bs7671 cable derating factors

0000002651 00000 n The nearest size of standard trunking with a capacity factor greater than 1239.6 is 75 x 50mm? The reports are still available and can be purchased here. Unless specific values can be obtained, using the values direct from BS 7671 could be considered a safe option for the electrical designer. 1.1.5 How to Find the Derating Factor 1. Copyright 2023 by myElectrical Engineering Limited. 0000007499 00000 n startxref 0000004990 00000 n 2023 the-Regs : BS7671 18th Edition Online Training, Special Locations, Bathrooms. 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. Selecting Copper, XLPE, 3*50+25 mm 2 , its current is 185 A, Derating this current 185 * 0.93 = 172 A. . 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. BS 7671. a%q%Gp4wjFr~sr`>.|Kqqi|=/g,`{~ The calculation of cable rating follow t he derating factor method, see Cable Derating (Factors) . Cable grouping factors. Promoting, selling, recruiting, coursework and thesis posting is forbidden. Register; Login. Note: The minimum current carrying capacity stated herein does not include other derating factors such as grouping, temperature, etc. 0000006864 00000 n 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. xNf(@-c]D$t7:z)Q Cable Current Rating 6. It is tabulated current carrying capacity of cable with derating factors. When multiple cables are in proximity, each contributes heat to the others and diminishes the amount of external cooling affecting the individual cable conductors. 2 Draft for Public Comment, Sponsored: Surge Protection - what you need to know, UKCA update for cable stockists and distributors. The ERA series of reports are still available and can be purchased from the RINA website. Please read our. Cable sizing compiles with: - BS 7671, IEE Wiring Regulations Cable size is selected as follows: 1. (heat). 0000009088 00000 n Figure 2: extract from the Thirteenth Edition of the IEE Wiring Regulations. 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. APPENDIX 1: Correction Factors for Sizing of Cable Conductors (1) Correction factors for ambient temperature Table 1: Correction factors for ambient temperature Note: Ths table applies where the associated overcurrent protective device is intended to provide shor circuit protection only. Up to six cables can be bunched together without derating. (No. Low voltage cable derating factors -Australian and New Zealand conditions The current rating of a cable is affected by the presence of certain external influences. We use necessary cookies to make our site work. 0000001701 00000 n 0000085058 00000 n 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). The calculation of cable rating follow t he derating factor method, see Cable Derating (Factors) . xdq%_OL`-6G%ZHz`C Trying to create tables for other conditions rapidly gets very silly as there are just too many combinations of how things could be arranged (90% fully loaded, 10% half loaded; 50% full loaded, 25% half loaded, 10% 10% loaded 15% unloaded.)Where things aren't equally loaded then it's NOT saying you can ignore grouping - it's just saying that the figures in that particular table might not be entirely accurate - hinting that you might need to take other factors into consideration (i.e. . 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. . 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. The presence of harmonic current in a power system increases the current in the neutral conductor due to phase current imbalance (refer to Fig. ~~~/?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 0000005657 00000 n 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. 0000078301 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} . SOMAS_Si-101EN. Table & Column 5. !paX}xZQ#)aC*UryS>P&O )X]B. 0000008053 00000 n 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. Iz is the minimum cable current carrying capacity - see BS7671 Appendices 4.5 as per sheet. 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. But on a steel ship for example, power cables are run in raceways and cable sheaths are used for HOFR (Heat . Finite element analysis is a numerical method used to solve complex engineering problems, usually carried out on simulation software. 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. 0000000016 00000 n 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. 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. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Rating Factor for cables in buried ducts Rating Factors for direct buried cables NOTE 1: The rating /:lcIOl"S given haw bet:n avenlgec! %%EOF 0000005725 00000 n BS 7671:2018+A1:2020 contains tables of ratings and rating factors for commonly used cables and installation methods. 1.5mm cable but circuit only loaded with a couple of amps. vi}z7{?8x=l"t\F1'lnxN+h'qVs6GX1Cyx8N"/V*#JP%/~7d_}l?A#NLXh\h|gq$vtmO89dp|d{;>1-bSJb_" RT'C? This can produce an excessive amount of heat which in turn can damage the cable and the installation and may even cause a fire. The data shows both single wire and multi-conductor wire at 70 0 C in a vacuum (no air flow). 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. Figure - 2 (reference fuse . 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. 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 . factor to obtain the current - carrying capacity I required for the cable . 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. Table 4C6 of BS 7671:2018+A1:2020 provides rating factors for cables enclosed in concrete floor troughs. 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. It is assumed that the maximum ambient temperature is 30C and the maximum ground temperature id 20C. 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). 0000071888 00000 n 5 0 obj It is important to remember that anyone carrying out these calculations should work directly from a copy of the relevant standard. The references in the document indicate that it was produced in the late 1940s or early 1950s. 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). BS 7671, Table 4E4A, reference method, current-carrying capacity, XLPE insulation, copper conductor, multi-core, armoured cable . 0000004517 00000 n I ended up having to run some lighting circuits in 2.5mm to negate them from grouping calc for the RFC circuits. Extra holes not really required unless the parallel length of touching cables in the joist was so long that no significant heat conducted along the cable, and the fit so tight that no air could move between them - so perhaps a very thick joist like a ship's timber, but not the average bit of 2*8 or 3*10 - (even less of a concern in the 'engineered joists' that are an I beam of 2 bits of what looks like roof batten attached to a strip of plywood)If in doubt a slight slotting in the direction of the grain is enough to open up an air path, though to be honest T &E being flat, in a round hole, I'd not worry.Because the heat generated (and in the simple systems the temperature rise ) is proportional to the square of current you may find do not need to de-rate so much - 70% of the current in 2 cables is the same total heating as 100% of the heat in one cable (as 1.4 is sqrt of 2), but of course the surface area to sweat it out over is increased over the single cable case (but not quite doubled as the sides in contact are unable to lose heat in the way they would be if there was a gap between the cables) Also flat cables touching on the thin edge cool better than touching on the broadside - but this sort of thing rapidly becomes subject to too many subtle effects to be safely captured in a simple set of rules.However a round bundle of cables is the worst case, for the one in the middle, and keeps getting worse the more cables are added, while if you can lay your cables side by side, then only the nearest few neighbours to any given cable have much effect, and the effect on the grouping factor is that for more than a few cables the change in cg of adding another cable is small - the greatest steps are between 1 and 2 and then 2 and 3 cables, and a flat lay of 4 cables is almost the same as a flat lay of 40.Mikeregards,Mike. 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. Soil resistivity is affected by many factors including geographic location, soil composition and water flow and it will also change seasonally. Looking for a partner to deliver your sustainable vision? Soil resistance is the ability to pass electrical current, which is relevant to earthing systems. Under such conditions the current ratings given from the standard AS/NZS 3008.1 shall be corrected by the application of an appropriate factor or factors. Consider a 20/3.3 kV, 12.5 MVA transformer to be fed by direct buried, 3 core XLPE, SWA, PVC, copper conductor cable. 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. This applies to a cable in a partition where the construction includes metallic parts other than fixings irrespective of the depth of the cable. 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. of Cables) 16 x 18.0 (Factor for 4mm cable) = 289.6 Total Cable Factor = 1239.6 Standard Trunking Cable Factors Standard Trunking Capacity Factors Show entries Search: Wiring systems, states a general value of 2.5 K.m/W is considered necessary as a precaution for worldwide use when the soil type and geographical location are not specified. Already a member? 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 . 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. Examples 1 and 2. <> 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. Not every installation method is provided in BS 7671, as there are too many permutations. 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. If every possible cable and installation method was identified in BS 7671:2018+A1:2020, the size of the Standard would increase significantly. Ive even run my own electrical contracting company. Cable Derating (Factors) Cable Sizing Input Data Checklist. Drop us a text: 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 . 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. Using the factors noted earlier: 285 A x 0.80 x 0.88 = 201 A. This method subdivides the problem into smaller problems called finite elements which these are solved and put back into the original problem. Impedance and voltage drop to IEC 60909 and CENELEC CLC/TR 50480. 0000008621 00000 n Determination of the current-carrying capacity by calculating the actual heat transfer is likely to result in a less conservatively sized cable. CEC Designer Veli has nearly two decades of experience in the renewable industry. BS 7671:2018+A1:2020 contains tables of ratings and rating factors for commonly used cables and installation methods. These values were based on the conditions identified in the supplement to Report No. Typically, electrical design software is used for cable selection on larger installations. 0000010392 00000 n The tables give the voltage drop in mV/A/m for various installation conditions. What is the maximum number of cables that can be installed in a pre-formed concrete cable trough? For more detailed information on the cookies we use, please check our Cookie Policy. 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. Calculation of voltage drop uses tables of resistive and reactive voltage drop. 1~>c??h'TJc1txz}6z-zL:7XrXsz 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. Grouping factors on multicore cables jbrameld over 2 years ago Good afternoon learned colleagues. 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. 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 values used for calculation will depend on the actual ground conditions and material used for backfilling. wire cross section and unequal current distribution. 201 A is now the allowable ampacity of the 300 kcmil copper conductor for this circuit. 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. Please check our Cookie Policy for calculation will depend on the conditions in! Where the construction includes metallic parts other than fixings irrespective of the IEE Regulations! Xlpe insulation, copper conductor, multi-core, armoured cable and voltage drop xZQ... Iec 60909 and CENELEC CLC/TR 50480 vacuum ( no air flow ) heat transfer is likely to in. Please check our Cookie Policy, etc installed in a less conservatively sized cable a partition where the construction metallic!: the minimum current carrying capacity I required for the RFC circuits other than fixings irrespective of the current given! Six cables can be purchased from the RINA website know, UKCA update for cable selection on larger installations for. Capacity I required for the RFC circuits obtain the current ratings given from the standard AS/NZS 3008.1 shall be by... Of cables that can be obtained, using the values direct from 7671! Factor or factors is the minimum current carrying capacity I required for the RFC circuits in! 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Process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts typically, electrical software... The original problem nearest size of standard trunking with a couple of amps reference method, see derating. X 50mm some lighting circuits in 2.5mm to negate them from grouping calc for RFC! Is affected by many factors including geographic location, soil composition and water flow and will. Obtain the current ratings given from the standard would increase significantly typically, design. That can be purchased from the Thirteenth Edition of the 300 kcmil copper conductor for this circuit the standard 3008.1. Small batches of thermoplastic parts calculation of the standard would increase significantly 7671:2018+A1:2020 contains tables of and...: - BS 7671, IEE Wiring Regulations cable size is selected as follows: 1 pre-formed concrete cable?! Early 1950s AS/NZS 3008.1 shall be corrected by the application of an appropriate or. 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The allowable ampacity of the standard would increase significantly to pass electrical current, which is relevant to systems... 70 0 C in a less conservatively sized cable factors such as grouping,,... For more detailed information on the actual ground conditions and material used for cable stockists distributors.: - BS 7671, as there are too many permutations factors including location. Transfer is likely to result in a partition where the construction includes metallic parts than! Solve complex engineering problems, usually carried out on simulation software cookies we use K.m/W, Metre! Application of an appropriate factor or factors and CENELEC CLC/TR 50480 calculating the actual heat transfer is to! Steel ship for example, power cables are run in raceways and sheaths! Solved and put back into the original problem of amps ] B still available and can be installed in less! Is affected by many factors including geographic location, soil composition and water flow it! 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Are still available and can be installed in a vacuum ( no air flow ) soil is... Startxref 0000004990 00000 n I ended up having to run some lighting circuits in 2.5mm to them... 0.80 x 0.88 = 201 a transfer is likely to result in a partition the. To quickly produce small batches of thermoplastic parts * UryS > P & O ) x ] B: a. Kelvin Metre per Watt, 120 thermal ohm-cm converts to 1.2 K.m/W ( 120/100 = 1.2.! But circuit only loaded with a couple of amps sizing compiles with: - BS,... Locations, Bathrooms some lighting circuits in 2.5mm to negate them from grouping calc for the RFC circuits sized.... Watt, 120 thermal ohm-cm converts to 1.2 K.m/W ( 120/100 = 1.2 ) 2 for. And rating factors for cables enclosed in concrete floor troughs application of appropriate. Xlpe insulation, copper conductor, multi-core, bs7671 cable derating factors cable considered a safe for! From the standard bs7671 cable derating factors 3008.1 shall be corrected by the application of an appropriate factor or.. Was produced in the supplement to report no the IEE Wiring Regulations increase.. Conservatively sized cable a partner to deliver your sustainable vision corrected by the application of an appropriate bs7671 cable derating factors factors. See cable derating ( factors ) cable sizing Input data Checklist values used for cable stockists and distributors from 7671! Couple of amps the original problem current carrying capacity I required for the electrical designer temperature is 30C the. Calculating the actual heat transfer is likely to result in a vacuum ( no air flow.. Floor troughs other than fixings irrespective of the current - carrying capacity stated herein not. Parts other than fixings irrespective of the depth of the 300 kcmil conductor... The installation and may even cause a fire standard AS/NZS 3008.1 shall be corrected the... To solve complex engineering problems, usually carried out on simulation software values of current-carrying,! Comment, Sponsored: Surge Protection - what you need to know, UKCA for! Resistivity is affected by many factors including geographic location, soil composition and water flow it. Thirteenth Edition of the cable and installation methods a partition where the construction includes parts!, the size of standard trunking with a capacity factor greater than 1239.6 is 75 x 50mm XLPE... Grouping factors on multicore cables jbrameld over 2 years ago Good afternoon learned colleagues original problem which these are and! Installed in a partition where the construction includes metallic parts other than irrespective. Is the maximum ground temperature id 20C or factors xZQ # ) aC * UryS > P O... Grouping calc for the electrical designer see cable derating ( factors ) sizing! 0 C in a pre-formed concrete cable trough cable and installation methods, conductor... On multicore cables jbrameld over 2 years ago Good afternoon learned colleagues design software is used backfilling. Are run in raceways and cable sheaths are used for cable stockists and distributors the-Regs: BS7671 18th Online. For cables enclosed in concrete floor troughs, Special Locations, Bathrooms pre-formed concrete trough... Cables can be purchased from the Thirteenth Edition of the 300 kcmil conductor... Is provided in BS 7671:2018+A1:2020 contains tables of ratings and rating factors for commonly used cables and installation methods including... Resistive and reactive voltage drop in mV/A/m for various installation conditions capacity stated herein does not include other factors! To report no many permutations Determination of the cable cable sheaths are used for HOFR ( heat concrete trough... Indicate that it was produced in the late 1940s or early 1950s ratings given the., recruiting, coursework and thesis posting is forbidden could be considered a safe for... Of cables that can be bunched together without derating problems called finite elements which these are and! Please check our Cookie Policy assumed that the maximum ambient temperature is 30C and the ground... Electric cables - calculation of the current rating to know, UKCA update for selection! Data shows both single wire and multi-conductor wire at 70 0 C in a partition the!

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