--- title: "Tower dimensions for 220 kV double circuit line" subject: "Transmission and Distribution System Design" chapter: 4 key: "semester_8/transmission_and_distribution_system_design:tower-dimensions-220kv-double-circuit" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tower-dimensions-220kv-double-circuit site: BCT Engineering --- # Tower dimensions for 220 kV double circuit line Transmission and Distribution System Design · Chapter 4: Conductor and Support Selection - Asked in: 2070 Bhadra, 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-double-circuit - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-4 ## Question For a 220 kV double 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 max. swing of insulator as: 45°) ## Short answer Cross arm length: 4.20 m; insulator string length: 2.34 m (14 discs); vertical separation: 4.88 m; horizontal separation: 9.89 m; one earth wire; earth wire height above top conductor: 8.57 m. ## Answer The tower dimensions follow from the minimum air clearance, the insulator string length, the 45° swing of the string, and the shielding requirement for lightning. **Data and assumptions:** maximum system voltage 245 kV (phase rms 141.45 kV); 254 × 146 mm disc insulators; maximum swing of the suspension string 45°; minimum air clearance 6.5 inch per 10 kV (phase rms) plus 8 inch safety, as given with IOE papers; tower body width at cross-arm level about 1.5 m. #### Minimum air clearance (a) $$ a = 6.5\times\frac{141.45}{10} + 8 = 99.94\ \text{inch} = 2.54\ \text{m} $$ #### Insulator string length (l) - Number of discs: the standard 220 kV suspension string has **14 discs**. - Check against the withstand and flashover tables: the 1-min wet withstand of 395 kV and the impulse withstand of 900 kV each need 10 discs. Extra discs are added for switching surges, pollution and one defective disc, giving 13–14. - Length: $$ l = 14\times0.146 + 0.3\ (\text{hardware}) = 2.344 \approx 2.34\ \text{m} $$ #### Cross arm length (X) The swung string must keep the conductor at least $a$ from the tower body: $$ X = a + l\sin\theta = 2.54 + 2.344\sin45^\circ = 4.20\ \text{m} $$ measured from the tower face to the point of suspension. #### Vertical separation (Y) The conductor hanging under one cross arm must clear the cross arm below by $a$: $$ Y = l + a = 2.344 + 2.539 = 4.88\ \text{m} $$ #### Horizontal separation In a double-circuit tower, each circuit is arranged vertically on one side of the tower. The horizontal distance between the conductors of the two circuits at the same level is: $$ S_h = 2X + w = 2(4.196) + 1.5 = 9.89\ \text{m} $$ #### Height of earth wire above the top conductor Use one earth wire at the tower peak with a shielding angle of at most 30° to the outer top conductors. The horizontal offset is $X + w/2 = 4.946$ m, so $$ h_e = \frac{X + w/2}{\tan30^\circ} = \frac{4.946}{0.577} = 8.57\ \text{m} $$ This is about 6.2 m above the top cross arm, since the conductor hangs 2.34 m below that cross arm. ```text E earth wire /|\ 30 deg shielding / | \ *------o--+--o------* top arm, conductors | | hang l below arm *------o-----o------* Y = 4.88 m | | *------o-----o------* X=4.2m w X=4.2m ``` #### Number of earth wires **One earth wire** is used. Lines up to 220 kV, which have a modest cross-arm spread, are adequately shielded by a single peak earth wire within a 30° shielding angle. Two earth wires are normally needed only for 400 kV and above, where horizontal configurations are wide. In very high-lightning hilly areas, two earth wires may also be used for a 220 kV double-circuit line to reduce the shielding angle. | Item | Value | |---|---| | Cross arm length | 4.20 m | | Insulator string length | 2.34 m (14 discs) | | Vertical separation | 4.88 m | | Horizontal separation | 9.89 m | | Number of earth wires | 1 | | Earth wire above top conductor | 8.57 m | **Answer: X ≈ 4.2 m, l ≈ 2.34 m, Y ≈ 4.9 m, horizontal spacing ≈ 9.9 m, one earth wire about 8.6 m above the top conductor.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is a computed? A. 6.5 inch per 10 kV of line voltage B. 6.5 inch per 10 kV of phase rms voltage plus 8 inch C. 8 inch per kV D. 6.5 inch fixed Answer: B. a = 99.94 inch = 2.54 m. (Key point: Air clearance) 2. What is the cross arm length? A. X = a + l = 4.88 m B. X = l sin45 = 1.66 m C. X = a + l sin45 = 4.20 m D. X = 2a = 5.08 m Answer: C. String swings out by l sin45. (Key point: Cross arm) 3. How are vertical and horizontal separations found? A. Y = 2l; horizontal = X + w B. Y = a; horizontal = 2X C. Y = l + a; horizontal = X + w = 5.7 m D. Y = l + a = 4.88 m; horizontal = 2X + w = 9.89 m Answer: D. Horizontal spacing spans both cross arms plus the tower width. (Key point: Separations) 4. What earth wire arrangement is chosen and why? A. One peak earth wire within 30 deg shielding angle; two only for 400 kV and above or high lightning areas B. Two earth wires always C. No earth wire for 220 kV D. One earth wire at 60 deg shielding Answer: A. he = 4.946/tan30 = 8.57 m. (Key point: Earth wire) ## Close variants set in other exams - [Tower dimensions for 220 kV single circuit line](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atower-dimensions-220kv) — 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 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 - [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 - [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 - [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 - [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