I am relocating a subpanel to a new location and the existing #4 wire is too short by a few feet. I would rather splice a new piece of wire to it an extend the wire than replace the whole cable as its pricey. What do I use to accomplish this? Is this permitted by the NEC?

Thanks.

Yes, splicing wire is allowed by the NEC, providing the splices are made up properly using the correct wire connectors, contained in a properly size junction box with the proper cover, and that the box is accessible.

If you need to know what parts are needed, we need to know exactly what type of wire, raceway, etc you have or want to extend.

It is 3c#4 copper wire sized for a 70 amp breaker. It is encased in a grey plastic jacket. Does that describe it well?

Need a 8" x 8" x 6" junction box, PVC would work fine.

Connectors for the cable you have, 1" two screw squeeze.

Insulated mechanical wire connectors with two ports sized to handle two #4 copper wire. Various styles are available in different areas, so this needs to be confirmed. Connecting these wires are the critical part of the job. Loose, poor connections create power failures and fires.

You need to be sure all cable connectors are tight, but not crushing the cable, no exposed copper after inserting into the mechanical wire connectors.

The box needs to be accessible, not hidden in any way.

You mention 3 wire, I hope this is plus ground for a total of 4 wires in the cable. Code now require 4 wire cable for this feeder.

You could use #2-4 SER cable with aluminum wire, follow instructions for terminating, and replace the entire cable. Aluminum works fine and is much cheaper than copper.

This is wise code requirement to consider.