[Cu0278] Fire Resistant Cable Sizing

Sizing of conductors supplying electrical equipment that shall remain functional during a fire.

The integrity and functionality of the electricity supply cables is vital for keeping safety services operational during a building fire. Choosing a cable that is tested and classified as fire resistant is a first step when designing an electrical circuit supplying equipment that shall remain functional during a fire. The second step – equally important – is to calculate the minimum cable conductor cross section. This requires particular attention because of the sharp increase in electrical resistance at high temperatures.

To calculate the electrical resistance under fire conditions, the fire temperature shall be known. A first step is to determine for how long the safety services must remain operational, which is expressed by the fire resistance class – 30, 60, 90 or 120 min. Knowing this time span, the fire temperature can be derived from the standard temperature-time curve of a cellulosic building fire. From the fire temperature, the electrical resistance under fire conditions can be obtained by applying the Wiedemann-Franz law. This will result in an electrical resistance correction factor that facilitates the calculation of the voltage drop and current-carrying capacity under fire conditions.

The voltage drop over the entire length of the supply cables must be limited to ensure that fire safety equipment will maintain its functionality for the required duration. Usually, the maximum voltage drop will be specified in the user guide of the equipment. If this is not the case, a maximum voltage drop of 10% must be considered.

Because buildings are often compartmentalised into fire zones, cables feeding fire protection equipment are rarely exposed to fire temperatures over their entire length. The part of the cable not affected by the fire will operate at normal temperature, while that exposed to fire has increased resistance. The total resistance over the length of the cable is calculated by applying the electrical resistance correction factor only to that part of the cable length that is affected.

Knowing the maximum voltage drop of the fire safety equipment, the electrical resistance correction factor for the relevant fire resistance class, and the compartmentation of the cable route, the maximum electrical resistance the cable is allowed to have at normal temperature (20°C) can be calculated. The minimum conductor cross section can be derived from this via Tables 1 to 4 of the international standard EN/IEC 60228, or equivalent tables in national standards.

A second calculation is that of the current-carrying capacity. Fire resistant cables are not tested for the potential additional heat production caused by the increased electrical resistance at high temperature. Moreover, any additional temperature rise above that of the standard cellulosic curve could bring the cable close to the melting temp of copper. For these two reasons, the internal heat production inside the cable due to electric losses must be limited. This means that the current-carrying capacity should be limited in a similar way as for cables working under normal operating conditions, but adjusted by a correction factor for the increased electrical resistance at high temperature.

The cross-section to be chosen is the highest value obtained from the maximum voltage drop and minimum current-carrying capacity calculations.