Voltage Drop Calculator

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 17th Edition.

kW
Volts
Metres

Cable Types

Cable Type Description Conductor Temperature
Twin & Earth or Three Core & Earth

House wiring cable

70 °C
SWA XLPE
(TLC Standard SWA)

SWA with XLPE thermosetting insulation giving an increased current carrying capacity.

90 °C
SWA PVC SWA with standard PVC insulation. 70 °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

Applications

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.

Notes:

Examples

An immersion heater rated at 240 V, 3 kW is to be installed using win 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