the breaker or RCBO protecting the circuit.
3. Installation Method
The way the cable is installed has a major impact on its rating.
For example, a cable clipped direct can carry more current than the same cable installed in insulation or grouped with other cables in trunking.
Typical installation factors include:
clipped direct
in trunking
in conduit
on tray
buried
surrounded by insulation
4. Ambient Temperature
Higher ambient temperatures reduce cable current-carrying capacity.
This is why temperature correction factors are applied during design where necessary.
5. Grouping
If multiple circuits are installed together, the cables can heat each other up.
Grouping factors reduce the effective rating of the cable, which may mean a larger cable is required.
6. Voltage Drop
Longer cable runs increase voltage drop.
Even if a cable can carry the current safely, it may still need to be upsized to keep voltage drop within acceptable limits.
7. Fault Protection
The cable also needs to withstand fault conditions long enough for the protective device to disconnect the supply.
This is a key part of cable sizing and is often overlooked by people trying to size circuits by rule of thumb.
The Main Checks in BS 7671 Cable Sizing
In practice, cable sizing is usually verified against several key checks.
Current-Carrying Capacity
The cable must be able to carry the design current without overheating.
This is one of the basic checks in every design.
Voltage Drop
The total voltage drop from the source to the load must be within the permitted limits for the installation.
This becomes especially important on long runs or heavily loaded circuits.
Earth Fault Loop Impedance
The circuit must allow sufficient fault current to flow for the protective device to disconnect in the required time.
Adiabatic / Fault Withstand
The conductor and CPC must be able to withstand the thermal effects of a fault until disconnection occurs.
Why Rule-of-Thumb Cable Sizing Is Risky
Many contractors have a feel for what cable size is “usually used” for certain circuits. That experience is valuable, but it is not a substitute for calculation.
A cable that works on one job may be wrong on another because of:
longer route length
higher ambient temperature
different installation method
grouping with other circuits
different protective device characteristics
This is why proper cable sizing calculations are so important on commercial and industrial projects, and especially on bespoke systems.
Common Cable Sizing Mistakes
Here are some of the most common issues seen in electrical projects:
Choosing the cable size from breaker rating alone
A 16A breaker does not automatically mean 2.5mm² is correct in every situation.
Ignoring installation method
The same cable installed in trunking, insulation, or on tray can have very different performance.
Forgetting voltage drop on long runs
A cable may pass current capacity checks but still fail on voltage drop.
Not checking fault protection
Cable sizing is not complete without checking fault conditions and disconnection times.
Assuming supplied route lengths are exact
Where lengths are estimated, good design should make sensible assumptions and document them clearly.
When Cable Sizing Becomes More Important
Cable sizing becomes especially critical when you are dealing with:
long cable runs
higher load circuits
three-phase distribution
motor loads
automated systems
grouped circuits in trunking or containment
projects requiring formal design responsibility
This is often where proper calculations save time, reduce rework, and avoid problems later in the job.
Cable Sizing for Distribution Boards and Final Circuits
For new distribution boards and associated final circuits, cable sizing is one of the core parts of the design package.
A typical design review will include:
submain cable sizing
final circuit cable sizing
protection device suitability
voltage drop calculations
fault protection checks
assumptions and limitations
That is why cable sizing should always be treated as part of the overall electrical design, not as an isolated calculation.
Why Use a Specialist Electrical Design Service?
Using a specialist electrical design service means the cable sizing is checked properly in the context of the whole system.
This gives contractors:
confidence before installation starts
clearer information for procurement
reduced design risk
better documentation for compliance
Where required, a full package can also include DB schedules, single line diagrams, protection verification, and supporting notes.
Need Help with Cable Sizing Calculations?
If you have a project that needs cable sizing, DB design, or electrical design sign-off, we can help.
We provide contractor-ready electrical design packages including:
cable sizing calculations
load assessment
voltage drop and fault protection checks
distribution board schedules
supporting documentation
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