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Cable sizing for the Effects of Fire Temperature

Guide to BS 8519:2010



A guide to the selection of cable types in accordance with the recommendations of BS 8519:2010 “Selection and installation of fire-resistant power and control cable systems for life safety and firefighting applications - Code of practice” 



(Photo Credit Prysmian Website)




BS 8519 is a Code of practice giving guidance and recommendations for the “Selection and installation of fireresistant power and control cable systems for life safety and fire-fighting applications”.

It specifically covers high rise and complex buildings and recognises that the fire engineered solutions developed for such buildings require a high level of performance from components of the building services including electrical supplies. 


  • It is intended for designers, contractors, regulators and enforcers, fire authorities and inspectors. 
  • It identifies electrical loads defined as life safety or fire-fighting and recommends minimum categories for particular applications. 
  • It identifies three categories of circuits (Categories 1, 2 and 3) with fire survival times of 30 min, 60 min and 120 min.
  • It identifies appropriate cable tests for the categories. 
  • It aims to ensure that the level of circuit integrity of the cables is not compromised by other electrical system components. 



Cable sizing for the effects of fire temperature 


The conductor temperature of a cable in a fire will rise above the maximum conductor temperature upon which tabulated current rating and voltage drop data is based. This will have implications for voltage drop and the effects of fault currents, which may be significant in certain applications. Annexes of BS 8519 give guidance on the review of these factors together with a recommendation that the advice of the cable manufacturer should be sought. 

Sizing of circuit protective conductors (CPCs) is also addressed in BS 8519. Where supplementing of the cable armour with a separate external CPC is necessary to achieve the correct earth fault impedance, it is recommended that the external CPC should be not less than 25% of the cross-sectional area of the phase conductor.


A Current ratings and voltage drop for cables in a fire: 


When a cable is involved in a fire, the conductor temperature will rise above the maximum conductor temperature upon which the tabulated current rating and voltage drop data given cable data sheets is based. In carrying a set current, a cable with its conductor temperature at 950 o C will experience a greater temperature rise due to current loading than a cable with its conductor temperature at 90 o C. 

However, the additional temperature rise due to this factor will be less that 50 o C and is not significant in relation to the temperature rise caused by fire. The voltage drop at typical fire temperatures will be higher that at 90 o C and this may be significant for certain types of load. Assuming a worst case of the total length of the cable run in the fire, it may be necessary to increase the conductor size as illustrated in the following examples. 


Voltage drop calculations for cables in a fire: 


The process of calculating voltage drop of a cable is normally straight forward. Tabulated values are multiplied by the length of run and current to be carried, to give the expected voltage drop. It should be noted that the tabulated values assume that the cable conductor temperature is at its maximum permitted normal operating temperature. If the cable is involved in a fire, the conductor temperature and hence the resistance would be higher, therefore the voltage drop would be higher. 

The problem in determining the voltage drop for a run of cable in a fire is to know the conductor temperature at each point along its length. Therefore assumptions have to be made in calculating what the voltage drop would be. 

To illustrate the effect of assuming different lengths of cable being involved in a fire, two sets of examples are given, one based on 5 A and the other based on 200 A load. 


Example 1.0: 


Assume a 2 core 2.5mm2 FP400 cable carrying 5 A over 50 m. 

In normal operation the voltage drop would be: 19 x 0.001 x 5 x 50 = 4.75 V. Where 19 is the mV/A/m for FP400 2.5mm2 at 90 o C


Example 1.1: 


Assume a 2 core 2.5mm2 FP400 cable carrying 5 A over 50 m. 

Assume 2 m are at 750 o C and the rest of the cable is at 90 o C. Volt drop would be: (19 x 0.001 x 5 x 48) + (19 x 0.001 x 3.0342 x 5 x 2) = 5.14 V 


Example 1.2: 


Assume a 2 core 2.5mm2 FP400 cable carrying 5 A over 50 m. 

Assume all 50 m are at 750 o C. Volt drop would be: 19 x 0.001 x 3.0342 x 5 x 50 = 14.41 V. 


Example 2.0: 


Assume a 2 core 120mm2 FP600S cable carrying 200 A over 50 m. 

In normal operation the volt drop would be: 0.42 x 0.001 x 200 x 50 = 4.2 V Where 0.42 is the mV/A/m for FP600S 120mm2 at 90 o C. 


Example 2.1: 


Assume a 2 core 120mm2 FP600S cable carrying 200 A over 50 m. 

Assume 2m are at 750 o C and the rest of the cable is at 90o C. Volt drop would be: (0.42 x 0.001 x 200 x 48) + (0.42 x 0.001 x 3.0342 x 200 x 2) = 4.45 V. 


Example 2.2: 


Assume a 2 core 120mm2 FP600S cable carrying 200 A over 50 m. 

Assume all 50 m are at 750 o C. Volt drop would be: 0.042 x 0.001 x 3.042 x 200 x 5 = 12.74 V 

As can be seen from these examples, although the voltage drop has increased from normal operation, with part of a cable or all the cable in a fire, the percentage drop from a 240 V single phase supply does not increase significantly. This to say : Example 1, would give 1.98%, 2.14% and 6% respectively. Example 2, would give 1.75%, 1.9% and 5.3% respectively. From these percentage volt drop values, it would seem unlikely that a fire would have a significant effect on most equipment being supplied by the cable, even in the example of the worst case given above. 

However if it is required to limit the volt drop to 4% for the example when the whole length of cable is in the fire, e.g. motors running fire fighting water pumps; then the cable sizes in examples 1.2 and 2.2 would have to be increased.

This article has bee extracted from "FP Guide to BS 8519:2010" by Prysmian Group, the full guide can be obtained free from their website. 



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