A surprisingly small amount of ice can start damaging power lines, but truly destructive failures usually happen once ice thickness gets around half an inch or more.

Quick Scoop: How much ice breaks power lines?

Rule-of-thumb numbers

  • Around 0.25 inch (about 6 mm) of ice:
    • Utilities consider this the point where damage can begin , with noticeable sagging and small outages, especially from tree branches snapping onto lines.
  • Around 0.25–0.50 inch :
    • Often classified as a “disruptive” ice storm , with increasing tree damage and more frequent power outages.
  • More than 0.50 inch :
    • Often called a “crippling” ice storm ; this level commonly leads to severe damage to trees and lines and widespread, long-lasting outages.
  • Around 1 inch or more :
    • Can be catastrophic , with many lines and poles failing and multi-day to multi-week outages in the hardest-hit areas.

Why a little ice is so dangerous

  • Ice is heavy: On a roughly 300‑foot span, about 0.5 inch of ice can add hundreds of pounds of extra weight to a line; 1 inch can add well over 700 pounds.
  • Trees become wrecking balls: Ice can make tree branches up to dozens of times heavier , so they snap and fall onto lines that might otherwise have survived the ice alone.
  • Wind multiplies stress: With ice loading, even 20–25 mph winds can make lines sway enough to break or hit each other, causing faults and outages.
  • Melting can also trip power: If ice melts unevenly (for example, from a lower neutral line first), lines can shift and touch and cause short circuits.

Why the exact “breaking amount” varies

How much ice it takes to actually snap a line or pole depends on:

  • Line design (thickness, material, tension, span length).
  • Pole type and age (wood vs. steel, maintenance history).
  • Local standards (regions with frequent ice storms often build to higher load ratings).
  • Surroundings (trees nearby, terrain funneling wind, etc.).

So there is no single universal “exact” ice thickness that always breaks power lines, but utilities often treat 0.25 inch as the danger threshold , 0.5 inch as serious/crippling , and 1 inch or more as potentially catastrophic.

Simple example

Imagine a 300‑foot residential line:

  • Light glaze to 0.25 inch : line sags more, some small branches break, spotty outages.
  • Around 0.5 inch plus moderate wind: hundreds of extra pounds on the span, multiple branches and maybe a pole or crossarm fail, neighborhood‑scale outages.
  • Close to 1 inch with wind and heavily iced trees: many spans fail, multiple poles break, city‑ or region‑scale outages that can last days.

Typical ice amounts and effects on power lines

[8][1][3][9] [8][7] [1][7] [10][7][9][1]
Ice thickness on lines Typical classification Likely effects
~0.25 inch Onset of damaging ice Noticeable sagging, broken small branches, scattered outages begin.
0.25–0.50 inch Disruptive ice storm Numerous outages, significant tree damage, hazardous travel.
>0.50 inch Crippling ice storm Severe damage to trees and lines, widespread outages that can last days.
~1 inch or more Catastrophic loading Lines and poles may fail in many places, large regions dark for days or longer.

Information gathered from public forums or data available on the internet and portrayed here.