Winter View from Mt. Baker, photo by Seattle City Light employee, Elham Naguib
Seattle City Light LARRY WEIS, General Manager and CEO
Vegetation Management
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Tree Trimming Program

What - Why - How - When - Who

City Light's Distribution Tree Trimming Program maintains and monitors over 300,000 trees with the potential to grow/fall into our power lines and critical infrastructure. We work along 1,770 miles of overhead lines in Seattle and parts of Burien, Lake Forest Park, Normandy Park, Renton, SeaTac, Shoreline, Tukwila and Unincorporated King County.

WHAT Do You Provide Clearance From?

City Light's Distribution Tree Trimming Program maintains clearances from our primary and secondary power lines, system "neutral", poles, transformers, guy-lines and the first 10 feet of customers' service lines. Sound complicated? This image should help clear things up - click on a letter to get information on the system component, what it does, how it ties into the rest of the system and what City Light's vegetation management responsibilities are.

Distribution System Outline

A    B    C    D    E    F    G    H    I    J   

  Name Function How it Connects Clearance Required Who Maintains
A Primary Distribution Lines also "Primaries" or "High Voltage Lines" To move power throughout the system at high voltages. Can be strung singly, in pairs, or in 3's. They carry 4,000V, 15,000V, or 27,000V of electricity. Move power from the substations throughout the system. Minimum 10' clearance for slow growing species.

Minimum 15' clearance for fast growing species.
ONLY qualified powerline clearance tree trimmers can work around these lines. We maintain a 4 year trim cycle and will prune more frequently as needed.

  Name Function How it Connects Clearance Required Who Maintains
B Transformer Serves as the gate-keeper between the high voltage Primaries and the lower voltage Secondaries. Primary voltages are high to allow electricity to travel long distances, but must be "stepped down" for everyday use. Transformers connect the primary to the secondary lines. Minimum 10' clearance for slow growing species.

Minimum 15' clearance for fast growing species.
ONLY qualified powerline clearance tree trimmers can work around transformers. We maintain a 4-year trim cycle and will prune more frequently as needed.

  Name Function How it Connects Clearance Required Who Maintains
C Secondary Distribution Lines or "Secondaries" To distribute electricity brought in on the Primaries throughout a neighborhood. These lines run from pole-to-pole and are comprised of two 120V lines and the system neutral. Secondaries distribute electricity brought in on primaries and "stepped-down" through transformers. Slow growing species: minimum 3 feet.

Fast growing species: minimum 5 feet.
ONLY qualified powerline clearance tree trimmers can work around these lines. We maintain a 4-year trim cycle and will prune more frequently as needed.

  Name Function How it Connects Clearance Required Who Maintains
D "Service Line" "Service Drop" or "Power Drop" To connect a residence/business to the distribution system. They are the same as the "Secondaries" but run from pole-to-structure instead of pole-to-pole. Older lines are arranged parallel, newer lines are braided. Most often black with a silver neutral, these lines are sometimes gray. Service Lines connect the secondaries to your residence and/or business. For the first 10 feet off of the utility pole:

minimum 10' clearance for slow growing species.

minimum 15' clearance for fast growing species.  
City Light maintains the first 10 feet of these lines off the utility pole. From the 10 feet mark to the structure is the property owner's responsibility to maintain.

  Name Function How it Connects Clearance Required Who Maintains
E Guy Wire Provides additional structural support to poles. Not a power delivery component, guy wires are considered critical infrastructure. Guy wires must be unobstructed clearly visible and accessible. City Light will clear vines and brush off/away from guy wires during our regular maintenance cycle.

  Name Function How it Connects Clearance Required Who Maintains
F Utility Pole Holds the lines and transformers off the ground and away from people. Not a power delivery component, utility poles are considered critical infrastructure. 3 feet. Poles must be unobstructed and clearly visible. City Light will clear poles during our regular maintenance cycle.

  Name Function How it Connects Clearance Required Who Maintains
G Telecommunication Lines or "Telecomm" lines To transfer telephone, internet, fiber optic and cable TV data to residences and businesses. These lines are NOT connected to our system. None These lines are owned, operated and maintained by the telecomm companies and City Light is not responsible for their maintenance or vegetative clearances.

  Name Function How it Connects Clearance Required Who Maintains
H Telecommunication Service Line These lines bring the telecomm services into your house or business. These lines are NOT connected to our system. None There are no legal clearance requirements.

  Name Function How it Connects Clearance Required Who Maintains
I Streetlight The streetlights illuminate the street and sidewalks Streetlights are powered by our system but do not serve a distribution function. Streetlights must be accessible for maintenance and repair. City Light maintains vegetative clearances for maintenance and repair access, but not for illumination.

  Name Function How it Connects Clearance Required Who Maintains
J Meter To measure the electricity usage of a given house, residence or business. The meter reads customers' wattage use from the Service Drop. Meters must be unobstructed, clearly visible and accessible. It is a property owner's responsibility to ensure the meters are unobstructed, clearly visible and accessible to meter readers.

People often call concerned about limbs in the thick, black telecommunication lines (G). They are owned, operated and maintained by the private companies and City Light is not involved in managing the vegetation around them. This often results in a "shelf" of untouched branches extending out from an otherwise significantly pruned tree. Not only is this beyond our control, but leaving these limbs in place is better for the tree since it reduces the number of cuts required and increases the amount of foliage remaining for photosynthesis.

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WHY Do You Have to Prune the Trees?

Because live tissue is such a good conductor of electricity we need to keep it away from our power lines. This means vegetative tissue (trees, shrubs, vines) that might fall onto, grow into, or hang up on our lines, as well as human tissue that might come in direct or indirect contact. Two facts highlight the importance of maintaining our clearance distances:
  1. Most injuries/fatalities from power lines come from indirect contact - someone touching a conductor (like a tree) that is touching an energized line;
  2. Vegetation contact is the #1 cause of power outages for City Light.
As you can see, this is a very important job that must be proactively addressed before an accident or outage occurs.

You don't need to make contact to get into trouble. Electricity will jump - or arc - from an energized line to a conductor that gets too close to it. How close "too close" is depends on the voltage of the lines. The higher the voltage, the farther away you need to stay to be safe. But it's not just distance that dictates when a line will arc; the humidity of the air, the ground elevation, how wet your skin is and the temperature outside all contribute to how close "too close" is.

We have established clearance distances based on industry best practice, the American National Standards Institute's A300 Pruning Standards for Utility Pruning of Trees, in accordance with Washington Administrative Code 296.24.960, and the Revised Code of Washington 64.12.035. The minimum clearances are determined by the voltage of the lines and may be adjusted based on the species of tree, its growth rate, and the condition of the limbs and/or trunks. ISA Certified Arborists determine which trees need to be trimmed and to what degree, with an eye towards minimizing the impact to the tree while prioritizing power safety and reliability. The objective is to ensure that the vegetation will not come close enough to the lines to potentially cause a safety risk and/or interrupt power, for the duration of the pruning cycle – currently 4 years. Our pruning clearance objectives are:

City Light Facility
Distribution Power Lines
4000 Volt, 14,000 Volt, or 26,000 Volt
System Neutral
"Service lines", "Service Drops" or "Power Drops" - two 120 Volt lines and a "neutral" line.
Clearance Objectives
10' clearance for all tree species
3' for slow growing species:

5' for fast growing species:
Seattle City Light trims branches applying significant, direct pressure to these lines, or which come within 10' of the nearest utility pole.
Transmission Power Lines

Based on line-engineering, species, trim-cycle, and right-of-way width
Underground Vaults
Power Meter
Utility Poles
Vaults must be accessible
Meters must be accessible

10' clearance for all tree species

3 feet

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HOW Do You Prune the Trees?

Our Distribution Tree Trimming program clears over 500 miles of overhead lines each year which allows us to trim our entire service rea in a four-year cycle. Every time we prune a tree we look three years into the future at how the tree will respond to our trimming, and remove enough to keep the lines clear until we come back again.
Directionally pruned trees grow away from the lines

Different tree species respond in different ways and at different rates, so our pruning must also be somewhat species-specific.

This explains the different distances that we require when trimming conifers vs. deciduous trees, for example. With some of the fastest growing species in the country calling the Seattle area home, we have a lot to keep up with.

City Light and our contractors use directional pruning methods to train trees to grow away from our lines. We selectively remove branches that are growing towards our lines and infrastructure and leave branches that grow away from them. This results in smaller caliper cuts that the tree has an easier time closing over. Because directional pruning focuses on the branches that cause problems and leaves the others alone, the tree is less impacted by our management efforts while the safety and reliability of the lines are ensured.

Construction Diagram
Directional Side Prune
Directional V-Prune
Directional Side Prune
Directional V-Prune
Construction Diagram
Directional Side Prune
Directional Side Prune
Top Prune

While we're not always able to maintain a tree's natural shape when pruning, directional pruning ensures we take the fewest branches possible and do the least harm to the tree.

Creative, but not property pruned!
circle cut through tree for power lines
City Light's management practices are guided by The American National Stantdards Institute's A300 Standards for Tree Maintenance, which is recognized and approved by the ISA and the USDA Forest Service. Directional pruning meets these guidelines and is in keeping with industry best practices for the utility line clearance and vegetation management industry.

When a medium/large tree grows directly underneath our lines there is little that we can do to avoid a conflict. Even trees that are not directly underneath the lines, but are too close to directionally prune must be repeatedly and aggressively cut for their entire lives.

This is an inefficient and unattractive way to maintain safety and reliable power delivery - and it is very bad for the tree's health. While topping and rounding were once favored as a vegetation management practice, modern arboriculture (tree care) tells us that it is extremely bad for the tree. City Light no longer tops trees if it can be avoided - we would rather remove and replace a tree than top it. We do, however, re-top trees to previous cuts to control re-sprouting limbs.

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WHEN Do You Prune the Trees?

The electricity delivery system is divided up into "feeders" which are sections of line that run from one switch to another. The feeders don't conform to neighborhood or even street boundaries. You might be on one feeder while your neighbor is on another. Each calendar year is divided into three trimesters and City Light awards contracts for feeders according to this schedule. Contractors then have that trimester to complete their awarded feeders. All the feeders in City Lights' service area are pruned by the end of our four-year cycle.

When it's time to prune a feeder, City Light contracted notifiers first assess which trees will require maintenance or removal and to what degree. Once the workload is established the contract for that feeder is then opened for bidding among our contractors. The notifiers then contact property owners either in person or by leaving a door hanger card with their contact information. Notifiers will explain exactly what work is to be performed, on which trees, and for what reason. There is typically a lag of three to four months between when customers are notified and when work begins, but sometimes the lag can be as long as 6 months.

At this time City Light will seek to remove "wrong trees" poorly suited for their location under our lines, and evaluate requests by property owners to remove trees. When we identify trees for removal, notifiers first obtain written permission from the property owner or his/her agent, and explain our tree replacement options. All trees in the right of way are posted for a 10-day public comment period. City Light removes and replaces trees at no additional cost to property owners, and processes all brush leaving all burnable firewood on site and cut to 18-24" lengths.

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WHO Prunes the Trees?

City Light contracts with three companies that specialize in utility line clearance: Kemp West Inc. and The Davey Tree Expert Company. Under City Light guidance these contractors trim and remove trees in accordance with the ANSI A300 and Z133 arboricultural standards. This includes the use of personal protective equipment, proper arboricultural cuts and directional pruning methods.

According to WA state and Federal OSHA laws only trained, qualified power line clearance tree trimmers may work around power lines. City Light requires all of our contractors be Certified Line Clearance Tree Trimmers or apprentices actively enrolled in a recognized apprenticeship program. This training ensures the tree trimmers working near our systems understand the special hazards involved. UNDER NO CIRCUMSTANCE should any unqualified person attempt to trim or remove a branch or tree that COULD fall within the Minimum Safe Approach Distances of our distribution or transmission systems.

If you have, ANY question as to whether or not you are qualified to work around power lines, contact City Light before beginning any work. It's more likely you are not qualified than that you are.

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Want to Know More?

City Light is committed to ensuring safe and reliable power delivery to our customers. In addition to this commitment, we are required by state and federal laws to ensure that our lines are free of vegetation, and to minimize the potential for power interruptions from vegetative interference. While we are always happy to inform, educate and explain our management practices and standards to the public, in the end we must achieve and maintain our vegetative clearances for everyone's safety.

If you have any questions or comments please contact us at: or call:
Arboriculturist, Power Line Clearance 206-684-3857
Arboriculturist 206-386-1650
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