AN A320 FLT STEERS THE WRONG COURSE FOR A SECOND WAYPOINT AFTER THE CREW HAD FAILED TO DETECT THE NAV ERROR WITHIN THE COORDINATES PLACED INTO THE FMS ON THE RAMP AT SJC; CA.

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WE HAD A NAV DEV ERROR CLBING OUT PAST SAC FROM SJC. WE HAD COMPLETED THE DANVILLE 2 DEP AND WERE ENTERING THE ENRTE PHASE OF THE FLT. AFTER SAC; OUR CLRNC WAS AS FILED; WHICH WAS FROM SAC TO THE 29SAC; A COMPANY WAYPOINT WHICH WAS N3955.4; W11932.4. THE PREVIOUS WAYPOINT WAS SAC WHICH WAS N3826.6; W12133.1. THE ERROR OCCURRED DURING PREFLT LOADING OF THE FMS. I INADVERTENTLY LOADED THE N3826.6 FOR SAC AS THE FIRST PART OF THE WAYPOINT AND THE W11932.4 OF THE 29SAC AS THE PROPER ENTRY FOR THE SECOND PART OF THE WAYPOINT. IN OTHER WORDS I ENTERED N FROM THE PREVIOUS WAYPOINT OF SAC INSTEAD N FROM THE 29SAC WAYPOINT. THIS GAVE US A S DEV FROM OUR COURSE. OUR NEXT WAYPOINT WAS A COMPANY WAYPOINT FOR TURB AVOIDANCE CALLED BAMNW; WHICH WAS PROPERLY LOADED. THIS GAVE US A MODERATE BEND ON OUR COURSE LINE. HAVING BEEN BRIEFED BY OUR DISPATCHER PRIOR TO THE FLT; WE ASSUMED THIS WAS NORMAL AS HE SAID HIS MOST TURB RPTS WERE IN THIS AREA; THUS THE DIFFERENCE IN ROUTING. WHEN CHKING ON WITH ZLC; THE CTLR ASKED IF WE WERE ON AN ASSIGNED HEADING. WE SAID NO; THAT WE WERE ON A DEV RTE AND I READ BACK THE WAYPOINT TO THE CTLR. THE CTLR THEN INFORMED US THAT WE SHOULD CHK OUR NAV EQUIP AS SHE SHOWED US DEVIATING FROM THE PLANNED COURSE. AFTER CHKING; WE DISCOVERED THE MISTAKE. THE CTLR THEN GAVE US A 20 DEG TURN TO THE L AND DIRECT TO OUR NEXT WAYPOINT; THE BAMNW. NO CONFLICTS WITH OTHER TFC WERE RPTED. THE FOLLOWING WERE CONTRIBUTING FACTORS: 1) THE ACFT WAS LATE; AND WE HAD ACCESS TO THE COCKPIT ABOUT 14 MINS BEFORE DEP. THE STATION BOARDED QUICKLY TO GET AN ON-TIME DEP; SO WE WERE RUSHED. DURING THE COCKPIT PREFLT; WE HAD NUMEROUS INTERRUPTIONS FROM THE FLT ATTENDANTS AND NOT THE LEAST BEING THE EXTERNAL PWR PLUG FELL OUT; KILLING ELECTRICAL PWR TO THE ACFT. THE APU HAD NOT YET BEEN STARTED. I WAS INTERRUPTED AT LEAST 3 TIMES WHILE LOADING THE FLT PLAN; INCLUDING HELPING AN ELDERLY MAN WHO COULD NOT SEE WELL. 2) THE FLT PLAN WAS FOLDED SO THE SAC AND 29SAC WAYPOINTS WERE ON EITHER SIDE OF THE FOLD. 3) THE FO NORMALLY DOUBLE-CHKS MANUALLY INPUT WAYPOINTS; BUT HE SAID HE DID SO QUICKLY; AS WE WERE VERY RUSHED. HE MADE THE EXACT SAME MISTAKE AS I DID. IT'S AMAZING THAT WE BOTH MADE THE SAME ERROR. LOOKING AT THE FLT PLAN WHILE TYPING THIS; I INADVERTENTLY DID THE EXACT SAME THING; BUT NOT BEING UNDER PRE-DEP HURRY UP; I CAUGHT IT. WOW! 4) AFTER ALL THESE YRS OF FLYING; I HAVE TRIED TO LEARN (AND RELEARN) THAT THE FASTER YOU GO; THE BEHINDER YOU GET. FROM NOW ON; I WILL FOLLOW MY OWN ADVICE THAT I USUALLY GIVE TO THE FO ON THE PREFLT BRIEF. THAT IS; THE MORE PRESSURE WE ARE UNDER TO HURRY; THE SLOWER WE WILL GO. THIS IS THE FIRST NAV DEV IN MY CAREER AND HOPEFULLY THE LAST.