I'm not in the IEEE anymore, but their IAS used to have such stuff. Still, I say this only as an experienced electrician (37 yrs), and can't refer you to any papers or studies to support my recommendations, although I'm sure some exist. If your area has flooding or other factors present even rarely, PVC conduits in particular can float and move enough to damage your "duct bank", which could nullify any initial savings. A multi layer formation would make compaction of dirt difficult, I'd recommend something similar to "pea gravel" in the conduit formation with a denser cover layer and, if flotation is possible, at least planks of concrete, no less than 4" thick with some re-bar in the planks.Ĭonsider your soil conditions carefully. I would definitely advise considering possible damage in the area, and would also say that 2" of concrete wouldn't even be worth the pour! I admit this is only my opinion, but 2" wouldn't bear any weight or impact, or prevent any flotation. Duct banks in loose back fill or low strength CDF/FTB may flex instead of fracturing.I've been told, whenever I've asked, that it's simply "what's been spec'd" but these locations have always been where other utilities were nearby, and heavy industrial conditions and occupancies were present. There seemed to be some concern that inadequately reinforced concrete duct banks could facture during earth movements. The 41.6 load factor represents a standard commercial building. A customer uses 15,000 kWh with a maximum demand of 1,500 kW. Assume a one-day billing period for a total of 24 hours. I have heard various discussions from civil engineers about which duct protection approach is best for handling movement from earthquakes. For example, load factor kWh in period/kW. SEE CONSTRUCTION STANDARD CU 1600-4A AND WORK ORDER. CET 500.1 - Cable Television Duct Bank Attached to Base Pavement (Typical). The reinforcing steel is installed longitudinally at each corner of a duct bank (in cross section) and along the top and bottom. In some applications red dye is also added as an attempt to discourage dig-ins. ALL CONDUITS IN A CONCRETE ENCASED DUCT BANKS SHALL BE GRAY PVC TYPE DB-100 MINIMUM, PER ASTM. In general, all concrete-encased electric conduit duct banks contain steel reinforcement throughout their entire length. Sand and pea gravel have horrible thermal properties, so these materials are only used for conduits carrying instrumentation cables or power cables with negligible thermal loading.įTB and CDF both provide some protection again accidental dig-ins, while still allowing some ability to expose conduits if needed. FTB is very similar to CDF but is specifically designed to have good thermal transfer. Depending on the thermal properties of the native soil, we may require the use of Fluidized Thermal Backfill (FTB) for installations with thermal constraints. I assumed it is to limit corrosion of the galvanized conduit in the concrete per NEC 344.10 (B). Greater depths also require that the ampacity be calculated under engineering supervision. Our projects involve building services, site lighting, electric vehicle. Re: Re: UG Conduits Separation NEC recommended Distance The other thing than many miss is that Figure 310.60 specifies a maximum burial depth of 36' to the top of a cable or 30' to the top of the duct bank. The conduit we install is used for both telecommunications / data and electric / power services. We specialize in both concrete encased and direct buried duct bank. For large power cables, thermal considerations are really important. My designer reviewing our companys installation specifications asked me why we spec an anti-corrosion wrapping on RGS conduits in a concrete duct bank. We service projects in Charleston, Greensboro, Raleigh, Greenville, Asheville and Upstate South Carolina. 12.the minimum separation between the 12kv and 69kv duct bank shall be as follows. 11.the contractor shall install approved, 3m type locator ball in all conduit stub-outs. for special pumping applications, use 565-e-2000p. One important distinction is thermal constraints. 10.concrete class of conduit encasement shall be 450-c-2000 per greenbook section 201, table 201-1.1.2.(a).
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