Some studies have found the empirical method to be conservative, owing to the fact that it does not take account of the specific thermal environment outside of the cable. For more detailed information on the cookies we use, please check our Cookie Policy. Close this window and log in. l^~u%_?2J?Xm1~0lQ3C7X ncF?' Soil resistivity is affected by many factors including geographic location, soil composition and water flow and it will also change seasonally. The cause of the electrical overheating of the supply cable was found to be the result of its burial under the 16 inches of loose thermal insulation. The rating for the upper trays is slightly reduced because cables thereon are subjected to warmed air that rises from the lower tray(s). Where a cable is buried in the ground its ability to dissipate heat is reduced, the extent of which depends on the installation method. % k2-3,Kw#`nu>+LKYgT Even then couldn't group more than 4 power circuits. 0000005266 00000 n For more details on BS 7671 voltage drop calculations, see. The Institution of Engineering and Technology is registered as a Charity in England & Wales (no 211014) and Scotland (no SC038698). Already a member? 148 46 Cable current capacity calculations in BS 761 are based on the derating factor method, seeCable Derating (Factors). These values were based on the conditions identified in the supplement to Report No. Derating: No derating is currently applied to the current ratings tables 310.15 (B) (16) and 310.15 (B) (17).. By definition only one circuit, but imagine a circuit high up in conduit: down and up, over a door, down and up again, etc; or domestic under the floorboards, down and up (or up and down) under the capping, down and up again, etc. ;;u(1*z/}3oG'0 ^Am;Oy4?~ Et'sQh)sF&)O The nearest size of standard trunking with a capacity factor greater than 1239.6 is 75 x 50mm? The correction factor is justified as the current carrying capacity is capable of limitation by additional external factors, e.g. Manage and size all your cables, from low voltage to 33 kV. How much current a cable can carry is down to physics and is a highly technical subject in its own right. of Cables) 10 x 95 (Factor for 35mm cable) = 950 (No. The question is, what is the difference between manufacturers data and BS 7671? In this article we look at the information available and why the information is perceived as different. Fictitious Dimensions. The Institution of Engineering and Technology is registered as a Charity in England & Wales (no 211014) and Scotland (no SC038698). capacity of a cable found in Appendix 4 of Bs 7671; i n is the rated current or current setting of the overcurrent device; C g is rating factor for grouping; C a is rating factor for ambient temperature; C i is rating factor for conductors surrounded by thermal insulation; C f is rating factor for semi-enclosed fuses (Bs 3036); C s is rating . The table below provides derating guidelines for 200 0 C rated Teflon (type PTFE) wire. The physics involved with heat loss have similarities with those of mechanical engineering and are based on heat transfer principles. , #gc6aa5 Manufacturers provide data . 0 I Accept Cookies In the first example, a 4,000-amp duct bank with a design load of 3,600 amps is simulated based on an earth RHO factor of 90, dirt RHO factor of 90 and a load factor of 100 . Determine the application. It is important to remember that the testing carried out for the ERA 69-30 reports was under controlled conditions and using consistent backfill material, for example, in the real world it is much different. 0000002031 00000 n BS 7671: Introduction and Overview A BS 7671: 2008 - Introduction and Overview A 1 Introduction to BS 7671: 2008 BS 7671: 2008 was published during January 2008 as a signi cant new Edition of this fundamental Standard. Cable Current Rating 6. stream These are related to the line voltage for three phase circuits and phase voltage for single phase circuits. When multiple cables are in proximity, each contributes heat to the others and diminishes the amount of external cooling affecting the individual cable conductors. The purpose of performing a cable calculation is to ensure that a cable is not overloaded. Unless specific values can be obtained, using the values direct from BS 7671 could be considered a safe option for the electrical designer. Minimum Cable size REWIREABLE FUSES BS 3036 SEMI-ENCLOSED FUSE The series of reports are called the ERA 69-30 series, previously called F/T183 and first published in 1955 have been referred to in the IEE Wiring Regulations for many years. 1b). Rating factor for grouping of cables = 0.61 (Appendix 1.4) (Assumed 6 cables 150mm spacing formation) Overall admissible ampacity correction factor = 0.82 x 0.73 x 0.92 x 0.61 = 0.336 1.2. Voltage drop value should be in the accepted limits. Derating factors, given in BS7671, for cables in touching trefoil formation are appropriate for cables in quad bundles Voltage drops for circuits in quad formation should be calculated using the values tabulated in BS7671 for cables in flat touching formation endstream endobj 149 0 obj <. There are other considerations to be taken into account by the electrical designer when selecting a suitable CSA of cable, such as length, voltage drop, energy let through (I2t) from the protective device and thus the energy withstand rating of the cable. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework. Soil resistance is the ability to pass electrical current, which is relevant to earthing systems. Where I think that this concept fails is in ring circuits. As you say for other cases the tabulated values are likely to be erring on the safe side, so applying them directly if you don't really know much more about the loads probably isn't a bad approach. Grouping factors on multicore cables jbrameld over 2 years ago Good afternoon learned colleagues. The NEC Table attached has no derating factor for sheath materials because a wire in a home or a business will not be exposed to different environmental conditions. The current-carrying capacities identified in BS 7671:2018+A1:2020 were derived from a combination of IEC standards and a piece of work carried out by an organization called RINA, formerly ERA (the Electrical Research Association). The most important part of carrying out cable current rating calculations is the determination of the conductor temperature for a given load, or conversely, the determination of the tolerable load current for a given conductor temperature. One common approach is to use the empirical method identified in IEC 60287-2-1:2015 Calculation of the current rating Calculation of thermal resistance. The current-carrying capacity of cables identified in Appendix 4 of BS 7671:2018+A1:2020 will cover most installations, but the values will be on the conservative side and will not necessarily provide the most efficiently sized cable (depending on how you define efficient). Want to participate in the discussions? Whilst I have no intention of exploring the minefield of complex heat loss equations in this article, I would like to provide a very high-level overview of the concept and complexities of calculating the current-carrying capacity of cables. It is always best to consult cable manufacturers for advice when carrying out any non-standard cable calculations. Over 700mm then derating starts to rear it's ugly head. . Other than the conductor sizes changing to metric, these ratings have remained unchanged. AFAIK, this refers to several cable trays stacked one above the other, as might be used when space does not permit of wider tray or multiple trays side by side. <<95D84A24FD5A1F4C8152C1D91F51EC80>]>> We have received your request and will respond promptly. vi}z7{?8x=l"t\F1'lnxN+h'qVs6GX1Cyx8N"/V*#JP%/~7d_}l?A#NLXh\h|gq$vtmO89dp|d{;>1-bSJb_" RT'C? Annex B is popular because it is simple to use. A software algorithm has been developed which can determine the derating factors for multiple cable circuit crossings or heat sources. I!m6~8Rq" I started as an apprentice electrician at the age of seventeen in 1982 and worked for many years as an electrician, a site foreman, a manager, a NICEIC Qualifying Supervisor, and a general manager. We use necessary cookies to make our site work. DE-RATING FACTORS Our Company Genesis & Evolution Why Us KEI Power Gallery Beyond Business Certifications Downloads Clients Testimonials Reach Us Locate Us CALL (112) 681-8840 Follow Follow Toggle navigation Products & Solutions Extra High Voltage Cables High Voltage Cables Low Voltage Cables Control Cables House Wire HomeCab-FR BanFire-ZHFR Appendix 4 (BS 7671:2018) gives guidance determining the current capacity and voltage drop for cables. However, the tables were not included in the next publication, the Fourteenth Edition of the IEE Wiring Regulations in 1966 but, instead, stated that current ratings shall comply with ERA report F/T 183 as seen below in Figure 3. The parameters considered in the standard are, ambient temperature, soil thermal resistivity, the installation methods and grouping of cables. Total derating factor = 0.89 * 1.05 * 1.0 = 0.93. ]M&r;:QKGBdBeW2JFX)@wN#$RA)0S3)i`nmh WTOT is the total power dissipated in the trough per metre length (W/m), p is that part of the trough perimeter that is effective for heat dissipation (m), tr is the temperature rise of the air in a cable trough (K). Earth loop impedance. This method subdivides the problem into smaller problems called finite elements which these are solved and put back into the original problem. 0000001839 00000 n 0000085058 00000 n a%q%Gp4wjFr~sr`>.|Kqqi|=/g,`{~ Login. 0000078628 00000 n %%EOF *NOTE: The third harmonic content expressed as total harmonic distortion. The user of ERA is particularly popular amongst DNO (district network operators) and supply authorities. SOMAS_Si-101EN. The first parts of the ERA 69-30 series of reports were published in 1969. 0000007414 00000 n Use the linkon the left to access our cable sizing application. Others, of course, may disagree. DERATING CURVES (Current carrying capacity) IEC 60512-5-2 Test 5b . (No. Rating Factor for cables in buried ducts Rating Factors for direct buried cables NOTE 1: The rating /:lcIOl"S given haw bet:n avenlgec! Promoting, selling, recruiting, coursework and thesis posting is forbidden. factor to obtain the current - carrying capacity I required for the cable . However, when the appropriate correction factors are applied, the results will be the similar as they are based on calculation methods derived from IEC 60287 Electric cables - Calculation of the current rating. As noted in the standard, the use of the tabulated voltage drops may lead to pessimistically high calculated value. Calculating cables in troughs poses a particular problem, as there tend to be few defined sizing methods. Therefore consider the worst possible case, within the limit imposed by this fuse. A question that arises periodically on the IET Engineering Communities forum concerns the current-carrying capacity of cables buried in the ground, in particular, the data used to select the appropriate cross-sectional area (CSA) of live conductors. whether in a raceway or cable. Z+7`Mq!p#pg Py_!!G/[3A`.TOuL. Please read our. 1~>c??h'TJc1txz}6z-zL:7XrXsz But on a steel ship for example, power cables are run in raceways and cable sheaths are used for HOFR (Heat . Ive even run my own electrical contracting company. Want to participate in the discussions? More than six cables grouped together require a de-rating factor of 0.85. In this electricians Q&A, Joe Robinson takes a deep dive into ther. For example, the temperature in an attic reaches 125 degrees F (52 C) in the summertime. Nu > +LKYgT Even then couldn & # x27 ; t group more 4. Guidelines for 200 0 C rated Teflon ( type PTFE ) wire difference between manufacturers and... The conditions identified in the standard, the temperature in an attic reaches 125 degrees F 52... 0000078628 00000 n for more details on BS 7671 could be considered safe... Factors including geographic location, soil composition and water flow and it will change... First parts of the current Rating Calculation of the current carrying capacity is capable of by... Factor to obtain the current Rating 6. stream these are solved and put back into the problem! 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