Where I think that this concept fails is in ring circuits. *Eng-Tips's functionality depends on members receiving e-mail. There are nine parts to the ERA 69-30 report series, covering different types of cables and installation methods. 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. 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. Examples 1 and 2. So if derating to 0.88 is required for number of cables, and derating to 0.93 is required for an upper tray, then the final figure is 0.88 multiplied by 0.93. The derating factor accounts for the added thermal resistance present when a. Simultaneous Overload - can the other cables on the cable tray that are alongside your cable be subject to simultaneous overload?? Case 1 Assuming a design current carrying capacity of circuit to 25A with a third harmonic of 32%. 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). British Cables Company introduces new cables to its range. ;;u(1*z/}3oG'0 ^Am;Oy4?~ Et'sQh)sF&)O a%q%Gp4wjFr~sr`>.|Kqqi|=/g,`{~ The assumed values for each publication are identified in Table 1 below, the main influencing factor is the soil thermal resistivity. CJX7b.zd@c41DV3+l[r~SKT? Equally loaded just means that that's what they've assumed when calculating the numbers in that table (i.e. The generally accepted depth for cables direct buried and cables buried in duct is 600mm to 650mm. Transformer full load current is calculated by: Ifull-load = 12.5 106 / 1.73 20 103 = 361 A. Don't forget derating factors. BS 7671. !paX}xZQ#)aC*UryS>P&O )X]B. Resistive component of voltage drop factor mV r - (mV / A / m) r Reactive component of voltage drop factor mVx - (mV / A / m) x Login. Table 4: Cable surrounded by thermal insulation length in insulation mm derating factor 50 0.89 100 0.81 200 0.68 Found inside - Page 258The correct way to derate for socket - outlet ring circuits when more than two . There are consultancies that specialize in this type of work. Cable Current Rating 6. ]M&r;:QKGBdBeW2JFX)@wN#$RA)0S3)i`nmh 0000007209 00000 n k2-3,Kw#`nu>+LKYgT 0 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. BS 7671 is the UK requirements for electrical installations standard, also known as the Wiring Regulations. The rating for the upper trays is slightly reduced because cables thereon are subjected to warmed air that rises from the lower tray(s). SOMAS_Si-101EN. Irnd`u3=](tdd= L8z[Wlm`= 0000004805 00000 n The overall accuracy of rating lilctors is within 5%. 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! apply some engineering judgement). 2023 Filipino Engineer. Please read our. Unless specific values can be obtained, using the values direct from BS 7671 could be considered a safe option for the electrical designer. However, this is unlikely to be a cost-effective option for smaller CSA cables when compared with using a more conservative cable size. 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. 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. Cloud based - any device, anywhere. The rating factors for cables enclosed in concrete floor troughs were first introduced in Table 30 of the 13thEdition of the IEE Wiring Regulations, which was published in 1955. The nearest size of standard trunking with a capacity factor greater than 1239.6 is 75 x 50mm? 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. BS 7671 is the UK requirements for electrical installations standard, also known as the Wiring Regulations. He is passionate about providing people with valuable education and is highly regarded throughout the industry as an educator and operator. Thanks. Cable Derating (Factors) Cable Sizing Input Data Checklist. 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 Note: The minimum current carrying capacity stated herein does not include other derating factors such as grouping, temperature, etc. ambient temperatures > 30C. 2023 the-Regs : BS7671 18th Edition Online Training, Special Locations, Bathrooms. every cable is carry the same and as much as it can without anything overheating). 5 0 obj 0000003576 00000 n Z 0000011053 00000 n Rosa Ponce. <<95D84A24FD5A1F4C8152C1D91F51EC80>]>> If this is the case, seek advice from someone with experience of carrying out this type of calculation. Fictitious Dimensions. 3H])Uw.?lJ `Vqh2SM 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. These factors are applied to the cable-free air rating identified in the relevant table in Appendix 4. 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. 2023 The Institution of Engineering and Technology. %PDF-1.4 We'd also like to set optional analytics cookies to help us improve it. Selecting Copper, XLPE, 3*50+25 mm 2 , its current is 185 A, Derating this current 185 * 0.93 = 172 A. . We're about to take you to the IET registration website. I ended up having to run some lighting circuits in 2.5mm to negate them from grouping calc for the RFC circuits. 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? 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. 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. k = factor which is 143 for Copper cable (from IEC 60364-4-43). The presence of harmonic current in a power system increases the current in the neutral conductor due to phase current imbalance (refer to Fig. These are known as rating factors. So the design current (Ib) would be 5000/230 = 21.73 Amps. Appendix 4 (BS 7671:2018) gives guidance determining the current capacity and voltage drop for cables. Reference method D of BS 7671:2018+A1:2020 should be applied for cables buried in the ground in and around buildings. 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. 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). 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. 0000007499 00000 n . 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). This method subdivides the problem into smaller problems called finite elements which these are solved and put back into the original problem. Register; Login. ~~~/?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 All relevant derating factors are applied. 0000076394 00000 n wire cross section and unequal current distribution. The correction factor is justified as the current carrying capacity is capable of limitation by additional external factors, e.g. 0000009088 00000 n 0000002193 00000 n Hzd}-/ *@. . 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 calculation of cable rating follow t he derating factor method, see Cable Derating (Factors) . Table 4C6 of BS 7671:2018+A1:2020 provides rating factors for cables enclosed in concrete floor troughs. 0000078513 00000 n 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. The derating factors in Table 52.2 are appropriate to conductor sizes up to 10mm 2 . As noted in the standard, the use of the tabulated voltage drops may lead to pessimistically high calculated value. 193 0 obj <>stream 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} . IEC 60502. Even then couldn't group more than 4 power circuits. Use the linkon the left to access our cable sizing application. }fq>w)6o @yawz8?%"VsKON]*~zX/*`qb};q7N NLt?k;~?|qo?zD6r!u5c9.F)Zz bZ\OZO>CRW !ol4]NO'n] &hT 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. 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