These values were based on the conditions identified in the supplement to Report No. 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. Bs7671 cable derating factors: CEF TechTalks are back - with a focus on renewables and regulations. 0000076394 00000 n I Accept Cookies The IEC 60502 standard is in two parts; part 1 for voltages up to 1 kV and part 2 for voltages from 1 kV to 30 kV. How much current a cable can carry is down to physics and is a highly technical subject in its own right. 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. 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. BS 7671:2018+A1:2020 contains tables of ratings and rating factors for commonly used cables and installation methods. Appendix 4 (BS 7671:2018) gives guidance determining the current capacity and voltage drop for cables. 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. There are nine parts to the ERA 69-30 report series, covering different types of cables and installation methods. 0000008621 00000 n These factors are applied to the cable-free air rating identified in the relevant table in Appendix 4. UTg-3[f;"v,u|==:vao~|?m{?=?;9#<=}?^7||o}=oOOKo~4o04qzt:o>oO{? 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. Table & Column 5. The user of ERA is particularly popular amongst DNO (district network operators) and supply authorities. Correction Factor 4. Where We use necessary cookies to make our site work. %%EOF Unless specific details are known, the electrical designer should use the tabulated values provided in BS 7671:2018+A1:2020. 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. 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 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. 0000071888 00000 n The assumed values for each publication are identified in Table 1 below, the main influencing factor is the soil thermal resistivity. Cable sizing compiles with: - BS 7671, IEE Wiring Regulations Cable size is selected as follows: 1. This method subdivides the problem into smaller problems called finite elements which these are solved and put back into the original problem. The data shows both single wire and multi-conductor wire at 70 0 C in a vacuum (no air flow). Three criteria are given for cable selection: (a) Current-carrying capacity (b) Voltage drop (c) Short-circuit temperature rise This standard provides sustained current-carrying capacities and voltage drop values for those types of electrical cable and installation practices in common use in Australia. 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} . The thermal resistivity of soil refers to its ability to dissipate heat, which is relevant to the current-carrying capacity of cables. Determine the application. Cable Current Rating 6. In some cases, it may be necessary to perform detailed heat transfer calculations to determine a cables current-carrying capacity. Following on from my previous post about the use of 'YY' type cables, our esteemed client has stated that BS7671 does not have grouping factors for multicore cables. Knowing this we can select the size of the protective device (In) and in this instance we are going to use a 32A circuit breaker. A derating factor is introduced that is defined in terms of the ampacity of power cables in open-top trays. Finite element analysis is a numerical method used to solve complex engineering problems, usually carried out on simulation software. 0000004990 00000 n For higher temperatures, a derating will have to be applied according to NEC. Registration on or use of this site constitutes acceptance of our Privacy Policy. Dose bicc comply? CEC Designer Veli has nearly two decades of experience in the renewable industry. P1{:f["ypc &@8o:a!`r At this point, I would usually provide some examples of calculations; however, they are far too complicated for this type of article. 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). HD 60364-5-52 Selection and erection of electrical equipment. 0000005657 00000 n 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. The ERA was commissioned to produce a series of reports on the current-carrying capacities of cables. The generally accepted depth for cables direct buried and cables buried in duct is 600mm to 650mm. apply some engineering judgement). <> 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. obtained from the cable manufacturer. I ended up having to run some lighting circuits in 2.5mm to negate them from grouping calc for the RFC circuits. 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. For more details on BS 7671 voltage drop calculations, see. Should that not be treated as two circuits? A software algorithm has been developed which can determine the derating factors for multiple cable circuit crossings or heat sources. % So the design current (Ib) would be 5000/230 = 21.73 Amps. 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. Use the linkon the left to access our cable sizing application. Some electrical design software packages specifically refer to BICC cable data but other manufacturers data also exists, other software packages refer to ERA 69-30. 0000078301 00000 n The ERA 69-30 series are still referred to in BS 7671:2018+A1:2020 today. Where a cable is buried in the ground its ability to dissipate heat is reduced, the extent of which depends on the installation method. 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 Myth Busters #7 - Out with the old, in with the new. <<95D84A24FD5A1F4C8152C1D91F51EC80>]>> Failing that, speaking to the manufacturers of the cable is always a good place to start. 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. 0000002616 00000 n ~~~/?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 The calculation of cable rating follow t he derating factor method, see Cable Derating (Factors) . 2023 the-Regs : BS7671 18th Edition Online Training, Special Locations, Bathrooms. Examples 1 and 2. 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 xNf(@-c]D$t7:z)Q 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. 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. Note: The minimum current carrying capacity stated herein does not include other derating factors such as grouping, temperature, etc. 0000011561 00000 n 148 46 These are related to the line voltage for three phase circuits and phase voltage for single phase circuits. It is always best to consult cable manufacturers for advice when carrying out any non-standard cable calculations. 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. 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. 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. Consider a 20/3.3 kV, 12.5 MVA transformer to be fed by direct buried, 3 core XLPE, SWA, PVC, copper conductor cable. The calculation of cable rating follow t he derating factor method, see Cable Derating (Factors) . 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. 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. 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 . 0000002193 00000 n The correction factor is justified as the current carrying capacity is capable of limitation by additional external factors, e.g. 0000003576 00000 n Impedance and voltage drop to IEC 60909 and CENELEC CLC/TR 50480. Total derating factor = 0.89 * 1.05 * 1.0 = 0.93. 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. 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.