Wire to vegetation versus wire to ground: measuring the right distance
GO 95 Table 1, GO 95 Appendix E, PRC 4293 and the NERC MVCD all measure from a conductor to vegetation. None measures from the ground. A tree height and a conductor height are taken from two different points on the ground, so subtracting them produces a clearance only on level terrain.
A vegetation inventory row for a 12 kV span in Tier 2 carries three numbers about the same canyon live oak. Tree height, 14 feet. Conductor height above ground, 41 feet. Clearance, 27 feet. The third number is the first two subtracted. The oak stands on a cut bank 10 feet to the side of the span. The top of that bank is 30 feet above the ground under the wire. The actual distance from the conductor to the nearest foliage is 10 feet 5 inches.
Every figure in that row was measured honestly. Two of the three were measured up from the ground, and the rule is measured out from the wire. The arithmetic that joined them assumed the ground under the tree and the ground under the conductor sat at the same elevation. That assumption is the most common defect in a clearance file, and the file itself will never show it.
Every clearance in the stack runs from the conductor
Four instruments set vegetation clearance on a California span. Each names its own two endpoints, and the wording differs enough to matter.
Instrument | Near end, as written | Far end, as written | Condition on the reading |
GO 95 Table 1, Case 13 | bare line conductors | tree branches or foliage | normal annual weather variations, footnote (ddd) |
GO 95 Table 1, Case 14 | bare line conductors | vegetation in the Fire-Threat District | normal annual weather variations, footnote (ddd) |
GO 95 Appendix E | the energized conductors and associated live parts | the vegetation | at time of trimming, where practicable |
PRC 4293, via 14 CCR 1256 | all conductors which are carrying electric current | all vegetation | any position of the conductor at adjacent air temperature of 120 degrees Fahrenheit or less |
NERC MVCD | conductors | vegetation | line operating within its Rating and all Rated Electrical Operating Conditions |
Four distinct near ends and two distinct far ends, across five rows. Ten endpoints, and every one of them is either a conductor or a plant. A height above ground is a real and enforceable quantity in California, and it belongs to a different rule, which the rest of this page separates out.
The conditions column is where each rule's own page picks the subject up. Three of them are worth reading whole: footnote (ddd) and normal annual weather variations, the 360-degree arc and the sway clause in 14 CCR 1256, and the six conductor positions the MVCD has to survive. What follows is the other half of the measurement, which is the two ends of it.
Which conductor, exactly
The near end of Cases 13 and 14 is defined twice over. It has to be bare, which the by-voltage page works through. It also has to be a line conductor, and Rule 20.9-C of General Order 95 is specific about what that is:
Line Conductor means an overhead conductor which extends from the last point of support on one overhead line structure to the first point of support on another overhead line structure.
The tabulated vegetation clearance runs to the span, between its two points of support. Rule 20.9 separately defines a Lateral Conductor and a Vertical Conductor, so the order is distinguishing among them deliberately.
Take a deadend pole on a 12 kV circuit, with a jumper loop carrying current around the structure and a limb sitting 10 inches off that jumper. The jumper sits outside the definition, because it runs around the structure rather than between two of them. Case 13's 18 inches reaches the span and stops at the last point of support.
Appendix E reaches "the energized conductors and associated live parts," so the limb is inside the guideline. PRC 4293 reaches "all conductors which are carrying electric current," so the limb is inside the statute. One limb, three documents, and two of them have a number for it.
The Commission has published no reading that settles this, and GO 95's own Appendix I pulls the other way. That appendix, Examples of Rule 18 Priority Levels and Safety Hazards, gives as a Level 1 condition "Vegetation contacting or nearly contacting high voltage conductor," dropping the word line entirely.
So the defensible position is a finding written against the instrument whose near end reaches the hardware, with the hardware named. "Limb at 10 inches from east jumper, deadend structure" survives a second reader. "Limb at 10 inches" leaves that reader guessing.
Surface or centerline, and the half inch inside the number
GO 95 knows how to answer this question. It answers it twice, in the two rules that follow the one carrying Table 1. Rule 38, on the clearance of wires from other wires:
All clearances of less than 5 inches shall be applied between surfaces, and clearances of 5 inches or more shall be applied to the center lines of such items.
Rule 39, on the clearance of wires from signs:
All clearances of more than 5 inches shall be applicable from the centerlines of conductors concerned. Lesser clearances shall be applicable from conductor surfaces.
Table 1 itself uses the word where it means it. Case 10 is titled "Radial centerline clearance of conductor or cable (unattached) from non-climbable street lighting or traffic signal poles or standards." Cases 13 and 14 are titled "Radial clearance of bare line conductors from tree branches or foliage" and "Radial clearance of bare line conductors from vegetation in the Fire-Threat District." Both say radial and stop there, and Rule 35 adds only that the clearance is "measured between line conductors and vegetation."
14 CCR 1256 comes closest, for PRC 4293's distances: they are "measured at a right angle to the conductor axis at any location outward throughout an arc of 360 degrees." That phrase fixes the direction of the measurement, perpendicular to the wire rather than vertical or horizontal. Whether the distance then starts at the axis or at the outside of the strand is left open.
Here is what the choice is worth, using Southwire's published nominal diameters:
Conductor | Nominal OD | Half the OD | Measured against |
1/0 6/1 ACSR Raven, a common distribution primary | 0.398 in | 0.20 in | 48 inches under Case 14, column E |
795 kcmil 26/7 ACSR Drake, a common 115 kV conductor | 1.107 in | 0.55 in | 120 inches under PRC 4293(c) |
A fifth of an inch will not decide a finding. What it will do is make two honest readings of the same span disagree, and make a re-measure next cycle read as growth when the only thing that changed was whose tape it was. A program that writes its convention into the field procedure and prints it on the form removes the ambiguity for its own records. The regulation leaves it open, and on a contested span the Commission decides what the number meant.
Foliage, or wood
The far end moves further than the near end does, and the two cases disagree with each other. Case 13 reaches "tree branches or foliage." Case 14 reaches "vegetation." Inside the High Fire-Threat District the far end is therefore broader on its face. A stand of chamise under a span is vegetation, and whether it is a tree branch is a question Case 13 invites and leaves open. PRC 4293 covers both, since it reaches "all vegetation" everywhere it applies.
Then the exemptions change the far end again, and they change it to wood. Rule 35's Exception 4:
Mature trees whose trunks and major limbs are located more than six inches, but less than the clearance required by Table 1, Cases 13E and 14E, from primary distribution conductors are exempt from the minimum clearance requirement under this rule.
14 CCR 1257(a)(3) runs the same way for PRC 4293. Its first two criteria are that "the tree or limb must be six (6) inches or more from the line at all times" and that "the size of the tree or limb at the conductor level must be at least six (6) inches in diameter."
So a valley oak standing 30 inches off a 12 kV phase produces two measurements from one visit. To the trunk and the major limbs, 30 inches, which is the figure that decides whether either exemption is available. To the nearest leaf on the nearest sprout, 8 inches, which is the figure Case 13 is written against.
A record that carries one number and calls it the clearance has thrown away the number the other question needed. What each exemption then costs to keep and how the same tree files two different ways under the two rules are their own subjects.
Wire to ground is its own requirement, on its own conditions
Height above ground is in the same table, four rows up. Rule 37 carries it:
Clearances between overhead conductors, guys, messengers or trolley span wires and tops of rails, surfaces of thoroughfares or other generally accessible areas across, along or above which any of the former pass; also the clearances between conductors, guys, messengers or trolley span wires and buildings, poles, structures, or other objects, shall not be less than those set forth in Table 1, at a temperature of 60° F. and no wind.
For supply conductors of 750 to 22,500 volts, column E, Table 1 Case 4 requires 25 feet above ground along thoroughfares in rural districts or across other areas capable of being traversed by vehicles or agricultural equipment. Case 5 requires 17 feet above ground in areas accessible to pedestrians only.
Those are construction and sag obligations. Trimming a tree moves the Case 13 reading and leaves Cases 4 and 5 exactly where they were. They also sit on a different measurement convention from the vegetation cases, in two respects.
Temperature and wind. Rule 37 states the whole of Table 1 at 60 degrees Fahrenheit and no wind. Footnote (ddd) then lifts Cases 13 and 14 off that benchmark and onto normal annual weather variations.
Tolerance. Rule 37 allows the Case 2 through Case 6 clearances to be reduced by up to 10 percent below the tabular values because of temperature and loading as specified in Rule 43. A 17-foot Case 5 clearance is compliant at 15 feet 3 inches on a hot afternoon.
The same rule then holds Cases 11, 12 and 13 to the tabular values exactly. That sentence names those three and stops, and Case 14's absence from it is a gap worth knowing about rather than relying on, since footnote (ddd) reaches both vegetation cases and Rule 35 states both as a shall.
The sag behind that tolerance is substantial. Rules 43.1-C and 43.2-C both require that "a conductor temperature of at least 130°F shall also be assumed for computing sag and its effect on structural loads due to weight span." A conductor height recorded at 60 degrees is near the top of the range the same order designs for.
What subtracting two heights above ground actually gives you
Back to the span at the top of the page. Lay out what was actually measured.
Number in the file | What it was measured from | What it was measured to |
41 feet | the ground directly beneath the conductor | the conductor |
14 feet | the ground directly beneath the oak, 30 feet higher | the top of the oak |
27 feet | nothing | nothing |
The third row subtracts two figures taken from different datums, which makes it arithmetic rather than a distance. Do the geometry instead. The top of the oak sits 30 plus 14, or 44 feet, above the datum the conductor height was taken from. The conductor sits at 41. The foliage is therefore 3 feet above the wire and 10 feet to the side of it, and the straight-line distance is the square root of 3 squared plus 10 squared: 10.44 feet, or 10 feet 5 inches.
Two things follow that the subtraction hid. The span is short of Appendix E's 12-foot time-of-trim guideline for a Case 14 conductor under 72 kV, though it clears Table 1's 48-inch minimum comfortably; the difference between those two figures is what the trim specification was budgeted against. And the closest approach is in the sector above the conductor, which is inside the 360-degree arc 14 CCR 1256 requires and outside what a side trim produces.
Slope is ordinary in California, and a utility's own program documents say so. PG&E's Operations and Maintenance Plan for Electric Facilities on National Forest System Lands, December 2018, names three drivers of corridor width:
The trim zone and the hazard tree zone are variable in width depending primarily on the height of trees adjacent to the distribution line, the topography, and the sway of the conductors at mid span as determined by the tension and length of the span.
Topography sits in that sentence beside tree height for a reason.
The one place a clearance rule measures from the ground
14 CCR 1257(a)(3) sets three criteria for an Exempt Tree, and the third is ground-referenced: the tree "must not have 'scaffold branches,' below eight and one-half feet from the ground." That is the only clearance distance in the California stack taken up from the soil rather than out from the wire, and the regulation leaves open which soil.
On a tree standing on a cut bank or a side slope, the uphill and downhill measurements to the same branch differ by whatever the slope gives up across the trunk, and Article 4 leaves the choice open.
The section says who decides. All three criteria are confirmed by "a Certified Arborist or a Registered Professional Forester," and the confirmation is recorded against a tree held in the utility's Exempt Tree database. So the answer on any given tree is the named professional's, and the file should carry the side the measurement was taken from, because the next annual inspection is a comparison against this one. What else that database has to hold, and when it is filed is the section's own subject.
The other ground-referenced obligation on the same pole is PRC 4292's firebreak, which clears a space around the structure rather than setting a distance to the wire. Each of the two is measured from the soil, and each answers a question about the space around a structure or a trunk. The distance to the conductor stays a separate reading, taken separately.
What remote sensing hands back, and which field it is
Aerial data formalizes the distinction rather than dissolving it. A LiDAR collection over a corridor yields three separate products: a bare-earth surface, a vegetation surface, and a model of the conductor. A height is the distance between the first two. A clearance is the distance between the third and the second. They come out of the same flight and land in different columns.
PG&E describes its own process as using both sources: "the vegetation inspection process entails ground patrols and the use of Light Detection and Ranging and advanced analysis techniques." Both halves earn their place, because CAL FIRE's field guide is direct about which findings an overflight can close and which it cannot, and a measured conductor clearance is on the wrong side of that line.
The conductor model is the part that carries an assumption. A modeled clearance is a clearance at whatever conductor temperature and loading the catenary was solved for, which is a choice somebody made in a processing specification. A deliverable that prints that assumption can be tested against footnote (ddd), against PRC 4293's 120-degree clause and against the MVCD's Rated Electrical Operating Conditions. One that omits it leaves all three open, since every one of them is written for a conductor in motion.
The ground side carries the same problem, and it surfaces again in hazard tree work, where the height that matters is measured from the hinge rather than from the soil. Even the quantity everyone treats as obvious has a datum question inside it.
What the record has to name
A clearance figure becomes testable when the file says what the two ends of it were. Each item below is a field somebody who was never on the span can ask for by name.
Both endpoints, named the way the rule names them. Which conductor, identified as a phase and a position rather than as "the wire," and whether it is a line conductor, a jumper or a riser. Which part of the vegetation, whether foliage, a branch, a major limb or a trunk. On a tree that may qualify under Rule 35's Exception 4 or 14 CCR 1257(a)(3), both the distance to wood and the distance to foliage.
The convention: axis or surface. One word on the form, applied the same way on every span. Cases 13 and 14 leave the choice to the program, so the program should make it in writing once rather than in the field each time.
The datum, for any figure measured from the ground. Whose ground. For a scaffold branch measurement under 1257(a)(3), the side of the trunk. For a tree height, the point at the base it was taken from.
The conductor condition at the time of reading. Ambient temperature and wind for a field measurement; the modeled temperature and loading for a LiDAR-derived one. It is what lets the next cycle net conductor movement out of growth.
Which instrument the number is being tested against. The case and column, the statutory subdivision, or the MVCD row. The same measured distance passes one and fails another, and the file has to say which question it answered.
Measurement is where the four instruments in the opening table stop being interchangeable. Which agency can act on each of them, and what the surrounding span record holds, are separate questions. This one is settled at the tape.
Further reading
PRC 4293: radial clearance in State Responsibility Areas: The 360-degree arc, and the conductor and vegetation positions the distance has to survive.
Minimum vegetation clearance by voltage in California: The Table 1 numbers this page tells you how to measure against.
Documenting clearance work: Where a measurement becomes a record somebody else can test.
Frequently asked questions
Is vegetation clearance measured from the conductor or from the ground?
From the conductor. GO 95 Rule 35 requires the Table 1 minimums "measured between line conductors and vegetation," PRC 4293 requires clearance "between all vegetation and all conductors which are carrying electric current," and the NERC MVCD is a distance between conductors and vegetation. Table 1's above-ground cases set how high a conductor hangs, which is a separate obligation under GO 95 Rule 37.
Is the clearance measured to the conductor's surface or to its centerline?
GO 95 does not say for Cases 13 and 14. It answers the question for wires from other wires in Rule 38 and for wires from signs in Rule 39, both at a five-inch threshold, and it writes "radial centerline clearance" into Table 1 Case 10. The vegetation cases use none of those words. 14 CCR 1256 says the PRC 4293 distance is measured at a right angle to the conductor axis, which fixes the direction of the measurement.
Can a tree height be subtracted from a conductor height to get a clearance?
Only where the tree and the pole stand on ground at the same elevation. Both figures are measured up from the ground beneath them, so on any slope they have different datums. The rules ask for a distance from the wire to the nearest vegetation in any direction, which on a side slope can be a fraction of the difference between the two heights.
Does any California clearance rule measure from the ground?
One criterion does. 14 CCR 1257(a)(3) requires that an Exempt Tree have no scaffold branches below eight and one-half feet from the ground. PRC 4292's pole firebreak is also ground-referenced, but it clears a space around the structure rather than setting a distance to the wire.
