NSI Polaris Insul-Tap Connector, for Two Wires and in-Line Splicer/Reducer, ISR Series, 250-6 AWG Wire Range, 4.00" x 1.19" x 1.90", Pack of 4: Screw Terminals: Amazon.com: Industrial & Scientific
NSI IT-750 750-250 MCM POLARIS INSULATED TAP CONNECTOR (DUAL SIDED ENTRY) | Gordon Electric Supply, Inc.
Polaris 4-14 AWG Bagged Insulated Tap Connector, Black IT-4B - The Home Depot