Polaris C-400 Tinned Copper Splice- Long Barrel, 400 Mcm
https://nsiindustries.com/product/polaris-c-400-tinned-copper-splice-long-barrel-400-mcm/
Splicing conductors are the professional’s choice for making quick in-line connections and repairing damaged wiring. Use POLARIS splice connectors for bonding solid or fine stranded UL-listed wires in harsh environments.