IL HB4989

Defines "underground" or "undergrounding" as routing and constructing electric transmission lines and associated components below ground consistent with sound engineering and utility practices.

Requires public utilities to underground any 138 kilowatt or greater electric transmission lines constructed after this Act's effective date in two scenarios: within 26,000 feet of an airport in a municipality with population between 70,000-80,000 in or contiguous to a county with over 1,000,000 residents, or in 4 or more contiguous municipalities with population density of 1,000 per square mile or more.

Directs the Illinois Commerce Commission to allow public utilities to recover all reasonable and prudent costs of undergrounding transmission lines from all retail customers in the utility's service territory, not solely from affected municipalities.

Requires utilities to record and defer undergrounding costs as a regulatory asset included in the utility's total rate base and amortized over a period equal to the expected life of the transmission line.

Takes effect immediately upon becoming law.