Fire resistance and cable sizing

Fire risks, a reality with high potential consequences

Fire safety is a high concern all over the world. Every year  in Europe, 1.1 million of fires are declared leading to more than 4000 deaths and 134 000 injuries. Fires also have economic consequences, total costs can amount to more than 25 billion euros per year.

To mitigate the risks, each country has developed its own regulation regarding potential hazards in sensitive buildings like public buildings (hotel, cinema, theatre, schools, retirement homes…) or high-rise buildings. All concepts are based on the reliability and efficiency of fire safety systems including emergency lighting, smoke detectors, alarms, voice speakers, smoke ventilation systems… allowing for a safe evacuation of people and supporting the firefighter rescue teams. Fire safety is the priority!

In this respect, the contribution of cables is key to warrant the supply of energy and the transmission of information to security systems. Fire resistant cables ensure their functionality even at extreme temperatures.

• Therefore, selecting a cable range that is classified as fire resistant is mandatory when designing fire safety electrical circuits. 
• Next, the cable size, its cross section shall be defined. The application calls for a different calculation than the one used for cables operating in normal conditions (70°C or 90°C being used for the maximum conductor temperature). When considering fire resistant cables, the sharp increase of the electrical resistance with temperature shall be considered. This can lead to voltage drops downgrading the performance of the electrical systems. 

The following inputs must be first determined:

1/ Maximum cable temperature during fire

The operational duration of safety equipment is given by the national regulations or specifications and can depend on the applications. It is usually given by a fire resistance class – 30, 60, 90 or 120 minutes. The related fire temperature the cable can be exposed to can be deduced from the temperature-time curve ISO 834. This curve serves as the reference when assessing fire resistance of building products. After 30, 60, 90, 120 minutes, the graph clearly indicates the fire temperature to be expected in a room corresponding to the given duration; respectively 842, 945, 1005 and 1050°C.

2/ Maximum cable length affected by the fire

The full length of an electrical line is rarely exposed to fire in a building; It is supposed to be stopped by fire proof walls. The walls split the floors in independent “cells” which means that only a portion of the cable is really affected. By knowing the building architecture, the cable lengths that could be damaged can therefore be determined for each individual cell. The longest length exposed to fire is of interest for the calculation.

3/ Equipment technical specifications

Complying with the technical specifications of fire safety device is crucial in order to maintain their performance. The voltage drop must be limited to ensure great reliability and efficiency. Installation manuals usually provide a tolerance value, otherwise a maximum value of 10% shall be considered.
 

Taking into account the 3 elements listed above and applying the Wiedemann-Franz law will help determine a correction factor applicable for the longest electrical line in the fire zones. The latter will then allow to calculate the value of the maximum permissible electrical resistance of the cable at normal temperature (20°C). Lastly, the minimum conductor cross section will be defined based on the tables given in national standards like IEC 60364-52.

As a leading cable manufacturer and fire safety expert, Nexans strongly recommends to follow such method and adapt the corresponding voltage drop calculation for fire resistant cables taking into account fire conditions, compartmentalization and equipment specifications. This principle is stated in the Wiedemann Franz formula based on the laws of physics. The Wiedemann-Franz formula is only a possible approach to the problem and in no way implies disapproval of other rules of good practice.