PVT PLT OF AN SMA SEL PENETRATED CLASS B AIRSPACE DUE TO A MAGNETIC COMPASS ERROR BELIEVED TO BE CAUSED BY THE NAV LIGHTS TURNED ON.

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DEPARTED RWY 9R AT LAL. INSTRUCTED TO PROCEED 3 MI ON RWY HDG THEN PROCEED ON COURSE. ON COURSE HDG WAS 347 DEGS. AT 3 MI TURNED TO HDG 347 DEGS USING WET COMPASS. EXPECTED HVY TFC IN AREA DUE TO SPECIAL EVENT. DID NOT LOOK DOWN INTO COCKPIT TO OBSERVE GPS MAP. COMPASS HAS ALWAYS BEEN ACCURATE. ALT 3000 FT. COMPASS IS ON TOP PANEL FACILITATING TFC SCAN AND MONITORING HDG. WATCHING FOR TFC AND OBSERVING WET COMPASS; APPEARED TO BE ON COURSE. NOTICED DOWNTOWN TAMPA DEAD AHEAD; APPROX 15 MI. IMMEDIATELY DSNDED TO 2500 FT. CHKED GPS WHICH SHOWED COURSE TO BE 272 DEGS TRUE. COMPASS STILL SHOWED HDG 347 DEGS. IMMEDIATELY TURNED N. I WAS AT 2500 FT -- 500 FT BELOW FLOOR OF CLASS B AIRSPACE. PROCEEDED N OVER X39 AND BKY. DISTANCES AND GND TRACK WERE RECORDED ON THE GPS MAP AND RECALLED FOR THIS RPT. NAV LIGHTS WERE ON AS I DEPARTED AT DAWN. TURNED NAV LIGHTS OFF. COMPASS SWUNG L FROM ERRONEOUS HDG TO ACTUAL HDG. I HAVE NEVER FLOWN THIS ACFT WITH THE NAV LIGHTS ON. SUGGEST ALL PLTS VERIFY VARIOUS ELECTRICAL LOADS DO NOT EFFECT THE WET COMPASS.