solarken
NABCEP PVIP
- Location
- Hudson, OH, USA
- Occupation
- Solar Design and Installation Professional
Designing a 2.15MW system at Industrial site. I need to interconnect on the customer side of the CT cabinet that the utility requires and probably cannot connect in that cabinet due to utility co rules. So looking at installing a Tap cabinet adjacent to the CT cab and connect there.
There will be 12 x 400kcmil conductors per phase in from the CT cabinet (utility feed), 12 x 400kcmil conductors per phase out to the 4000A panelboard in the building, and 8 x 500kcmil conductors per phase from the PV inverters. Conductors are aluminum.
Looking at 2023 NEC Annex B, Figure B.2(1) shows duct bank for 9 conduits (3 row, 3 col) and has ampacity for 500kcmil at 375A for RHO 60 LF 50. Then it drops severely to 240A for RHO 90 LF 100, and marginally further to 220A for RHO 120 LF 100. The notes say this is for copper conductors.
I have several challenges I hope someone can help me with:
Copper conductors for this would be cost prohibitive as the run between inverters and Tap cabinet will be about 400ft. Any suggestions on extrapolating the Annex B.2(2) Ampacities to Aluminum?
The few examples in the Annex are not granular, obviously. But for a PV inverter, we calculate continuous current as 1.25 x Max inverter current. However, everyone knows generally an inverter may output it's max for only several minutes, to a few hours max, depending on DC/AC ratio. How does LF in this context relate to the inverter current? Do I use the LF 50 table entry and compare that to the datasheet max inverter current? or compare it to the calculated inverter continuous current? And over what time interval is the Load factor calculated for solar?
And the Annex Design criteria lists Rho of 60 with LF of 50. As I understand, Rho of concrete is typically 55, and average soil is 90. But the design criteria in the Annex shows Rho Concrete = Rho Earth - 5. What does that mean? And it lists Rho of PVC, cable insulation, and cable jacket. How are we to interpret how to use that info?
I know this is a complex heat transfer problem, how have you all approached this? I requested pricing on Ampcalc, but am thinking it may be too expensive.
-Ken
There will be 12 x 400kcmil conductors per phase in from the CT cabinet (utility feed), 12 x 400kcmil conductors per phase out to the 4000A panelboard in the building, and 8 x 500kcmil conductors per phase from the PV inverters. Conductors are aluminum.
Looking at 2023 NEC Annex B, Figure B.2(1) shows duct bank for 9 conduits (3 row, 3 col) and has ampacity for 500kcmil at 375A for RHO 60 LF 50. Then it drops severely to 240A for RHO 90 LF 100, and marginally further to 220A for RHO 120 LF 100. The notes say this is for copper conductors.
I have several challenges I hope someone can help me with:
Copper conductors for this would be cost prohibitive as the run between inverters and Tap cabinet will be about 400ft. Any suggestions on extrapolating the Annex B.2(2) Ampacities to Aluminum?
The few examples in the Annex are not granular, obviously. But for a PV inverter, we calculate continuous current as 1.25 x Max inverter current. However, everyone knows generally an inverter may output it's max for only several minutes, to a few hours max, depending on DC/AC ratio. How does LF in this context relate to the inverter current? Do I use the LF 50 table entry and compare that to the datasheet max inverter current? or compare it to the calculated inverter continuous current? And over what time interval is the Load factor calculated for solar?
And the Annex Design criteria lists Rho of 60 with LF of 50. As I understand, Rho of concrete is typically 55, and average soil is 90. But the design criteria in the Annex shows Rho Concrete = Rho Earth - 5. What does that mean? And it lists Rho of PVC, cable insulation, and cable jacket. How are we to interpret how to use that info?
I know this is a complex heat transfer problem, how have you all approached this? I requested pricing on Ampcalc, but am thinking it may be too expensive.
-Ken