70amp solar on a 200 amp main

[mooreaaryan]

Very confused on installing solar. If I have a 200 amp main is there a way to technically put a 70amp solar system to feed this system?
I have been told putting a 175amp main will allow me to install a 70amp system nit sure how that actually changes anything.

 

[GoldDigger]

One option for sizing solar backfeed as a line side connection in a panel or subpanel is the "120% rule".
You find out the amp rating of the panel bus bars (which in your case will be at least 200A, but may be 225A if you are lucky. You need to find out for sure for your panel. )
You then multiply that by 120%. For you, that is a minimum of 240A.
The sum of the main breaker size and the nominal amperage of the solar installation must not exceed that number. Unless you have a 225A bus or reduce the size of the main breaker, that limits you to 40A. If you have a 200A bus, even a 175A main will not allow you to put in 70A of solar.

The motivation behind that rule is that by putting the main breaker and the solar backfeed breaker at opposite ends of the ;panel bus you can avoid overloading the bus if you have more than 200A of connected loads. The reason it is a 120% rule rather than a 200% rule is that the Code makers wanted to be totally conservative with a configuration that was not part of the panel testing.

 

[bwat]

The reason it is a 120% rule rather than a 200% rule is that the Code makers wanted to be totally conservative with a configuration that was not part of the panel testing.

Can you expand on this part? Not clear to me why 200% could have made more sense than 120%. I view them both as arbitrary numbers that a CMP could choose, but the way you said it made it sound like 200% might make more sense from an actual mathematical sense, but chose 120% to be conservative.

 

[jaggedben]

Very confused on installing solar. If I have a 200 amp main is there a way to technically put a 70amp solar system to feed this system?
I have been told putting a 175amp main will allow me to install a 70amp system nit sure how that actually changes anything.

Assuming the busbar is rated 200A (not 225A)...

(200×1.2)-175=65.
The rule states that 125% of inverter output cannot exceed that number, i.e. 65 in this case.

So if your inverter output is 52A or less (52×1.25=65) you can put it on a 70A solar breaker and it's okay with the 175A main.
If your inverter output is between 52 and 56A, the 70A solar breaker is okay but you need to downsize the main to 150A, or change something else (like, downsize the system.)

 

[jaggedben]

Can you expand on this part? Not clear to me why 200% could have made more sense than 120%. I view them both as arbitrary numbers that a CMP could choose, but the way you said it made it sound like 200% might make more sense from an actual mathematical sense, but chose 120% to be conservative.

A straight application of theory (Kirchoff's law) finds that when the two sources are at opposite ends of the busbar, the max amps that can flow through any part of the busbar is whichever source is larger.

However, Kirchoff's law doesn't directly relate to what causes a panelboard bus to be overloaded. What actually has to happen is that heat needs to be transferred out of the enclosure before the plastic parts melt (not the busbar itself). So since panels are not tested for such scenarios, the code making panel chose a very conservative limit.

 

[wwhitney]

Can you expand on this part? Not clear to me why 200% could have made more sense than 120%. I view them both as arbitrary numbers that a CMP could choose, but the way you said it made it sound like 200% might make more sense from an actual mathematical sense, but chose 120% to be conservative.

It's about heating from the busbar connections. With a 200% rule, you potentially have double that heating. With a 120% rule, the worst case is only 104% of the previous worst case, given the square in I²*R.

Cheers, Wayne

 

[ggunn]

Can you expand on this part? Not clear to me why 200% could have made more sense than 120%. I view them both as arbitrary numbers that a CMP could choose, but the way you said it made it sound like 200% might make more sense from an actual mathematical sense, but chose 120% to be conservative.

The reasoning does not matter. Read 705.12(B)(3) and pick your method of qualifying the bus (the 120% rule is option 2), or connect on the line side of the main service disconnect per 705.11. Ours is not to reason why; ours is to install per code or fail the inspection.

 

[don_resqcapt19]

It's about heating from the busbar connections. With a 200% rule, you potentially have double that heating. With a 120% rule, the worst case is only 104% of the previous worst case, given the square in I²*R.

Cheers, Wayne

Would that not be quadruple heating?

 

[wwhitney]

Would that not be quadruple heating?

Not if we require that the current in every connection to the busbar is limited to the busbar rating. The worst case with one source is one equal load of maximum value, so you get heating from two full current connections. With a hypothetical 200% rule, you could have 4 full current connections, two sources and two separate loads, or double the heating.

As far as I can see, a hypothetical 200% rule doesn't increase worst case I²*R heating in the busbar itself. That worst case is that the whole busbar carries full current for its full length, either way.

Cheers, Wayne

 

[Joethemechanic]

Feeding the bus from opposite ends should decrease I2R heating of the bus. I got to be missing something here

 

[ggunn]

Feeding the bus from opposite ends should decrease I2R heating of the bus. I got to be missing something here

It really makes no difference in the the real world. The code is the code; design to it or die.

 

[winnie]

Feeding the bus from opposite ends should decrease I2R heating of the bus. I got to be missing something here

If you have the same total power delivered to to various branch breakers, then you are correct; delivery current from opposite ends of the bus, and reduce heating.

But by adding sources at both ends of the bus you could have a condition where more total power is delivered to the load.

Jon

 

[Carultch]

Feeding the bus from opposite ends should decrease I2R heating of the bus. I got to be missing something here

The issue is that loads panelboards in general are routinely populated with more load breakers than the busbar capacity, on the underlying assumption that not all of them will draw full power at once. When you feed it from a source at both ends, this is the ideal thing to do from the point of view of Kirchhoff's current law, to avoid too much current accumulating at any individual section of the busbar. However, the issue is mutual heating among the breakers, rather than only the current on the busbar. It's true that current on the busbar is an issue, but it isn't the only issue.

Supplying the panelboard from multiple sources means that there is the unlikely event that all the branch breakers draw more load current than the main supply's trip rating, which creates a blindspot for the main breaker that is intended to trip when this unlikely event happens. This creates the possibility of too much mutual breaker heating. The 120% rule is an industry compromise, to allow some headroom for taking credit for Kirchhoff's current law, but not full credit for it (as a hypothetical 200% rule would allow).

 

[jaggedben]

It really makes no difference in the the real world. The code is the code; design to it or die.

You don't have to keep saying this. We can all tell the difference between a discussion of current code requirements, vs. a discussion of the reasoning behind them (or lack thereof). Some people here are involved in the code making process (or want to be). It's not off topic.

 

[jaggedben]

Would that not be quadruple heating?

My understanding, for what it's worth...

Current UL testing puts a load breaker(s) right next to the main breaker, and they turn on load equal to the busbar rating. (Also they raise the temp to whatever the max temp the panel is supposed to operate at, etc.)

Presumably they put the supply and load right next to each other because this concentrates the heating in essentially one location and tests the dissipation from the concentrated area.

If we had a '200% rule', the testing procedure might involve putting the second source and load at the opposite end. Or, concentrating all the load in one area.

If the opposite end, then it's just twice the heating; an equal amount of heating added, in a different location.

If they concentrate all the load in the middle or something, then it could be 4 times the heating for the load, and twice the heating for the supplies? Thus three times overall? (6/2) But your panelboard might have restrictions on branch breaker size that make this not a real situation anyway.

I still think there should be an expanded standard that allows listing to specific configurations, and a code exception for that. Not that I'm in a position to make it happen.

 

[wwhitney]

Current UL testing puts a load breaker(s) right next to the main breaker, and they turn on load equal to the busbar rating. (Also they raise the temp to whatever the max temp the panel is supposed to operate at, etc.)

Interesting--that would spatially concentrate the heating but not maximize the total heating within the enclosure, as it does not utilize the full resistance of the busbar. But perhaps experience shows that localized hot spots dominate.

If they concentrate all the load in the middle or something, then it could be 4 times the heating for the load, and twice the heating for the supplies? Thus three times overall?

Only if you could have one load that is double the size of the main breaker rating. Which wouldn't happen (how can you supply a single 400A load from a 200A panel?) Instead you'd have at worst two loads of current equal to the main breaker rating, and twice the connections points, so you'd double the load heating and double the supply heating, for double overall.

I still think there should be an expanded standard that allows listing to specific configurations, and a code exception for that. Not that I'm in a position to make it happen.

The NEC now has some language in 705 to allow panels to be listed for multiple supplies and not have to comply with the 705.12 rules. Are you saying that UL 67 doesn't have any testing provisions for such a listing? I haven't checked.

Cheers, wayne

 

[bwat]

The reasoning does not matter. Read 705.12(B)(3) and pick your method of qualifying the bus (the 120% rule is option 2), or connect on the line side of the main service disconnect per 705.11. Ours is not to reason why; ours is to install per code or fail the inspection.

Strange response...

FWIW, I wasn't questioning the code was, I was asking GoldDigger to expand on why he picked 200% to compare to the code of 120%. Why not compare to 150% in his response. It sounded like there was a reason he suggested that 200% could have been a reasonable decision, rather than the 120% that they picked.

I got the answer from the other helpful responses.

 

[Joethemechanic]

The BUS is in RED.
Where is the amperage on the bus over 200 amps? Where is this I2R that exceeds ratings at??

 

[jaggedben]

Interesting--that would spatially concentrate the heating but not maximize the total heating within the enclosure, as it does not utilize the full resistance of the busbar. But perhaps experience shows that localized hot spots dominate.

I'm pretty sure that at least for plug on breakers the resistance and heating at the plug on connections dwarfs the resistance of the busbar. But I could be wrong. What I stated is based on an old John Wiles article that I've not otherwise verified.

Only if you could have one load that is double the size of the main breaker rating. Which wouldn't happen (how can you supply a single 400A load from a 200A panel?) Instead you'd have at worst two loads of current equal to the main breaker rating, and twice the connections points, so you'd double the load heating and double the supply heating, for double overall.

You could probably find some situations where by loading up multiple feeder breakers in a side by side format panel you could load up enough load on some stabs in the middle. But I agree this is pressing edge cases, and very unlikely in the field, as well as often prohibited by panel labeling for branch and feeder breakers.

The NEC now has some language in 705 to allow panels to be listed for multiple supplies and not have to comply with the 705.12 rules. Are you saying that UL 67 doesn't have any testing provisions for such a listing? I haven't checked.

You mean the informational note? I hadn't noticed that. And no, I have not checked any UL standards. But the IN refers only to UL 1741 and equipment designed to combine sources. This probably applies to Enphase and Tesla MID equipment and such (Enpower, Gateway). I would be surprised if UL 67 or UL1741 has progressed to the point that a typical panelboard for loads is also getting listed for sources in certain arrangements. But no, I haven't been looking at the details.

 

[jaggedben]

The BUS is in RED.
Where is the amperage on the bus over 200 amps? Where is this I2R that exceeds ratings at??

View attachment 2565923

Well, the black part, technically, could be the stabs on a plug-on bus.
But by and large your point is well taken, and is why Golddigger mentioned 200% in the first place.