Siemens Panel w/ QN breaker

[Boston1277]

Questions:

1. Is this a MLO panel
2. Is this 200A breaker considered the main breaker?,
3. The panel has a 200A bus, so how can this solar quad breaker backfeed the bus and comply with the 120% rule?

https://imgur.com/a/a6vsPEr

https://imgur.com/ufSfFb8

 

[electrofelon]

Yes that is a MLO panel. This would be a line side connection.

 

[ggunn]

Yes that is a MLO panel. This would be a line side connection.

I agree; the 120% rule does not apply.

 

[electrofelon]

I agree; the 120% rule does not apply.

As a side question, why? You can overload a bus in this case just like you can with a load side connection. Was this an oversight in the code?

 

[ggunn]

As a side question, why? You can overload a bus in this case just like you can with a load side connection. Was this an oversight in the code?

A bus connected on the line side of OCPD already has a virtually infinite amount of current available to it; the current from the PV is minuscule by comparison.

 

[electrofelon]

A bus connected on the line side of OCPD already has a virtually infinite amount of current available to it; the current from the PV is minuscule by comparison.

But We are not talking about available fault current. We are talking about overloading the busbar, that is connecting two sources of power which can result in exceeding the bus rating.

 

[wwhitney]

But We are not talking about available fault current. We are talking about overloading the busbar, that is connecting two sources of power which can result in exceeding the bus rating.

If there is only one load breaker (as in this case), it protects the bus against overload.

if there is more than one load breaker, then they have always been allowed to sum to more than the service conductor rating, and only the load calc protects the bus and service conductors.

Introducing multiple sources of supply to the bus doesn't change any of the above for an MLO service panel.

Cheers, Wayne

 

[electrofelon]

If there is only one load breaker (as in this case), it protects the bus against overload.

if there is more than one load breaker, then they have always been allowed to sum to more than the service conductor rating, and only the load calc protects the bus and service conductors.

Introducing multiple sources of supply to the bus doesn't change any of the above for an MLO service panel.

Cheers, Wayne

Right I understand the physics, I guess what I am trying to say is why the super overly conservative concerns for overloading a bus for load side connections, yet we can have multiple service disconnects whose sum exceeds the bus rating? Why the difference? It's like they want to provide for this one in 10 million chance of a bus overload for a load side connection but are like "well screw it, let it burn" if it's a line side in a MLO.

 

[ggunn]

But We are not talking about available fault current. We are talking about overloading the busbar, that is connecting two sources of power which can result in exceeding the bus rating.

But the bus is already exposed to much more current than it is rated for. The contribution from the PV system doesn't make any appreciable difference.

 

[ggunn]

Right I understand the physics,

Do you? Apply Kirchoff to the bus and show me where it is overloaded.

 

[electrofelon]

Do you? Apply Kirchoff to the bus and show me where it is overloaded.

230.90(A) exception #3 and 408.36 exception #1 allows for overloading of the bus.

Perhaps the article 705 people don't "like" these sections but have no control over them as it is a different CMP?

 

[wwhitney]

230.90(A) exception #3 and 408.36 exception #1 allows for overloading of the bus.

I'm having trouble understanding what you see as the conflict.

408.36 generally requires a panelboard bus to have individual protection. The possibility of a panelboard having multiple sources of supply makes that more complicated. So 705.12(B) has rules to handle that, corresponding to the general case of 408.36.

408.36 exception #1 is a special case where you don't need to have individual protection. That special case corresponds to 705.11, which therefore doesn't need to have special rules on protecting the bus.

So 408 and 705 seem aligned to me.

Cheers, Wayne

 

[electrofelon]

I'm having trouble understanding what you see as the conflict.

408.36 generally requires a panelboard bus to have individual protection. The possibility of a panelboard having multiple sources of supply makes that more complicated. So 705.12(B) has rules to handle that, corresponding to the general case of 408.36.

408.36 exception #1 is a special case where you don't need to have individual protection. That special case corresponds to 705.11, which therefore doesn't need to have special rules on protecting the bus.

So 408 and 705 seem aligned to me.

Cheers, Wayne

Sorry, I guess I am having a hard time articulating my position. I am not saying there is a conflict, it's more of a philosophical issue on the wide disparity in the amount of conservativeness in the code rules in different situations.

Basically we have the 705 load side rules, which require a setup so that every theoretically possible way of overloading a bus bar is avoided (well there is the 120% rule, but IMO that is highly overly conservative). Then on the other end of the spectrum, we have the rules I mentioned in post #11. Why this broad disparity in philosophy? Why can't we analyze a load side connection with a load calc like we can for a service MLO?

 

[wwhitney]

Why this broad disparity in philosophy?

My only comment is that the disparity you identify is already reflected in 408.36 and its exception #1. So as that section presumably predates 705, it is to be expected that 705 reflects that disparity.

In other words, it feels like what you're asking is equivalent to asking "why do we have 408.36 at all, why not allow any bus to be protected by a load calc only?" Which is a question I have no answer to.

Cheers, Wayne

 

[electrofelon]

My only comment is that the disparity you identify is already reflected in 408.36 and its exception #1. So as that section presumably predates 705, it is to be expected that 705 reflects that disparity.


Cheers, Wayne

Right, certainly the 408.36 and 230.90 sections existed long before 705 came along..... I guess I would have liked to see more consistency when they wrote 705, like perhaps considering that load calcs have been an accepted way for many decades of protecting a bus...... Or rework those pre-existing sections to match the conservativeness of the 705 sections. I just don't like the inconsistency.

 

[ggunn]

230.90(A) exception #3 and 408.36 exception #1 allows for overloading of the bus.

Perhaps the article 705 people don't "like" these sections but have no control over them as it is a different CMP?

I have no dog in that hunt; I just know that a line side connected PV system doesn't add to bus loading.

 

[electrofelon]

I have no dog in that hunt; I just know that a line side connected PV system doesn't add to bus loading.

Well it doesn't in the sense that one of the sources to the bus is already very high with no OCPD to limit it. My gripe is that different CMP's have different views on bus protection. 230 allows the sum of all MLO breakers to exceed the busbar rating and the busbar to be protected by a load calc. Article 705 does not.

 

[ggunn]

Well it doesn't in the sense that one of the sources to the bus is already very high with no OCPD to limit it. My gripe is that different CMP's have different views on bus protection. 230 allows the sum of all MLO breakers to exceed the busbar rating and the busbar to be protected by a load calc. Article 705 does not.

MLO MDPs are out now, anyway, aren't they? Moot point.

 

[electrofelon]

MLO MDPs are out now, anyway, aren't they? Moot point.

yes in many places. In NY we are still on 2017.

 

[letgomywago]

MLO MDPs are out now, anyway, aren't they? Moot point.

Most of the country is behind on code cycles and lots have delayed that rule due to supply chain issues. It was delayed here for a while. I think they just extended it this month again but I'm waiting on clarification.