I am splicing some lamp cables. The cable is SPT-2 18-2 lamp wire. First I will be using some vinyl insulated butt splices rated for 22-18 AWG and 600v to make the connection. Then, I want to cover the splices with heat shrink tubing. I'm having a hard time finding definitive results on what size tubing to use, though I am inferring from various sources that it's probably 3/4" polyolefin tubing that I need. Can anyone confirm or deny this, or otherwise make a suggestion?
You should not be splicing lamp wires outside the fixture or lamp. If you need to splice in the middle of a run you should replace the cord.
Inside a fixture or lamp no heat shrink is required or needed.
Heat shrink tubing just needs to fit over the area to be covered before shrinking.
Article 400 Flexible Cords and Cables
Flexible cord shall be used only in continuous lengths without splice or tap where initially installed in applications permitted by 400.7(A). The repair of hard-service cord and junior hard-service cord (see Trade Name column in Table 400.4) 14 AWG and larger shall be permitted if conductors are spliced in accordance with 110.14(B) and the completed splice retains the insulation, outer sheath properties, and usage characteristics of the cord being spliced.