Noob needs more help (pics inside)

[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.

[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.

[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.

[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.

[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.

[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]

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.

[diegodog]

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.

[bcroe]

I do realize this is not authoritative discussion, I'm just trying to bring board's attention to over-
conservative (IMO) approach behind 120% rule. Just from my personal perspective I don't see why the limit is set that low.

Remember the heat into a conductor resistance is proportional to the SQUARE of the current. So a 120% current
would produce 144% heating, more if the temp caused the resistance to increase. I would not want this to happen.

HOWEVER the currents can add together only with the sources toward one end of the bus bar, loads at the other.
Since the PoCo and solar feeds are requested to be at opposite ends of the bus, loads in the middle, there is
NO POINT at which the currents add. IF the placement rule is adhered to, the 120% rule is quite conservative. Bruce Roe

[sensij]

Neutral would remain the way it is now- one coming from service would 'collect' its unbalanced currents and the one from inverters- theirs, I don't see a problem there.

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.

[max2k]

Are you sure that is the worst case? Isn't heat generated at each of the contact points potentially more damaging than the bulk resistance of the busbar? Maybe someone, somewhere, has a model for that which is informing code? I truly don't know, but I think your take on this is overly simplistic.

Edit: another potential consideration is protection of the neutral feeder. If all of the worst case situations we can imagine were loaded onto a single phase in some kind of fault scenario, the 120% rule could be one way to help ensure that the neutral wouldn't fail. Again, it is way outside my expertise, but I don't think this discussion is really looking at the big picture.

Quick search indicates busbar rating calculations are concerned with heat produced by current flowing through it and cooling conditions. I was unable to find rating affected by the contact resistance. For example: http://bralpowerassociate.blogspot.c...lculation.html

Neutral would remain the way it is now- one coming from service would 'collect' its unbalanced currents and the one from inverters- theirs, I don't see a problem there.

I do realize this is not authoritative discussion, I'm just trying to bring board's attention to over- conservative (IMO) approach behind 120% rule. Just from my personal perspective I don't see why the limit is set that low.

[sensij]

most likely false, if each of the sources <= 200A not the worst case though



true, if the load is somewhere in the middle but that's not the worst case scenario either. Worst case scenario in the normal setup without PV is when busbar has single 200A load on the end opposite to the main breaker. That would heat up the busbar the most and it is still rated for that. Now, if we add 200A PV source to the same spot instead of load and place loads in between the sources no part of the busbar will be subject to the current > 200A while potentially can supply 400A of loads still heating up less than in the original 200A worst case scenario as 200A currents will be present only at the ends of the busbar closest to the 2 sources and less everywhere else.

Are you sure that is the worst case? Isn't heat generated at each of the contact points potentially more damaging than the bulk resistance of the busbar? Maybe someone, somewhere, has a model for that which is informing code? I truly don't know, but I think your take on this is overly simplistic.

Edit: another potential consideration is protection of the neutral feeder. If all of the worst case situations we can imagine were loaded onto a single phase in some kind of fault scenario, the 120% rule could be one way to help ensure that the neutral wouldn't fail. Again, it is way outside my expertise, but I don't think this discussion is really looking at the big picture.

[max2k]

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.

most likely false, if each of the sources <= 200A not the worst case though

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.

true, if the load is somewhere in the middle but that's not the worst case scenario either. Worst case scenario in the normal setup without PV is when busbar has single 200A load on the end opposite to the main breaker. That would heat up the busbar the most and it is still rated for that. Now, if we add 200A PV source to the same spot instead of load and place loads in between the sources no part of the busbar will be subject to the current > 200A while potentially can supply 400A of loads still heating up less than in the original 200A worst case scenario as 200A currents will be present only at the ends of the busbar closest to the 2 sources and less everywhere else.

[sensij]

PV backfeed makes it actually easier on the busbar from the heat point of view as loads get their energy from shorter paths given now we have 2 sources instead of one. I understand this is not up to Code I'm just trying to see the reasoning behind those rules.

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 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.

In any "worst case scenario" I can think of for the original busbar, I think adding the PV as an additional source makes that worst case scenario worse.

I'm not trying to defend 120% or any other number, just acknowledging that it takes much more knowledge and experience than what I possess to figure it out.

[max2k]

So, the short answer is, I don't know. The longer answer is that I could see how the temperature of the busbar would be sensitive to the total current passed along it, even if the peak current at any one point was within the rating. The 120% rule [2014 NEC 705.12(D)(2)(3)(b)] specifically requires the PV be end-fed, so the rule to some extent considers the point you are trying to make. The more general rule that allows PV to be put anywhere limits the sources to 100% of the busbar rating [2014 NEC 705.12(D)(2)(3)(a)].

PV backfeed makes it actually easier on the busbar from the heat point of view as loads get their energy from shorter paths given now we have 2 sources instead of one. I understand this is not up to Code I'm just trying to see the reasoning behind those rules. I thought it was brilliant to end feed the busbar from the side opposite to main breaker but they 'deregulated' it right after. IMO they put too much safety margin into 120% rule, it has much more 'potential' .