NESC (IEEE C2) · Overhead line clearances

How to read NESC clearance tables

Vertical clearance over ground and roads (232-1), clearance to buildings (234-1), and clearance between conductors (235-5) — plus where the NESC ends and the NEC begins.

Need the actual tables? The NESC is sold by IEEE Standards → — also check your serving utility’s construction standards.

NESC or NEC? Who governs which wire

Situation	Usually governed by	Why
Utility distribution and transmission lines, up to the service point	NESC	Installations under the exclusive control of the utility
Premises wiring on the customer side of the service point	NEC (NFPA 70)	NEC scope covers premises wiring
Customer-owned overhead distribution on a campus, plant, or solar/wind site	Depends on the state and AHJ — often NEC, sometimes NESC by adoption	Check the local adoption and the project spec
Communication lines of a telecom utility on joint-use poles	NESC	NESC covers communication utility lines too
Crane or equipment working near any overhead line	OSHA 29 CFR 1926 Subpart CC / 1910.269 / 1926 Subpart V	Worker-safety rules sit on top of design clearances

Our own summary, not the code text. The NESC (IEEE C2) is published by IEEE — consult the edition adopted by your state public utility commission or AHJ, and the serving utility's construction standards, which are often stricter. Not engineering advice.

Which clearance table answers which question

Question	Table	What it is organized by
How high must the line be above the ground, road, rail, water, or a field?	232-1	Rows = surface underneath; columns = type of wire/cable and voltage class
How far must the line stay from a building, sign, tank, bridge, or pool?	234-1 (plus companion tables for bridges and pools)	Rows = structure and horizontal vs vertical; columns = wire type and voltage
How far apart vertically must conductor levels be on the same support (e.g., primary above secondary above communication)?	235-5 (other Rule 235 tables cover horizontal spacing and span sag)	Rows = upper and lower conductor classes; columns = voltage of the conductors

How to read a NESC clearance table, step by step

Step	What you do	Watch out for
1	Identify the wire type: communication, neutral / insulated supply cable, open supply conductor	Effectively grounded neutrals often get the lower-voltage column
2	Identify the voltage — to ground, or phase-to-phase, as the table heading states	Tables switch between phase-to-ground and phase-to-phase; read the heading
3	Pick the row for the surface or structure (road, driveway, pedestrian-only, roof, wall, window)	"Subject to truck traffic" means vehicles taller than 8 ft are normally expected
4	Read the basic clearance, then read every footnote attached to that cell	Footnotes add or reduce clearance for spans, guys, and special cases
5	Apply the loading/sag condition the rule requires (max conductor temperature or ice loading)	Clearance is measured at worst-case sag, not on the day you string it
6	Add voltage adders for lines above the table's base voltage range	Higher-voltage lines add clearance per kV above the base

Worked example — 12.47 kV primary crossing a county road

Input / step	Result	Source
Wire type and voltage	Open supply conductor, 12.47 kV phase-to-phase (7.2 kV to ground)	Line design
Surface	Road subject to truck traffic	Table 232-1 row
Column	Open supply conductors, over 750 V to 22 kV to ground	Table 232-1 column
Minimum vertical clearance	18.5 ft at final sag, worst-case loading	Table 232-1
Field check	Measure mid-span at the lowest point; correct for the day's temperature vs the design max	Rule 232 sag conditions

Worked example value only. Confirm against the NESC edition your state enforces and the utility's standard — many utilities and DOTs require more.

Common field mistakes

Mistake	What to do instead
Applying NESC clearances to customer-owned premises wiring (or NEC to utility lines)	Establish the service point first; that sets which code governs
Measuring clearance at stringing temperature on a cool day	Clearance applies at the design sag condition — hot or iced conductors hang lower
Ignoring footnotes in Table 232-1	Footnotes change the value for driveways, pedestrian areas, and oversize-vehicle routes
Treating OSHA crane clearances and NESC clearances as the same thing	NESC = where the line is built; OSHA = how close workers and equipment may come
Forgetting that later construction (a new roof, sign, or grade change) can break an existing clearance	Check clearances when the structure or grade changes, not just when the line is built

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Built-in clearances for overhead lines

Where to get the tables. The National Electrical Safety Code is published by IEEE as ANSI/IEEE C2 and is available from IEEE Standards. This page explains how to use the clearance tables; it does not reproduce them. Your serving utility’s construction standards are the next place to look — they usually state the clearances they will accept.

NESC vs NEC. The NESC is the design and construction code for utility-controlled supply and communication lines, substations, and their workers. The NEC picks up at the service point and covers premises wiring. The gray zone is customer-owned overhead distribution on plants, campuses, and solar farms: some states and AHJs apply the NEC there, some adopt NESC clearances by reference, and many project specs call out NESC for overhead work regardless. Settle it before design.

Reading the tables. All three are organized the same way: find the kind of wire (communication, grounded neutral or insulated cable, open supply conductor), the voltage class, and the thing you are clearing — a road, a roof, a window, another conductor. Then read every footnote. Table 232-1 covers vertical clearance over surfaces; 234-1 covers buildings and other structures; 235-5 covers vertical spacing between conductor levels on the same support. Clearances apply at the worst-case sag the rules specify, which is why lines strung tight in winter can be out of compliance on a hot August afternoon.

Worked example. A 12.47 kV primary crosses a county road. The phase conductors are open supply conductors in the over-750 V to 22 kV class, and the road is subject to truck traffic, so Table 232-1 calls for 18.5 ft of vertical clearance at final sag. Measure mid-span at the lowest point and account for conductor temperature — then check the county or state DOT permit, which may require more.

Design clearances are only half the story: when a crane, boom lift, or dump truck works near the line, OSHA’s approach distances apply — see the crane power line clearance chart (1926.1408) and the NFPA 70E approach boundaries guide. For underground runs, see the burial depth chart. Field PM keeps line-crossing permits, as-built sag measurements, and photos with the job.

FAQ

What is the NESC minimum clearance for a power line over a road?+

It depends on the wire type and voltage. As one example, an open supply conductor between 750 V and 22 kV over a road subject to truck traffic requires 18.5 ft in Table 232-1, measured at the worst-case sag condition. Other wire types and voltages differ, so consult the NESC edition your state has adopted and the utility's standard.

What is the difference between the NESC and the NEC?+

The NESC (IEEE C2) covers utility-controlled supply and communication lines and substations up to the service point. The NEC (NFPA 70) covers premises wiring on the customer side. Customer-owned overhead lines on large sites can fall under either depending on local adoption.

Which NESC table covers clearance to buildings?+

Table 234-1 covers clearances of wires, conductors, and cables from buildings, signs, tanks, and similar installations — both horizontal (to walls and windows) and vertical (over roofs and balconies). Companion rules and tables cover bridges and swimming pools.

Is the NESC free to read?+

The NESC is a copyrighted IEEE standard and is sold by IEEE; there is no NFPA-style free viewer. Many utilities publish their own construction standards that summarize the clearances they require, which are often more conservative than the NESC minimums.

How often is the NESC updated?+

The NESC is revised on a five-year cycle; the 2023 edition is current as of this writing. States adopt editions on their own schedules, so confirm which one your public utility commission enforces.

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