Electrical Design



a) Parallel resistive cable CWM/EST

Voltage 230 – 400 V
Output 10 – 36 W/m

At a normal electrical installation within the industry the fuses most often is maximised to 16Amp. The fuses should have C-characteristic. This means that you per circuit can load maximum 3680W/230V and 6400W/400V 2-phase. It is thought realistic to use 80% of the fuse capacity, when the fuse at group mounting on a bar emit self heat which influences the release circuit and partly to be able4 to do smaller supplements in the application without changing the sizes of the fuses and supply cables.

Max. capacity at 230V will be 3680 x 0.8 = 2945 W. I.e. when EST-10 has been used with a maximum length:

L1 = 2945 = 294 m L² = 5120 = 512 m
10 10

Voltage drop in the conductors:

UL = RI /UL=L x W copper x number of conductors x current

At 230V

UL1 = 294 x 0.017 x 2 x 12.8 = 51 V

At 400V

UL2 = 512 x 0.017 x 2 x 12.8 = 89 V

It means an decrease of the output in the end of the circuit on approx.:

U% = (1-U²end) x 100
At 230V At 400V
(1 -179²) X 100 = 39 % (1 -311²) x 100 = 39%
230² 400²

It is too much, in case that we allow a 10% voltage drop, it should mean a voltage loss of 19%.

10% voltage drop = 23V/40V.

If you allow a higher voltage drop this can bring a too low output in the end of the cable.

b) Self-limiting cables (RSL, RSM)

When using self-limiting heating cables the connection lengths are limited as a function of their start currents and the fuse of the circuit.

The start current is instantaneously operating and the max. lengths stated as a result of the starting currents are therefore not be the cause of any considerable power loss in the end of the cable.

Please note! Use fuses which manages the requirements of the industry.

VärmeKabelTeknik uses fuses from Siemens in their control cabinets.

Type of cable Nominal output
Start current/m A*.Start current factor
RSM3-2CT (10 W/m) 0.091 A/m 1,6
RSM5-2CT (16 W/m) 0.153 A/m 1,8
RSM8-2CT (26 W/m) 0.195 A/m 1,5
RSM10-2CT (33 W/m) 0.296 A/m 1,8
RSM15-2CT (49 W/m) 0.421 A/m 1,8
RSM20-2CT (65 W/m) 0.5 A/m 1,5

The starting current is based on the temperature -20°C at 230V.

Type of cable Nominal output
Start current/m Start current factor (-20°C)
RSL3-2 (10 W/m) 0.12 A/m 2,8
RSL5-2 (16 W/m) 0.266 A/m 3,8
RSL8-2 (26 W/m) 0.421 A/m 3,8
RSL10-2 (33 W/m) 0.5 A/m 3,5

The starting current is based on the temperature -20°C at 230V.

Supply voltage


The available supply voltage have some importance when also parallel resistive cables in spite of that they is specified to a certain output and the supply voltage can be connected to varying supply voltages to obtain varying output.

Parallel resistive cables can be used from 100 V up to 400 V by following calculation:

U² disposable x Pcable = Pemitted (W)

You have to consider not exceeding the highest stated rated voltage for the cable and the maximum output, which the cable withstands considering the max. operating temperatures.

See further on the cable data sheets.

e.g. CWM/EST 10/36 gives 10 W/m on 230V
gives 36 W/m on 400V
CWM/EST 24 gives 24 W/m on 230V
CWM/EST 36 gives 36 W/m on 230V

Self-limiting heating cables (SRL, SRM) can only be connected to 230 V (+-20V) (can be specially ordered for 110V). The material in the cable core is adapted to the stated voltage. You can not, regarding self-limiting heating cables, use a 230V cable on a 400V installation.

For series resistive cables the conditions are the reversed since the output is a function of:

a) the resistance of the cable (Ohm/m)
b) The length
c) the connection voltage

Regarding series resistive cables can the required lengths and connection voltages be varied free to obtain required lengths with adapted outputs. Though, you have to consider the test voltage of the cable and the recommendations from the supplier regarding output per meter at varying temperatures and way of installation.

Note. Lengths of series resistive type are finished on the factory, which limits the flexibility for this product in contrary to the both earlier mentioned cable types.




It is quit normal that you within a classified area must choose material that meets requirements available. It means that all cables, connection boxes, fittings, end terminations, must be approved for this range of application.

To be more specific:

The cable must be Ex-approved and meet the temperature class required for the surface. The connection in the junction box must be Ex-approved and have required IP-class. Even components that are used inside the boxes shall be Ex-approved. Clips, clamps and fittings must be approved and have the same class as the boxes. In short, the whole application is rated after the weakest link. It is not of any help to use a classified junction box if you use a standard fitting.
In the most countries in Europe it is not permitted to do T-joints under the insulation. It should be done in a box outside the insulation so that it is easy to reach. In some installations the thermostat is placed outside the application. Thermostat and enclosure should in these case be Ex-approved.

Thermostats and other automatic equipment can very well be placed outside the Ex-zone, if the thermostat do not have Ex-approval, this requirement can be provided with a Zener-barrier on the conductor.



Circuit breakers of standard type have a breaking curve, which allows an instantaneously operating current, three to five time its nominal value. The breaking curve admits only one very short current pulse.

This means if consideration is not taken to the starting current of a self-limiting cable, the fuse will unconditionally switch off in the moment of switching on.

As soon as the current in for example a self-limiting cable pass the limit for the field where magnetic release of the fuse is done the current is switch off instantly.

This can be solved in two ways:

  1. Over dimensioned fuses (five times larger than the operating current). This causes a negative effect since the areas of the supply cables must be increased with much more than the ones required for the operating current.
  2. Limitation of the lengths together with switch delay between the cable groups gives an economical design of the heating cable application.

Cable lengths


Max. lengths as a result of voltage drop in the electric conductor of the heating cable extend both self-limiting and parallel resistive cables. If you exceed the stated max. length in the data sheet, will the voltage drop give a lower output in the end of the cable.

When using self-limiting heating cable should the voltage drop in the electrical conductors be calculated at the output at operating temperature. As a rule, the voltage drop in the conductor is not determining the length for a self-limiting heating cable when starting currents related to fusing and supply cables limits the cable lengths to relatively moderate number of meter.

This does not influence heating cables of series resistive type when the same current traverses the whole loop irrespective of length. It should be mentioned that with series resistive lengths with build-in return-wire, considerations should be taken to the resistance of the return-wire at lengths over approx. 200m and high outputs.

When using series resistive at high temperatures (normally mineral insulated heating cables, MI-cable) where some types have high copper content in the heating conductor, have an increasing resistance as a result when the conductivity of the copper is decreased with rising temperature. Formula and temperature coefficient is available in the data sheets for these cables.