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I am feeding a subpanel from a main panel, and the subpanel will have a main breaker installed. Do I need to feed the subpanel from a breaker in the main panel, or can I use subfeed lugs?

I understand from this question that my subpanel on a different building needs to have a "main" breaker, so the feeder will have a breaker at the load side regardless. The question is whether or not I need it on the line side or if subfeed lugs are appropriate.

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    Wait. The subpanel feeds a sub-sub-panel. Don't you need a breaker in the subpanel to protect the wiring to the sub-sub-panel? Does that not suffice? Also, just as a general thing, how are you handling "don't electrocute your linemen" gen/utility interlocks, since the battery-solar system is kind of a generator? – Harper - Reinstate Monica Feb 1 '18 at 9:24
  • @Harper I think your answer here (diy.stackexchange.com/questions/132150/…) works to answer that question. Also, modern grid-tied inverters are utility-interactive, requiring powerline communications carried on a 60Hz, 240VAC supply. WIthout that, they shut themselves down. Built-in safety for linemen. – Hari Ganti Feb 1 '18 at 18:30
  • Also, I am going to edit this into a more broadly applicable question. – Hari Ganti Feb 1 '18 at 18:30
  • OK. I was confused because your earlier edit mentioned "battery", which implies a more complicated affair than a simple grid-tie (which I'm sure you know dies when the grid fails). – Harper - Reinstate Monica Feb 1 '18 at 19:41
  • Oh, I forgot to mention, but some of these systems use batteries with the same utility-interactive microinverters, meaning they can be programmed to draw energy at certain times and redistribute it at others, but they can't provide offline power because the battery inverters are also grid-tied. – Hari Ganti Feb 1 '18 at 20:24
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If the wire size from the sub panel is the size that the main breaker in the panel being fed from you don't have to have the breaker but this means if you have a 200 a main breaker in that panel the wire ampacity will need to be that large. Most subs are smaller and it saves $ to install a breaker and feed the sub with smaller wire. Not sure about the comment on not electrocute the line man, is this for a generator or photovoltaic system? I don't see that in the question.

Edit to clarify from comment. A 125 amp main panel with a 125 amp main braker can direct feed anything larger than 125 A with 2/0 since 2/0 at 75 column is rated for 175a. The run between panels in my area need to be in Ridgid metal conduit because I was replacing the original panel. Some AHJ will limit the distance the panels can be seperated if this is the case. I have done this several times when preparing to replace the original panels (60 & 100 amp to 200) In one case the AHJ did require a 125 amp breaker in the sub, until the service was replaced I thought this was silly because the main breaker in the main panel is still the limiting factor. I did use large enough pipe so when I was ready to convert the service I just had to pull new feeder wire to the meter base so power to the house was only off for a few minutes. Green tag and final prior to meter connection since everything else was already run.

  • I edited my question. It was originally about a subpanel that I am using to aggregate solar inputs, but I decided it was better to keep the scope broad. – Hari Ganti Feb 1 '18 at 20:25
  • Also, can you clarify what you mean a bit with an example? I am still a little confused. Let's say I have a 125A panel feeding a subpanel rated for 200A (because I do). Can I put subfeed lugs in the 125A panel, wire it with 2/0 AWG Cu, and feed a 200A breaker in the 200A subpanel? – Hari Ganti Feb 1 '18 at 20:28
  • Editing again to further clarify question intent. – Hari Ganti Feb 1 '18 at 20:28
  • Now I understand. Essentially, if the main feeds a larger subpanel with larger wires than feed the main, you don't need an additional breaker. – Hari Ganti Feb 2 '18 at 2:48

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