Time of trim clearance versus maintained clearance

GO 95 Rule 35 carries two clearance figures. Table 1, Cases 13 and 14, is the enforceable minimum that must hold at all times, and Appendix E's larger distances are recommended clearances to be established at time of trimming. Appendix E ties its numbers to holding compliance until the next scheduled maintenance.

Rule 35 gives a span two clearance figures and calls neither of them by the name the industry uses. A crew works to one, an inspector measures against the other, and the two are separated on a 12 kV span inside the High Fire-Threat District by 96 inches.

That gap is not slack. It is a budget, it is spent by growth and conductor movement between one visit and the next, and the size it needs to be is a calculation the Commission asks each utility to do and never does for anyone.

Two numbers, and the sentence that joins them

Rule 35's opening sentence is ungrammatical, and the reason is that two clauses were spliced together:

Where overhead conductors traverse trees and vegetation, safety and reliability of service demand that certain vegetation management activities be performed in order to establish necessary and reasonable clearances the minimum clearances set forth in Table 1, Cases 13 and 14, measured between line conductors and vegetation under normal conditions, shall be maintained.

Two verbs, and they are the whole subject of this page. Activities are performed to establish clearances. The Table 1 minimums shall be maintained. Establishing is something a crew does on a date. Maintaining is a condition that has to hold on every date until the crew comes back.

Appendix E supplies the establishing number, and its first two sentences say exactly what it is for:

The radial clearances shown below are recommended minimum clearances that should be established, at time of trimming, between the vegetation and the energized conductors and associated live parts where practicable.

Reasonable vegetation management practices may make it advantageous for the purposes of public safety or service reliability to obtain greater clearances than those listed below to ensure compliance until the next scheduled maintenance.

The second sentence is the connective tissue of the whole regime, and it is skipped in almost every summary of Rule 35. It says the guideline exists to hold the other number, it says the horizon is the next scheduled maintenance, and it says in advance that the figures printed underneath it may not be enough.

Both that sentence and the list of factors that follows it were added by Decision 12-01-032 on January 12, 2012. The version before that decision recommended greater clearances in a shorter sentence that never said what they were for.

Neither text uses the terms a program actually runs on. Appendix E says "at time of trimming" and Rule 35 says "shall be maintained." Time-of-trim clearance and maintained clearance are operational shorthand, which is fine in a work order and a problem in a compliance file, because a finding has to be written against a case and a table rather than against a phrase the order does not contain.

Only one of the two can be cited

Appendix E's distances are conditioned three times over: recommended, should be established, and where practicable. Table 1 is a shall. Rule 35 and its four exceptions hang the obligation on Table 1, Cases 13 and 14, and there is no offense in GO 95 called falling short of Appendix E.

That has an asymmetry inside it worth stating plainly, because it is where programs go wrong. A span trimmed to 7 feet against a 12 foot specification, inside the district at 12 kV, is at 84 inches against a 48 inch minimum. On the day the crew leaves it conforms, comfortably.

It generates no nonconformance, so it enters no priority level, so it gets no correction date under GO 95 Rule 18A, and nothing in the compliance machinery will look at it again until the next scheduled visit or the next patrol. The short trim is invisible to every control that exists, and it becomes visible only as a Table 1 failure two or three years later, on somebody else's shift.

There is one place where the guideline is not merely a guideline, and it is geographic. Appendix E's Case 13 column prints 4, 6 and 10 feet on the same voltage bands PRC 4293 uses, which means that on State Responsibility Area land outside the High Fire-Threat District, the guideline figure and the statutory floor are the same distance. There the margin is zero by construction, and the whole of this page's arithmetic has to be done above the guideline rather than between it and Table 1.

The gap, band by band

Appendix E prints its recommended clearances in two columns, headed Case 13 of Table 1 and Case 14 of Table 1, on four voltage bands of its own:

Line operating voltage	Case 13 column	Case 14 column
2,400 or more volts, but less than 72,000	4 feet	12 feet
72,000 or more, but less than 110,000	6 feet	20 feet
110,000 or more, but less than 300,000	10 feet	30 feet
300,000 or more	15 feet	30 feet

Set those against the Table 1 minimums for the same conductors and the budget appears as a subtraction.

A 12 kV distribution span inside the district: 12 feet recommended, 48 inches required, 96 inches of budget. The same span outside the district: 4 feet recommended, 18 inches required, 30 inches of budget. A 115 kV line inside the district: 30 feet recommended, 120 inches required, 240 inches of budget. The same line outside: 10 feet recommended, 19 inches required, 101 inches of budget.

Now divide by a cycle. On a four year cycle, the 96 inches inside the district is 24 inches a year of combined growth and conductor movement before Case 14 is breached. The 30 inches outside the district, on the same four years, is 7.5 inches a year.

Same species, same slope, same crew, and a budget more than three times tighter on the span that is not in the fire-threat district, because Appendix E's outside column was set to a statute's floor rather than to a growth model. Whether either figure is adequate is a question about the species and the site, and Appendix E's second sentence exists precisely because the answer is often no.

The two numbers are not measured the same way

The subtraction above is slightly generous, because it treats both figures as though they were the same kind of quantity. They are not.

Only one footnote in Table 1, (ddd), reaches the vegetation cases, and it lifts them off the still air benchmark every other row is measured on: clearances in these cases shall be maintained for normal annual weather variations rather than at 60 degrees, no wind.

What that condition does to a reading is the rule's own subject. What matters here is that the maintained number is not a distance measured on a day. It is a distance that has to survive a year of ordinary heat and ordinary wind, which is to say it has to hold at the conductor's hottest sag and its furthest sway.

Appendix E attaches no condition at all. It says the clearance should be established at time of trimming, and time of trimming is whatever the weather was that morning. So a crew that measures 12 feet at 55 degrees in March, conductor at rest, has established 12 feet under one set of conditions and has to hold 48 inches under a different and worse set.

The difference between the March position of that conductor and its August position is spent out of the 96 inches before a single leaf grows, and on a long span at distribution voltage it is not a small fraction of the budget.

The practical consequence is that the trim reading and the audit reading are not comparable numbers unless the trim reading says what the conditions were. Two figures beside the distance, ambient temperature and wind, cost a crew nothing and are what let a later reader net the conductor movement out of the growth. What was measured, and from which surface is the other half of making the two readings comparable.

The cycle sets the number, and GO 95 does not set the cycle

Appendix E's third sentence lists what a utility may weigh in going beyond the printed figures:

Each utility may determine and apply additional appropriate clearances beyond clearances listed below, which take into consideration various factors, including: line operating voltage, length of span, line sag, planned maintenance cycles,

The list runs on for another nine items, through species type, prior experience with a species, "vegetation growth rate and characteristics," management standards, local climate, elevation, fire risk, and the trimming requirements applicable to State Responsibility Area land. The full list, and the pointer to the Public Resources Code sitting at the end of it, is worth reading whole once.

Thirteen factors, and three of them are the calculation itself: the planned cycle, the growth rate, and line sag. The rest are inputs to those three or are the site.

Then the Commission stops. No rule in GO 95 sets a vegetation maintenance interval. General Order 165's Table 1 intervals are inspection intervals for facilities, not for vegetation, and what a patrol can and cannot close is a separate question again. The vegetation interval is committed in the utility's wildfire mitigation plan under Public Utilities Code section 8386(d), which is a filed document rather than a general order.

That leaves a dependency running in one direction with nothing enforcing it. The trim specification is a function of the cycle. If the cycle moves from four years to five, the specification has to move with it or the maintained number will fail on the spans where the budget was already tight, and no rule, no table and no footnote will tell anyone that it moved.

The only place the two facts sit side by side is the utility's own program, which is what Rule 18A means when it requires that program to be auditable. An auditor reading a trim specification of 12 feet against a five year cycle and a documented growth rate of 26 inches a year is reading a program that has written down its own failure.

Where the same distinction is a requirement rather than a guideline

The Commission recommends the calculation. NERC requires it, in a numbered requirement with a violation risk factor attached, and audits the documentation of it. FAC-003-5 Requirement R3 reads:

Each applicable Transmission Owner and applicable Generator Owner shall have documented maintenance strategies or procedures or processes or specifications it uses to prevent the encroachment of vegetation into the MVCD of its applicable lines that accounts for the following: 3.1. Movement of applicable line conductors under their Rating and all Rated Electrical Operating Conditions; 3.2. Inter-relationships between vegetation growth rates, vegetation control methods, and inspection frequency.

Those two sub-parts are the two halves of the gap this page is about. R3.1 is the conductor movement that Appendix E's silence on measurement conditions leaves the reader to work out. R3.2 is growth rate against cycle. Requirement R7 then makes a change in expected growth rate an enumerated reason for amending the annual work plan, at 7.1, which puts the growth assumption in a document that gets revised on the record rather than in a specification nobody revisits.

What the MVCD is and how it is derived and which lines FAC-003 reaches are their own subjects, and the standard applies on top of GO 95 rather than instead of it.

The third regime on the same corridor goes the other way and has no time-of-trim number at all. PRC 4293 requires the utility to maintain four, six or ten feet, full stop, with 14 CCR 1256 folding conductor movement and vegetation sway into the measurement itself. There is no statutory guideline sitting above it. A program that treats PRC 4293's four feet as a trim specification has set its trim number equal to its floor, which is the one configuration guaranteed to fail before the next visit.

Where practicable is the guideline's exit, and it does not move the floor

Appendix E's own conditionality is real and gets used. A crew cannot always reach 12 feet:

• The tree stands outside the width the utility actually controls.

• The owner has refused the cut.

• An adjacent structure blocks the aerial device.

• The reduction the specification implies would leave the tree unsound.

None of that is a nonconformance. Appendix E says where practicable, and impracticability is exactly what it contemplates. Table 1, though, does not move by an inch. What actually happened on such a span is that the entire growth budget was spent on the day of the trim, and the span's next due date is therefore not the cycle's due date.

So a short trim is a scheduling event, not a compliance event, and it has to leave the field as one. The span identifier, the achieved clearance, the reason it fell short, and a re-visit date derived from the shortfall rather than from the cycle.

Where the reason is a refusal, Exception 2's two artifacts are a separate file that answers a separate question. PRC 4295.5's right of entry after notice may reach the same tree on other land with the utility's own specification, not the statutory minimum, as the distance it authorizes.

What the trim record proves two years later

Take a 12 kV span in Tier 2, found at 30 inches to the nearest limb on August 14, 2026, which is 18 inches inside the Case 14 minimum. It is a Level 2 nonconformance with a 12 month correction clock, and the utility now has to say how it got there.

If the trim record for that span reads "cleared to specification," it says nothing. It cannot distinguish a crew that achieved 12 feet from one that achieved 6, and it therefore cannot distinguish a growth model that was wrong from a work quality failure, which are different problems with different fixes and different exposures.

If it reads "12 kV, Case 14, west phase, achieved 141 inches at the closest approach, 58 degrees, calm, March 3, 2023," it does real work. It dates the failure, it fixes the elapsed encroachment at 111 inches over 41 months, roughly 32 inches a year, and it lets the program compare that against the rate it assumed when it set the specification.

One span produces a rate. Fifty spans of the same species on the same cycle produce a defensible reason to change either the specification or the cycle, and a paper trail showing when the utility knew.

That is the whole practical case for measuring the trim rather than certifying it. The maintained number is proved forward from the trim date, and a record that carries only the specification has no forward to prove it from.

What the record has to name

Four things distinguish a trim entry that can carry a compliance argument from one that cannot.

The specification, and the case it was derived to hold. Twelve feet is not self-explanatory. Twelve feet against Table 1 Case 14 column E, inside the High Fire-Threat District, is.

The clearance achieved, measured, per span and per phase. Not the specification restated. Where the achieved figure is below the specification, the reason and a re-visit date that follows from the shortfall.

The conditions of the reading. Ambient temperature and wind, because Table 1 footnote (ddd) puts the maintained number on annual weather variations and the trim reading was taken under one set of them.

The cycle assumption in force on that date, and the growth rate it was built on. This is the field almost nobody carries, and it is the one that turns a stack of trim records into evidence that the program was reasoned rather than inherited. Appendix E names planned maintenance cycles and vegetation growth rate as factors; a file that names the specification without them has recorded a conclusion and thrown away the working.

What a full clearance record contains, and the retention period each artifact in it has to clear, are their own subjects. The contribution this distinction makes is narrower and it is the one an evaluator tests first: the number in the file has to say which of the two clearances it is, because the same figure passes as one and fails as the other.

Further reading

• CPUC GO 95 Rule 35: the vegetation clearance rule, explained: The rule both numbers sit inside, and the exceptions that move the smaller one.

• Overhead power line clearance by voltage in California: The Table 1 figures themselves, case by case and column by column.

• Growth rate and the trim cycle: The other input to the trim specification, and what happens when the cycle moves.

Frequently asked questions

Is it a GO 95 violation to trim to less than the Appendix E clearance?

Not on its own. Appendix E's distances are described in its own text as recommended minimum clearances that should be established, at time of trimming, where practicable. The enforceable minimum is GO 95 Table 1, Cases 13 and 14. On State Responsibility Area land the Appendix E Case 13 figures happen to coincide with PRC 4293's statutory minimums, and there falling short does breach a statute.

How much clearance should a crew actually cut to?

Enough that the Table 1 minimum still holds at the next scheduled maintenance. Appendix E says so directly: reasonable vegetation management practices may make it advantageous to obtain greater clearances than those listed to ensure compliance until the next scheduled maintenance. Its listed figures are a starting point that the rule itself anticipates being exceeded.

Does GO 95 set the trim cycle?

No. Appendix E ties its recommended clearances to the next scheduled maintenance and names planned maintenance cycles as a factor, but no rule in GO 95 sets an interval for vegetation work. General Order 165 Table 1 sets inspection intervals for facilities rather than for vegetation, and the vegetation interval is committed in the utility's wildfire mitigation plan under Public Utilities Code section 8386(d).

What does a trim record need beyond the specification?

The clearance actually achieved, measured, by span and phase, with the date. The maintained obligation is proved forward from the trim date, so a record saying only that a span was cleared to specification cannot show how much of the growth allowance was in place when the crew left, or how much has been consumed since.