Current Carrying Capacities of Electric Cables

An electric cable current carrying capacity is the maximum current the cable can withstand before damage will occur, such as overheating, melting or fire.

The current (Amps) carrying capacity of an electric cable, such as flat twin and earth, SWA or SY cables will depend on the cable installation method (also known as the Installation Reference Method) and any relevant correction factors.

Cable manufacturers publish current carrying capacity tables for each cable type and installation method, for example, the current carrying capacity of a flat twin and earth cable clipped direct to a surface is different to it being enclosed within trunking or conduit.

4mm Twin and Earth Current Carrying Capacity Table

Here is an example of a current carrying capacity table for a 4mm² twin and earth cable with different installation methods.

Ref	Method	Current Carrying Capacity
C	Clipped direct	37 Amps
B	Enclosed in conduit on a wall or in trunking etc.	30 Amps
102	In a stud wall with thermal insulation with cable touching the inner wall surface	27 Amps
100	Above a plasterboard ceiling covered by thermal insulation not exceeding 100mm in thickness	27 Amps
A	Enclosed in conduit or trunking in an insulated wall	26 Amps
101	Above a plasterboard ceiling covered by thermal insulation exceeding 100mm in thickness	22 Amps
103	In a stud wall with thermal insulation with cable not touching the inner wall surface	18.5 Amps

As you can see from the table above, the installation method greatly alters the current carrying capacity from 37 Amps down to 18.5 Amps.

For full installation methods refer to BS 7671 Table 4A2.

In addition to the maximum cable current carrying capacity (ccc) and installation method combination, other factors also need to be considered, such as, Correction Factors, Voltage Drop, Fault Current and the maximum permitted earth fault loop impedance (Max Zs).

Correction Factors for Cable Current Carrying Capacity.

Correction factors which affect the maximum current carrying capacity of a cable include (but not limited to),

• Ci – Thermal Insulation: Is the cable covered (or partly covered) by thermal insulation?
• Cd – Buried: Is the cable buried in the ground or duct?
• Cg – Grouping: Is the cable grouped with other cables?

There are many correction factors specified in BS 7671 which will need to be taken into consideration when designing an electrical circuit, as correction factors will alter the current carrying capacity rating of a cable.

For SWA cables, see SWA Current Carrying Capacity.

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