The NFIRS Incident Type for a power line down is 444. (Four hundred and forty-four.)
Note: Code 444 - Power line down is appropriate for when the threat is confined to a powerline down (perhaps with a little charring), but when that becomes the ignition source for a wildland, brush or grass fire, use those NFIRS outdoor fire codes instead.
See all the NFIRS fire incident types to determine the correct NFIRS code for your incident.