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Metered Source panel is 200 Amp, what size direct bury cable do I need for a shop subpanel 400 ft away, Using a 100 amp breaker in the source and panel 80 amp in sub panel. Want 240V at sub-panel cable. Will 1/0 1/0 1/0 aluminum with ground be OK - I assume the sub-pane will be grounded with a grounding rod.

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  • I'm presuming by "with ground" you mean the cable has a fourth wire, right? Also, why limit the subpanel to 80A? Commented Oct 17, 2021 at 0:24
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    Direct burial looks like a great cost savings the first time. After that, not so much.
    – Ecnerwal
    Commented Oct 17, 2021 at 0:45
  • yes 4th wire is ground
    – ken
    Commented Oct 17, 2021 at 2:24
  • Keep in mind that direct burial avoids the cost of conduit but adds (depending on equipment/labor needed) the cost of a deeper trench than with conduit. Commented Oct 17, 2021 at 2:32
  • Conduit adds the complication if pulling the wire into conduit too. Commented Oct 17, 2021 at 5:32

2 Answers 2

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You've got the breakers backwards.

The breaker in the source (main panel) needs to be small so that it protects the wires.

The breaker in the destination (subpanel) needs to be large to avoid nuisance trips. An extreme example would be a 40A breaker on a subpanel connected to a 100A breaker in the main panel. Go to 80A (allowed for continuous usage on a 100A feed) and you'll generally get a trip in less than a minute.

In reality, they are often the same - e.g., 100A for both, which is perfectly fine because for most people, except for a true short-circuit situation, rarely trip the main breaker of a subpanel or of a main panel. Even with a short-circuit, almost always the branch circuit breaker will trip first.

However, you could, for example, use a "main panel" as a large subpanel, that comes with a 200A main breaker. That's perfectly fine to use with a 100A feed from the actual main panel, as the 200A breaker will function as a disconnect switch but not actually be needed for over-current protection.

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By NEC you could protect 1/0 AL at 100A at both ends. Loading it to 80A would result in a 5.3% voltage loss, which would leave you in great shape. Loading #1 to 80A would result in 6.7% loss, which is more than "recommended" by the NEC, and if branch circuit length in the shop are excessively long you could suffer greater loss that could effect performance.

Since you're using an increased size for voltage loss you will need to upsize the ground too, 75°C 1/0 Al is rated for 120A, so #6 copper or #4 Aluminum ground.

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