IEEE 422:2012 pdf free download – IEEE Guide for the Design of CableRaceway Systems for Electric Generating Facilities
The type of load (resistive,capacitive，inductive,or combination thereof) and ambient temperature areimportant design parameters because they determine the continuous current-carrying capability (ampacity)of a cable for a given size in a particular type of installation. In areas where temperatures exceed statedambient, cables require ampacity de-rating or special types of insulation and jacket materials to be used.
The ampacity of low and medium voltage power conductors may be determined from tables provided inNEC Articles 310.15(B) or 310.60(C) or under engineering supervision as provided in NEC Articles310.15(C) or 310.60(D).
The ampacity of medium voltage power cables in cable trays with single layer or maintained spacingshould be in accordance with NEC Articles 392.22(C) and 392.80(B).
Power cables should be designed to carry normal and emergency load currents.IEEE Sid 835TM[B25]provides cable ampacity tables for various cable constructions and methods of installation.These tables arebased on 40 °C ambient air and 25 C ambient earth and include data for various conductor temperatures.The ampacity ratings for the cable shown in the tables of IEEE Std 835 [B25] will require adjustment if thesite ambient air or carth temperatures are different.Appropriate factors for cable and conduit grouping arealso given， as well as an adjustment formula for change in parameters. Ampacity for cable in anunderground duct bank is based on all power cable ducts being peripherally located and having the cableshields of single-conductor, non-triplexed, medium voltage cables grounded at one point.
Ampacities for non-spaced, random filled cables in open top trays should be determined from ICEA P-54-440/NEMA WC51 [B11], unless the circuit is routed in both underground and open top trays. For this case,the most conservative ampacity should be used.However, normally the ampacity tables listed in IEEE Std835[B25] are not applicable for the non-spaced, random filled cables in open top tray applications. Thepublications listed in Annex A also provide guidance for de-rating the cable ampacity. There is an ampacitydifference for the same size cable between the NEC(NFPA 70) and ICEA P-54-440 [B11] where a singielayer with no spacing is used.IEC 60287-1-1 [B16] provides a simplified formula for the ampacity ofcables in a trench.Trenches are complicated from the heat transfer perspective, and more sophisticatedcalculation techniques may be required.
Where cables are routed through several types of installation conditions (buried,outdoors exposed tosunlight, exposed conduit, covered cable trays，wireways,near hot steam lines,etc.), the conductor sizeshould be selected for the most severe thermal condition. The application of fire-retardant coverings,penetration fire stops, etc. may also affect cable ampacity and may require further de-rating.
Guidance for fault current and short circuit characteristics of insulated conductors is provided in ICEA P-32-382 [B9].Fault current capabilities of metallic shields and sheaths are provided in ICEA P45-482 [B10]J.
Voltage regulation requirements should be considered when selecting the conductor size.Motor feedervoltage drop under starting and running conditions should be limited to allow the motor to operate withinits design specifications.
System nominal voltage level and the type of grounding (solidly grounded,resistance grounded,orungrounded)determine the rating and insulation level (i.e.，100%，133%,or 173%) of medium voltagecable. AEIC specification CS8-07 [B1] and ICEA S-73-532 [B12],ICEA S-93-639 [B13], ICEA S-95-658[B14], and ICEA S-97-682 [B15] provide guidelines for the proper selection of cable rating and insulationlevel as well as the overvoltage capabilities associated with cable voltage ratings.