Means to satisfy 80% bus loading in side-tap

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greens

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In a solar expansion/rework, with an existing PV side-tap, we are faced with a PV combiner where the current is just over the 80% rule.

The customer is willing to disconnect a single existing small inverter, which would just satisfy 80% loading. However, since there are two arrays of panels on two ag buildings, with different insolation, it occurred to us that the max current actually produced by the equipment would be about 67.5%, two to four days of the year.

So I got called in, and was asked to find a way to "make this work." My inclination is to have the production analysis, and to also change the main on the PV combiner to 75%. The idea is that the derated main should not have nuisance trips, but the bus is protected by the derated main breaker.

This is at a residential / small agricultural operation, and if we upsize the PV combiner, we would have to upgrade the service feed, involving a long trenching job, a new combiner box, and the utility is balking at upsizing the transformer.
 
Can't you qualify the bus of the combiner under 705.12(B)(3)(3) (or whatever is the equivalent in earlier code cycles) where you have the breakers contacting the bus totaling to 100% or less of its rating, disregarding the main breaker? Of course, you still have to adhere to 705.12 for all the panels between the PV system and the interconnection.
 
greens said:
In a solar expansion/rework, with an existing PV side-tap, we are faced with a PV combiner where the current is just over the 80% rule.

The customer is willing to disconnect a single existing small inverter, which would just satisfy 80% loading. However, since there are two arrays of panels on two ag buildings, with different insolation, it occurred to us that the max current actually produced by the equipment would be about 67.5%, two to four days of the year.

So I got called in, and was asked to find a way to "make this work." My inclination is to have the production analysis, and to also change the main on the PV combiner to 75%. The idea is that the derated main should not have nuisance trips, but the bus is protected by the derated main breaker.

This is at a residential / small agricultural operation, and if we upsize the PV combiner, we would have to upgrade the service feed, involving a long trenching job, a new combiner box, and the utility is balking at upsizing the transformer.
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Can you provide more details about the feeder, feeder breaker, panel bus rating, and inverter current? I'm unclear on the setup, but first thought that jumps to mind is to use a 100% rated breaker to serve the combiner.
 
electrofelon said:
Can you provide more details about the feeder, feeder breaker, panel bus rating, and inverter current? I'm unclear on the setup, but first thought that jumps to mind is to use a 100% rated breaker to serve the combiner.
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Feeder is 4/0 Al, less than 200 ft. Breaker for combiner is presently 200A, panel bus is 200A (Siemens) and inverter max current is 164A. However there are two arrays, and they are at different angles (roof mount). One is 40deg and the other is 18deg, and the arrays are different sizes. The max current out of the largest array is 116A, and when at the maximum, the smaller array outputs 33A. This means that the max input current will not exceed 149A. That will happen for just a few minutes a few days out of the year. (That's actually why I got called in on this job.) More nominal currents are in the total of 120A combined from both arrays.
 
ggunn said:
Can't you qualify the bus of the combiner under 705.12(B)(3)(3) (or whatever is the equivalent in earlier code cycles) where you have the breakers contacting the bus totaling to 100% or less of its rating, disregarding the main breaker? Of course, you still have to adhere to 705.12 for all the panels between the PV system and the interconnection.
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Thank you for pointing me to that. 2020 NEC states in 705.12(B)(3)(3):
(3) The sum of the ampere ratings of all overcurrent devices on panelboards, both load and supply devices, excluding the rating of the overcurrent device protecting the busbar, shall not exceed the ampacity of the busbar. The rating of the overcurrent device protecting the busbar shall not exceed the rating of the busbar. Permanent warning labels shall be applied to distribution equipment displaying the following or equivalent wording:

WARNING: THIS EQUIPMENT FED BY MULTIPLE SOURCES. TOTAL RATING OF ALL OVERCURRENT DEVICES EXCLUDING MAIN SUPPLY OVERCURRENT DEVICE SHALL NOT EXCEED AMPACITY OF BUSBAR.
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Except when installing inverters, we increment up to the next size OCPD, which results in (upsizing to next breaker size) adding an additional 10A for each inverter (7 in this case). The inverters output a max 29A for 4 of the seven inverters, applying the 125% rule on solar OCPD sizing, this requires 40A OCPD.

In this installation, the max production of the largest array aggregates to 116A, due to array positioning and solar angle, however the aggregate breaker sizing would be 160A.
 
Unfortunately I see no provision for adjusting the total inverter output current due to differential shading or any other reason. Your conductors are undersized. One work around may be to run them at their 90 degree ampacity and change to larger 75 degree conductors for the terminations.
 
electrofelon said:
Can you provide more details about the feeder, feeder breaker, panel bus rating, and inverter current? I'm unclear on the setup, but first thought that jumps to mind is to use a 100% rated breaker to serve the combiner.
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My first thought would be to qualify the combiner bus under a different subsection of 705.12(B).
 
On another subject: When did we start having commercials in our threads?
 
wwhitney said:
So why does section 705 have the 125% factor on the power source output current? If it's because it's continuous, why isn't there an exception for 100% rated breakers?

Cheers, Wayne
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Hmmm I'm not sure....I won't have access to a code book until tomorrow, and I think I would need to look at it before I could comment meaningfully
 
jaggedben said:
It's in there somewhere. Or at least it has been.
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It's in 2017 705.60(B) or 2020 705.30(B). But it's not present in 705.12. So unless those sections are implicitly modifying 705.12, the allowance for 100% rated breakers is useless in a grid-tied system. As every conductor between the service and the inverter has to satisfy 705.12, which requires the 125% factor.

Cheers, Wayne
 
My Siemens distributor seems to be unable to find a 100% breaker for this application. Only for large switchgear.
 
wwhitney said:
It's in 2017 705.60(B) or 2020 705.30(B). But it's not present in 705.12. So unless those sections are implicitly modifying 705.12, the allowance for 100% rated breakers is useless in a grid-tied system. As every conductor between the service and the inverter has to satisfy 705.12, which requires the 125% factor.
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Where does 705.12 require a 125% factor for 'every conductor between the service and the inverter'? It only requires 125% of inverter current to be used in calculations for particular situations. Also you can connect a system under 705.11 (formerly 705.12(A)).
 
jaggedben said:
Where does 705.12 require a 125% factor for 'every conductor between the service and the inverter'? It only requires 125% of inverter current to be used in calculations for particular situations. Also you can connect a system under 705.11 (formerly 705.12(A)).
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So, are you saying that if one were to interconnect a PV system that per 690 requires a set of #1 conductors in a subpanel that it would be OK if the conductors feeding the sub from the main were smaller than #1? I can't say that I have ever encountered a situation like that, but I certainly would never design a system that way. If I need #1 to the sub, I would be sure that there was at least #1 all the way back to the service.
 
ggunn said:
So, are you saying that if one were to interconnect a PV system that per 690 requires a set of #1 conductors in a subpanel that it would be OK if the conductors feeding the sub from the main were smaller than #1? I can't say that I have ever encountered a situation like that, but I certainly would never design a system that way. If I need #1 to the sub, I would be sure that there was at least #1 all the way back to the service.
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I am not saying that. We are talking only about the exception to 705.30(B). It might apply, say, to a supply side connection to a single inverter via a single overcurrent device with no panelboard or tap involved.

I think the OPs stuff is just not rated high enough, for multiple reasons.
 
jaggedben said:
Where does 705.12 require a 125% factor for 'every conductor between the service and the inverter'?
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OK, I spoke a bit too broadly, let me reconsider. The main issue I guess is that (2020) 705.12(B)(1) has no exception for 100% rated breakers. So if you want to interconnect at a feeder that is protected by a 100% rated breaker, you are still limited to 80% of the feeder ampacity for your inverter maximum current. There's no reason I see that for that 80% limit in that case.

Cheers, Wayne
 
greens said:
It appears that the issue may not be the feeder size but rather the bus size.
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There's no question that with 164A of inverter maximum output current, your combiner panel busbar would need to be rated at least 205A, so you need to upsize your combiner panel. Unless there's a PCS system you can install that will monitor the relevant currents and disconnect an inverter if necessary to keep the total inverter current at most 160A. Which disconnection would never actually be required, given the PV geometry you've described.

Then there's the 4/0 Al feeder, which has a 75C ampacity of only 180A. For normal conditions you'd have to limit the maximum inverter output current on that feeder to 144A. Which again could be done via a hypothetical PCS that could disconnect inverters as required. Otherwise, you'd have to upsize the feeder to have an ampacity of 205A. If replacing the 4/0 Al is to be avoided, there are still a couple options:

Electrofelon mentioned using the 90C ampacity of the 4/0 Al, which is 205A. To do that on the grid end, you'd replace your 200A breaker with a 225A breaker, use a short section (4' is typical) of 75C 205A rated conductors to go to a splice box, and use a 90C splicing method to splice to the 4/0 Al. On the combiner end, I'm not sure if MLO lugs are already 90C rated? If so, nothing would be required. If not, you'd again use a splice box and a short section of 75C 205A conductors.

There's also the 100% rated breaker option. As I understand it, a 200A 100% rated breaker would need to be in its own enclosure. So you'd still have the 225A breaker supplying a short section of 75C 205A rated conductors to the 100% rated breaker, at the grid end of the 4/0 Al feeder.

Cheers, Wayne
 
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