Do I need a fused combiner box for more than 2 strings?

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bbartmasse r

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Someone just told me that if I have more than 2 strings I will need a fused combiner (I presume on the roof) for those strings.

This is in regards to a straight grid-tied SolarEdge system

Is this correct? What is the code reference for this. I live in Washington State and we are on 2014.

I have never don't this before, don't want to start now.

Thanks
 
bbartmasse r said:
Someone just told me that if I have more than 2 strings I will need a fused combiner (I presume on the roof) for those strings.

This is in regards to a straight grid-tied SolarEdge system

Is this correct? What is the code reference for this. I live in Washington State and we are on 2014.

I have never don't this before, don't want to start now.

Thanks
Click to expand...

In general, yes.

For SolarEdge arrays, no.
https://www.solaredge.com/sites/default/files/string_fusing_requirements.pdf

At one point by omission of this information, 3-string SolarEdge arrays did require fusing. But with the more recent testing and listing of their products, they do not anymore.
 
bbartmasse r said:
Someone just told me that if I have more than 2 strings I will need a fused combiner (I presume on the roof) for those strings.

This is in regards to a straight grid-tied SolarEdge system

Is this correct? What is the code reference for this. I live in Washington State and we are on 2014.

I have never don't this before, don't want to start now.

Thanks
Click to expand...

SolarEdge has released a white paper report that shows that (in their opinion) string fuses are not required in a SE three string system: https://www.solaredge.com/sites/default/files/string_fusing_requirements.pdf

That said, one AHJ I deal with a lot said that they will not accept a document from SolarEdge as sufficient evidence of this.
 
There have been a number of inverter manufacturers in the last year or so who are saying some requirement in the NEC does not apply to their equipment because of how it is designed. They get a letter written by someone well known in the code community to back them up and maybe do a white paper describing how it works. But it is up to the AHJ if they accept it as providing equivalent safety to the NEC requirement and they are under no obligation to accept it.

The NEC does not lend itself to innovative solutions and alternatives. This is one of the reasons I push for stated allowances in the NEC to allow alternative solutions under engineering supervision. On the other hand, some of the stuff I have seen manufacturer's promoting as alternatives are not well documented and lack transparency on the part of the manufacturer about how the equipment works. Take SolarEdge as an example, does the use of DC optimizers remove the need to have fused combiners? Maybe, it depends on if the DC optimizers provide isolation for the strings and what the fault current rating of the DC optimizers is on the inverter side. Since this information is not provided and as far as I know it's not part of the UL Standard that the DC optimizers are listed too I don't know. Should I just trust them?
 
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pv_n00b said:
There have been a number of inverter manufacturers in the last year or so who are saying some requirement in the NEC does not apply to their equipment because of how it is designed. They get a letter written by someone well known in the code community to back them up and maybe do a white paper describing how it works. But it is up to the AHJ if they accept it as providing equivalent safety to the NEC requirement and they are under no obligation to accept it.
Click to expand...

Bill Brooks has provided SolarEdge with exactly that letter. I'd provide it, but I cannot upload a PDF. At one point, SolarEdge built fuses in their larger inverter models, where 3 strings are anticipated. In some of those same inverters, they have since removed them, with paralleled DIN rail terminal blocks instead. I stumbled upon one with the fuses removed, and received that white paper and the Bill Brooks letter.
 
Carultch said:
Bill Brooks has provided SolarEdge with exactly that letter. I'd provide it, but I cannot upload a PDF. At one point, SolarEdge built fuses in their larger inverter models, where 3 strings are anticipated. In some of those same inverters, they have since removed them, with paralleled DIN rail terminal blocks instead. I stumbled upon one with the fuses removed, and received that white paper and the Bill Brooks letter.
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Thanks for that information. The next time I get a three string SolarEdge design in my queue, I'll contact Bill.
 
The SolarEdge system connects a single module to an optimizer, and then connects the optimizers in series. PV modules are not in parallel even if strings are in parallel. The situation requiring series overcurrent devices [such as 2014 NEC 690.9(A) Handbook comment] does not exist.
 
The rated output of the SE Optimizers is 15-amps. So in theory with 3-string dc-to-dc converter source circuit system, couldn't you have the 2-string total of 30-amps feeding towards the 3rd string in a shortcircuit or a groundfault?

Having the scenario above, as long as the ampacity of wire is sized to carry the fault current no fusing is needed according to 690.9(A)...

So, depending on how those dc-to-dc converter source circuit conductors are routed, fusing may not be required. But, if the 3 source circuits are in the same raceway and derating is applied to the wire at a value lower than the ampacity of the fault, fusing would be needed IMO.
 
shortcircuit2 said:
So in theory with 3-string dc-to-dc converter source circuit system, couldn't you have the 2-string total of 30-amps feeding towards the 3rd string in a shortcircuit or a groundfault?
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In theory, yes, but in practice the ground-fault interruption kicks in and the optimizers shut down and stop feeding the fault, possibly before any fuse would act. At least, that is what SolarEdge is saying.
 
For the two AHJ's I deal mostly with, I decided not to continue the fight. I fuse SE strings when there must be three of them on the same input. One must choose one's battles; discretion is the better part of valor.
 
jaggedben said:
In theory, yes, but in practice the ground-fault interruption kicks in and the optimizers shut down and stop feeding the fault, possibly before any fuse would act. At least, that is what SolarEdge is saying.
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Yes...that is the same story I hear too...from the manufacturer of the product and from their engineers on the payroll who write them white papers.

Thing is, the Code doesn't recognize "ground-fault protection" as "overcurrent protection".
 
This has been an ongoing discussion in some code circles, can electronics provide the same level of protection as our standard fuses and circuit breakers? Currently, the code does not support this at 1,000V and below but maybe someday it will. Until then if the contractor and the AHJ accept the word of the manufacturer on how these systems will operate then it's an alternative method and material exemption allowed by the NEC. If these systems start having trouble in the field, i.e. bursting into flame, then it will be the contractor and AHJ on the line for not complying with the code.

I was looking over a system the other day that used SunPower 435 modules with an Isc of 6.43A that have a 20A string fuse rating. Three strings in parallel would only back feed 2*6.43*1.25= 16.1A into a faulted string so no fuse needed. So there are some modules out there that can have more than two strings in parallel without fusing.
 
BillK-AZ said:
The SolarEdge system connects a single module to an optimizer, and then connects the optimizers in series. PV modules are not in parallel even if strings are in parallel. The situation requiring series overcurrent devices [such as 2014 NEC 690.9(A) Handbook comment] does not exist.
Click to expand...

That would be true if the DC optimizers provided electrical isolation between the module and the string conductors. But the manufacturer has not come out and said that and I have not tested them so I don't know. Maybe you have tested them.
 
bbartmasse r said:
Someone just told me that if I have more than 2 strings I will need a fused combiner (I presume on the roof) for those strings.

Is this correct? What is the code reference for this. I live in Washington State and we are on 2014.

Thanks
Click to expand...

With respect to NEC references, 690.9(A) requires overcurrent protection for most PV circuits including source circuits, but provides exceptions that apply if there are only 1 or 2 PV source circuits.

The disconnect reference is in 690.16(A) which requires that "disconnecting means shall be provided to disconnect a fuse from all sources of supply if the fuse is energized from both directions. Such a fuse in a PV source circuit shall be capable of being disconnected independently of fuses in other PV circuits." The wording is ambiguous but generally interpreted as requiring a disconnecting means between the fuse and the combiner, but not between the fuse and the array. Also it is generally interpreted as meaning touch-safe fuse holders can be used as the disconnecting means, not necessarily a blade disconnect. The 2014 NEC Handbook includes a note here that "Switches, pullouts, or similar devices that have suitable ratings may serve as means to disconnect fuses from all sources of supply." If using touch-safe fuse holders, they should be marked "Do not open under load," as in 690.16(B).
 
SolarAlternativesDesign said:
With respect to NEC references, 690.9(A) requires overcurrent protection for most PV circuits including source circuits, but provides exceptions that apply if there are only 1 or 2 PV source circuits.

The disconnect reference is in 690.16(A) which requires that "disconnecting means shall be provided to disconnect a fuse from all sources of supply if the fuse is energized from both directions. Such a fuse in a PV source circuit shall be capable of being disconnected independently of fuses in other PV circuits." The wording is ambiguous but generally interpreted as requiring a disconnecting means between the fuse and the combiner, but not between the fuse and the array. Also it is generally interpreted as meaning touch-safe fuse holders can be used as the disconnecting means, not necessarily a blade disconnect. The 2014 NEC Handbook includes a note here that "Switches, pullouts, or similar devices that have suitable ratings may serve as means to disconnect fuses from all sources of supply." If using touch-safe fuse holders, they should be marked "Do not open under load," as in 690.16(B).
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But his question is whether he needs fusing when he is combining three strings of SolarEdge optimizers. SolarEdge says that he doesn't, but some AHJs, including the ones I deal with the most, disagree.
 
SolarAlternativesDesign said:
Yes I understand the question of whether the AHJ will accept the SolarEdge solution. It's good to see the SolarEdge documentation - should give the AHJ a better basis for deciding.
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The AHJs I deal mostly with both said that they would need documentation from an ITL to be convinced that not fusing three strings would be safe and code compliant, not just a white paper from the company that makes and sells the gear. I could not find such documentation, so I chose not to pursue it any further.
 
ggunn said:
The AHJs I deal mostly with both said that they would need documentation from an ITL to be convinced that not fusing three strings would be safe and code compliant, not just a white paper from the company that makes and sells the gear. I could not find such documentation, so I chose not to pursue it any further.
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That’s my problem with white papers and engineering support letters. If the manufacturer really wants to use the equipment in a configuration that is not supported by the NEC or the Listing they need to go to a NRTL and have the equipment Listed to do what they want it to do. Not go out and pay an engineer, who I doubt will actually test the equipment in a lab to verify the manufacturer’s claims, to write a letter of support, or write their own white paper describing how they are different than the rest of the industry and should be granted an exemption.

Basically based on my experience, most manufactures who do this are looking for a cheap workaround of the listing requirements and the NEC. I have had some pretty knock down discussions with manufacturers over various workarounds over the years. It has always come down to the same thing, they don’t want to pay to have it tested but they do want to tell contractors their system is cheaper to install using some off the book method. I’m not saying SolarEdge is doing this, just that I’ve seen manufacturers doing it over the years.
 
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