SOMAS_Si-101EN. 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. lol!!! Derating: No derating is currently applied to the current ratings tables 310.15 (B) (16) and 310.15 (B) (17).. 5;b^VL477/8:m}*lXIw{%;YOol*V5@r,;VkYNM|&Q@VVGC`i(c cB&0s!|8~%q$sJ(2% cL*qOX_Q>H+i.3@qhJ Use the linkon the left to access our cable sizing application. In some cases, it may be necessary to perform detailed heat transfer calculations to determine a cables current-carrying capacity. To ascertain that the current-carrying capacity of the circuit is sufficient factoring the presence of harmonics in the system, Table 4Aa (BS7671 - 2011) provides some factors in determining the cable sizes. However, this is unlikely to be a cost-effective option for smaller CSA cables when compared with using a more conservative cable size. 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. Appendix 4 (BS 7671:2018)gives guidance determining the current capacity and voltage drop for cables. 0000009088 00000 n Typically, electrical design software is used for cable selection on larger installations. 0000003576 00000 n 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. Note the unit used for thermal resistivity of the soil in the Thirteenth Edition of the IEE Wiring Regulations was thermal ohm-cm as seen in Figure 2. Determination of the current-carrying capacity by calculating the actual heat transfer is likely to result in a less conservatively sized cable. 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. 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. 0000076394 00000 n 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. 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 The electrical designer must choose which is the most appropriate and apply the relevant correction factors. - 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. 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. The derating factors of Section 310.15 (B) (2) (a) apply only to multiconductor cables of three or more current carrying conductors in cable, not in cable tray. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework. 3H])Uw.?lJ `Vqh2SM 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. . vi}z7{?8x=l"t\F1'lnxN+h'qVs6GX1Cyx8N"/V*#JP%/~7d_}l?A#NLXh\h|gq$vtmO89dp|d{;>1-bSJb_" RT'C? 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. The table below provides derating guidelines for 200 0 C rated Teflon (type PTFE) wire. However, I see some publications that apply these factors to ccts, in trunking, for example, with no reference to cables being equally loaded (as though it's a belt-and-braces approach and apply the factors . Myth Busters #7 - Out with the old, in with the new. Thank you for helping keep Eng-Tips Forums free from inappropriate posts.The Eng-Tips staff will check this out and take appropriate action. Back to the Forum - Current-carrying capacity of cables buried in the ground, Electrical Safety Standards in the Private Rented Sector. 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. startxref CJX7b.zd@c41DV3+l[r~SKT? Electric power & transmission & distribution Forum, https://files.engineering.com/getfile.aspx?folder=13c66bdb-39dd-4639-a3e2-7, https://files.engineering.com/getfile.aspx?folder=94810acb-102b-494c-ad52-a, Low-Volume Rapid Injection Molding With 3D Printed Molds, Industry Perspective: Education and Metal 3D Printing. 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). 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. I Accept Cookies 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. It is important to remember that anyone carrying out these calculations should work directly from a copy of the relevant standard. ambient temperatures > 30C. P1{:f["ypc &@8o:a!`r trailer Whilst technology has advanced significantly since then, the laws of physics remain unchanged. Selecting Copper, XLPE, 3*50+25 mm 2 , its current is 185 A, Derating this current 185 * 0.93 = 172 A. . Bs7671 cable derating factors: CEF TechTalks are back - with a focus on renewables and regulations. 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 . Over 700mm then derating starts to rear it's ugly head. SOMAS_Si-101EN. The generally accepted depth for cables direct buried and cables buried in duct is 600mm to 650mm. stream 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. For further information, see the IET Wiring Matters article Amps per pound. I ended up having to run some lighting circuits in 2.5mm to negate them from grouping calc for the RFC circuits. When considering energy efficiency and the cost associated with a cable throughout its lifetime, it is important to remember that a smaller CSA cable will have higher energy losses (I2R), when compared with larger CSA, this is because more heat is dissipated. Reference method D of BS 7671:2018+A1:2020 should be applied for cables buried in the ground in and around buildings. More than six cables grouped together require a de-rating factor of 0.85. For example, suppose a cable had an ambient temperature derating factor of k amb = 0.94 and a grouping derating factor 5/12/13Cable Sizing Calculation - Open Electrical Cable Sizing Calculation From Open . If a detailed heat transfer calculation is carried out, however, a more accurate analysis of the whole cable route can be performed. Cable grouping factors. 148 46 It is the electrical designers responsibility to apply the appropriate factors to ensure the cable is sized adequately, it would not be deemed acceptable to blame the electrical design software for any errors. 0000004805 00000 n 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. If the tray is covered for more than 6 ft. with a solid unventilated cover, the ampacities allowed are 95% of those shown in the tables. 1.5mm cable but circuit only loaded with a couple of amps. *NOTE: The third harmonic content expressed as total harmonic distortion. For example ambient temperature (Ca), buried cables (Cc), Semi-enclosed fuse to BS3036 (Cf) etc (Take a look at Appendices 4 Section 3 of BS7671 for more rating factors that may apply). Most cable manufacturers data is based on the values used in the ERA 69-30 series but not all, so it is worth checking. 0000085335 00000 n 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. You have to look at using the <30% of the grouped rating for the cable i.e. 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. The current-carrying capacities of cables buried in the ground used in BS 7671:2018+A1:2020 and the ERA 69-30 report series are derived from the same calculation methods identified in the relevant parts of IEC 60287. Cable sizing compiles with: - BS 7671, IEE Wiring Regulations Cable size is selected as follows: 1. Although the document is a British Standard, it is also known as (and jointly labelled as) the IEE Wiring So the design current (Ib) would be 5000/230 = 21.73 Amps. the-Regs : BS7671 18th Edition Online Training, BS 7671: 18th Edition Wiring Regulations Study when you want and pass your exam when you're ready. Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a parts function at the center of their design considerations. 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). It is assumed that the maximum ambient temperature is 30C and the maximum ground temperature id 20C. The Institution of Engineering and Technology is registered as a Charity in England & Wales (no 211014) and Scotland (no SC038698). Irnd`u3=](tdd= L8z[Wlm`= Failing that, speaking to the manufacturers of the cable is always a good place to start. 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