I am installing a 200A main breaker panel as my service disconnect. The Eaton BR816B200RF panel will have feed thru lugs on the bottom. I want to feed a 150A sub panel from those lugs. The subpanel will be 50 ft away and have it's own 150A breaker.

I would rather not run 4/0 Al SER cable if 2/0 Al is sufficient to feed the 150A subpanel. Not sure why the cable would have to be sized for 200A if it will never see current greater than 150A. This work will be inspected but in any case I want to follow the code. Can I use the 150A cable? Thanks.

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    Short answer: we don't wire things for the best case but rather the worst case. – Harper May 5 '17 at 16:56
  • How many circuits will the sub-panel accommodate? Wish I had feeder lugs on my GE panel. My main, and only, panel (150 A) holds a total of 20 slots (split-bus w/ four full-sized 2-pole above and 16 half-sized below) and it is completely full. The few people in my neighborhood of tract houses who have upgraded don't install a sub panel; they just put in a new and much larger main panel, usually 200 A with 40 one inch slots. – Jim Stewart May 6 '17 at 0:05
  • What are the considerations that favor main plus sub-panel vs a single large panel? Is the modern approach all 20-A branch circuits or a mixture of 15-A and 20-A? – Jim Stewart May 6 '17 at 0:24
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    I am rewiring a large house and the subpanel must be far away from the disconnect. More spaces are needed at the sub than at the main. I think it will be a cleaner installation to have one cable running through the attic than dozens of small ones running in multiple directions. – Phil F. May 6 '17 at 13:23
  • That's what I figured but wanted it confirmed. How many slots will be in the subpanel? Will you be putting in both 15-A and 20-A circuits? – Jim Stewart May 6 '17 at 13:50

The conductors will have to be full size to the 200A breaker. Remember what protects the conductor is the 200A breaker if you reduce the conductor size there must be an additional breaker to protect the the current carrying conductor. NEC does allow for the feeder to be reduced for the final 1/3 distance of a feeder which is probably impractical, but it does allow the subpanel to have 150A buss.

  • Did you get that that the SER cable conductors will have a breaker on each end? 200A on one and 150A on another. Is there a way where the current could exceed 150A? Certainly no one is going to tap into it. – Phil F. May 5 '17 at 22:07
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    Yes, there are scenarios which would allow more than 150 amps to flow, it's called a short circuit or ground fault. The wires will be subject to current flow up to the rating of the breaker protecting them if the current goes somewhere upstream of your 150amp panel. Then the wire heats up and fails, then you have a fire. – Jimmy Fix-it May 5 '17 at 22:36

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