# Thread: Differences between data sheet and the calculation.

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## 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!

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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.

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Originally Posted by INGMRS
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.

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Originally Posted by ggunn
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.
Hi,

Well, is a data taken directly from the datasheet of the inverter.

Regards

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Originally Posted by INGMRS
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.

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Originally Posted by Carultch
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.

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Originally Posted by ggunn
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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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.

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Originally Posted by pv_n00b
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.

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Originally Posted by ggunn
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.

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