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My original plans have changed and I now am stuck with having direct burial cable 4/0-4/0-2/0-with 1/0 ground. My new plan is now to run sub panel from my home 200 amp main breaker panel to the shop. My problem is I am using a 100 amp breaker and the larger cable wires will not fit into the 100 amp breaker. Can I use the larger cable for burial and use connectors inside a junction box to splice in the last few feet of wire to fit the breaker ?

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Make the splice in the panelboard itself

I'd use a two-pole aluminum rated power distribution block of suitable size listed to UL 1953 and rated to IP20 (such as a pair of Eaton/Cooper PDBFS303s, dovetailed together) to make the splice inside the panelboard cabinet instead of in a separate junction box. Then you can run a pair of 2/0 jumpers from the distribution blocks to the panelboard terminals.

This takes up a bit more panelboard cabinet space, but is neater and cleaner than having splice lugs hanging loose in the panelboard or having a separate box just to splice wires. Check with your AHJ first though, as there is the off chance that they're not OK with mounting things to a panelboard cabinet that the manufacturer did not originally intend to go there.

(Making the splice in a separate box is also possible, just slightly silly as you'd need more wire and parts to do that vs. doing the splicing in the panelboard cabinet.)

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    I agree with TPE, if you have the room for some terminal blocks in the panel. Another option is a screw-cover-junction-box attached to the panel with a short nipple, with the taps in there. You could then use #3 copper lead wires from the 100 amp breaker as long as the terminals are rated for copper AND aluminum. Good luck! – ArchonOSX Jul 1 '17 at 23:45
  • Can the downvoter here please explain what's wrong with my suggestion? – ThreePhaseEel Jul 5 '17 at 22:12
  • I have used similar power blocks to change the size of feeders and tap feeders. I always always always use an oxide inhibitor if aluminum wire or lugs are involved. If there is room in the panel this would be the best place for the junction because it would require at least an x6" box or larger depending on the hole size for the wire if it was an angle pull x8" for straight with 4/0 that is going to be a big box so inside the panel is the cheapest way. – Ed Beal Jan 12 '18 at 16:38

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