--- title: "Tower dimensions for 220 kV single circuit line" subject: "Transmission and Distribution System Design" chapter: 4 key: "semester_8/transmission_and_distribution_system_design:tower-dimensions-220kv" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tower-dimensions-220kv site: BCT Engineering --- # Tower dimensions for 220 kV single circuit line Transmission and Distribution System Design · Chapter 4: Conductor and Support Selection - Asked in: 2079 Chaitra, 8 marks - Video: [LECTURE-8: 220KV TOWER (SIGLE AND DOUBLE LINE)](https://www.youtube.com/watch?v=2cWhpfT-bWc) — CBST TECHNOLOGY - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tower-dimensions-220kv - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-4 ## Question For a 220 kV single circuit transmission line, suggest the dimensions for following: (i) Cross arm length (ii) Insulator string length (iii) Horizontal and vertical separation of conductors (iv) Number of earth wires (v) Height of earth wire from the top most conductor. (Take maximum swing of insulator as: 45°) ## Short answer Using a 2.54 m air clearance and 2.0 m insulator string (11 discs): cross-arm length 4.0 m; vertical conductor separation4.6 m; horizontal separation9.5 m; one earth wire; earth wire height above top conductor8.3 m. ## Answer The tower dimensions follow from the minimum air clearance, the insulator string length and the 45° swing. They are taken from the given appendix rules and common IS practice. **Data assumed:** maximum system voltage $V_m = 245$ kV. Tower width at cross-arm level is 1.5 m. Discs are 254 × 154 mm. #### Minimum air clearance $a$ Using 6.5 inch per 10 kV of maximum phase rms voltage plus 8 inch: $$ \begin{aligned} V_{ph} &= \frac{245}{\sqrt{3}} = 141.45\ \text{kV} \\ a &= 6.5 \times 14.145 + 8 = 99.94\ \text{in} = 2.54\ \text{m} \end{aligned} $$ As a check, 1 cm per kV of phase peak voltage plus 30 cm gives $200 + 30 = 230$ cm. The larger value, $a = 2.54$ m, is used. #### (ii) Insulator string length For 220 kV, 10 discs meet the wet (395 kV) and impulse (900 kV) withstand levels. One spare disc is added, giving **11 discs**: $$ l = 11 \times 0.154 + 0.30\ (\text{hardware}) \approx 2.0\ \text{m} $$ #### (i) Cross-arm length When the string swings by $\theta = 45°$, the conductor must stay at least $a$ from the tower: $$ \begin{aligned} L_{ca} &= a + l\sin\theta \\ &= 2.54 + 2.0 \times 0.707 = 3.95\ \text{m} \end{aligned} $$ Take **4.0 m** from the tower face. #### (iii) Conductor separation - **Vertical** (between cross-arms): the conductor hanging from the upper arm must keep clearance $a$ from the arm below. $$ V = l + a = 2.0 + 2.54 = 4.54\ \text{m} \approx 4.6\ \text{m} $$ - **Horizontal** (conductors on opposite sides of the tower): $$ H = 2L_{ca} + w = 2 \times 4.0 + 1.5 = 9.5\ \text{m} $$ This is well above the phase-to-phase clearance (about $1.15a \approx 2.9$ m). #### (iv) Number of earth wires A 220 kV single-circuit line in a vertical or delta configuration is narrow enough for **one earth wire** at the peak to give a 30° shielding angle. #### (v) Height of earth wire above the top conductor For a shielding angle $\alpha = 30°$, the horizontal distance from the tower centre to the outermost conductor is $4.0 + 0.75 = 4.75$ m. So: $$ y = \frac{4.75}{\tan 30°} = 8.23\ \text{m} \approx 8.3\ \text{m} $$ | Item | Value | |---|---| | Air clearance | 2.54 m | | String length | 2.0 m (11 discs) | | Cross-arm length | 4.0 m | | Vertical spacing | 4.6 m | | Horizontal spacing | 9.5 m | | Earth wires | 1 | | Earth wire above top conductor | 8.3 m | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The cross-arm length is found from: A. a + l sin θ (air clearance plus string swing) B. a + l cos θ C. l only D. a only Answer: A. At maximum swing the conductor must still keep clearance a from the tower. (Key point: Cross-arm length) 2. The vertical separation between conductors is: A. a alone B. l + a (string length plus air clearance) C. 2l D. l - a Answer: B. V = 2.0 + 2.54 = 4.54 m, about 4.6 m. (Key point: Vertical spacing) 3. The height of the earth wire above the top conductor is found from: A. The shielding angle (30°) and horizontal distance to outermost conductor: y = d/tan 30° B. The string length only C. The number of discs D. The tower width only Answer: A. y = 4.75/tan 30° = 8.23 m. (Key point: Earth wire height) 4. How many discs are used for 220 kV in this answer? A. 9 B. 10 C. 11 (10 plus one spare) D. 14 Answer: C. 10 discs meet withstand levels, plus one spare. (Key point: String length) ## Close variants set in other exams - [Tower dimensions for 220 kV double circuit line](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atower-dimensions-220kv-double-circuit) — Transmission and Distribution System Design, chapter 4, asked 2× ## Related in Transmission and Distribution System Design - [Air clearances for 220 kV double circuit line](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aair-clearance-220kv-double-circuit) — Transmission and Distribution System Design, chapter 4 - [Air clearances, 220 kV line for 200 MW](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aair-clearances-220kv-200mw-120km) — Transmission and Distribution System Design, chapter 4 - [Number of insulator discs for 220 kV line](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Ainsulator-discs-220kv-appendix) — Transmission and Distribution System Design, chapter 3 - [400 kV lines: configuration, V-string, earth wires](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Ajustify-400kv-config-vstring-earthwire) — Transmission and Distribution System Design, chapter 4 - [Air clearances for 132 kV double circuit line](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aair-clearance-132kv-double-circuit) — Transmission and Distribution System Design, chapter 4 - [Ratio of vertical separation, 132 vs 220 kV](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Avertical-separation-ratio-132-220kv) — Transmission and Distribution System Design, chapter 4 ## Related in other subjects - [Tower top potential and insulator string voltage](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Atower-top-potential-insulator-voltage) — High Voltage Engineering, chapter 3, asked 2× - [True/false: cross-arm length and string efficiency](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Atf-cross-arm-string-efficiency) — High Voltage Engineering, chapter 5