NEC 2017 PV source circuit current using 690.8(A)(1)(2)

Electrical PE

Member
Location
MA, USA
Hi all,

I have a question about calculating the PV source circuit current by using the System Advisor Model in accordance with NEC 2017 690.8(A)(1)(2).

After the highest 3-hour average irradiance is calculated for a specific PV system, can this number be multiplied by the Imp current or should it be multiplied by the Isc? Knowing that the Imp would be the actual current that is going through the conductors under normal operation conditions and not Isc.

Thank you
 

ggunn

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

I have a question about calculating the PV source circuit current by using the System Advisor Model in accordance with NEC 2017 690.8(A)(1)(2).

After the highest 3-hour average irradiance is calculated for a specific PV system, can this number be multiplied by the Imp current or should it be multiplied by the Isc? Knowing that the Imp would be the actual current that is going through the conductors under normal operation conditions and not Isc.

Thank you
As far as I know the DC Imax for a PV source circuit is always 1.25 X Isc.
 

pv_n00b

Senior Member
Location
CA, USA
You want the highest three hour average Isc for the year.

There is an article about this in Solar Professional magazine 11.4 (July-Aug 2018) that covers the calculations using software. SAM can output Isc and Voc for the system but the article author chooses to calculate these values from other outputs. He seems to feel the values provided by SAM are not accurate.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
You want the highest three hour average Isc for the year.

There is an article about this in Solar Professional magazine 11.4 (July-Aug 2018) that covers the calculations using software. SAM can output Isc and Voc for the system but the article author chooses to calculate these values from other outputs. He seems to feel the values provided by SAM are not accurate.
Is the system >100kW AC? If not, the above does not apply; see 690.8(A) (1) and (2). The OP implied with his question that it is >100kW but he didn't say.
 

pv_n00b

Senior Member
Location
CA, USA
Is the system >100kW AC? If not, the above does not apply; see 690.8(A) (1) and (2). The OP implied with his question that it is >100kW but he didn't say.
The OP did say the question was calculation of the current in accordance with 690.8(A)(1)(2). I gave an answer that matched the request.
 

Electrical PE

Member
Location
MA, USA
You want the highest three hour average Isc for the year.

There is an article about this in Solar Professional magazine 11.4 (July-Aug 2018) that covers the calculations using software. SAM can output Isc and Voc for the system but the article author chooses to calculate these values from other outputs. He seems to feel the values provided by SAM are not accurate.
Thanks pv_n00b on your reply. I really appreciate it!
I did a comparison today between the current calculated by using equation 2 from the SolarPro article, and the Isc that SAM outputs.
I got 10.8A from Equation 2 and 10.65A from SAM. Just for reference, if I use the traditional method [690.8(A)(1)(1)] I get 12.71A.

Which method would you recommend using? Why do you think the article author decided not to use the SAM Isc?

Thank you
 
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pv_n00b

Senior Member
Location
CA, USA
My guess is that he did a curve fit of the data and did not like it versus the same data plot using his calculated values. I have not had a chance to compare the results of the two methods myself yet so I can't recommend one way or the other.
 

Electrical PE

Member
Location
MA, USA
My guess is that he did a curve fit of the data and did not like it versus the same data plot using his calculated values. I have not had a chance to compare the results of the two methods myself yet so I can't recommend one way or the other.
Just following up on this. I ended up using the equation 2 in the SolarPro article after noticing that it is the same formula also in the SAND 2004-3535 report from Sandia National Labs, which is referenced in the informational note of 690.8(A)(1)(2).

Thank you
 
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