PV Output Circuit with 100% Rated CBX

[bwat]

Unless you're using 100% rated equipment, or other considerations, your PV source and PV output circuit must abide by the "156%" rule. 125% for the solar insolation above STC, and then another 125% for continuous use considerations.

If you use a DC combiner (CBX) that has at 100% rated disco on PV output side, do your conductors on the PV output still have to be 156% x Isc x number of strings? Or is just 125%? It's not clear to me in 690. Perhaps it could, but maybe you have to consider the fuse at the (central) inverter?

If putting in a CBX with 100% rated disco really saves you from having to increase all of your PV output circuits by 125%, and there's no other "gotcha", this seems like it's really the only option most should consider.

 

[jaggedben]

What code cycle?
Above or below 100kw?

There have been some subtle changes but I believe that 690.8(A) has always held that circuit current is 125% of module Isc if you're below 100kw. Now you can use another method over 100kW. Regardless, any allowances for 100% continuous rated stuff is after you determine that circuit current. That's how I read the 2017 NEC.

 

[bwat]

What code cycle?
Above or below 100kw?

There have been some subtle changes but I believe that 690.8(A) has always held that circuit current is 125% of module Isc if you're below 100kw. Now you can use another method over 100kW. Regardless, any allowances for 100% continuous rated stuff is after you determine that circuit current. That's how I read the 2017 NEC.

2017 NEC (I thought it was still in my signature, but maybe it’s not showing up)

Over 100kW. Small utility scale.

We could do something like the Sandia method, but assuming we weren’t, would output of combiner to the inverter have to be 125 or 156% of Isc? I’m reading it as 156% even if combiner disco is 100% rated at the 125% Isc.

(Is another answer here that anything in MW scale, everybody uses Sandia method?)

 

[wwhitney]

We could do something like the Sandia method, but assuming we weren’t, would output of combiner to the inverter have to be 125 or 156% of Isc? I’m reading it as 156% even if combiner disco is 100% rated at the 125% Isc.

Does the combiner have OCPD? If it does, and it's 100% rated, and everything else in the circuit has a sufficient continuous rating, then 690.8(B)(1) Exception tells you that you don't need a second 125% factor. If it does and it's not 100% rated, then you definitely need the second 125% factor.

But if it doesn't have OCPD, the situation is a bit unclear if everything in the circuit has a sufficient continuous rating. It depends on whether you interpret the phrase "an assembly, together with its overcurrent device(s)," to imply that the assembly has to have an OCPD in it. Or if you read it to implicitly mean "overcurrent device(s), if any".

Cheers, Wayne

 

[bwat]

Does the combiner have OCPD? If it does, and it's 100% rated, and everything else in the circuit has a sufficient continuous rating, then 690.8(B)(1) Exception tells you that you don't need a second 125% factor. If it does and it's not 100% rated, then you definitely need the second 125% factor.

But if it doesn't have OCPD, the situation is a bit unclear if everything in the circuit has a sufficient continuous rating. It depends on whether you interpret the phrase "an assembly, together with its overcurrent device(s)," to imply that the assembly has to have an OCPD in it. Or if you read it to implicitly mean "overcurrent device(s), if any".

Cheers, Wayne

Thanks.

If the CBX has OCPD, and it's 100% rated, but if there's also a fuse or CB at the inverter, would this inverter OCPD have to be 100% in order for the conductors to not be an additional 125%? The way I understand some of the 125% continuous rule is that it's because of heat dissipation in the OCPD. If you don't have enough surface area of the wire when running at near the rating, that's a problem with the OCPD. I think this is what you meant by everything in the circuit. I would say this includes any OCPD at the inverter.


FWIW, it's looking like the CBXs won't have OCPD on output side, just an un-fused disco, so we're into that gray area. (Somewhat) glad to hear it's not clear for you either in this scenario.

 

[wwhitney]

Thanks.

Quick correction to my previous post: I assumed that any OCPD in the circuit would be at the combiner. But you indicate that there could be OCPD at the inverter. So really my post should have referred to OCPD in the circuit, rather than at the combiner. And certainly "everything else" would cover other OCPD in the circuit, if there is more than one.

Anyway, this is just my reading of the exception--I have no relevant experience with this type of equipment.

The way I understand some of the 125% continuous rule is that it's because of heat dissipation in the OCPD.

To my understanding, the only reason that there is a 125% continuous factor in the NEC is due to OCPD limitations. If all OCPD were 100% rated, then all those 125% continuous factors would go away. [But not the 125% factor in 690.8(A).]

Cheers, Wayne

 

[ggunn]

Thanks.

If the CBX has OCPD, and it's 100% rated, but if there's also a fuse or CB at the inverter, would this inverter OCPD have to be 100% in order for the conductors to not be an additional 125%?

I do not think that a 100% rating (or not) of the OCPD changes the minimum size of the conductors.

 

[wwhitney]

I do not think that a 100% rating (or not) of the OCPD changes the minimum size of the conductors.

What about 690.8(B)(1) Exception which we have been discussing?

"Exception: Circuits containing an assembly, together with its overcurrent device(s), that is listed for continuous operation at 100 percent of its rating shall be permitted to be used at 100 percent of its rating."

Cheers, Wayne

 

[ggunn]

What about 690.8(B)(1) Exception which we have been discussing?

"Exception: Circuits containing an assembly, together with its overcurrent device(s), that is listed for continuous operation at 100 percent of its rating shall be permitted to be used at 100 percent of its rating."

Cheers, Wayne

What about the terminals on the inverter? If they are 75 degree rated I don't see how you can avoid the rule that 125% of the inverter output current cannot exceed the 75 degree ampacity of the conductors.

 

[wwhitney]

What about the terminals on the inverter? If they are 75 degree rated I don't see how you can avoid the rule that 125% of the inverter output current cannot exceed the 75 degree ampacity of the conductors.

If the inverter doesn't incorporate OCPD, what reason is there to believe that their terminals aren't "listed for continuous operation at 100 percent" of their rating? In which case the exception is telling you that the conductors only need to have an ampacity matching the inverter output current.

Cheers, Wayne

 

[ggunn]

If the inverter doesn't incorporate OCPD, what reason is there to believe that their terminals aren't "listed for continuous operation at 100 percent" of their rating? In which case the exception is telling you that the conductors only need to have an ampacity matching the inverter output current.

I don't know that they aren't but the burden of proof is on the inverter manufacturer. If it isn't documented that the terminals are thus rated I have to assume that they are not.

 

[wwhitney]

I don't know that they aren't but the burden of proof is on the inverter manufacturer. If it isn't documented that the terminals are thus rated I have to assume that they are not.

Agreed that it should be confirmed with the manufacturer. Personally, I would be surprised if the inverter thermal design is dependent on the extra heat sinking you would get if the wire terminations on the DC side are upsized by a 125% continuous use factor. Would be curious if anyone has ever inquired.

Cheers, Wayne

 

[bwat]

Why did we switch over to talking about inverter output? Is this related or similar to the circuit from CBX to inverter?

I do not think that a 100% rating (or not) of the OCPD changes the minimum size of the conductors.

What it sounds like you're saying here is that per the exception in 690.8(B)(1), if your CBX, which may or may not have OCPD, is fully 100% rated, then the CBX rating itself doesn’t need the additional 125%, it can be only Isc x 125%, but it doesn’t mean the PV Output circuit wires don’t have to still be Isc x 156%. Is that what you're suggesting?


This confusion is where I stared. If your CBX is not 100% rated, it seems pretty clear that PV Output wires have to be 156% x Isc. But if the CBX is 100% rated, and it's just an un-fused disconnect, how big are the conductors supposed to be? Re-posting the key point from OP.

If you use a DC combiner (CBX) that has at 100% rated disco on PV output side, do your conductors on the PV output still have to be 156% x Isc x number of strings? Or is just 125%? It's not clear to me in 690. Perhaps it could, but maybe you have to consider the fuse at the (central) inverter?

 

[ggunn]

This confusion is where I stared. If your CBX is not 100% rated, it seems pretty clear that PV Output wires have to be 156% x Isc. But if the CBX is 100% rated, and it's just an un-fused disconnect, how big are the conductors supposed to be? Re-posting the key point from OP.

At the outset I have to say that I have not dealt with this issue before, but when I am calculating conductor size the OCPD does not enter into it except to check that the conductors are compliantly protected by the OCPD.

 

[jaggedben]

I do not think that a 100% rating (or not) of the OCPD changes the minimum size of the conductors.

I think it can, potentially, in theory. That's how I read 690.8(B)(1) Exception, which is in the section on conductor size.

(Note the exception is only for the 125% factor for continuous use, and does not apply to adjustment and correction factors.)

What about the terminals on the inverter? If they are 75 degree rated I don't see how you can avoid the rule that 125% of the inverter output current cannot exceed the 75 degree ampacity of the conductors.

What I hear you saying is that the inverter end of things may make the 100% continuous rating of a combiner box a moot point. I agree, it may. One probably needs an inverter with the same kind of listing to actually take advantage of the exception.

 

[jaggedben]

See also discussion here:

Reason for 125%

On a continuous load I know we have to use the 125% to size the conductors and then the OCPD. But if the OCPD is designed to hold at 100% and the conductor is rated for that current, remind me again what was the purpose for the 125%? Thank you

forums.mikeholt.com

 

[wwhitney]

One probably needs an inverter with the same kind of listing to actually take advantage of the exception.

That's basically the question: the NEC specifies that by default a breaker is not rated for 100% continuous duty, and you need a special listing if you want to use one that way and avoid upsizing conductors.

But it doesn't say that about any other equipment. So my initial take is that by default other equipment would have terminations rated for 100% continuous duty, and you don't need any special listing to take advantage of that. If the equipment has particular conductor sizing minimums because its thermal design relies on heat sinking from upsized conductors, then its listing or specification should indicate that.

Of course, I don't know what is typical as far as PV inverter specs.

Cheers, Wayne

 

[ggunn]

I misspoke earlier, or at least I had old information. Apparently the SE80KUS and SE120KUS can connect to a delta service, but I don't know what their abbreviations mean.

Supported grids are WYE: TN-C, TN-S, TN-C-S, TT, IT; Delta: IT.

 

[bwat]

I misspoke earlier, or at least I had old information. Apparently the SE80KUS and SE120KUS can connect to a delta service, but I don't know what their abbreviations mean.

Supported grids are WYE: TN-C, TN-S, TN-C-S, TT, IT; Delta: IT.

I'm assuming you intended this for a different thread?

 

[pv_n00b]

While this is an interesting theoretical discussion, I am not aware of any combination of existing PV DC side equipment that will allow a 100% rated installation. One end of the conductor or the other will always land on something that is 80% rated or undefined and has to be assumed to be 80% rated and that will define the conductor rating.