I installed 2 TL, dual MPPT inverters on a residence.

[So Cal]

So there are 2 seperate systems roof. I combined all of the wiring into one conduit that splits again at the inverter location. The EGC is common as well. It connects all of the panels & rails from both systems to one wire that splits again to both inverters. Is there a good reason to have seperate conduits & EGCs for each inverter? Is it different for non TL inverters?

 

[GoldDigger]

So there are 2 seperate systems roof. I combined all of the wiring into one conduit that splits again at the inverter location. The EGC is common as well. It connects all of the panels & rails from both systems to one wire that splits again to both inverters. Is there a good reason to have seperate conduits & EGCs for each inverter? Is it different for non TL inverters?

If both arrays ate operated ungrounded, this creates the possibility that the voltage between the DC conductors of the two arrays could ns twice the line-to-line voltage of each array. Are the DC wires insulated to a high enough voltage to allow them to run in the same conduit given that?
Similar to what happens with a bi-polar array, inly happening only if the twp TL inverters are connected to different AC phases.

 

[shortcircuit2]

Would it also be important to fuse the strings being that they are in the same conduit? (Assuming that there are 4 strings between the 2 dual input TL inverters.)

 

[GoldDigger]

Would it also be important to fuse the strings being that they are in the same conduit? (Assuming that there are 4 strings between the 2 dual input TL inverters.)

As long as the array wires are not connected at either end, and no fusing is required for PV array protection (since there are two or fewer strings connected in parallel in each array), then I don't see a separate requirement for fusing just because they are in the same conduit.
In other words, NO.

 

[jaggedben]

Is there a good reason to have separate conduits & EGCs for each inverter?

In my opinion, no. AFAIK the NEC does not require it. However I believe I have heard of some state codes (Pennsylvania?) that require it.

Is it different for non TL inverters?

In my opinion, no.

If both arrays ate operated ungrounded, this creates the possibility that the voltage between the DC conductors of the two arrays could ns twice the line-to-line voltage of each array. Are the DC wires insulated to a high enough voltage to allow them to run in the same conduit given that?
Similar to what happens with a bi-polar array, inly happening only if the twp TL inverters are connected to different AC phases.

I don't see what difference it makes if the systems are grounded or not. Theoretically if there are four PV source circuits in a conduit, then no matter what is on the other end (one or multiple inverters, grounded or ungrounded) there is a very slim possibility that multiple faults could lead to one or more source circuits adding their voltages together. But in my opinion this danger isn't any more likely with an ungrounded system, particularly since most grounded systems are configured such that all conductors become ungrounded when a ground-fault is detected.

 

[GoldDigger]

In my opinion, no. AFAIK the NEC does not require it. However I believe I have heard of some state codes (Pennsylvania?) that require it.



In my opinion, no.



I don't see what difference it makes if the systems are grounded or not. Theoretically if there are four PV source circuits in a conduit, then no matter what is on the other end (one or multiple inverters, grounded or ungrounded) there is a very slim possibility that multiple faults could lead to one or more source circuits adding their voltages together. But in my opinion this danger isn't any more likely with an ungrounded system, particularly since most grounded systems are configured such that all conductors become ungrounded when a ground-fault is detected.

Since it is a transformerless inverter, the array conductors which are not explicitly grounded will end up referenced during operation to one or both of the AC output terminals of the inverter. That means that the voltage to ground of one of the wires of that subarray at any given moment could be the DC output of the array plus or minus the instantaneous voltage at one AC output terminal.
Same applies to each array.
Now if the design is such that one DC lead will be near the voltage of on AC lead and the other will be offset in the direction of the other AC lead by the Vmp voltage, the result should be that the voltage on the other DC lead will still be less than Vmp from ground.
But I am not convinced that you will not potentially have voltages between two conductors of different arrays that are more than Vmp if the two inverter's AC outputs are on different AC phases.
As long as each conductor is insulated for 2 x Vmp, that should be OK. I just feel bad about it.
I am not at all convinced yet that the voltage between any two leads will never be more than Vmp
With a grounded non-bipolar array you can guarantee that relationship.

 

[shortcircuit2]

Theoretically if there are four PV source circuits in a conduit, then no matter what is on the other end (one or multiple inverters, grounded or ungrounded) there is a very slim possibility that multiple faults could lead to one or more source circuits adding their voltages together.

This same slim possibility is what I was thinking as far as fuse protection of the 4 strings in the same conduit. So if the fault weren't to ground and between strings, wouldn't this be a fire hazard?

 

[GoldDigger]

This same slim possibility is what I was thinking as far as fuse protection of the 4 strings in the same conduit. So if the fault weren't to ground and between strings, wouldn't this be a fire hazard?

Usually not, because the PV arrays are current limited and the conductors themselves should be sized to accommodate that limit value.
If you have a short between two arrays combined with an open circuit somewhere else, it can produce twice the nominal voltage and could generate an arc based on that voltage, but the current will still not exceed the calculated current and your fuses would not have any reason to blow. So they would not give any added protection.

 

[jaggedben]

Since it is a transformerless inverter, the array conductors which are not explicitly grounded will end up referenced during operation to one or both of the AC output terminals of the inverter. That means that the voltage to ground of one of the wires of that subarray at any given moment could be the DC output of the array plus or minus the instantaneous voltage at one AC output terminal.
Same applies to each array.
Now if the design is such that one DC lead will be near the voltage of on AC lead and the other will be offset in the direction of the other AC lead by the Vmp voltage, the result should be that the voltage on the other DC lead will still be less than Vmp from ground.
But I am not convinced that you will not potentially have voltages between two conductors of different arrays that are more than Vmp if the two inverter's AC outputs are on different AC phases.
As long as each conductor is insulated for 2 x Vmp, that should be OK. I just feel bad about it.
I am not at all convinced yet that the voltage between any two leads will never be more than Vmp
With a grounded non-bipolar array you can guarantee that relationship.

I am not at all convinced yet that any reference as you describe could actually occur through the power electronics of multiple grid-tied inverters, even transformerless ones. On the other hand, I am convinced that I didn't entirely follow your line of thinking.

But ultimately, I am fairly convinced, by the OPs mention of a residence in the original subject line, that the systems in question are almost certainly both connected to a single phase service, so your concern, if a real one, wouldn't arise.

 

[jaggedben]

This same slim possibility is what I was thinking as far as fuse protection of the 4 strings in the same conduit. So if the fault weren't to ground and between strings, wouldn't this be a fire hazard?

To add to what GoldDigger said...

The reason we fuse PV source circuits is to prevent the fault current from multiple strings from flowing onto the conductors of a single string which are sized too small. Where multiple strings are deliberately combined onto common buses, such a fault current could occur from a single line-line fault on a single source circuit. Neither the inverters control circuits nor GFDI would interrupt the current flow, but opened fuses will at least isolate the faulted string from the rest of the system. Where multiple strings are not combined on the DC side of inverter electronics, as in the system described by the OP, it's more less a certainty that the same line-line fault on a single string will not conduct the fault current of all the other strings (particularly those connected to other inverters). A modern inverter will likely shut down before it backfeeds current from other strings.

 

[shortcircuit2]

To add to what GoldDigger said...

The reason we fuse PV source circuits is to prevent the fault current from multiple strings from flowing onto the conductors of a single string which are sized too small. Where multiple strings are deliberately combined onto common buses, such a fault current could occur from a single line-line fault on a single source circuit. Neither the inverters control circuits nor GFDI would interrupt the current flow, but opened fuses will at least isolate the faulted string from the rest of the system. Where multiple strings are not combined on the DC side of inverter electronics, as in the system described by the OP, it's more less a certainty that the same line-line fault on a single string will not conduct the fault current of all the other strings (particularly those connected to other inverters). A modern inverter will likely shut down before it backfeeds current from other strings.

ok thanks jaggedben.