Bus Backfeed options - 120% rule

[bcroe]

120%

I have always assumed that MPPT works the best when the panels
are identical with the same exposure. I am not sure I want to risk impacting the inverter
efficiency bty having two strings with widely different exposures. Plus one string is on a pole
mount vs the new string is on a roof so the temps will be quite different.

I'm not convinced your step up transformer idea would work. It would raise the impedance the
inverter sees at the line, possibly tripping the anti island function. It would be more likely to work
(in this regard) and be twice as efficient as a 120V- 240 autotransformer.

Your secondary panel idea allowing a single breaker to your main, sounds attractive if it doesn't
break any rules. Certainly less expensive than buying an inverter.

My paralleled arrays idea fails if there is significant overlap of peak output times.

My rule of thumb, is the MPP of a properly operating array is about 78% of Voc. This voltage
doesn't vary a lot over the range of useful power output, so strings with the same Voc but
different sun intensity can still be efficient in parallel. If the instantaneous Voc were to shift
on one string due to temp, a common MPP output would be reduced a somewhat less percentage.

If a panel in a string were shaded, bypass diodes could allow the string to still produce useful
power at a lower voltage. But if another string in parallel continued with no shade, a common
MPP inverter would have to choose between running at much reduced voltage for all, or running
at full voltage for the unshaded string and receiving very little current from the string with a
shaded panel. Either way would reduce power much more than having separate MPP inverters
for each string. The argument can be extended to a micro for each panel. Bruce Roe

[peakbagger]

I have always assumed that MPPT works the best when the panels are identical with the same exposure. I am not sure I want to risk impacting the inverter efficiency bty having two strings with widely different exposures. Plus one string is on a pole mount vs the new string is on a roof so the temps will be quite different.

[bcroe]

Exposure

You say your arrays have different exposures? Does that mean, different orientation,
with different times of peak power? If 2 arrays were the same voltage and peaked at
widely different times, you could parallel them to one inverter, which would have less
capacity than the 2 separately. That would reduce the apparent total to closer to the
actual possible power. Bruce Roe

[inetdog]

Install a subpanel for PV only, have it feed a 40 amp breaker in the main panel.? This wouldnt be so bad but I am not sure the code allows this?

So anyone have any suggestions or thoughts besides shutting down the 1 KW system?

As long as the subpanel meets the 120% rule also (1.2 x bus rating greater than 45 + 40 main if you can find one.), that is fine. A 100A bus subpanel with a 50A or 75A main and three 15A feeds would be OK since the total is less than 120A.
And the main panel is OK with all of the back feed coming through the single 40A breaker.
Unfortunately the code does not let you just add the maximum power output of the GTIs when looking at the 120% rule.

Oddly enough, if the inverters were hybrid GI instead of just GTI, you would be able to add up 125% of their individual rated GI outputs (1.25 x 8.35 = 10.44 each) instead of adding up the breaker sizes, and you would not have a problem.

[peakbagger]

I am getting ready to install an addtional 2 KW grid tie system and want to avoid a line side tap. The local area really doesnt have any master electricans with solar experience so I need to work with one and point him in the right direction . The local inspector has no electrical background and basically has told me that a master has to sign off on the install. The area uses NEC 2011

Here is what I have

200 Amp breaker panel, there is no bus rating so I will assume it is 200 AMP bus

One 2 KW grid inverter - 8.35 max amps hooked to a 2 pole 15 amp breaker

One 1 KW grid inverter - 8.35 max amps hooked to a 1 pole 15 amp breaker

New 2 KW grid tie inverter - 8.35 max amps hooked to 2 pole 15 amp breaker.

The breaker amperage ratings are all the manufacturers recomended sizing and down sizing to 10 Amp breakers would exceed the 80% continuous rating of the breaker

120% backfeed is 40 AMPs maximum based on breaker ratings, If I look at the potential load per bus leg , I have an unbalanced backfeed with 30 Amps on one leg and 45 amps on the other. I assume this will not fly?

I do see references to 705.12(D)(2) Exception for battery sourced multimode inverters, that is real tempting as then all I have to do is add up the inverter ratings and multiply by 1.25. This yields 31.3 and 20.8 amps. Looks like this works but since I have battery less units I expect this wont fly ? Arguably either unit is limited on its output by internal circuitry but the code is the code


Options
All three arrays have different exposures, I wouldnt mind buying the new inverter with dual MPPT but they seem to have fallen out of favor.

Install a 110 to 220 volt step up transformer on the 1 KW unit and switch to a smaller 2 pole breaker? This balances the load and gets me below 40 amps if I install a 10 amp 2 pole breaker. At that point I would probably do a line size tap due to the cost and loss of the transformer.

Install a subpanel for PV only, have it feed a 40 amp breaker in the main panel.? This wouldnt be so bad but I am not sure the code allows this?

So anyone have any suggestions or thoughts besides shutting down the 1 KW system?