Bus duct-internal and external firestops: Complying either with building OR electrical code, not both

Discussion created by ahering on May 25, 2018

The following Wikimedia Commons Category:


showcases fire testing per ASMT E814, which highlights a problem for bus ducts. Bus ducts are qualified to UL 857. 857 includes mandatory testing for ampacity derating. Nothing prevents a bus duct manufacturer from testing a bus duct with internal firestops. But usually, they don’t. They used to. Here is an example:


Then a switch came from separated bus bars to sandwiched ones, which cause some bus duct vendors to just drop the internal firestop, because there was no longer space between the plastic wrapped bars to put a firestop. However, that still leaves the space between bus bar sandwiches, and, it also leaves the covalent wrappings of the bars, which are great fuel for fire and smoke migration. A sprinkler system cannot put a bus duct internal fire out. The above-mentioned Commons category shows two tests that confirm what should actually be completely obvious. 130°C “rated” mylar plastic wrappings are excellent fuel for an 1100°C building fire. Because of the sheet metal box that the bus bars sit in, no sprinkler system would be triggered by such a fire unless combustibles were nearby, which could auto-ignite. The smoke and heat coming off these bus bar wrappings is serious. Also, the bus bars are held in place with plastic parts that disappear in a fire. In the first of 2 tests, the plastic holders let go and the bus bars crashed south to be caught by a completely arbitrary sheet metal cap, which is customary to do in E814 testing. In the second test, the bars were fastened with self-drilling sheet metal screws to hold them in place during the test. Bus ducts get so hot that only thermally insulated systems are capable of attaining a T rating, which can be mandatory in buildings. Firestop manufacturers cannot install internal firestops in bus ducts for type approval testing because this violates the UL 857 listing of the bus duct itself, unless it was tested with an internal firestop in the first place. In Germany, this is mandatory, for obvious reasons. In North America, it is one of those “grey areas”. Check out UL firestop listings and see how many of those have a T rating more than a few minutes. You will find that C-BJ-8028 has decent T-ratings. Here is the problem: It used the oldest firestop trick in the book to get a T-rating: External insulation. That ALSO violates the UL 857 listings for the bus ducts, because when those bus ducts were tested, they did not have that amount of insulation, which would completely alter the ampacity derating. For back-up of the fact that firestops and circuit integrity fireproofing can affect ampacity derating, just check out this standard:


IEEE 848 exists for the purpose of quantifying the ampacity derating effects of firestopping and fireproofing. So, if you need a T-rated bus duct firestop to comply with the building code, you can’t do that unless you violate the electrical code, which demands compliance with UL 857 listed bus ducts. You cannot currently comply with both codes. Cable bus are different. One could easily make an argument that an internally firestopped cable bus system, pick any of hundreds of UL or Intertek listed systems, are covered by existing firestop listings. The only caveat there is the compensation for internal cable motion – plus: the internal firestop needs to have been included in the original type approval test for that cable bus. If you are an electrical engineer specifying the use of bus ducts, it may help to require internal bus duct firestops, rated both to UL 857 AND ASTM E814. If not, you now know you will violate at least one code.