St. Luke's officials say the power outage was caused by a break in the underground cable between the pole and switch gear.
Hospital officials say patients haven't been evacuated or harmed.
Mid-American Energy is working to restore power to the hospital as soon as possible.
The hospital has back-up generators working at full capacity to provide power to priority areas.
Sioux City Fire Rescue says the power outage caused multiple false fire alarms today.
They say the ventilation systems also weren't working.
There was an odor that officials say may have been cause by the power outage or the heat.
Sioux City Fire Rescue officials say they were at the hospital at least four times today.
"We've been up here a number of times today for fire alarms, because of the power outage," said Mike McClennen of Sioux City Fire Rescue Captain. "This last call, they called us because they were having some odors and and they were concerned about them. So, they wanted us to come up and check them, and we didn't really find anything."
Hospital officials say patient safety is the priority and they are taking extra steps as a caution.
They are currently evaluating the needs of each patient and developing their patient care plan over the next several hours.
St. Luke's says they are collaborating with surrounding health care facilities to determine if they need to transfer patients to other facilities.
Anyone who needs immediate or emergent medical care is advised to go to Mercy Medical Center at this time.
Classes at UnityPoint Health St. Lukes are canceled Friday Night.