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I’m going to be installing a 200A meter base with a 200A breaker in it, which acts as an external disconnect (required in California). Here’s the spec sheet for it.

The load center I’m using (model LP422-MB) has a 225A main breaker in it. Here’s the spec sheet for it.

I’m pretty sure that between the load center and the meter base I need 250 MCM aluminum SER cable. Right?

But where I start to get fuzzier is between the meter base and the service drop. The 200A meter is rated for 200A continuous load but something like 250A max load. And the load center is rated for 225A. The amperage for aluminum XHHW is:

  • 200A - 4/0
  • 250A - 300 MCM
  • 225A - 250 MCM

So… which one do I go off of? 😅

Thanks for your help! 🙏🏼

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  • I take it the loadcenter and the meter/main are in the same structure? Jul 2, 2022 at 1:39
  • Yep! The meter is just on the outside and the load center is inside (in the garage). Jul 2, 2022 at 1:52
  • I take it the main breaker is just there to provide a local disconnecting means for the panel then? Jul 2, 2022 at 13:55

1 Answer 1

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This is a 310.15(B)(7) question.

Generally, ampacity must be taken straight off Table 310.15(B)(16), rounding disfavorably. The 75C thermal rating is used, since the breaker panel is rated for 75C (but not 90C) and every modern cable (except NM or UF, rarely used for feeder) are rated 75C.

This brings us to 250 kcmil for 200A.

However. Services to a dwelling get a discount.

When service wires power an entire dwelling, because of the load factors typical of dwellings, they get a 17% discount (83% favorable derate) - so e.g. a 200A service only requires 166A wire.

When we select wires from Table 310.15(B)(16), 4/0 wire (180A) is the minimum size (exceeding 166A).

It also says feeders never need to be larger.

You don't have service wire. You have feeder. However, here's what they don't do: allow you 4/0 for service wire but require 250 kcmil for the feeder coming off that. That would be stupid.

So 310.15(B)(7) says your feeder never needs to be larger than your service wire is allowed to be. Since 4/0 is acceptable as service wire, it's acceptable as feeder for any circuit.

What this doesn't do is use the 83% favorable derate on smaller feeders. If you run a 100A subpanel, you must run proper 100A wire, not "83A" wire.

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  • This was incredibly helpful! Thank you so much for the detailed explanation! ❤️ Jul 4, 2022 at 4:45

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