The　objective　of　this　paper　is　to　determine　arcing　contact　gap　of　a　126-kV　horseshoe-type　bipolar　axial　magnetic　field　vacuum　interrupter.　Drawn　arc　experiments　were　carried　out　in　a　demountable　vacuum.　Experimental　current　ranged　from　12-to　40-kA　rms.　Based　on　the　vacuum　arc　behavior,　minimum　arcing　contact　gap　D-1　and　maximum　arcing　contact　gap　D-2　were　defined.　When　average　opening　speed　within　20　mm　(v(1))　was　1.9,　2.3,　and　2.6　m/s,　D-1　was　determined　as　15.3,　16.8,　and　14.3　mm,　respectively.　In　addition,　D-1　reflected　a　peak　effect　with　v1.　When　average　opening　speed　within　40　mm　(v(2))　was　2.6,　2.7,　and　2.8　m/s,　D-2　was　determined　as　38.5,　36.7,　and　36.1　mm,　respectively.　Uncontrollable　arc　mode　appeared　when　current　was　higher　than　32　kA　with　arc　duration　of　9　ms.　The　critical　contact　gap　to　avoid　uncontrollable　arc　mode　decreased　as　a　liner　function　of　arc　current.　Based　on　the　experimental　results,　quantitative　design　method　of　three-interval　opening　velocity　for　the　126-kV　horseshoe-type　vacuum　circuit　breaker　was　proposed.　For　short-circuit　current　interruption　test　duty　T100s,　lower　limit　value　of　v(1)　and　upper　limit　value　of　v(2)　were　determined　as　2.5　and　2.9　m/s　with　a　prospective　minimum　arcing　time　of　7　ms.