Differences between data sheet and the calculation.

Hello,


I am confused with the calculation of the main disconnection of a photovoltaic inverter, in the inverter data sheet it is said that the maximum AC output current is 145A, then, as I understood, the way to calculate the OCPD is to multiply this current value by 125%, and the current would be 181.25A and the switch 200A.


But when I review the data sheet, the maximum external AC overcurrent protection is: 225A, how is that possible?


Could someone help me clarify this dobut,


Thank you!
 

jaggedben

Senior Member
That's a slightly unusual situation, but the two things are not in conflict. The 200A calculation is a minimum. The 225A data sheet spec is a maximum. You can thus pick which you want. However, if you choose 225A then you'll need conductors and a switch that are higher rated. I'd pick 200A to save money.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Hello,


I am confused with the calculation of the main disconnection of a photovoltaic inverter, in the inverter data sheet it is said that the maximum AC output current is 145A, then, as I understood, the way to calculate the OCPD is to multiply this current value by 125%, and the current would be 181.25A and the switch 200A.


But when I review the data sheet, the maximum external AC overcurrent protection is: 225A, how is that possible?


Could someone help me clarify this dobut,


Thank you!
I'm confused. You can interconnect an inverter through as big an OCPD as you like as long as the conductors and switches between the inverter and the OCPD are adequately protected. The NEC specifies a minimum OCPD but not a maximum.
 

Carultch

Senior Member
Location
Massachusetts
Hello,


I am confused with the calculation of the main disconnection of a photovoltaic inverter, in the inverter data sheet it is said that the maximum AC output current is 145A, then, as I understood, the way to calculate the OCPD is to multiply this current value by 125%, and the current would be 181.25A and the switch 200A.


But when I review the data sheet, the maximum external AC overcurrent protection is: 225A, how is that possible?


Could someone help me clarify this dobut,


Thank you!
One reason you might use a higher-than-necessary OCPD, is if there are multiple inverter ratings in your system, and you choose to unify on the breaker you use for each. That way you don't need a special breaker and a special wire size, for the "runt of the litter".

An inverter manufacturer might specify a maximum OCPD, higher-than-necessary for the NEC, because a family of inverters are all built to withstand up to that amount of amps (for the largest unit in the family), and smaller units within the family could use a smaller OCPD on the output. There are few (if any) cases where you actually have to override the NEC's OCPD sizing calculation, and the manual would explicitly tell you if you did. Do make sure you are using the datasheet amp rating, rather than calculating it from kW/Voltage(/sqrt(3)), as some inverters have extra current headroom in the event of lower than nominal voltage.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
One reason you might use a higher-than-necessary OCPD, is if there are multiple inverter ratings in your system, and you choose to unify on the breaker you use for each. That way you don't need a special breaker and a special wire size, for the "runt of the litter".

An inverter manufacturer might specify a maximum OCPD, higher-than-necessary for the NEC, because a family of inverters are all built to withstand up to that amount of amps (for the largest unit in the family), and smaller units within the family could use a smaller OCPD on the output. There are few (if any) cases where you actually have to override the NEC's OCPD sizing calculation, and the manual would explicitly tell you if you did. Do make sure you are using the datasheet amp rating, rather than calculating it from kW/Voltage(/sqrt(3)), as some inverters have extra current headroom in the event of lower than nominal voltage.
I still don't get it. The OCPD has nothing to do with the output of the inverter other than to open a current window large enough to avoid nuisance tripping. The OCPD is there to protect the conductors and switchgear from fault current coming from the service, not current from the inverter; the inverter is current limited and cannot endanger properly sized conductors. You could connect a 1kW PV system through a 100A breaker as long as the conductors and switchgear were rated high enough.

I don't know what you mean by "withstand" in that context.
 
I still don't get it. The OCPD has nothing to do with the output of the inverter other than to open a current window large enough to avoid nuisance tripping. The OCPD is there to protect the conductors and switchgear from fault current coming from the service, not current from the inverter; the inverter is current limited and cannot endanger properly sized conductors. You could connect a 1kW PV system through a 100A breaker as long as the conductors and switchgear were rated high enough.

I don't know what you mean by "withstand" in that context.


Hi ggunn,


In my design, i have 5 inverters of 120kW, and I am collecting them in a switchboard of 1600A, 480/277V 3ph, they are intalled in the roof in different places, with distances between between inverters and swichboard of 150-480 ft, then, i am placing an OCPD for each inverter at the end of the conductors of each one.


I was sizing this OCPD, acording to the 125% of the maximum current, because the radio of the inverter has been defined as 120%, and in probability several hours we are going to be working at full capacity. the datasheet says that this current is 145A, that as I understand is the result of (120000kW)/(480V*sqroot(3))=144.33A.



Acccording to my knowlege, the OCPD size should be for this maximum current AC = 200A, but again, they says that is 225A, and this is my doubt.

Regards,

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pv_n00b

Senior Member
Location
CA, USA
The maximum AC OCPD rating is part of the UL 1741 listing so the manufacturer has to provide a maximum value. That just means you can use any OCPD rating between the NEC calculated minimum and the inverter manufacturer supplied maximum. The manufacturer is not requiring that you use the OCPD rating they supply. Don't over think this.

Be glad it's not the old days where occasionally the manufacturer supplied maximum was less than the NEC required minimum, that used to happen.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
The maximum AC OCPD rating is part of the UL 1741 listing so the manufacturer has to provide a maximum value.
Are you sure about that? I just went through a slew of inverter data sheets and not one of them specifies a maximum AC OCPD. I cannot see how or why an inverter company or UL would care about such a thing. We routinely interconnect inverters through OCPD 3X or more the maximum inverter current for various reasons.

But in the event an inverter company specifies a max OCPD for some reason I guess you have to comply in order not to void their warranty. Stupid, IMO.
 

Carultch

Senior Member
Location
Massachusetts
I still don't get it. The OCPD has nothing to do with the output of the inverter other than to open a current window large enough to avoid nuisance tripping. The OCPD is there to protect the conductors and switchgear from fault current coming from the service, not current from the inverter; the inverter is current limited and cannot endanger properly sized conductors. You could connect a 1kW PV system through a 100A breaker as long as the conductors and switchgear were rated high enough.

I don't know what you mean by "withstand" in that context.
Suppose there is internal wiring and connections inside the inverter. That is what would need to "withstand" a fault coming from the utility, in the event that the OCPD doesn't trip due to it.
 
Are you sure about that? I just went through a slew of inverter data sheets and not one of them specifies a maximum AC OCPD. I cannot see how or why an inverter company or UL would care about such a thing. We routinely interconnect inverters through OCPD 3X or more the maximum inverter current for various reasons.

But in the event an inverter company specifies a max OCPD for some reason I guess you have to comply in order not to void their warranty. Stupid, IMO.
Well,


In my case, because I have to take into account the voltage drop, I have increased the wire size to 4/0, so it is fine for 225A protection. but as pv_n00b says, I'm not going to to worry about this anymore.


Thanks for your support.


Regards,
 

PWDickerson

Senior Member
Location
Clinton, WA
Most of the residential inverters I work with (SMA, Solectria, SolarEdge, Fronius) specify a max OCPD on the AC side. This is generally found in the installation manual, but not the spec sheet. The max OCPD specified in the manual is often larger than the minimum OCPD calculated per the NEC. For instance, the SolarEdge HD Wave inverters from 3.0 kW to 7.6 kW all have a max OCPD of 50A. When installing a 3.0 kW inverter with a max rated AC current of 16.5A, the installer has the option to install a 20A, 25A, 30A, 35A, 40A, 45A, or 50A breaker.
 

Carultch

Senior Member
Location
Massachusetts
Well,


In my case, because I have to take into account the voltage drop.
Voltage drop is one reason you might want to strategically increase the OCPD size to larger than what you need, if allowable. As an example, suppose you have a circuit which would ordinarily be a 30A breaker with #10 Cu wire & #10 Cu EGC. If you need to use a #6 Cu wire to curtail voltage drop, and keep the 30A OCPD, this would mean a #6 Cu EGC. However, if you can use a 60A OCPD, then #6 Cu is already the starting point size, and there's no need to upsize beyond the #10 Cu EGC. It's counterintuitive that you can do this, but it is what the NEC allows, should your inverter manual also allow it.
 

pv_n00b

Senior Member
Location
CA, USA
Are you sure about that? I just went through a slew of inverter data sheets and not one of them specifies a maximum AC OCPD. I cannot see how or why an inverter company or UL would care about such a thing. We routinely interconnect inverters through OCPD 3X or more the maximum inverter current for various reasons.

But in the event an inverter company specifies a max OCPD for some reason I guess you have to comply in order not to void their warranty. Stupid, IMO.
Yup, I'm sure. Some inverter manufacturers of central inverters include an internal OCPD that satisfies the requirement and then will not have to specify an external OCPD rating. But if they don't then the external OCPD max rating has to be given. It's almost never listed on the data sheet but is in the installation manual somewhere, usually in the section on AC interconnection. If it's not given somewhere then the NRTL that listed the equipment failed to verify the documentation.

It's necessary to limit the fault current into an inverter in the event there is an internal fault. If the fault current exceeds the inverter containment rating then the box could let the fire out during a fault. So if you have been exceeding the manufacturer supplied max OCPD rating you have been creating unsafe conditions in your installations. Not good.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Yup, I'm sure. Some inverter manufacturers of central inverters include an internal OCPD that satisfies the requirement and then will not have to specify an external OCPD rating. But if they don't then the external OCPD max rating has to be given. It's almost never listed on the data sheet but is in the installation manual somewhere, usually in the section on AC interconnection. If it's not given somewhere then the NRTL that listed the equipment failed to verify the documentation.

It's necessary to limit the fault current into an inverter in the event there is an internal fault. If the fault current exceeds the inverter containment rating then the box could let the fire out during a fault. So if you have been exceeding the manufacturer supplied max OCPD rating you have been creating unsafe conditions in your installations. Not good.
When we are designing with microinverters the maximum number of inverters on a branch is 12 @1.33A per, which maxes out a 20A OCPD, but we interconnect with a 20A breaker whether the inverter count is 12 or 1. Also, if it were a safety issue, you'd think the inverter companies would display it more prominently than burying it somewhere in a manual.
 
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