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Thread: Splitting up AC output after meter base?

  1. #11
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    Quote Originally Posted by bbartmasse r View Post
    Thanks for the replies everyone.

    These are two, 200 amp services on the side of the residence fed form a single 400 amp service with one meter. Sorry for the confusion, no matter how hard I try there is always something I leave out that leads to more questions.

    I understand I can downsize one of the panels to a 175 main and be on my way but that was the point of the question, to see if there were other ways that I was overlooking. Its a shame downsizing the main when you have that much bussing right in front of you.

    This is a good lead in to line side taps........I live in a place that does not allow them, which means I have never done one. Where do you actually "feed in" when doing a line side tap? Also, I know our inspectors always look for the breaker to be installed(in service panel) at opposite end of where the service is fed, this way the solar can theoretically supply the loads in house first, before heading to grid.
    In a line side situation, am I right thinking there is no way to accomplish this. That might be part of why they do not allow it, Im not sure. I will ask the utility why.

    Thanks
    The placement of a backfed breaker at the opposite end of the bus from the main doesn't have anything to do with preferentially supplying loads, it's to conform with 705.12(D)(2)(3)(b) and prevent hot spots on the bus. The sources and loads are all in parallel; if the bus OCPD and 125% of your inverter current sum to less than 100% of the bus rating, you can put the backfed breaker anywhere on the bus because then you would comply with 705.12(D)(2)(3)(a).

    With a supply (line) side connection your point of interconnection is between the meter and the first OCPD. In a case where you have a meter supplying two MDPs with no OCPD upstream of them, maybe there is a gutter where the service conductors split to feed the panels. If so the gutter would be a logical place to put your connection.

  2. #12
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    400A feeder goes to the meter and splits into a Y branching off into two 200A panels, correct?

    If you feed from the bottom, its like you're helping push a load from the bottom that is being hoisted from the top which means you can't overload the rope. If you join at the top, the rope carries your effort as well as the crane's and make it possible for rope to carry load beyond the crane's full load rating. This is the reasoning for sandwiching the load breakers between utility supply and solar supply.

    You could back feed at the bottom of either panel and it makes no practical difference. If your generation at the time is 4kW and combined power usage on customer side of meter is 3kW, the net export is 1kW.

  3. #13
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    Quote Originally Posted by Electric-Light View Post
    400A feeder goes to the meter and splits into a Y branching off into two 200A panels, correct?

    If you feed from the bottom, its like you're helping push a load from the bottom that is being hoisted from the top which means you can't overload the rope. If you join at the top, the rope carries your effort as well as the crane's and make it possible for rope to carry load beyond the crane's full load rating. This is the reasoning for sandwiching the load breakers between utility supply and solar supply.
    The problem with an analogy is that, well... it's just an analogy. In this case I believe that is is easier to understand what is actually going on than to make the leap from the physical to the electrical, especially since all of us in here have at least some understanding of electricity.

    If a backfed breaker (BFB) feeds onto a bus between the main breaker and some loads, then the loads belyond the BFB can draw from both the main breaker and the BFB through a section of the bus and overload it if the sum of the available current from the two breakers is greater than the bus rating.

  4. #14
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    Quote Originally Posted by Electric-Light View Post
    If you feed from the bottom, its like you're helping push a load from the bottom that is being hoisted from the top which means you can't overload the rope. If you join at the top, the rope carries your effort as well as the crane's and make it possible for rope to carry load beyond the crane's full load rating. This is the reasoning for sandwiching the load breakers between utility supply and solar supply.
    I don't think that's the reasoning. The reasoning is that no one tests for the thermal effects of a panelboard busbar being fed from two different sources, and it is conservatively assumed that having the sources spread out will also spread out the thermal load. There's some Kirchoff's law involved in that theory as well, since if the sources are at opposite ends you can't have more current than the busbar is rated for at any point.

    But this is drifting off topic.

  5. #15
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    Quote Originally Posted by jaggedben View Post
    I don't think that's the reasoning. The reasoning is that no one tests for the thermal effects of a panelboard busbar being fed from two different sources, and it is conservatively assumed that having the sources spread out will also spread out the thermal load. There's some Kirchoff's law involved in that theory as well, since if the sources are at opposite ends you can't have more current than the busbar is rated for at any point.

    But this is drifting off topic.
    Ya think?

  6. #16
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    Quote Originally Posted by jaggedben View Post
    I don't think that's the reasoning. The reasoning is that no one tests for the thermal effects of a panelboard busbar being fed from two different sources, and it is conservatively assumed that having the sources spread out will also spread out the thermal load. There's some Kirchoff's law involved in that theory as well, since if the sources are at opposite ends you can't have more current than the busbar is rated for at any point.

    But this is drifting off topic.
    That is the reasoning. If you feed a duplex with two identical pairs of #12 and connect them both to 20A breakers on the same phase you could have 40A total at the receptacle but if one pair carries more than it should, the breaker on that side will trip and neither pair of wires can overheat.
    However, if you "tap" a 15A solar into a box somewhere in between the branch and the receptacle, the section between the receptacle and junction box could carry 35A without anything abnormal being detected by either breaker.

    If you add a

  7. #17
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    Thanks everyone,

    So are line-side taps allowed in most places? Why would they not be allowed? Perhaps it's just a matter of talking with them.

    thanks

  8. #18
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    From your description it sounds like you have a 320A line current metered service that feeds two 200 amp rated main breaker panels each with a 200 amp service disconnect breaker. I am also in WA, and I see this configuration at larger residences all the time. There is a way to install up to 200 amps of solar backfeed (160 amps of inverter output) on the load side of one of the main breakers, although you will have to relocate the breaker and reconfigure one of the main panels.

    If you swing one set of service conductors out of the meter base over to a 200 amp fused disconnect or a 200 amp circuit breaker enclosure, and then feed one of the two main panels from there (you will have to remove the bonding jumper, add an EGC, and separate the grounds and neutrals at the existing 200 amp panel), this provides the opportunity to tap the feeder conductors between the new service disconnect the 200 amp panel (which is now a sub-panel) and use the tap as the solar interconnection point. Because there is no bussing upstream of the interconnection point, the 120% rule does not come into play. Because you have the 200 amp main breaker in the existing 200 amp panel, you won't need to upsize the conductors between the tap and the existing panel. I have never done this, but I believe it is permitted under the code and would still be considered a load side connection.

  9. #19
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    Quote Originally Posted by bbartmasse r View Post
    So are line-side taps allowed in most places? Why would they not be allowed? Perhaps it's just a matter of talking with them.
    Allowed everywhere I've worked.

    Of course it's not possible on all equipment. Residential meter/main combos with factory bussing between the meter and main breaker are basically a no-go. Tapping in a crowded panel or a small ancient disconnect may violate wiring space and bending rules and just be a terrible idea.

    But in principle, it should be allowed.

    I would encourage talking to 'them', if you haven't already.
    Ask simple, respectful questions, such as:
    'Is this actually a policy or just a rumor I heard from someone?'
    'Where is the documentation for your policy on this?'
    'What is the reasoning behind your policy?'
    'How is this different from other supply side taps which are allowed (e.g. 230.40 exception 2)?'

  10. #20
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    There is a lot of good information to learn from here:

    https://www.nyserda.ny.gov/-/media/N...ility-Grid.pdf

    I don't dispute the theory and understand what they're talking about but personally think of their reasoning are far too conservative.
    From utility's perspective whenever someone mess around upstream of the main breaker, it increases the possibility of blowing or tripping utility side protective device if someone mess up and causing outage to other customers.
    Last edited by Electric-Light; 09-09-17 at 12:43 AM.

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