Noob needs more help (pics inside)

[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

[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)

[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)].

[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.

[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.

[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.

[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]

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.

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.

[sensij]

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.

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.

[max2k]

There you go. 200 A, so 7700 W inverter max if you leave the 200 A main breaker. You'd have to drop all the way to a 125 A main to install 3 x 6 kW inverters. A pair of 7.7 kW inverters would have been reasonably well matched to those panels, and would have been ok with 150 A main.

Looks like line side tap is probably the way to go for you, if you have access to the conductors between the meter and the panel.

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.

[sensij]

Here is the product data sheet for the MSP.

There you go. 200 A, so 7700 W inverter max if you leave the 200 A main breaker. You'd have to drop all the way to a 125 A main to install 3 x 6 kW inverters. A pair of 7.7 kW inverters would have been reasonably well matched to those panels, and would have been ok with 150 A main.

Looks like line side tap is probably the way to go for you, if you have access to the conductors between the meter and the panel.

[diegodog]

Here is the product data sheet for the MSP.

[diegodog]

I will try to grab more pictures tonight with better shots of the MSP and stickers. I've been scanning online and if I had to guess I would say the bus bar is 200AMP. But that's simply a guess.

Yeah I wish the code book had some pretty pictures with my exact set up with a step by step guide. The MSP panel issue I'm running into was unexpected. I thought it was as easy as 3x 30 AMP double pole breakers. LOL!! I also get the feeling once you have done this a few times its in actuality somewhat simple. Its that first go around that is the problem and all those little nagging questions that slow things down like "Hey you can't hang those the electricity has no where to go..", Can I just wing nut it?

As you can imagine there are not many people around here. The electrician is a guy named Bob. He works with no shirt on no matter the weather, smokes about a pack an hour and leaves one to wonder if he works up here due to no inspections. I will say though the barn has had no fires or explosions due to his previous work so that's a relief! I would assume this is not that complicated for a professional electrician but I want to have a general idea of what's going on as my family works in the barn.

The electric company told me "we don't care what you do on your side we only care about our side" I'm not originally from around here so the answer was surprising but I didn't push it as I like the freedom to blow myself up if I so choose so I support the electric company in allowing me to do that if I want to. I assume they would place their meter out there along with the disconnect that's currently there and drive off. They never inspected anything other than outside at the meter when the main service was brought in. It surreal almost after dealing with big city rules in the past. I know I'm out my league here so I would never even think about hooking any of this up but I want to know what Bob is doing at all times as well. The less Bob touches the better! I just want him for the dangerous parts!!

Yeah I agree on going a bit further and doing a nice junction box.

Hopefully I have some pictures and data sheets out here on this panel I have in the next 24hrs or so.

Also excited to watch some videos and do some reading on the Tap ButchDeal is talking about. Sounds fun and dangerous. Sounds like maybe a 2nd panel box and a piece of rebar connecting the two would do the trick!

Was going to paint the tops of the PT wood frames today to put the first layer in between the cooper in the wood and the aluminum frames but we got rain! The EPDM that I'm placing on top of the painted wood will be here this weekend. Once I find some decent junction boxes hopefully this part can go quickly.

Thanks again!

[max2k]

You guys are awesome!!

I spent the entire night trying to find documentation on what my bus bar rating was and couldn't find that. Is there a way other than looking at the sticker (I'll get more pictures) to see what I have? I see 100A, 125A, 200A on the stickers. Nothing that clearly said bus bar rating = x. Any particular place I take get a close up photo of that would help identify it for sure. Would be very happy if Sensij was correct and it was 225A.

sensij is most likely correct but it would help if you could post more pics with your panel stickers at better angle.

I'm in a unique situation where there are 0 inspectors. There is 1 electrician.

there are 2 possible issues here:

- safety. While some parts of the Code may sound draconian it also reflects results of analysis of the previous problems so when making compromise you need to understand possible consequences. One problem with the Code is it tries to address every possible case out there and sometimes I think it makes things more complicated than they need to be. Code authors rightfully assume that the rules won't be understood, they just need to be followed. Like in your case- what bad could happen if you leave 200A main breaker and feed your PV from another end of the MSP busbar through 100A breaker? I'm aware of 120% limit but in this arrangement I don't see any part of bus bar to get under current load above 200A, ever. Any load can only be connected between those breakers and as such the current partially would come from main breaker side of bus bar and another portion of it - from PV breaker side, there won't be place on the busbar where they'd flow together. Given that load 'connections' to busbar are made through multiple breakers between 2 'source' breakers they will be spreading the total load on the busbar even further.

The Code though needs to assume nothing and prevent case where someone would actually try to feed some other subpanel with 300A breaker off the MSP busbar where at the point of contact the total 275A current will exceed busbar rating above safe limit. I possibly can be missing some other scenario here so hopefully other members would be able to come up with one where this would be a problem besides not being up to Code.

- how are you going to deal with POCO? They usually use inspections as the means to ensure the install was done up to the Code so they don't have to come out and look for themselves. Since you really need for your POCO to recognize your install and update your billing settings (so you could take advantage of net metering) I wonder how this will be handled in your case.

Can I use a simple junction box to make the splices between the PV wire and the THWN-2? Do I need anything fancy in there? I assume I need more than wingnuts and electrical tape.

I'd try to find something with the terminals/bolts rated for outdoor use and not just splices. I'd not use electrical tape for insulation, it doesn't last, especially outdoors. You want to build your system once and forget about it, not to fix some bad connections every 3 months.

[diegodog]

You guys are awesome!!

I spent the entire night trying to find documentation on what my bus bar rating was and couldn't find that. Is there a way other than looking at the sticker (I'll get more pictures) to see what I have? I see 100A, 125A, 200A on the stickers. Nothing that clearly said bus bar rating = x. Any particular place I take get a close up photo of that would help identify it for sure. Would be very happy if Sensij was correct and it was 225A.

I'm in a unique situation where there are 0 inspectors. There is 1 electrician. Closest Walmart is 100 miles away. So there are no issues in that respect. There is simply an old lady down at a small building that you send your property tax check to. I have yet to find a town without the property tax lady I have to visit once a year.

It looks like I downgrade the MSP or I can do the line side tap. I'll need to study up on what exactly the line side tap is and see if I grasp it.

Going back down to pre-school level again if you guys don't mind.

Can I use a simple junction box to make the splices between the PV wire and the THWN-2? Do I need anything fancy in there? I assume I need more than wingnuts and electrical tape.

Thanks again guys.