Noob needs more help (pics inside)

[sensij]

Have I covered this scenario as well?

Nope. Let's say L1 has 200 A of 120 V load circuits (10 x 20 A circuits, for example, not very different than what is the service panel of my old house), and a 100 A main breaker (we agree that it is typical for the sum of load breakers to exceed the main breaker). With a single source, we know that even in an overload condition, the 100 A main will trip.

Now let's add 100 A of PV to the opposite end of that bus because we think 120% is conservative and 200% sounds better. It would be *possible* for 200 A of load on L1 to be supplied (100 A from main, 100 A from PV) and no breakers tripped. All 200 A of that current would potentially travel to the transfomer through the neutral bus (which also has 100 A rating), neutral lug, neutral service feed and service drop (which have been sized, like the service panel, only for 100 A service), where 100 A could then return to the inverter on L2. Yes, L2 loads would reduce the neutral current, but there is nothing that requires L1 and L2 loads to be balanced, or for L2 loads to even be present if the load center was poorly arranged.

[max2k]

Nope. Let's say L1 has 200 A of 120 V load circuits (10 x 20 A circuits, for example, not very different than what is the service panel of my old house), and a 100 A main breaker (we agree that it is typical for the sum of load breakers to exceed the main breaker). With a single source, we know that even in an overload condition, the 100 A main will trip.

Now let's add 100 A of PV to the opposite end of that bus because we think 120% is conservative and 200% sounds better. It would be *possible* for 200 A of load on L1 to be supplied (100 A from main, 100 A from PV) and no breakers tripped. All 200 A of that current would potentially travel to the transfomer through the neutral (which had been sized, like the service panel, only for 100 A service), where 100 A could then return to the inverter on L2. Yes, L2 loads would reduce the neutral current, but there is nothing that requires L1 and L2 loads to be balanced, or for L2 loads to even be present if the load center was poorly arranged.

OK, you're correct here- 200A through L1 load and neutral and no breakers tripping.

Surprisingly all discussions related to the rule revolve around busbars without anyone ever bringing up problem of asymmetric load overloading neutral with people commonly upgrading their MSPs even though the service wires are often sized to support higher current such that only MSP gets replaced. Based on this discussion that is not really needed.

[bcroe]

not really possible to sum up: say we put load 200A on L1 only and PV is capable of supplying that current - the grid neutral would provide 'return' path but then grid L2 would carry 200A back to PV inverter to close the circuit.

This won't happen. The 240VAC PV is incapable of unsymmetric feed. What might happen is the PV feeds 100A
to L1 and L2, the power goes out to the pole transformer which as an autotransformer feeds back 200A 120V. With PV
supplying the load, I see on L1 100A from PV and 100A from main breaker (to PoCo trans) feeding load point,
200A from load return to PoCo trans, and in L2 100A from PV to main breaker (to PoCo trans). Bruce Roe

[foo1bar]

No, you are missing the point. You want to assert that the worst case without PV is a single 200 A load on the end of the busbar. Ok, now add PV to that specific case, and allow the load to increase to 250 A (which is possible now in a fault scenario, and no limit on the PV input). Don't you think that would be hotter? You'd have 200 A still flowing the length of the busbar to one slot above the PV, plus 50 A from the PV going to that load now. The finger from the bus that the breaker attaches to would be carrying all 250 A.

No, I don't think it'd be hotter (for the heat from current flowing through the bus itself).

It would be 200A flowing 90% of the length (instead of 100%)
And 50A flowing on the remaining 10% (instead of 200A on that portion)

If there were a 250A breaker (there isn't) it'd be across multiple fingers.
I believe all the 100A or 200A breakers are across multiple fingers. So if there were larger ones, they would be too.

[bcroe]

Its good to thrash this subject some. I have a few conclusions.

The idea of resistance at the breaker connection to the busbar causing a lot of heat
is new to me. However these are likely to fail "safe" as an open circuit, in fact likely
all breakers are designed to fail that way.

I conclude its best not to put very large "continuous" load breakers on buses; move them to their own
boxes. Lots of small breakers in the middle will not contribute to contact heat, with far less current
through just as robust a contact, even with more, the heat of each contact diminishes as the SQUARE
of current. Or consider it as many contacts in parallel having much less resistance than just one.

Certainly badly unbalanced L1 and L2 is not good. Bruce Roe

[sensij]

Surprisingly all discussions related to the rule revolve around busbars without anyone ever bringing up problem of asymmetric load overloading neutral with people commonly upgrading their MSPs even though the service wires are often sized to support higher current such that only MSP gets replaced. Based on this discussion that is not really needed.

There are several examples in the forum where the costs of PV motivated MSP upgrades were multiplied because new underground service wires would be required to handle the larger service, and the trenching, etc costs to upsize the conduit are significant. I just had to have the service conductors (overhead drop and feed) replaced on my old house a couple weeks ago, and had asked what they were putting in. The existing conduit would only support wire large enough for 125 A (2 AWG), but they stuck with the plans and replaced like for like using #4 for my 100 A service (no oversizing by default).

Anyway, even if you increase the size of the service conductor, the neutral bus+lug are still potentially getting overloaded. Even if you add in some 120 V loads on L2 to balance the service neutral conductor, the neutral bus could still have areas that are overloaded, depending on where each of the connections are made in the bus.

Clearly I'm out of my expertise here and just throwing stuff to the wall to see what sticks, and I'm fine with conceding the point on L1/L2 bus temperature for the scenarios you and foo1bar have walked through. I just think that is is generally a bad idea to try to get into the mind of the code panel and pick it apart on the basis of one particular aspect of the system (which might be the most obvious place to look), without considering the impact to the entire system. It is a slippery slope to attempt to qualify code with "too conservative" or "not conservative enough" without more training and experience than I think most of us here possess.

[max2k]

...
Anyway, even if you increase the size of the service conductor, the neutral bus+lug are still potentially getting overloaded. Even if you add in some 120 V loads on L2 to balance the service neutral conductor, the neutral bus could still have areas that are overloaded, depending on where each of the connections are made in the bus.

neutral overload can only happen if you start making changes to the loads after connecting PV and only when it actually producing electricity. Come cloud / night and the main breaker would trip (if the system makes that far after changes). If we stick to the original case of up-to-code MSP and then adding PV to it the only consequence is cooler busbar..

Clearly I'm out of my expertise here and just throwing stuff to the wall to see what sticks, and I'm fine with conceding the point on L1/L2 bus temperature for the scenarios you and foo1bar have walked through. I just think that is is generally a bad idea to try to get into the mind of the code panel and pick it apart on the basis of one particular aspect of the system (which might be the most obvious place to look), without considering the impact to the entire system. It is a slippery slope to attempt to qualify code with "too conservative" or "not conservative enough" without more training and experience than I think most of us here possess.

Don't get me wrong- I respect the Code for it providing consistent set of rules and accumulating measures which would prevent mistakes. OTOH I don't worship the Code for multiple reasons. It is written by people, not some tech gods. People are susceptible to politics, mistakes, etc. Just one such example you sited yourself: it took them 3 years between 2011 and 2014 cycles to realize the headroom in breaker current rating is not going to heat up the MSP busbar. When things like this get missed my desire to understand their reasoning behind other rules increases even more.