Adding additional 7 kW to existing 130 kW PV system

[brent8605]

I'm wondering if it is possible to add 7 kw more to an existing 130 kW system, being combined in a separate combiner box and landing it on the main DC busbar in the central inverter?

With the existing system using (2) 350A fused circuits coming into the DC bus of the inverter, and the additional circuit being strung to the same voltage as this, will I have a problem fusing the 7 kW addition with a 40 amp fuse, and landing it to the common DC bus in the inverter?

My concerns are the possibility of back feed from the existing large PV strings, to the smaller 40 amp circuits.

I'm using Uni-Solar PV-128W laminates, in strings of 11. The inverter is undersized now, so this is not a problem.

I would appreciate any input on this.

Thanks!

[inetdog]

I'm wondering if it is possible to add 7 kw more to an existing 130 kW system, being combined in a separate combiner box and landing it on the main DC busbar in the central inverter?

With the existing system using (2) 350A fused circuits coming into the DC bus of the inverter, and the additional circuit being strung to the same voltage as this, will I have a problem fusing the 7 kW addition with a 40 amp fuse, and landing it to the common DC bus in the inverter?

My concerns are the possibility of back feed from the existing large PV strings, to the smaller 40 amp circuits.

I'm using Uni-Solar PV-128W laminates, in strings of 11. The inverter is undersized now, so this is not a problem.

I would appreciate any input on this.

Thanks!

1. The fact that the existing strings are higher power (more panels in parallel) will not cause them to backfeed a smaller panel array as long as the voltage is the same.
2. If you have parallel strings of panels as part of each array, those individual strings should have their own fuses rated at or below the maximum series fuse size the panel manufacturer specifies. That will be enough to protect all of your panel strings.
3. If your only fusing or other overcurrent protection is after the combiner boxes, then you have just as much risk of a string of panels in your existing array being damaged by backfeed as you would with the additional array. And backfeed will normally only be a problem when one or more of the panels in a string are damaged or the wiring shorts out.
4. It is a good idea to have individual fuses after the combiners (or circuit breakers to make disconnecting easier), but they are not able to serve as the primary protection for your panels.

A 130kW system is small commercial size. Is there or was there an engineer involved in setting the system up in the first place? And will you need to get permits and inspections for the changes you will be making?