Distance Between Power Line Poles: What Determines the Span Length

Why there is no fixed span table

The Rules for the Arrangement of Electrical Installations (PUE) in the 2026 edition (the part on overhead power lines above 1 kV) do not set the span length as a fixed number by voltage class. Instead, PUE sets a calculation method: conductor sag, wind and ice loads, reliability factors depending on pole height, terrain type and the design recurrence period of climatic effects. The 2017 edition of PUE (order of 21.07.2017 No. 476) has been replaced by the 2026 edition (PUE:2026). The values given here were checked against its text, and the final values for a specific line are taken by the designer from the current text.

This means that even for the same voltage, the span can differ significantly depending on the conductor grade and cross-section, the wind region, the pole type and the route’s terrain.

Length of a tension section: what the PUE limits

PUE gives no fixed span length, but it does limit the length of a tension section, that is, the distance between tension poles (cl. 2.5.135). Where the tension poles go is decided by the conditions of building and operating the line and by the working conditions of the pole structure itself. The upper limits depend on voltage, on how the conductors are fixed and on the terrain:

Values from PUE (2026 edition), cl. 2.5.135. These are upper limits for a design, not typical distances. In mountainous or heavily broken terrain in ice regions 3 and above, tension poles are recommended at passes and at other points that rise sharply above the surrounding ground.

What determines the span length in practice

• conductor grade and cross-section — determine the allowable sag and mechanical strength;
• the route’s wind and ice region — the harsher the climatic loads, the more conservative the span margin;
• pole type — suspension, tension or angle poles carry loads differently (see the article Power Line Pole Types by Purpose);
• vertical clearance requirements — the height of conductors above the ground and distances to buildings and crossings with other objects (see the article Power Line Protection Zone);
• the route’s terrain and obstacles: rivers, roads, other lines.

Distance between conductors in a span: the PUE formula

Span length enters the PUE rules through the sag: the longer the span, the larger the sag, and with it the distance between conductors that their operation in the span requires (insulation distances and lightning-protection conditions are checked separately, cl. 2.5.96). For lines with supporting insulator strings (other than lines with covered conductors) and a horizontal arrangement of conductors, the minimum distance between conductors in the span is calculated by these formulas (cl. 2.5.97):

• up to 330 kV: d = 1.0 + U/110 + 0.6·√f;
• 500 and 750 kV: d = 1.0 + U/150 + 0.6·√f + 2r.

Here d is the horizontal distance between undeflected conductors, m; U is the line voltage, kV; f is the greatest sag of the conductor at the highest temperature or under ice without wind, taken for the clearance span, m; r is the radius of the conductor bundle in the phase, m.

The distances are taken for the sag of the clearance span (defined below, in the section on span types). If some individual spans that result from pole placement (no more than 10 % of all spans) are longer than the clearance span by no more than 25 %, the distance found for the clearance span need not be increased. With a greater excess the distances are checked separately by the formulas (cl. 2.5.96).

A conditional example (the sags are chosen arbitrarily). A 110 kV line with a sag f = 6 m: d = 1.0 + 110/110 + 0.6·√6 = 1.0 + 1.0 + 1.47 ≈ 3.47 m. A 35 kV line with a sag f = 4 m: d = 1.0 + 35/110 + 0.6·√4 = 1.0 + 0.32 + 1.2 ≈ 2.52 m.

On the same lines, with a mixed or vertical arrangement of conductors, the rules differ (cl. 2.5.98). On intermediate poles with sags up to 16 m the distance is taken from Table 2.5.22 (moderate galloping of conductors) or Table 2.5.23 (intense galloping); in ice regions 3–6 a check by formula (2.5.24) is added to Table 2.5.22, and the greater of the two values is used. On intermediate poles with sags over 16 m the distance is calculated by formula (2.5.24). On tension poles of 35–750 kV it is calculated by formulas (2.5.22) and (2.5.23), and the minimum horizontal offset of conductors in adjacent tiers should, as a rule, be no less than the values in Table 2.5.24.

Voltage class and the minimum conductor height above ground

Voltage class does not set the span length by itself, but it affects it through clearance: as the voltage rises, the minimum height of the conductor above the ground at the lowest point of the span, as a rule, increases (from 7 to 15.5 m in the table below). This is a clearance, not a ready-made span length: the longer the span, the greater the sag and the higher the conductor has to be hung. Pole heights are covered in Transmission Pole Height by Voltage Class; distances to buildings and the width of the protection zone are in Power Line Protection Zone.

For populated areas, the minimum vertical distance between a conductor and the ground surface for normal operation of the line, measured when the conductor sags the most, is set by PUE (2026 edition), Table 2.5.33. Values by voltage class:

Values from PUE (2026 edition). For unpopulated and hard-to-reach terrain, PUE allows smaller values (Table 2.5.30), so these figures do not apply to the whole route. There is no 750 kV row: 750 kV lines may not be routed through populated areas (cl. 2.5.169).

What else PUE requires in populated areas

PUE (2026 edition), cl. 2.5.169, requires lines of 35–500 kV, as a rule, to be routed outside the residential parts of settlements. Where a line passes over streets and access roads, a conductor whose aluminium part has a cross-section smaller than 185 mm² is also checked for the case of a conductor breaking in the adjacent span: at the mean annual temperature, without allowing for current heating of the conductor, the distance to the ground must then be at least 5.5 m (for lines up to 220 kV; cl. 2.5.172, Table 2.5.33). For lines in a specially designated city corridor, and for conductors of 185 mm² and more, this check is not required.

Types of span: clearance, weight, wind

Calculations use several distinct span concepts, and they should not be confused:

• clearance span — a span whose length is set by the required clearance of the conductor above the ground, assuming the poles stand on level ground;
• weight span — a notional length used to work out the vertical load on a pole from the weight of the conductor and ice (on level ground, half the sum of the two adjacent span lengths; with height differences between poles it is set by the positions of the lowest points of the conductor’s sag in the adjacent spans);
• wind span — a notional length for the pole’s horizontal wind-load calculation (usually half the sum of the adjacent span lengths, so on sloping ground or with height differences it can differ from the weight span).

A pole is designed for these calculated values, not simply for the visible distance to the next pole.

How span length affects the pole

For the same horizontal tension and the same load per metre of conductor, the sag grows roughly in proportion to the square of the span: a span twice as long gives about four times the sag. So long spans need higher suspension points, and as the wind and weight spans grow, so do the loads on the pole and foundation (for loads on overhead lines in general, see Loads on Overhead Lines and Transmission Poles).

A shorter span relieves each pole but adds to their number, so the span is chosen together with the pole type. Pole types are compared in Power Line Poles: Multi-sided, Lattice or Reinforced Concrete.

What data we need to select poles for your project

The span length is set by the line design. From the customer’s input data — voltage class, conductor grade and cross-section, number of circuits, wind and ice region, route type — we design the poles: we prepare KM/KMD drawings and carry out verification calculations. Send us this data or fill in the technical specification form and we will select the pole design for your project.