There are various types and gauge sizes of butt connectors available at www.princessauto.com . The butt connector yo will need will depend on the gauge and type of wire you will be splicing.
The largest butt connectors are for 10-12 gauge wires and have yellow insulation.
A butt splice with blue insulation is for wires that are 14-16 gauge.
Red insulated butt connectors are for 18-22 gauge wires.
It may even be necessary to use a step down butt splice that goes from a large to a small gauge wire.
Happy wiring!
-Princess Auto