Remove Combiner Panel and fuse - PV System

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inforaj

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After a couple of weeks of design and approval of the city, the subcontractor advised the management to remove the combiner panel, and add a circuit breaker to the main panel-board (Existing MDP) and also remove fused from safety switches ( means install non-fused) during the construction time to save construction money.
Would you please advise, is this 100% secure if these pieces of equipment are removed from the project?
 
Without knowing the details of the system it's impossible to say, but it sounds like someone decided that the maximum inverter output current X 1.25 is less than the difference between 120% of the MDP bus rating minus the rating of the main breaker.
 
I can't tell much from your description but I can say this, swapping a fused switch for an unfused switch behind a circuit breaker might be okay. Most unfused switches behind a circuit breaker of the same rating have an AIC rating of 10kA, so you need to know if that is a sufficient rating for the available fault current.
If multiple inverters are being combined in an AC combiner it would be difficult to replace that with a single circuit breaker.
 
ggunn said:
Without knowing the details of the system it's impossible to say, but it sounds like someone decided that the maximum inverter output current X 1.25 is less than the difference between 120% of the MDP bus rating minus the rating of the main breaker.
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These are 42 KW-AC systems. 480V
 
inforaj said:
These are 42 KW-AC systems. 480V
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That still doesn't tell me much. Was the PV system, including the interconnection, designed by a qualified person? It sounds like the design is being changed in the field from a supply side to a load side interconnection; does whoever made that decision know what he or she is doing? If the answer is yes to both I would say that your concerns are probably unfounded. Generally speaking, supply side interconnections require fused disconnects while load side interconnections do not.
 
ggunn said:
That still doesn't tell me much. Was the PV system, including the interconnection, designed by a qualified person? It sounds like the design is being changed in the field from a supply side to a load side interconnection; does whoever made that decision know what he or she is doing? If the answer is yes to both I would say that your concerns are probably unfounded. Generally speaking, supply side interconnections require fused disconnects while load side interconnections do not.
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It's a viable design but does not look like the best design to me based on the info in this drawing. If there is only one inverter there is no need for the AC combiner. If there is a CB position available in the existing panel it might be a better POI than tapping the bus. The callout on the switch says it's fused but the fuse is not shown in the switch. Don't skimp on the details and hope someone reads the text. Anyway, the contractor might be correct and they "fixed" the design. But I would check their work to make sure it complies with the NEC.
 
ggunn said:
Hmmmm... Well, it looks to me like there may indeed be some problems with this design, but it's not my project. Is there a PE responsible for reviewing, revising, and sealing the design packet for this project? There certainly should be.
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Not sure what kinds of problems you found with this design. PE and the city have already approved this drawing. If possible, can you please let me know?
 
pv_n00b said:
It's a viable design but does not look like the best design to me based on the info in this drawing. If there is only one inverter there is no need for the AC combiner. If there is a CB position available in the existing panel it might be a better POI than tapping the bus. The callout on the switch says it's fused but the fuse is not shown in the switch. Don't skimp on the details and hope someone reads the text. Anyway, the contractor might be correct and they "fixed" the design. But I would check their work to make sure it complies with the NEC.
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Here is the PDF file snapshot.
 
It sounds like this is a value engineering situation. You have a PE and AHJ approved design and the contractor value engineered it. This happens all the time. There is probably nothing wrong with your design, it was just more expensive than what the contractor thought was needed. The thing I do in this situation is to make sure that what the contractor did still complies with the code. It sounds like you are in the position to approve or not approve what the contractor suggested, so it's up to you what you want to do. The management will probably want to know why you think the more expensive design is necessary.
 
pv_n00b said:
It sounds like this is a value engineering situation. You have a PE and AHJ approved design and the contractor value engineered it. This happens all the time. There is probably nothing wrong with your design, it was just more expensive than what the contractor thought was needed. The thing I do in this situation is to make sure that what the contractor did still complies with the code. It sounds like you are in the position to approve or not approve what the contractor suggested, so it's up to you what you want to do. The management will probably want to know why you think the more expensive design is necessary.
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That's true. The management are thinkings about the design.
 
The SMA Core1-50-US inverter has a max output current rating of 64A. Times 1.25 equals 80A, which meets the 120% rule in the 400A rated MDP. So it looks like there is no issue with interconnecting at a breaker in the MDP as long as there is space in the panel opposite the main breaker. If that is the case, it looks like the fused disconnect and the combiner panel are unnecessary.

It almost looks like someone was designing for future expansion.
 
PWDickerson said:
The SMA Core1-50-US inverter has a max output current rating of 64A. Times 1.25 equals 80A, which meets the 120% rule in the 400A rated MDP. So it looks like there is no issue with interconnecting at a breaker in the MDP as long as there is space in the panel opposite the main breaker. If that is the case, it looks like the fused disconnect and the combiner panel are unnecessary.

It almost looks like someone was designing for future expansion.
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I don't see that it is interconnecting through a backfed breaker. The drawing looks like it is coming in on a tap on the bus, and it's not clear that it would be at the opposite end of the bus from the main. It's not drawn well. The OP said that they had removed the fuses, but the inverter output conductors must be protected at the utility end either with a fused disco or a backfed breaker. The earlier white on black drawing showed neither. I would have never sealed either of these drawings.

There is no room for expansion on the load side of the MDP.
 
I didn't mean to suggest that the drawing indicated the system was backfed at a breaker, only that the design could be easily changed to interconnect at a breaker (if space is available), which is what I suspect the contractor is pushing for. If the system were to backfeed a breaker in the MDP, the redundant OCPD's in the fused disconnect and the AC combiner panel could be removed.

One could convert to a line-side interconnection in order to expand the system, and there is tons of space in the conduits to pull larger wires, but the fused disconnect and AC combiner panel don't have much additional capability, so on second thought, that probably wasn't the goal.
 
PWDickerson said:
I didn't mean to suggest that the drawing indicated the system was backfed at a breaker, only that the design could be easily changed to interconnect at a breaker (if space is available), which is what I suspect the contractor is pushing for. If the system were to backfeed a breaker in the MDP, the redundant OCPD's in the fused disconnect and the AC combiner panel could be removed.
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The way the MDP is drawn, I cannot tell how the system is interconnected. As I said, I would never seal a drawing that looked like that.
 
ggunn said:
The way the MDP is drawn, I cannot tell how the system is interconnected. As I said, I would never seal a drawing that looked like that.
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My guess would be feed through lugs from the note. But it does leave the details to the contractor.
 
pv_n00b said:
My guess would be feed through lugs from the note. But it does leave the details to the contractor.
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That would be my guess as well, but as I said i would never draw or seal a drawing that left that "detail" out.
 
ggunn said:
That would be my guess as well, but as I said i would never draw or seal a drawing that left that "detail" out.
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I can understand that. But I see a lot of sealed drawings where the circuit terminates at an existing bus or something with a note to the effect that, "Contractor to install in compliance with codes and equipment manufacturer's requirements." No details were provided. So many, and on all size systems.
 
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