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:

Cable Sizing

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

由於Circuit會有Harmonics, 所以參照翻EMSD BEC 2021, 7.6.2.2, Table 7.6.2, 我地既Harmonic Current最大可以去到以下Percentage:

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 🙂