Noob needs more help (pics inside)

[DanS26]

just to satisfy my curiosity - have you seen anywhere explanation why PV cannot be simply fed to the side of the busbar opposite to the main breaker? IMO such connection would never exceed 200A busbar rating all the way up to 200A PV output even with 200A main breaker in place.

If that doesn't present any danger OP could just install that Midnight AC Disco/combiner next to his MSP, wire its output to the end of busbar and would be done with this interconnect dilemma.

max....I don't think you are thinking like the code writers who see Mr. (what can go wrong will go wrong) Murphy. For example...a panel has a defective 80A breaker on a large #2 or larger wire that will not trip no matter what the load.....a 260A fault and no protection....in this case a 200A main and 75A coming from the solar array no matter where located on the buss....will melt that 225A buss bar in no time and cause a major fire.

The NEC was written to protect lives and property......do not violate its rules.

[max2k]

max....I don't think you are thinking like the code writers who see Mr. (what can go wrong will go wrong) Murphy. For example...a panel has a defective 80A breaker on a large #2 or larger wire that will not trip no matter what the load.....a 300A fault and no protection....in this case a 200A main and 75A coming from the solar array no matter where located on the buss....will melt that buss bar in no time and cause a major fire.

The NEC was written to protect lives and property......do not violate its rules.

I see, so the intent of the rule here is to protect the connection point of the busbar where that short happens as using my exaggerated numbers it will be subject to 400A current. From that point to the sides it would still be 200A currents so the rest of the busbar would be safe. This also assumes there were no other loads on the busbar at the time, just that one short otherwise they would reduce current 'available' to the short by amount they consumed. It also assumes that the short is current limited to 400A just so it could melt the busbar otherwise it would already tripped one of the 'source' breakers. That's deep wisdom of the Code . The probability of all that happening at the same time is close to 0 IMO.

[foo1bar]

This is only true if the sum of the load breakers does not exceed 200 A. Usually, the sum of the breakers is higher since the loads aren't all concurrent, but crucially, if all the loads did fire off at once, the main breaker should protect the panel. Since the load side of the equation isn't designed to respect the 200 A bus-bar rating, it means the source side must.

It's still true if the sum of the load breakers exceeds 200A.
Let's say I have 200A breaker at one end of the bus bar, 200A breaker at the opposite end (one is PV, other is POCO))
And loads in the middle that are each less than 200A, but total can be way more (ex. 8 50A breakers running right at 50A)
At no cross-section of that bus can there be more than 200A flowing.
Closest to the ends it is 200A flowing.
But as you get to the middle it diminishes and at some spot in the middle it is zero. (Has to be - because the currents are coming from opposite directions on the bus and will cancel each other out)
If you draw it out and use Kirchoff's law you can see that there's no point with more than 200A flowing.
(Although maybe there could be on the neutral if they aren't at opposite ends of the neutral bus bar.)

What I've read as a reasoning for the 120% rule is that allowing greater than that would be of a concern because of heat dissipation. That the bus-bars need to be able to dissipate heat from the 100s of amps flowing. So heat from 240A flowing is still OK when it was designed for 200A; but the heat from 400A flowing might not be.

I think that argument's not a real great one - I think you'd have very close to the same heat generated by doing a 200A main at one end and a 200A load at the opposite end. Maybe there'd be less because there'd be less heat where the breakers contact the bus.

[foo1bar]

There you go. 200 A,

I'd probably email the manufacturer and ask.
I wouldn't be surprised if they use the same bus bar for 200A and 225A and just install a different main breaker.

[sensij]

sure, let's say we have 4x 100A load breakers installed between 200A PV output connection and 200A main breaker on some abstract system. If they all 100 A loads consume at full rating 200A PV out would go to the closest pair and 200A from main breaker- to the pair closest to it. I'm having trouble to picture where along the busbar more than 200A would flow. In my mind the worst case would be single 400A breaker as it would at least need 400A rated lugs to connect to the bus bar.

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)].

[sensij]

I'd probably email the manufacturer and ask.
I wouldn't be surprised if they use the same bus bar for 200A and 225A and just install a different main breaker.

I was looking through the Schneider catalog after the OP posted the information, and the data sheets seem to indicate this is not the case, showing what panels and breakers are allowed to be mixed and matched (page 9, for example)

[max2k]

It's still true if the sum of the load breakers exceeds 200A.
Let's say I have 200A breaker at one end of the bus bar, 200A breaker at the opposite end (one is PV, other is POCO))
And loads in the middle that are each less than 200A, but total can be way more (ex. 8 50A breakers running right at 50A)
At no cross-section of that bus can there be more than 200A flowing.
Closest to the ends it is 200A flowing.
But as you get to the middle it diminishes and at some spot in the middle it is zero. (Has to be - because the currents are coming from opposite directions on the bus and will cancel each other out)
If you draw it out and use Kirchoff's law you can see that there's no point with more than 200A flowing.
(Although maybe there could be on the neutral if they aren't at opposite ends of the neutral bus bar.)

What I've read as a reasoning for the 120% rule is that allowing greater than that would be of a concern because of heat dissipation. That the bus-bars need to be able to dissipate heat from the 100s of amps flowing. So heat from 240A flowing is still OK when it was designed for 200A; but the heat from 400A flowing might not be.

I think that argument's not a real great one - I think you'd have very close to the same heat generated by doing a 200A main at one end and a 200A load at the opposite end. Maybe there'd be less because there'd be less heat where the breakers contact the bus.

exactly, the case is even easier on the bus compare to main breaker on one end and single rated 200A load - on the opposite. Shorts are actually the blessing from safety point of view as they tend to be not current limited and trip a breaker somewhere along the path. In terms of melting/protecting stuff here's the picture of one of my approved/rated/listed breakers which pretty much burnt out at the point it contacted the busbar. In my experience things burn much more often from the bad contacts like in this case- spring instead of bolt on 40A double circuit. The resistance of the contact surface increases but the current still continues to flow generating a lot of heat at the place of bad contact making it even worse fairly quickly leading to this:
BadBreaker2.jpg

[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' .

[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]

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]

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]

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]

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.

[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

[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.