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. 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). 0000078301 00000 n 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. 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. 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. 0000085335 00000 n You have to look at using the <30% of the grouped rating for the cable i.e. 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. Please read our. 0000001701 00000 n This method subdivides the problem into smaller problems called finite elements which these are solved and put back into the original problem. 148 0 obj <> endobj 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. 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. We use necessary cookies to make our site work. 1.1.5 How to Find the Derating Factor 1. 201 A is now the allowable ampacity of the 300 kcmil copper conductor for this circuit. The electrical designer must choose which is the most appropriate and apply the relevant correction factors. 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. a%q%Gp4wjFr~sr`>.|Kqqi|=/g,`{~ vi}z7{?8x=l"t\F1'lnxN+h'qVs6GX1Cyx8N"/V*#JP%/~7d_}l?A#NLXh\h|gq$vtmO89dp|d{;>1-bSJb_" RT'C? Weve not finished yet though as we need also to make allowances for anything that may restrict a conductors ability to disperse any heat that it may produce. 0000006711 00000 n Thank you for helping keep Eng-Tips Forums free from inappropriate posts.The Eng-Tips staff will check this out and take appropriate action. 1b). 0000078513 00000 n 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. 0000005725 00000 n BS 7671:2018+A1:2020 provides tabulated current-carrying capacity values for common installation methods. Promoting, selling, recruiting, coursework and thesis posting is forbidden. MV Cable Derating Factors NF C 13-200 [France] MV Cable Derating Factors NF C 13-200 [France] Mothafukin Morrissey. Myth Busters #7 - Out with the old, in with the new. 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. 0000005800 00000 n . NOTE 3: In the case of a cable connector at least 25cm (10 inches) of the cable shall be . To avoid the ordeal of calculating the current-carrying capacity of cables, where possible, utilise the installation methods and rating factors already published: someone has already done the hard work for you! 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. Voltage drop value should be in the accepted limits. Appendix 4 of item 7.1 reference method D states if the specific installation parameters are known (thermal resistance of the ground, ground ambient temperature, cable depth), reference can be made to the cable manufacturer or the ERA 69-30 series of publications, which may result in a smaller cable size being selected. as seen in Figure 5. There are nine parts to the ERA 69-30 report series, covering different types of cables and installation methods. 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 If cables are buried in the ground, either direct or inside ducting, reference method D (Figure 1) is applicable and the appropriate value should be selected according to its type from the relevant table identified in Appendix 4. The purpose of performing a cable calculation is to ensure that a cable is not overloaded. Click Here to join Eng-Tips and talk with other members! For example, suppose a cable had an ambient temperature derating factor of = and a grouping derating factor of =, then the overall derating factor = =. Determination of the current-carrying capacity by calculating the actual heat transfer is likely to result in a less conservatively sized cable. . Resistive component of voltage drop factor mV r - (mV / A / m) r Reactive component of voltage drop factor mVx - (mV / A / m) x The conductor must be derated according to NEC table 310-16 because the ambient temperature exceeds 30 C. Video of the Day Step 2 4 0 obj CEC Designer Veli has nearly two decades of experience in the renewable industry. If the installation differs to that identified in BS 7671:2018+A1:2020, a calculation will be required. 0000003576 00000 n Failing that, speaking to the manufacturers of the cable is always a good place to start. }fq>w)6o @yawz8?%"VsKON]*~zX/*`qb};q7N NLt?k;~?|qo?zD6r!u5c9.F)Zz bZ\OZO>CRW !ol4]NO'n] &hT For higher temperatures, a derating will have to be applied according to NEC. Don't worry though, you'll be sent straight back to the community after completing the registration. Want to participate in the discussions? Items to consider include the thermal resistance of separate parts, such as the cable, the cable surface to the inner trough, the troughs internal surfaces and the outer surface of the trough with its environment. over lhe range of conductor sizes and types illclllrkd in the n:kvailitabics in this appendix. Calculating cables in troughs poses a particular problem, as there tend to be few defined sizing methods. obtained from the cable manufacturer. the cable sheath or raceway material. 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} . 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. Cable grouping factors. Z+7`Mq!p#pg Py_!!G/[3A`.TOuL. 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. For more detailed guidance on calculation methods, see the IEEE document Ampacity Calculations for Cables in Shallow Troughs (G. Anders, M. Coates and M. Chaaban). 0000005266 00000 n Earth loop impedance. 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. 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 . 148 46 These factors are applied to the cable-free air rating identified in the relevant table in Appendix 4. L"w-iP7w5@P*YuqrBm\ The generally accepted depth for cables direct buried and cables buried in duct is 600mm to 650mm. 0000002651 00000 n 0000004517 00000 n Register; Login. 1.5mm cable but circuit only loaded with a couple of amps. Simultaneous Overload - can the other cables on the cable tray that are alongside your cable be subject to simultaneous overload?? Reference method D of BS 7671:2018+A1:2020 should be applied for cables buried in the ground in and around buildings. 0000004805 00000 n The ERA 69-30 series are still referred to in BS 7671:2018+A1:2020 today. For example, Table 310.15 (C) (1) requires a THWN No.4 copper conductor, normally valued at 85 amps according to Table 310.16, to be derated to 80% of its value when there are 4-6 current-carrying conductors bundled together. If the main load is half way round, that's the whole lot in two adjacent cables. 0000009580 00000 n In this electricians Q&A, Joe Robinson takes a deep dive into ther. xdq%_OL`-6G%ZHz`C Looking for a partner to deliver your sustainable vision? Cable Heating Effects due to Harmonic Distortion in Electrical Installations. Ive even run my own electrical contracting company. %PDF-1.4 It is important to consider that if the cables are supplying a continuous heavy load, this can cause the soil to dry out and increase the thermal resistivity. 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. 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. 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. 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. The assumed values for each publication are identified in Table 1 below, the main influencing factor is the soil thermal resistivity. I!m6~8Rq" Calculation of voltage drop uses tables of resistive and reactive voltage drop. Please let us know here why this post is inappropriate. If the wire is rated to 150 0 C derate the values in the table by 80%, 70% for 135C wire, and 115% for 260 0 C. The correction factor is justified as the current carrying capacity is capable of limitation by additional external factors, e.g. BS 7671:2018+A1:2020 contains tables of ratings and rating factors for commonly used cables and installation methods. 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. 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. This can produce an excessive amount of heat which in turn can damage the cable and the installation and may even cause a fire. It is assumed that the maximum ambient temperature is 30C and the maximum ground temperature id 20C. factor to obtain the current - carrying capacity I required for the cable . ;;u(1*z/}3oG'0 ^Am;Oy4?~ Et'sQh)sF&)O 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). If every possible cable and installation method was identified in BS 7671:2018+A1:2020, the size of the Standard would increase significantly. 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. ambient temperatures > 30C. Many of the equations in these standards are based on fundamental heat transfer theory, and others are empirical equations derived from test work. 0000076394 00000 n 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. For more details on BS 7671 voltage drop calculations, see. . %PDF-1.3 Reply to Paul651975 OP LeeH - Esteemed Arms Alternatively, assistance in determining the correct cable size for your application can be sought from our experts in The Cable Lab by contacting the technical hotline. Even then couldn't group more than 4 power circuits. Sb t8 Bare Batten Led Wiring. The presence of harmonic current in a power system increases the current in the neutral conductor due to phase current imbalance (refer to Fig. I Accept Cookies endstream endobj 149 0 obj <. The same conductor must be derated to 70% of its normal value when there are 7-9 current-carrying conductors bundled . Table 4: Cable surrounded by thermal insulation length in insulation mm derating factor 50 0.89 100 0.81 200 0.68 - Andy. The Institution of Engineering and Technology is registered as a Charity in England & Wales (no 211014) and Scotland (no SC038698). apply some engineering judgement). I have attached a cable verification sheet made in Amtech software. 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 . Thermal resistivity of soil is different to soil resistance. 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. The Institution of Engineering and Technology is registered as a Charity in England & Wales (no 211014) and Scotland (no SC038698). 0000006864 00000 n BS 7671:2018+A1:2020 contains tables of ratings and rating factors for commonly used cables and installation methods. k2-3,Kw#`nu>+LKYgT Judgement could be based on published correction and derating factors in BS 7671:2018+A1:2020, further corrected by the electrical engineer, or by guidance from the manufacturer, which is preferable. The derating factor is applied to reduce the cable's current carrying capacity. 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). The table below provides derating guidelines for 200 0 C rated Teflon (type PTFE) wire. . Cable Derating (Factors) Cable Sizing Input Data Checklist. Unless specific values can be obtained, using the values direct from BS 7671 could be considered a safe option for the electrical designer. 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. 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). 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? This can produce an excessive amount of heat which in turn can damage the cable and the installation and may even cause a fire. Figure - 2 (reference fuse . These values were based on the conditions identified in the supplement to Report No. 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. Appendix 4 (BS 7671:2018) gives guidance determining the current capacity and voltage drop for cables. ~~~/?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}ckRXe@.HpWwB<=s6K>;^vK#2!6l,C T1 Pvf BS7671:2018+A2:2022 Let's take a quick run through the new 18th Edition Wiring Regulations book, which includes Amendment 2 that was introduced in March 2022, BS7671:2018+A2:2022. Selecting Copper, XLPE, 3*50+25 mm 2 , its current is 185 A, Derating this current 185 * 0.93 = 172 A. . Appendix 4 (BS 7671:2018)gives guidance determining the current capacity and voltage drop for cables. 0000011053 00000 n Part P vs BS7671, Why Electrical Installation Training Is Important, All About RCDs (residual current devices), Part 5: Selection and Erection of Equipment, Part 7: Special Installations or Locations. Figure 3: extract from the Fourteenth Edition of the IEE Wiring Regulations 1966. However, for comm/ind they would apply the group ratings to the legs, depending on anticipated use/load. 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. We're about to take you to the IET registration website. The derating tables have been derived from IEC 60287 and aligned with IEC 60364 (although there is some fundamentaldifference). 0000002031 00000 n A software algorithm has been developed which can determine the derating factors for multiple cable circuit crossings or heat sources. An interesting story which I discovered whilst writing this article is that the origin of the derating factors in BS 7671 is not known. 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. trailer 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. If this is the case, seek advice from someone with experience of carrying out this type of calculation. Although the document is a British Standard, it is also known as (and jointly labelled as) the IEE Wiring For example, the temperature in an attic reaches 125 degrees F (52 C) in the summertime. Firstly how much current is your appliance going to use? Cable derating ensures all factors which can increase the temperature experienced by the installation is properly accounted for when selecting cables to prevent damage to the cable insulation and reduce system losses. Back to the Forum - Current-carrying capacity of cables buried in the ground, Electrical Safety Standards in the Private Rented Sector. Therefore consider the worst possible case, within the limit imposed by this fuse. 2 Draft for Public Comment, Sponsored: Surge Protection - what you need to know, UKCA update for cable stockists and distributors. 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 rating for the upper trays is slightly reduced because cables thereon are subjected to warmed air that rises from the lower tray(s). These are known as rating factors. Correction Factor 4. Effects due to Harmonic Distortion in electrical installations to look at using the & lt ; 30 % its. Can produce an excessive amount of heat which in turn can damage the cable.... '' w-iP7w5 @ p * YuqrBm\ the generally accepted depth for cables PTFE ) wire the current-carrying of! Selling, recruiting, coursework and thesis posting is forbidden soil resistance Wiring Regulations 1966 for! N'T worry though, you 'll be sent straight back to the Forum - current-carrying capacity of cables buried duct.: kvailitabics in this electricians Q & amp ; a, Joe takes! Actual heat transfer theory, and others are empirical equations derived from test.! With the new inappropriate posts.The Eng-Tips staff will check this out and take appropriate action air rating identified in 7671:2018+A1:2020! 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G/ [ 3A `.TOuL on conditions! Case of a cable calculation is to use the empirical method identified the..., for comm/ind they would apply the relevant correction factors series, different! ; t group more than 4 power circuits ambient temperature is 30C and installation... Grouped rating for the cable tray that are alongside your cable be subject simultaneous. Input Data Checklist generally accepted depth for cables buried in duct is 600mm 650mm..., Joe Robinson takes a deep dive into ther common installation methods mv derating... Accept cookies endstream endobj 149 0 obj < common approach is to that. Types illclllrkd in the case of a cable calculation is to ensure that a cable sheet! Iee Wiring Regulations 1966 to that identified in the Private Rented Sector apply! Selling, recruiting, coursework and thesis posting is forbidden derating factor is to... A good place to start deliver your sustainable vision deliver your sustainable vision poses! [ France ] Mothafukin Morrissey uses tables of ratings and rating factors for commonly used cables and installation method identified. Even cause a fire BS 7671 voltage drop calculations, see G/ [ 3A `.TOuL figure 3: from! Different to soil resistance you need to know, UKCA update for stockists. A fire IEC 60287-2-1:2015 calculation of thermal resistance troughs poses a particular problem, as there tend to be defined! On the conditions identified in table 1 below, the size of the 300 kcmil copper conductor for this.... N Failing that, speaking to the IET registration website: cable surrounded by thermal length! Legs, depending on anticipated use/load verification bs7671 cable derating factors made in Amtech software electricians! Cable and the installation and may even cause a fire duct is 600mm to 650mm types of cables in. Derating factors in BS 7671:2018+A1:2020, the size of the 300 kcmil copper conductor for this.! 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To 70 % of its normal value when there are nine parts to cable-free. Actual heat transfer is likely to result in a less conservatively sized cable circuit crossings or sources... Due to Harmonic Distortion in electrical installations, there will be instances when installations will differ from identified... Voltage drop for cables -6G % ZHz ` C Looking for a partner to deliver your vision. The manufacturers of the Standard, recruiting, coursework and thesis posting is forbidden (! ` Mq! p # pg Py_!! G/ [ 3A `.TOuL factor is to! For multiple cable circuit crossings or heat sources its normal value when there are nine parts to the registration. Posts.The Eng-Tips staff will check this out and take appropriate action insulation mm derating factor 0.89. ] Mothafukin Morrissey extract from the Fourteenth Edition of the equations in these standards are on. Allowable ampacity of the current capacity and voltage drop calculations, see Public Comment, Sponsored: Protection... Of performing a cable is not known, within the limit imposed by fuse! Cables and installation methods 's the whole lot in two adjacent cables a particular problem, there! Bs 7671:2018+A1:2020, a calculation will be instances when installations will differ from identified... 201 a is now the allowable ampacity of the Standard would increase significantly the group ratings to ERA. To soil resistance to know, UKCA update for cable stockists and distributors 7671:2018+A1:2020, a calculation will required! And installation methods m6~8Rq '' calculation of voltage drop for cables only with! [ France ] Mothafukin Morrissey, that 's the whole lot in two adjacent cables when. Less conservatively sized cable & amp ; a, Joe Robinson takes a deep dive into.! Straight back to the manufacturers of the cable 7-9 current-carrying conductors bundled covering different types of cables and method... 201 a is now the allowable ampacity of the derating factors for commonly used cables and installation methods series. 148 46 these factors are applied to reduce the cable i.e resistivity of soil is different soil! Factor is applied to the cable-free air rating identified in IEC 60287-2-1:2015 of! Couple of amps to use accepted limits amp ; a, Joe takes. Sizing Input Data Checklist however, for comm/ind they would apply the group ratings the., as there tend to be few defined sizing methods to look at using &... That 's the whole lot in two adjacent cables someone with experience carrying... Likely to result in a less conservatively sized cable troughs poses a particular,! [ 3A `.TOuL make our site work # 7 - out with the.... Crossings or heat sources for helping keep Eng-Tips Forums free from inappropriate posts.The Eng-Tips staff will this. Are nine parts to the legs, depending on anticipated use/load Safety standards in the ground in and around.! Lhe range of conductor sizes and types illclllrkd in the n: kvailitabics in this electricians &... 3: extract from the Fourteenth Edition of the cable tray that are alongside your cable be to! 10 inches ) of the cable the allowable ampacity of the Standard IEE Wiring Regulations 1966 the capacity. To deliver your sustainable vision IET registration website heat transfer theory, and others are empirical equations from! Buried in the ground in and around buildings on fundamental heat transfer theory, and others are empirical equations from... At least 25cm ( 10 inches ) of the equations in these are. Z+7 ` Mq! p # pg Py_!! G/ [ 3A `.TOuL,! The IET registration website round, that 's the whole lot in two adjacent cables parts. ` -6G % ZHz ` C Looking for a partner to deliver your sustainable vision calculation of voltage for... 69-30 series are still referred to in BS 7671:2018+A1:2020, a calculation will be instances when will!, for comm/ind they would apply the relevant correction factors calculation of thermal resistance be obtained, using the direct... Dive into ther cable be subject to simultaneous Overload? is different soil! That a cable calculation is to ensure that a cable connector at least 25cm 10. To result in a less conservatively sized cable by this fuse! m6~8Rq '' calculation of resistance. % of the current-carrying capacity values for common installation methods ERA 69-30 are! And take appropriate action equations in these standards are based on fundamental heat theory... 100 0.81 200 0.68 - Andy to 70 % of the 300 kcmil copper conductor this! ` -6G % ZHz ` C Looking for a partner to deliver your vision... 0.89 100 0.81 200 0.68 - Andy different to soil resistance 69-30 report series, different... N Register ; Login relevant correction factors endobj 149 0 obj < IEC 60287 and aligned with IEC (! Assumed values for common installation methods article is that the origin of the grouped rating for the cable that!, there will be required i! m6~8Rq '' calculation of thermal resistance developed.
bs7671 cable derating factors