Noob needs more help (pics inside)

[max2k]

A grid tie inverter doesn't resolve an unbalanced load, and it's neutral is not required to be sized as a current carrying conductor, it is used for a voltage reference.

I see; in any case it doesn't affect busbar rating.

I think the Code tried to prevent the case when someone puts load breaker with rating exceeding main breaker and possibly melting busbar at the point of contact as both sources could possibly be working into this single 'sink'. The Code could prevent this with simple clause 'no breaker rating should exceed busbar rating' but it doesn't seem to be the case.

If we model busbar with load breakers as split into individual mini-bars all connecting to the main breaker it becomes clear that PV source on the other end can take some of those mini-bars out of original group and as it is closer to the loads those mini bars can be made shorter decreasing total amount of heat produced. Too bad this most common case didn't make into the Code.

[foo1bar]

True or false? A busbar that is supplying 250 A from two sources to 20 load circuits will be hotter than one supplying 200 A from a single source to the same 20 circuits.

True or false? A busbar that has 200A flowing from one end to the other will be hotter than one with two 200A coming in at each end but zero flowing near the middle.

True or false? A busbar supplying 250 A to a single 250 A load from two sources will be hotter than one supplying 200 A from a single source to the load.

Do you mean 200A from one end to that load and 50A from the other end?
vs. 200A over the entire length of the busbar?
I think the answer is 200A over the full length of the busbar is more heat.

[max2k]

You guys are awesome. I love the chat. Great information!! I'm reading intently!

I think I get My 1st option:

Inverter amp max 25A: 25A x 3 = 75A x 1.25 = 93.75 or 100 AMP breaker because they don't make 93's.

So the only way to get this to work with my existing setup is to take my main breaker down from a 200 to a 125. This will free up 75A + I can add that +40 amps (200x1.20) to get me the 100A I need.

Hopefully option 2 is better. The Line Side Tap!

I do have full access to every inch of the main wire from the Meter to the MSP.

sorry for hijacking your thread but these are the only 2 code complaint options as I see it. While line tap 'looks' ugly to me it would leave you with 200A main breaker for future loads. Wiring line tap into say Midnight AC combiner/disco output bus would give you AC disconnect and clean way to connect your inverters as it can have inverter breakers inside. According to Butch this is how it is done. I'm 'pushing' for that combiner simply because that's what I used to combine output of my 2 inverters, I liked it and I don't know anything else . It is pricey but DIY might allow you to use it.

Please confirm with your utility they're OK with line tap, some are not, depends on location.

[diegodog]

sorry for hijacking your thread but these are the only 2 code complaint options as I see it. While line tap 'looks' ugly to me it would leave you with 200A main breaker for future loads. Wiring line tap into say Midnight AC combiner/disco output bus would give you AC disconnect and clean way to connect your inverters as it can have inverter breakers inside. According to Butch this is how it is done. I'm 'pushing' for that combiner simply because that's what I used to combine output of my 2 inverters, I liked it and I don't know anything else . It is pricey but DIY might allow you to use it.

Please confirm with your utility they're OK with line tap, some are not, depends on location.

Sweet.

No hijacking at all. I love the knowledge so if its called hijacking hijack away.

Thanks again to everyone.

[ButchDeal]

Please confirm with your utility they're OK with line tap, some are not, depends on location.

Find a master electrician he will have to do the tap and have him review your plans. He will know what will pas code in the regin.
hint the older the master electrician the better. Everyone knows and respects the older ones and rarely question them. For rural DIY this can be helpful sometimes.

[max2k]

Find a master electrician he will have to do the tap and have him review your plans. He will know what will pas code in the regin.
hint the older the master electrician the better. Everyone knows and respects the older ones and rarely question them. For rural DIY this can be helpful sometimes.

OP said there's only single shirtless Bob available there who goes for everything electrical . Sounds like there are not so many people around there to begin with.

[sensij]

True or false? A busbar that has 200A flowing from one end to the other will be hotter than one with two 200A coming in at each end but zero flowing near the middle.


Do you mean 200A from one end to that load and 50A from the other end?
vs. 200A over the entire length of the busbar?
I think the answer is 200A over the full length of the busbar is more heat.

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.

[sensij]

I see; in any case it doesn't affect busbar rating.

Huh? So you are saying we should ignore the fact that the 120% rule for PV installation provides protection for the neutral conductor from a risk that can only be introduced by the PV system?

[max2k]

Huh? So you are saying we should ignore the fact that the 120% rule for PV installation provides protection for the neutral conductor from a risk that can only be introduced by the PV system?

please elaborate, I was under impression 120% rule is only intended to limit heat from the current flowing through the bus. It would be fairly far fetched to claim it is relevant to neutral conductor ampacity- why they didn't simply state that in neutral conductor requirements directly?

BTW, your example with singe 250A load is not very realistic unless someone deliberately installed 250A breaker/load on the 200A rated bus. If you meant possible short fault then it wouldn't limit itself to 250A and would trip one of the breakers. Multiple loads totaling to more than 200A don't present a problem as was shown above.

I think bcroe's comment is the closest to the reasoning- someone said 50% heat overload is acceptable, they took square root, got 1.22, rounded it down to 1.2 and 120% rule was born .

[sensij]

please elaborate, I was under impression 120% rule is only intended to limit heat from the current flowing through the bus. It would be fairly far fetched to claim it is relevant to neutral conductor ampacity- why they didn't simply state that in neutral conductor requirements directly?

BTW, your example with singe 250A load is not very realistic unless someone deliberately installed 250A breaker/load on the 200A rated bus. If you meant possible short fault then it wouldn't limit itself to 250A and would trip one of the breakers.

I'm sorry, where in code does it say what the requirement is *intended* to do? Code is taken as a collection, most parts don't work well alone if you don't assume other parts are also followed. The 120% rule is written to specifically only apply when one of the sources is the utility, with, presumably, all the controls that apply. The general case rules are more restrictive.

In a fault scenario, you don't know what the fault current will be. That is why you don't protect 14 AWG with a 40 A breaker... logic that says "a short will trip it in any case" is terrible.

[max2k]

...
In a fault scenario, you don't know what the fault current will be. That is why you don't protect 14 AWG with a 40 A breaker... logic that says "a short will trip it in any case" is terrible.

not really applicable here - as I stated if one connects 250A breaker to the 200A bus bar they're asking for it.

What we started with was code complaint MSP where all breakers are feeding correctly sized wires and the only change made was additional PV source on the busbar end opposite to the main breaker. It is still my opinion such addition would reduce heat load on the busbar during times PV produces power.

[sensij]

What we started with was code complaint MSP where all breakers are feeding correctly sized wires and the only change made was additional PV source on the busbar end opposite to the main breaker. It is still my opinion such addition would reduce heat load on the busbar during times PV produces power.

Right, and we've also covered some scenarios of risks that the PV system can introduce to that system that are protected against by the 120% rule. It is impossible to say whether that rule is conservative or not on the basis of your simplistic thermal analysis alone.

[sensij]

not really applicable here - as I stated if one connects 250A breaker to the 200A bus bar they're asking for it.

It is applicable to the neutral discussion. It is possible for the utility current and the PV current to be summed on the neutral conductor, no fault required, just unbalanced loading.

[max2k]

Right, and we've also covered some scenarios of risks that the PV system can introduce to that system that are protected against by the 120% rule. It is impossible to say whether that rule is conservative or not on the basis of your simplistic thermal analysis alone.

I haven't seen yet even 1 scenario where 120% rule would improve safety. My analysis is 'good enough' - it covers what is going on so no other complications would change the fact that current has to travel less distance along the bar with PV producing extra power. I guess, it just seems too counter intuitive to accept: extra source + extra power but less losses.

[max2k]

It is applicable to the neutral discussion. It is possible for the utility current and the PV current to be summed on the neutral conductor, no fault required, just unbalanced loading.

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. If we put some load on L2 as well it would reduce current through neutral and L2 coming from the grid as inverter current can now travel back over shorter circuit. When L2 load reaches 200A as well neutral or L2 from the grid won't have any current flowing through them. Now if we continue to increase L2 load asymmetrically L2 current from the grid can reach 200A on its own (to the total of 400A through the L2 load) creating 200A neutral current from the grid. Then we can increase load on L1 from 200 to 400A at which point point PV and grid would be symmetrically supplying 200A each with neutral conducting 0 current. If in any of those cases asymmetrical load exceeds 200A the main breaker would trip. Have I covered this scenario as well?