It is this requirement for separation between normal and emergency feeders that encourages the use of gypsum enclosures; yet the same effective separation can be achieved by using a fire-rated cable, fully rated for two hours, while avoiding all the issues surrounding drywall enclosures.

In our litigious society, there is also the inevitable question of who is liable in the event of a fire that is traced to a faulty enclosure? Is it the architect, engineer, contractor or, possibly, the manufacturer?

Economics of gypsum panel enclosures versus fire-rated cables

Despite the challenges of specifying and constructing gypsum board enclosures and the critical questions regarding their performance, a key question should be: Where do gypsum board enclosures stand in terms of economics?

With the present availability of polymer fire-rated cable, the question of economics is clearly a significant issue. Are gypsum panel enclosures really less expensive than polymer fire rated cable?

Hanscomb Limited, the producer of Yardsticks for Costing,  estimated costs for constructing life safety circuits in a typical 12-story high-rise building in a North American city. Two methods were analyzed:  one using a conventional cable within a gypsum panel enclosure; the other using fire-rated cable.

Pricing for installing 2-hour vertical and horizontal shaft wall assemblies was obtained from three contractors and averaged. Pricing of conventional cable and polymer fire-rated cable was also obtained from contractors and averaged .

The circuits included a 400A, 500 kcmil emergency feeder which runs horizontally from the generator to the electrical room in the basement. The others, 200A, 150A, and 100A, run horizontally and vertically.

 (kcmil is a 1000 cmils. A circular mil is the area of a circle with a diameter of one thousandth (0.001) of an inch. All wire sizes larger than 4/0AWG (American Wire Gauge) are given as kcmil sizes. Kcmil wire size is the equivalent cross sectional area in thousands of circular mils.)

While the fire-rated 500 kcmil cable cost approximately 85 percent more than conventional cable, its cost was reduced to 56 percent more than equivalent conventional cable after including conduit and fittings. As might be expected, fire-rated cable is more expensive than conventional cable; but the cost of the enclosure has to be taken into account when using conventional cable.

The costs of installing a 2-hour vertical gypsum enclosure with liner panel  (1.5-ft x 4-ft, three sided) and a
2-hour fire rated horizontal enclosure with liner panel (1.5-ft x 4-ft) were $10,140 and $11,507 respectively.

The bottom line shows that the cost of using several sizes of conventional wiring plus one horizontal run and one vertical shaft is more ($53,592) than using fire-rated cable ($50,030).

 

 

Costs will vary depending upon how many conductors are protected, their size and the length of the enclosures - and of course, local labor costs.  A one-hour enclosure is much simpler and less expensive to make, but in order to ensure that a reasonable protection is provided for critical circuits, it is prudent to require a  2-hour construction enclosure even where codes do not mandate it. What the calculations indicate is that using gypsum enclosures for meeting fire-protection codes is not automatically cost-effective and that estimating costs of alternatively using of fire-rated cable is worthwhile.