Conductor sizing

[erosroberts]

I need some help with clarification on conductors in a PV system. My understanding is that the conductors that come from the panels/strings to the dc combiner box are sized at 156% to account for the open circuit current adjustments (as well as the string fuses). Now to address the dc conductors from the combiner to the inverter. The information I have is that the conductors do not need to be rated at 156% because when the system is not under load there is no current flowing through the conductors so there is no need for the adjustment and the fuses will protect the strings. The only adjustment the dc conductors to the inverter need is a 125% multiplier for the continuous load factor. Is this line of thinking correct ? If is not where do I find the correct calculations and information at ?

 

[BillK-AZ]

I need some help with clarification on conductors in a PV system. My understanding is that the conductors that come from the panels/strings to the dc combiner box are sized at 156% to account for the open circuit current adjustments (as well as the string fuses). Now to address the dc conductors from the combiner to the inverter. The information I have is that the conductors do not need to be rated at 156% because when the system is not under load there is no current flowing through the conductors so there is no need for the adjustment and the fuses will protect the strings. The only adjustment the dc conductors to the inverter need is a 125% multiplier for the continuous load factor. Is this line of thinking correct ? If is not where do I find the correct calculations and information at ?

You find the calculations in Art 690.8(B)(2). You do at least two calculations per "Circuit conductors shall be sized to carry not less than the larger of 690.8(B)(2)a or (2)b. You also have to check on the temperature ratings of all terminals that these wires connect to and the expected temperature at those points. It is common to find that the terminals in a DC combiner and any DC disconnect are only rated at 75?C. If that is true, repeat the calculations using the 75?C column in the wire tables. This definitely applies to the cables between the DC combiner and the inverter.

The main purpose of the fuses in the DC combiner is to protect the PV modules from a back feed from parallel strings in case of a fault within a PV module.

 

[jaggedben]

...Now to address the dc conductors from the combiner to the inverter. The information I have is that the conductors do not need to be rated at 156% because when the system is not under load there is no current flowing through the conductors so there is no need for the adjustment and the fuses will protect the strings. The only adjustment the dc conductors to the inverter need is a 125% multiplier for the continuous load factor. Is this line of thinking correct ? ...

No, that line of thinking is not correct. It is the second time it's been suggested in this forum in a couple weeks, but it's still not correct.:happyno:

There is no difference in the calculations between source circuits (directly connected to panels) and output circuits (between combiner(s) and an inverter.) 690.8(B) is the relevant section, and it makes no such distinction.

If is not where do I find the correct calculations and information at ?

In addition to what Bill mentioned, you need to understand section 310.