好耐之前記得自己講過點做Cable Sizing, 如果未知Cable Sizing既Concept大家可以去翻以下呢個Post到睇翻:
今次同大家分享一個Worked Example, 呢個Worked Example係會包括點Size Protective Device, 點計Harmonic, 再去Size一個合適既Cable尺寸, 等大家可以睇下實際係點做. 問題如下:
A 380V three-phase sub-main circuit supplying for a 150kW remote distribution
board (DB) is to be designed. The sub-main power cable is protected by a MCCB
and is to be routed from the MCCB to the DB via concrete cable pipe duct. There
are various non-linear loads to be supplied via the DB such as fluorescent lighting
and computer equipment. Local capacitor compensation bank will be installed to
maintain power factor to be not less than 0.85 for the circuit itself. Appropriate
harmonic filter will be installed at the DB to comply with the Energy Code. The
cable will be mounted on a perforated cable tray together with three other similar
cables. The steel wire armour of the cable will be used as circuit protective
conductor. The length of power cable is 50m and the maximum temperature of the
installation environment is 40°C. Given that the maximum short circuit current at
the MCCB is 18 kA and the disconnection time of overcurrent protection of the
MCCB at 18kA is 0.5s, select suitable type of power cable and calculate the
required cable size to comply with statutory and Energy Code requirements.
以下係答案:
Step 1: Calculate Design Current
按照呢個Table, 我地可以重新計算新既Design Current:
因應呢個Current, 我地會選擇用300A既MCCB去做呢條Cable既Protective Device.
Step 2: Select Appropriate Cable Size
係未計之前我地首先睇下問題比左咩環境因素我地先:
- Ambient Temperature – 40°C
- Grouping Factor – Group with three other similar cables
- Cable will be mounted on perforated cable tray
因應呢堆環境因素, 我地可以計到個Correction Factor, 從而計算到Cable需要負載既Current Carrying Capacity (CCC). 以下係Corrector Factor既Formula:
首先睇Ca, 因爲Ambient Temperature係40度, 而根據EMSD CoP 2020 Table A6(8)裏面關於Amoured Cable 既 Operating Temperature 係90度, 可以由Table A5(1)得出Ambient Temperature Factor Ca = 0.91.
再睇Cg, 題目寫左會條Cable同時同另外3條Cable走埋一齊, 姐係總共有4條Cable. 由於我地知道呢條係Multicore Cable (因爲三相), 而又係Cable Tray上面走, 所以根據EMSD CoP 2020 Table A5(3) Item 3, 4條Cable一齊走既Grouping Factor會係0.77.
再睇Ci, 以前因爲有Overload用Fuse, 會影響到條Cable (問過前輩, 好似因爲Fuse會過熱, 原因有待證實), 所以會有Correction Factor. 但如果係Fuse以外, 根據EMSD CoP 2020 Table A5(5)一律都係1.
最後, 如果條Cable本身係裝係隔熱既牆身, 就要根據EMSD CoP 2020 Table A5(4)比翻相應既Derating Factor佢, 但由於今次個Case條Cable係裝係Cable Duct入面, 所以Cd = 1.
得出所有Factor後可以計到Correction Factor = 1 / (0.91 x 0.77 x 1 x 1) = 1.43
然後我地就將呢個數乘翻Protective Device既Rating, 姐係300A, 就可以計到條Cable需要既CCC係 = 300 x 1.43 = 428A.
再睇埋Reference Method, 由於Cable係Mount係Cable Tray, 所以應該係跟EMSD CoP 2020 Appendix 7裏面既Item 31個Reference Method, 所以Reference Method應該係 “E or F”.
跟住我地就可以去Table A6(8)裏面, 用Reference Method E揾一條CCC大過428A既Cable, 結果係 185mm2 4/C XLPE/SWA/PVC cable, 因爲根據個Table呢條Cable個CCC係463A.
Step 3: Validate Voltage Drop
當揀好Cable Size之後就要計該條Cable既Voltage Drop 同 Copper Loss. 我地係條問題到得知條Cable會行50m, 首先計左Volt Drop先, 我地首先要用Table A6(8)揾翻條Cable既Z Value. 由個Table可見, 185mm2 4/C XLPE/SWA/PVC cable既Z value係0.26 mv/A/m.
用翻我地之前個Post既Formula可以計到:
由於呢段係計緊Submain Cable, EMSD要求係成個Installation唔大過4%, 所以係Design上面通常要定左個Assumption先(例如Feeder Volt Drop 0.5%, Submain Volt Drop 2.5%, 跟住Final Circuit Volt Drop 1%), 所以都要靠個Designer定好左每段Cable個Max. Volt Drop係幾多先可以Check到. 呢個例子我地當個Max. Volt Drop係2.5%先, 所以0.93%係OK既.
Step 4: Validate Copper Loss
之後我地再計埋Copper Loss. 我地首先要揾Cable既Conductor Resistance, 由於我自己查過BS7671:2018已經冇左個Conductor Resistance表, 我地呢個Example就會用返舊時EMSD – Guideline on Energy Efficiency of Electricity of Electrical Installation, 2007先:
由呢個表可以睇到, 185mm2 4/C XLPE/SWA/PVC cable既conductor resistance係0.13 mOhm/m
跟住就要計埋Conductor Operating Temperature, 根據之前個Post既Formula:
跟手計埋個Ratio:
之後就可以計到個Copper Loss in kW:
最後, 計埋個Load既總Power:
只要將Copper Loss (kW)除翻Active Power就知道個Copper Loss Percentage:
Copper Loss(%) = 1.3/176.45 x 100% = 0.74%
根據EMSD BEC 2021, 細過100m既Cable既Copper Loss要求要係細過1.5%, 所以我地揀既185mm2 4/C XLPE/SWA/PVC cable係滿足到BEC既要求.
Conclusion:
根據我地計出黎既Result, 我地就可以總括呢條問題既答案就係185mm2 4/C XLPE/SWA/PVC cable. 當然, 問題其實仲有延申, 就係講緊MCCB既Short Circuit Current係18kA, 我之前個Post都有講, 有部分Consultant會繼續計埋條Cable頂唔頂到個Fault Current, 各位如果有興趣可以自己研究下點計, 如果計完想知答案既可以留Comment, 如果我睇到會答翻你有冇計錯. 但普遍Cable Sizing都只係做Volt Drop同Copper Loss.
今次就分享到呢度, 希望大家繼續多多支持同Share呢個Blog出去 🙂
Steven
可唔可以教下MCB排位同T&C🙏🏻
AC
可以, T&C你係想知General點做?定T&C for 某d equipment.
Stevenkwok
例如這些可否具體講講🙏🏻
1. 水線連續測試
2. 環形插蘇連續性測試
3. 絕緣電阻
4. 極性測試
5. 接地故障環路阻抗
6. 跳脫測試
樓主加油💪🏻你的分享很實用
AC
我比較少做T&C, 但會盡量分享~遲下睇下可唔可以寫多少少關於T&C野.
GC
請問可否講解下protective devices、 earthing、各類cables/busbar 用處、T&C and etc.
thanks for sharing. A highly recommended blog.
AC
可以, 遲少少我會寫個blogpost講下
Kn
Hi 關於voltage drop 我想問 feeder cable係咪指由transformer到normal/essential busbar既cable? 而submain係咪指由normal/essential busbar出去到各層mcb既cable? 咁既話final circuit通常係D咩?
AC
Feeder:牛去制櫃
Submain:制櫃上分層電錶房入面既MCCB/MCB
Final Circuit:由MCCB/MCB出既Cable, 去到Final Equipment. 常見既例子就係住宅樓分層電房去每個單位既配電箱個段Cable.
Jerry
師兄, copper loss當中, 我睇返上一章, 條公式係乘埋 power factor, 想請問下今次情況係咪唔需要 x 0.85? 同埋點解既? 如果乘左就 >1.5% 了
Jerry
Power transferred = root3 x 380 x loading in kVA x power factor /1000
AC
因爲題目比既150kW, 單位係kW.
kW = kVA x power factor, 所以已經乘左power factor. 如果題目係比kVA就需要再乘power factor.
idon'tknow
hi, AC 師兄, 咁點解計copper loss 既active power 係用 176.45kW，唔係用番15KW？
fl
係咪乘少左個power factor，VxA既unit應該係VA, not W
KK
我想請教一下, 如果同一條path有其他cirult但cable size同loading current都唔同, grouping factor要點選?
AC
一般都係睇同一條Path有幾多條Circuit去定. 當你有一條Path3組唔同Load既Circuit, Rating Factor都係Based on 3組Circuit去定.
KN
AC可唔可以講一下唔同cable type 既分別同幾時用? PVC 同XLPE 大概都清楚﹐但係就唔係幾知道single 同multicore ，同埋 armoured 同non- armoured 既分別….
AC
Sorry, 遲左reply你, single core主要用係single phase application(單相), multicore就會用係three-phase application(三相). 同埋通常Multicore用得係三相都係4 Core, 因爲傳統都係三相四線.
至於Armored同non-armoured分別, 通常main riser 或者戶外地底走既線都係用armoured, 平時走廊天花d trunking入面d細線或者其他分層EL ROOM出既線就會落non-armoured.
(Y_Y)
唔好意思, 想問下我見你計operating temperature個計法同BS 7671條式有d唔同,
BS7671條式係
Ct = (230 + tp – (Ca^2 * Cg^2 – (Ib^2/It^2)) (tp-30)) / 230 + tp
而家做緊房署相關project, 見佢地個excel都係跟呢條式計, 想問樓主個條式個出處? Thank
下面係房署個個Excel成個計法
1. t1 = tp – (Ca2*Cg2 – Ib2 / It2)*(tp – tr)
2. Rt / Rp = (230 + t1) / (230 + tp)
3. z = Ctcosq (tabulated mV/A/m)r + sinq (tabulated mV/A/m)x
4. VD = (z of each cable) * Ib of each cable * L
5. = VD / U
6. Active power (P = 1.73205 * UL * I1 * cosq for three phase) & (P= UL * I1 * cosq for single phase)
7. Pcopper = {(3 * Ib2 + IN2) * (Rt / Rp) * (r / 1.73205 / nos. of cable in parallel ) * L / 1000 for three phase} & {2 * Ib2 * (Rt / Rp) * (r / 2 / nos. of cable in parallel ) * L / 1000 for single phase}
8. = Pcopper / P
AC
Hello,
多謝你咁深入研究先, 其實我個計法都係取自EMSD 2007年個COP for Energy Efficiency 亦都係ASD Design Guide入面既Formula, 而呢條Formula都係取自BS7671:1992. 當然, BS7671已經進化到2018年所以我就唔去考究係咪update左啦.
我亦都用左你條Formula, 計出黎個Copper Loss都係差0.1 kW, Percentage都係差0.07%, 我相信應該問題不大. 不過既然BS7671 Update左係應該要跟BS7671:2018既, 但我公司既Template都係跟緊EMSD.
你可以Refer以下呢條Link:
https://www.emsd.gov.hk/filemanager/en/content_724/eleccop_2007.pdf
Karson fung
你好呀 因為我見emsd講400a or above既circuit先會apply maximum total harmoic distortion percentage既 所以我想問harmonic filter係呢度case其實係咪唔一定要裝？
(Y_Y)
Active Power個度係未有d問題?
Active Power = 1.732 * 380 * 150 /1000, 應該唔係150? 咁未變左 1.732 * V * P/1000?, 係未應該乘番design current?
Active Power = 1.732 * 380 * 268.1 /1000?
AC
多謝你提醒, 我的確打錯左, 我會改翻個答案.
WY
A = I√t / k
= 18,000√0.5 / 143
= 89 mm2
請問k有冇表查?
AC
Hello,
要跟Cable Supplier提供既表, 可以睇翻之前個Post個個表:
https://i1.wp.com/www.dinhaylo.com/wp-content/uploads/2021/05/image-16.png?w=1503&ssl=1
YY
do you have cable calculation for single core in trunking.
grouping factor seems too harsh for the calcuation.
AC
Hi,
Sorry for the late reply. For the single core cables that you have mentioned, you can still use EMSD COP’s Table A5(3) on the grouping factor, taking note that “If a group consists of n single-core cables it may either be considered as n/2 circuits of two loaded conductors or n/3 circuits of three loaded conductors.”, as stipulated in note 5 of the notes to table A5(3) by EMSD.
Karson
你好呀 因為我見emsd講400a or above既circuit先會apply maximum total harmoic distortion percentage既 所以我想問harmonic filter係呢度case其實係咪唔一定要裝？
AC
BEC係講大過400A既Circuit唔可以大過Table 7.6.2. 個個表係所有Cable都要Apply (Refer to 7.6.2.1), 同埋如果你望翻BEC 2021 7.4.6, EMSD講明如果計Copper Loss就要Cater埋Harmonic Distortion.
至於Harmonic Filter, BEC要求就係400A以上既Circuit要留Connection Point比將來用黎裝Harmonic Filter (你可以Day1做埋或者將來做都得). 呢個Case 細過400A所以連Connection Point都唔洗留. 詳情請睇翻BEC TG.
Jim
HI, 如果有1條100A feeder circuit 用2條 say, 16mm2 multi core run in the same cable tray, 係哩個Parallel conductor 情況下要唔要計grouping factor?
AC
Yes, 呢個情況唔洗Grouping Factor, 大過2條而又Touching就要開始Apply Grouping Factor
TW
參考 BS7671 Table 4C5, 如果1層cable tray 有3組以上既 single core trefoil circuit 行緊, 個CF 洗唔洗再fine tune? 另外cable tray 層與屠之間<300mm 又洗唔洗呢?
fl
點解conductor resistance 唔係用 Table A6(8) 既r value？
PM
Hi AC,
謝謝你嘅分享, 想請教一下你以上嘅計算有冇一啲rule of thumb 或software 之類可以做到? 或一定要續條慢慢計?
AC
冇, 最常見就係有個Excel Template續條入
sandals
請問AC師兄 2500A 3 phase armoured muti core XLPE cable點sizing?
跟步驟計到
2500x(0.92 x 0.72 x 0.51 x 1)=7564A….
又望下COP個表已經企左係度
最大先400 mm 4c 728A…….
但我查CLP既GuideToSupplyMetering_(v8) 講 2500A 上升總線係用 4 x 630mm
請問其實點計出黎?
AC
400mm 4c 728A 係一條線既CCC, 如果跟你計要7564A, 姐係main riser起碼要 11條400mm線先食到個電流 (11 x 728 = 8008). 寫法就會係 11 x 400mm2 4/C SWA/XLPE/PVC Cu. Cable.
至於CLP份Supply Guide, 留意翻佢係講緊每相起碼要4 x 630mm 單Core Cable. 姐係如果4 Core咁睇的話, 個線既數量應該係 16 x 630mm 1/C XLPE/PVC Cu. Cables.
sandals
多謝AC兄指教
我就係計到 每相11條400 mm^2 4/C
不過我係乘埋隔熱因數所以咁高A
依家隔熱因數=1 就成條數通哂
Joe
師兄你好！
有個位我比較混淆
Grouping factor個度cable tray 1條4/c xlpe swa pvc cu係等於一條載體？
呢個情況cg係用1？
還是4/c係當有4個載體要用0.77？
AC
1條4/C當1條Multicore Cable ->所以Cg = 1
Victor
想請問如果sub-main cable放超過1條線既話如何計算voltdrop%?
例子:
Design current: 300A,
cable: 3x120mmsq 4/C FR/XLPE/SWA/LSOH Cu.Cable,
Voltage drop per meter (mV/A/m): 0.370,
Estimate length: 90m
AC
我估你係講有3條120mm 4/C線, 咁你最後計個Correction Factor (Ct)要預翻個Grouping Factor, Grouping Factor 3條線預幾多就Refer EMSD CoP Table A5(3)
fl
點解 conductor resistance 唔係用table A6(8) 既 r value？
AC
因爲Impedance (Z) = R + jX
fl
why volt drop use table A6(8) then？ unit都係ohm/m。 我意思係如果計copper lost時 因為係計heat lost, 所以唔係淨係take R 既value就ok？