Guidance on the use of finite element methods for calculating cable current ratings is given in IEC TR 62095:2003 Electric cables Calculation of current rating Cable current rating calculations using finite element method. We'll go into more detail and get you ready to pass your 18th Edition exam when we go through the book together fully in your 18th Edition online course at . The current-carrying capacity of cables has often been discussed on the IET Engineering Communities Forum and we also receive several enquiries on the IET technical helpline on the topic. 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. 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. Reference method Exampleofinstallationmethod Relevanttablefrom BS7671:2008 Image. Equally loaded just means that that's what they've assumed when calculating the numbers in that table (i.e. Where The electrical designer must choose which is the most appropriate and apply the relevant correction factors. Iz is the minimum cable current carrying capacity - see BS7671 Appendices 4.5 as per sheet. Do you need to include all cables fixed to a cable tray when calculating grouping factors. 0000002160 00000 n Table & Column 5. Total derating factor = 0.89 * 1.05 * 1.0 = 0.93. SOMAS_Si-101EN. 2023 The Institution of Engineering and Technology. Other losses include dielectric losses from insulating materials, skin effect introduced by alternating current, and eddy current losses. apply some engineering judgement). These values were based on the conditions identified in the supplement to Report No. Not every installation method is provided in BS 7671, as there are too many permutations. The IET jointly publish BS 7671 so who better to confirm the changes. 0000005725 00000 n An interesting story which I discovered whilst writing this article is that the origin of the derating factors in BS 7671 is not known. Thank you for helping keep Eng-Tips Forums free from inappropriate posts.The Eng-Tips staff will check this out and take appropriate action. Dose bicc comply? These factors are applied to the cable-free air rating identified in the relevant table in Appendix 4. 0000015597 00000 n Should that not be treated as two circuits? 5K)xIOa[QfER]Q $x+X -"^Fqa! over lhe range of conductor sizes and types illclllrkd in the n:kvailitabics in this appendix. The ERA 69-30 report assumes a soil thermal resistivity of 1.2 K.m/W, which corresponds with typical soil thermal resistivity for the UK in Annex A (A.22.2) of BS IEC 60287-3-1 Electric cables - Calculation of the current rating: Operating conditions - Site reference conditions, although this is considered to be a broad-brush statement. 0000003576 00000 n Reference method D of BS 7671:2018+A1:2020 should be applied for cables buried in the ground in and around buildings. Consider a 20/3.3 kV, 12.5 MVA transformer to be fed by direct buried, 3 core XLPE, SWA, PVC, copper conductor cable. 0000011053 00000 n The overall accuracy of rating lilctors is within 5%. The reports are still available and can be purchased here. 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. Part III of the ERA 69-30 provides values of current-carrying capacity and calculation methods for, sustained current ratings for 600/1000 V and 1900/3300 V cables with 70C thermoplastic insulation (ac 50 Hz and dc). The values of current-carrying capacity tabulated in the report are in accordance with IEC 60287 Electric cables - Calculation of the current rating. Cable Heating Effects due to Harmonic Distortion in Electrical Installations. The ERA was commissioned to produce a series of reports on the current-carrying capacities of cables. BS 7671:2018+A1:2020 provides tabulated current-carrying capacity values for common installation methods. If any of the rating factors do not apply to your cable then simply remove them from the equation. Cable sizing compiles with: - BS 7671, IEE Wiring Regulations Cable size is selected as follows: 1. Both the load factor and the RHO value of the backfill also play large roles in determining the potential derating of the conductors in the duct bank. 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. Rating Factor for cables in buried ducts Rating Factors for direct buried cables NOTE 1: The rating /:lcIOl"S given haw bet:n avenlgec! 0000085058 00000 n 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. AbstractThis paper analyses the derating to the current rating of buried cables which are crossing with heat sources or other cables.Calculating the derating factors will avoid overheating and prevent premature cable failures. wire cross section and unequal current distribution. ~~~/?Ppx.F@ 0K^,spyi>}>o?g'{wo?nO?~~oLG`|0_N~m#dGg?6?w7ovv_ ~v`C#t%(7cP>$7=^JJKh$eewE1l?lnrOB%@?6x8u"51SbeHD4_/Ba_=}$A5(scQw|Az7cnw; Uk{j3$;g/?uQ o8qbE>9D)q~0;Uch%omWg}c[email protected]<=s6K>;^vK#2!6l,C T1 Pvf HD 60364-5-52 Selection and erection of electrical equipment. obtained from the cable manufacturer. well, it should be treated as two adjacent cables carrying the same current, and grouped, just the same as you would (I hope) for a hairpin of cables going up and down the wall to call in at a switch for a heater or similar on a radial as well. All relevant derating factors are applied. Please read our. The current-carrying capacities published in BS 7671:2018+A1:2020 are based on tabulated values in HD 60364-5-52, whereas the current-carrying capacities used for manufacturers data are based on the ERA 69-30 report series. Typically, cables installed in ducts will need to be larger than those buried direct in the ground as the air surrounding the cable is heated by thermal radiation from the cable, reducing heat dissipation. If the designer wants to avoid this conservatism, a full calculation taking into account the environment outside the cable would be required, typically using finite element analysis. Applying the BS7671:2008 de-rating rule would have resulted in that cable installation being limited to 107.5 amps, just over half of the measured current flow in the cable. Cable current carrying capacity. As noted in the standard, the use of the tabulated voltage drops may lead to pessimistically high calculated value. - Andy. Others, of course, may disagree. l^~u%_?2J?Xm1~0lQ3C7X ncF?' The same conductor must be derated to 70% of its normal value when there are 7-9 current-carrying conductors bundled . The user of ERA is particularly popular amongst DNO (district network operators) and supply authorities. When multiple cables are in proximity, each contributes heat to the others and diminishes the amount of external cooling affecting the individual cable conductors. %PDF-1.4 % - Table 52.2 Cable surrounded by thermal insulation, gives slightly reduced derating factors, to take account of the availability of material with improved thermal insulation. Impedance and voltage drop to IEC 60909 and CENELEC CLC/TR 50480. Click Here to join Eng-Tips and talk with other members! 0000078628 00000 n endstream endobj 149 0 obj <. <> I have attached a cable verification sheet made in Amtech software. Reply to Engineer54 Reply to reference method D in the Industrial Electrician Talk area at ElectriciansForums.net Post reply Similar Threads S Reference method 100/102 or A Hi everyone. The rating for the upper trays is slightly reduced because cables thereon are subjected to warmed air that rises from the lower tray(s). 2023 the-Regs : BS7671 18th Edition Online Training, Special Locations, Bathrooms. BS 7671:2018+A1:2020 contains tables of ratings and rating factors for commonly used cables and installation methods. It was not until the Seventeenth Edition of the IEE Wiring Regulations in 2008 that tabulated values were reintroduced as Table 4D4A (thermoplastic) and Table 4E4A (thermosetting). Table 33 (as it was identified back then) was dedicated to current ratings for PVC insulated cables to BS 3346 installed directly in the ground. BICC or BS7671 comply for design Anthony Sullivan over 4 years ago Hello IET guys I have a problem where a cable installed on site doesn't comply to Bs7671 current carrying capacity for underground cable reference method D the engineer who design the cable used amtech and data from bicc with reference method 110. BS 7671:2018+A1:2020 provides tabulated current-carrying capacity values for common installation methods. 0000005657 00000 n Cable Sizing Standards. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework. Just a point of interest if you are using twin and earth cable and your cable comes into contact with insulation as per any of the reference methods 100, 101, 102 or 103 then just use the table 4D5 and Ci = 1 in any calculations. 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. The Institution of Engineering and Technology is registered as a Charity in England & Wales (no 211014) and Scotland (no SC038698). Z Earth loop impedance. Sb t8 Bare Batten Led Wiring. Traditionally, tabulated values were not provided in the IEE Wiring Regulations, the current-carrying capacity ratings for cables buried directly in the soil first appeared in the 13th Edition of the IEE Wiring Regulations in 1955 as seen in Figure 2. It consists of one table, whereby the designer selects the size of cable conductor and whether it is a single-core, two-core or multi-core cable. The derating factors in Table 52.2 are appropriate to conductor sizes up to 10mm 2 . 0000010392 00000 n Note: myCableEngineering does not use the BS 7671 tables, but instead calculations the circuitimpedances in accordance with IEC 60909 and directly calculations voltage drop from this. If the installation differs to that identified in BS 7671:2018+A1:2020, a calculation will be required. every cable is carry the same and as much as it can without anything overheating). So, we have In/(Ca x Ci) or 32/(0.94 x 1) = 34.04A and looking at Table 4D5 column 4 Method 102 we can see that a 6mm cable gives us a current carrying capacity of 35A which is what well be using providing that the Volt drop is okay? The international standards that cover the current-carrying capacity of cables are the IEC 60287 series for steady-state conditions and IEC 60853 for cyclic conditions for most practical applications. . Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. There are nine parts to the ERA 69-30 report series, covering different types of cables and installation methods. If the appropriate correction factors are applied, the results will be the same. << /Length 5 0 R /Filter /FlateDecode >> We'd also like to set optional analytics cookies to help us improve it. (heat). The ERA series of reports are still available and can be purchased from the RINA website. Correction Factor 4. Selecting Copper, XLPE, 3*50+25 mm 2 , its current is 185 A, Derating this current 185 * 0.93 = 172 A. . capacity of the cable shall be reduced appropriately depending on the size of cable, length in Table 4 are appropriate to conductor size up to 10 mmsq in thermal insulation having a thermal conductivity greater than 0.0625 W/K.m. Cable grouping factors. We're about to take you to the IET registration website. By the time tabulated values for buried cables were reintroduced in the Seventeenth Edition of the IEE Wiring Regulations in 2008, the IEE Wiring Regulations had become a British Standard, BS 7671:2008, based on the European CENELEC HD 60364 series of standards. Got the answer from BS 7671, but still got another question on the previous attachment. What is the maximum number of cables that can be installed in a pre-formed concrete cable trough? Today we use K.m/W, Kelvin Metre per Watt, 120 thermal ohm-cm converts to 1.2 K.m/W (120/100 = 1.2). Had the derating factors for conduit fill and ambient not been required, a 3/0 copper conductor would have met the needs for this application. 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). The resulted current should be the new current carrying capacity for the cable. Other than the conductor sizes changing to metric, these ratings have remained unchanged. (No. lol!!! presumably then each leg should also be run separately in floor joists - at the risk of making Swiss cheese out of the joists with all the cables run in separate holes, or upping the cable size based on rating factors. trailer Firstly how much current is your appliance going to use? Cable Current Rating 6. The rating for the bottom tray is the same as for a single tray, since the cables on the bottom tray do not know that other trays containing other cables are above them. 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 . Unless specific values can be obtained, using the values direct from BS 7671 could be considered a safe option for the electrical designer. 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. Current Capacity Cable current capacity calculations in BS 761 are based on the derating factor method, see Cable Derating (Factors). All cable sizing methods more or le ss follow the same basic six step process: 1) Gathering data about the cable, its installation conditions, the load that it will carry, etc 2) Determine the minimum cable size based on continuous current carrying capacity 3) Determine the minimum cable size based on voltage drop considerations 4) Determine the a%q%Gp4wjFr~sr`>.|Kqqi|=/g,`{~ 1. 0000088434 00000 n 0 A software algorithm has been developed which can determine the derating factors for multiple cable circuit crossings or heat sources. 0000002651 00000 n 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. The method involves calculating a temperature rise of the air in the trough above its ambient, with the rating then being calculated as though it were in free air, but with the increased temperature. The current-carrying capacity of a cable is determined by the maximum permitted conductor temperature and the rate at which it dissipates heat into the environment, taking into account the surrounding material, which will have thermal properties and resist the dissipation of heat, this is called I2R losses. 2023 The Institution of Engineering and Technology, The Institution of Engineering and Technology is registered as a Charity in England and Wales (no 211014) and Scotland (no SC038698). Typically, electrical design software is used for cable selection on larger installations. Calculation of voltage drop uses tables of resistive and reactive voltage drop. Transformer full load current is calculated by: Ifull-load = 12.5 106 / 1.73 20 103 = 361 A. Don't forget derating factors. stream If several installation methods are adopted for a cable, such as being buried underground for part of the route, and on cable tray for the remaining part, it is common practice for the designer to select the worst-case scenario and apply that to the whole cable run. Cable is calculated on current . 193 0 obj <>stream 1a If it is a fan, centrifugal pump or centrifugal compressor use steps 2 to 4 to determine which graph applies from illustration 1.1 - 1.9. The document is not dated and no author is provided, but it is believed to have been prepared by one or more of the UK cable manufacturers. SOMAS_Si-101EN. %PDF-1.4 The references in the document indicate that it was produced in the late 1940s or early 1950s. Issues can arise when manufacturers cable data has been used by a consultant to complete the design, but when checked by the contractor against tables in BS 7671 directly, the sizing can appear to be inadequate. Hzd}-/ *@. To utilize temperature and power correction factors accurately, BS 7671 offers voltage drop factors separately in resistive and inductive components for cables above 16 mm 2. ambient temperatures > 30C. In this scenario, the designer will have to make an engineering judgement to determine the current-carrying capacity of the cable or seek advice from cable manufacturers. He is passionate about providing people with valuable education and is highly regarded throughout the industry as an educator and operator. NOTE 3: In the case of a cable connector at least 25cm (10 inches) of the cable shall be . For smaller projects, this may not have much of an impact, but for larger projects and high design currents, this could be extremely beneficial, in terms of both physical size and cost. It is important that these calculations are carried out by someone who is experienced in this field of work (which may not necessarily be an electrical engineer). Usually the derating factor is tabulated in a nation's wiring regulations. xZ[cs@k./0gqW5J$?Q /3=+uUW5mFN_zo++y.t^02:xr*ZK$7_Cw/~5a(vW~?jd64s/k=!W?fYZ`?x}h571t"TI?xf`D4i* k=VHYh$E#-Snf#9D?X 1?q9!Qkp"L$7+(dvirmlDMR[bvlz72Qbpqm>fs?8Y#f:S0SrTN:+IY+hY7wQ ot7bJ[!eN$KRv&%BiV,B[Zgbb+L5L o$+E0VY0B 0000006711 00000 n In this electricians Q&A, Joe Robinson takes a deep dive into ther. 0000008053 00000 n 0000004990 00000 n Promoting, selling, recruiting, coursework and thesis posting is forbidden. Ive also been qualified tutor since 2008 and have taught electrical installation C&G, EAL and Logic courses and weve been successfully teaching the Wiring Regulations online here at the-regs.co.uk since 2014 and have had hundreds of students pass 18th Edition exam why not join them today? 0000002031 00000 n The nearest size of standard trunking with a capacity factor greater than 1239.6 is 75 x 50mm? According to the IEEE document Ampacity Calculations for Cables in Shallow Troughs (G. Anders, M. Coates and M. Chaaban), a draft document has been found that derives a simple equation for the calculation of thermal resistance of a trough. 0000002616 00000 n BS 7671, Table 4E4A, reference method, current-carrying capacity, XLPE insulation, copper conductor, multi-core, armoured cable . The thermal resistivity of soil refers to its ability to dissipate heat, which is relevant to the current-carrying capacity of cables. 0000004403 00000 n 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 installation method takes into account assumed parameters, such as ambient ground temperature (Ca), soil thermal resistivity (Cs), depth of laying (Cd) and spacing factor (Cg), the correction factors for which can be found in Tables 4B2, 4B3, 4B4 and 4C2 respectively. Although the document is a British Standard, it is also known as (and jointly labelled as) the IEE Wiring Solar Radiation Effects. If every possible cable and installation method was identified in BS 7671:2018+A1:2020, the size of the Standard would increase significantly. Soil resistance is the ability to pass electrical current, which is relevant to earthing systems. We're about to take you to the IET registration website. Case 1 Assuming a design current carrying capacity of circuit to 25A with a third harmonic of 32%. Cables Derating Factor Explained for beginnersVisit my electrical site https://www.electrical4uonline.com/cables-derating-factors-explained-for-beginners/htt. 0000088221 00000 n 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 Current capacity to BS 7671, ERA 69-30 and IEC 60502. UTg-3[f;"v,u|==:vao~|?m{?=?;9#<=}?^7||o}=oOOKo~4o04qzt:o>oO{? Created for free using WordPress and, Common Mode Voltages: Causes, Effects and Mitigation. MV Cable Derating Factors NF C 13-200 [France] MV Cable Derating Factors NF C 13-200 [France] Mothafukin Morrissey. Finite element analysis is a numerical method used to solve complex engineering problems, usually carried out on simulation software. xdq%_OL`-6G%ZHz`C What does the number of trays mean in the IEC derating factors table? Want to participate in the discussions? Join your peers on the Internet's largest technical engineering professional community.It's easy to join and it's free. Most cable manufacturers data is based on the values used in the ERA 69-30 series but not all, so it is worth checking. Cable Calculations Part 1 - the-Regs : BS7671 18th Edition Online Training Cable Calculations Part 1 13 June 2016 by the Regs Guy The purpose of performing a cable calculation is to ensure that a cable is not overloaded. Reply to Paul651975 OP LeeH - Esteemed Arms The calculation method is complex and time-consuming, with additional possibilities of introducing errors, and thus the finite element method is the favoured approach of engineers today. This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. For more details on BS 7671 voltage drop calculations, see. 1.1.5 How to Find the Derating Factor 1. Total derating factors = K1*K2*K3 = 0.95*0.9*0.91 =0.77, Then multiply this value by the cable current. The physics involved with heat loss have similarities with those of mechanical engineering and are based on heat transfer principles. Manage and size all your cables, from low voltage to 33 kV. Appendix 4 of BS 7671 by Mark Coles Appendix4,Current-carrying capacity and voltage drop for cables and flexible cords,hasseensignificant . Current-carrying capacity of cables installed in concrete cable troughs, The impact of Amendment 2 of the 18th edition (BS 7671:2018+A2:2022), COVID-19 and how it has changed the way we work, Section 514 identification and notices in the Amendment No. The software is able to simulate and analyze both steady-state and transient conditions, seasonal changes and effects such as soil drying out, much more quickly than would be achieved by carrying out separate calculations. This can produce an excessive amount of heat which in turn can damage the cable and the installation and may even cause a fire. There are consultancies that specialize in this type of work. A derating factor is introduced that is defined in terms of the ampacity of power cables in open-top trays. xNf(@-c]D$t7:z)Q It is tabulated current carrying capacity of cable with derating factors. F/T 183 issued by the ERA. This is done by dividing the rating of the protective device (In) by any appropriate rating factor/s (Ca x Cc x Cd x Cf x Cg x Ci x Cs). Thermal resistivity of soil is different to soil resistance. Over 700mm then derating starts to rear it's ugly head. Register; Login. The parameters considered in the standard are, ambient temperature, soil thermal resistivity, the installation methods and grouping of cables. Derating is necessary for multiple cables in proximity. Don't worry though, you'll be sent straight back to the community after completing the registration. Within this session you can put your question directly to the IET. Fictitious Dimensions. 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. These standards contain methods for calculating the permissible current rating of cables from details of the permissible temperature rise, conductor resistance, losses and thermal resistivities, using analytical methods. 0000078800 00000 n For higher temperatures, a derating will have to be applied according to NEC. BS 7671 is the UK requirements for electrical installations standard, also known as the Wiring Regulations. BS 7671 is the UK requirements for electrical installations standard, also known as the Wiring Regulations. The assumed values for ambient ground temperature, soil thermal resistivity and installation depth used in each publication are different which will result in a different cable size being selected. These are related to the line voltage for three phase circuits and phase voltage for single phase circuits. 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. Appendix 4 (BS 7671:2018)gives guidance determining the current capacity and voltage drop for cables. From cables ampacity tables. Derating factors should be applied to the cable current. 0000005800 00000 n The ERA 69-30 series are still referred to in BS 7671:2018+A1:2020 today. We have received your request and will respond promptly. Using the factors noted earlier: 285 A x 0.80 x 0.88 = 201 A. vi}z7{?8x=l"t\F1'lnxN+h'qVs6GX1Cyx8N"/V*#JP%/~7d_}l?A#NLXh\h|gq$vtmO89dp|d{;>1-bSJb_" RT'C? %PDF-1.3 Z+7`Mq!p#pg Py_!!G/[3A`.TOuL. P1{:f["ypc &@8o:a!`r Note 1 of table 4C1 in Apdx 4, says that the factors are applicable to uniform groups of cables, equally loaded. Some studies have found the empirical method to be conservative, owing to the fact that it does not take account of the specific thermal environment outside of the cable. A. Non-sheathedcablesinconduitina Back to the Forum - Current-carrying capacity of cables buried in the ground, Electrical Safety Standards in the Private Rented Sector. Don't worry though, you'll be sent straight back to the community after completing the registration. 0000004805 00000 n Annex B is popular because it is simple to use. Derating factors given in BS7671 for cables in touching trefoil cleat formation are appropriate for cables in quad bundles; Voltage drops for circuits in quad formation should be calculated using the values tabulated in BS7671 for cables in flat touching formation For a cable with a base current rating of 42A, the installed current rating would For example, the temperature in an attic reaches 125 degrees F (52 C) in the summertime. The question that was asked, and is primarily responsible for publishing this article, was: The enquiry was related to an extensive electric vehicle (EV) charging project: the designer needed to consider derating factors for up to 60 cables installed in a concrete trench. k = factor which is 143 for Copper cable (from IEC 60364-4-43). fpo&Q)"oHu4D3]*?/ 23,A o?NTpZ4H=`y53,O5+1-WC8Fy"n\IGOS`] pEA L1xi1g]lG+@/GCJ&$y91lIOJ8aSu|)w)ue(,'k5J%Z'#MD"%#B x CJX7b.zd@c41DV3+l[r~SKT? Whilst BS 7671:2018+A1:2020 is extremely useful as a guide for determining the current-carrying capacity of cables for most electrical installations, there are situations when a detailed current-carrying capacity calculation might be required. Where a cable is buried in the ground its ability to dissipate heat is reduced, the extent of which depends on the installation method. Irnd`u3=](tdd= L8z[Wlm`= 0000076770 00000 n For a cable with a base current rating of 42A, the installed current rating would be I c = 0.799 x 42 = 33.6 A {\displaystyle I_{c}=0.799x42=33.6A} . Figure - 2 (reference fuse . Whilst BS 7671:2018+A1:2020 does cover most typical electrical installations, there will be instances when installations will differ from those identified in the Standard. L"w-iP7w5@P*YuqrBm\ An important note in the ERA 69-30 report Part III, states cables installed in and around buildings subject to the provisions of the IEE Wiring Regulations, BS 7671, should be rated in accordance with those Regulations., which is probably where the in or around buildings comes from in the description under reference method D in Table 4D4A as seen in Figure 6. This method subdivides the problem into smaller problems called finite elements which these are solved and put back into the original problem. 0000076394 00000 n Soil resistivity is affected by many factors including geographic location, soil composition and water flow and it will also change seasonally. !paX}xZQ#)aC*UryS>P&O )X]B. Login. For example, suppose a cable had an ambient temperature derating factor of = and a grouping derating factor of =, then the overall derating factor = =. The values used for calculation will depend on the actual ground conditions and material used for backfilling. British Cables Company introduces new cables to its range. 0000001839 00000 n However, for comm/ind they would apply the group ratings to the legs, depending on anticipated use/load. Rosa Ponce. The correction factor is justified as the current carrying capacity is capable of limitation by additional external factors, e.g. Iasked the nic about that, some time ago, and they said that for domestics they read it as for ccts, so wouldn't be concerned about individual legs if the ring was generally lightly loaded most of the time. Be sent straight back to the current-carrying capacities of cables 0000004990 00000 n that. Capacity of cables and types illclllrkd in the supplement to report No %?! Comm/Ind they would apply the relevant table in appendix 4 and installation methods calculations in 7671... For commonly used cables and flexible cords, hasseensignificant the overall accuracy of rating lilctors is within 5 % is... Heat loss have similarities with those of mechanical engineering and are based on the current-carrying and. 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