UNAUTH RWY XING -- ACR CROSSES AN ACTIVE RWY WITH CLRNC WHILE TAXING TO THE WRONG RWY.

Narrative

WHILE TAXIING FOR TKOF FROM CHARLESTON; S CAROLINA; (CLRED TO TAXI RWY 21; HOLD SHORT RWY 15) WE EXPERIENCED 2 MINOR CAP PANEL LIGHTS NEEDING SYS ATTN AND CORRECTION. WE HAD JUST LANDED ON RWY 15; WITH A FULL AIRPLANE; AND WERE RETURNING TO IAD WITH A FULL ACFT AND A JUMPSEATER. WE HAD LEFT IAD LATE BY SOME 25-30 MINS AND AS A RESULT WERE BEHIND SCHEDULE. I WAS CONCERNED WITH PAX CONNECTIONS AND WANTED TO GET AIRBORNE. AS WE WERE HEAVILY LOADED I WANTED TO USE RWY 15 FOR TKOF; BUT; NEVER VERBALIZED THIS DESIRE; AND HEARING RWY 15 IN OUR TAXI CLRNC AND BEING RUSHED I ASSUMED RWY 15 WAS THE CLRED TO TAXI RWY. RWY 21 ALSO HAD A 9 KT TAILWIND WHICH MENTALLY REINFORCED MY INCORRECT; RWY 15 ASSUMPTION. WHEN I REACHED THE END OF RWY 21; I CROSSED IT AS NECESSARY TO TAXI TO RWY 15. THE FO BEING BUSY WITH HIS DUTIES; HEAD DOWN AS IT WERE; DID NOT CATCH MY ERROR. THE CTLR'S QUERY BROUGHT MY MISTAKE TO OUR ATTN; I APOLOGIZED AND TOLD HIM I HAD WANTED RWY 15 AND THUS THOUGHT (INCORRECTLY) THAT WAS HOW WE HAD BEEN CLRED. THERE WAS NO OTHER TFC IN THE AREA OR ON GND CTL AT THE TIME; HOWEVER THE UNAUTH RWY INTRUSION AS ALWAYS HAS THE POTENTIAL FOR SEVERE DANGER! REASONS: ALLOWING MYSELF TO GET INTO A HURRY-UP MODE; VERY UNSAFE AND PRIMARY CAUSE. NOT REPEATING AND VERBALIZING OUR TAXI CLRNC INSTRUCTIONS AS I NORMALLY DO. THE DISTRS OF CLRING THE 2 MINOR SYS PROBS JUST AS WE BEGAN TO TAXI. CORRECTIVE ACTION: SLOW DOWN; VERBALIZE; COMMUNICATE BETTER.