This calculator will help determine the required cable size and approximate values for the voltage drop, percentage drop, current load and max cable load of the specified electrical circuit. The calculations are based on information provided in IET Wiring Regulations 18th Edition.
| Cable Type | Description | Conductor Temperature |
|---|---|---|
| Twin & Earth (Twin & Earth & LS0H) |
BASEC-approved house wiring cable |
70 °C |
| Three Core & Earth (Three Core & Earth & LS0H) |
BASEC-approved house wiring cable |
70 °C |
| SWA XLPE |
SWA with XLPE thermosetting insulation giving an increased current carrying capacity. |
90 °C |
| NYY-J Cable |
PVC insulated with Tough PVC thermosetting insulation giving an increased current carrying capacity. Suitable for building sites and some garden and other tough conditions. |
70 °C |
| Reference Method | Description |
|---|---|
| Method 100 Above a plasterboard ceiling with less than 100mm insulation. |
Installation methods for flat twin and earth cable clipped direct to a wooden joist above a plasterboard ceiling with a minimum U value of 0.1W/m K and with thermal insulation not exceeding 100 mm in thickness. |
| Method 101 Above a plasterboard ceiling with more than 100mm insulation. |
Installation methods for flat twin and earth cable clipped direct to a wooden joist above a plasterboard ceiling with a minimum U value of 0.1 W/m K and with thermal insulation exceeding 100 mm in thickness. |
| Method 102 Above a plasterboard ceiling with less than 100mm insulation. |
Installation methods for flat twin and earth cable in a stud wall with thermal insulation with a minimum U value of 0.1 W/m K with the cable touching the inner wall surface. |
| Method 103 In an insulated stud wall with the cable NOT touching the plasterboard. |
Installation methods for flat twin and earth cable in a stud wall with thermal insulation with a minimum U value of 0.1 W/m K with the cable not touching the inner wall surface. |
| In conduit in an insulated wall (including Thermalite). | |
| Clipped direct to a wall. |
Wherever practical, a cable is to be fixed in a position such that it will not be covered with thermal insulation.
When cables are grouped or bunched, any cables carrying less than 30% of the load may be ignored for the purpose of obtaining the rating for the rest of the group.
Where accurate calculations are required please refer to the IEE 17th Edition regulations.
An immersion heater rated at 240 V, 3 kW is to be installed using twin and earth cable. The circuit will be fed from a 16A MCB over 14m in a roof space which is insulated with glass fibre. The roof space temperature is expected to rise to 50°C in summer.
| Example | Description | Required Cable Size |
|---|---|---|
| A. |
Where it leaves the consumer unit and passes through a 50 mm insulation-filled cavity, the cable will be bunched with seven others. This adds considerably to the costs and would make it difficult to terminate such a large cable in the accessories. A more sensible option would be to look for a method of reducing the required cable size. |
10mm |
| B. | Eight cables leave the consumer unit in two bunches of four. | 6mm |
| C. | To attempt to make the cable size smaller, if the eight cables left the consumer unit in four bunches of two. | 4mm |
| D. | If it were possible to run the cable out on its own then no grouping factor would apply. | 2.5mm |
| E. | Finally, if the cable left the consumer unit on its own and the cable was not in contact with the glass fibre and clipped direct to the rafters in the loft space. | 1.5mm |