Feed through panels

[ggunn]

A design I am reviewing has a PV interconnection through a 70A breaker in a 225A (bus rating) MDP with a 200A main breaker. The designer has the PV breaker at the opposite end of the bus from the main, so it would be OK by the 120% rule except that the MDP is a feedthrough design with a 200A panel with a 200A main on the feed through lugs. I am inclined to direct him to move the subpanel to a 200A load breaker in the MDP. What say you?

 

[Smart $]

That's getting pretty darn nitpicky!

If we hold to the letter of the Code, moving to a load breaker would be required to use the opposite end of bus rule... as the feed thru conductors are a bus extension or tap.

 

[ggunn]

That's getting pretty darn nitpicky!

If we hold to the letter of the Code, moving to a load breaker would be required to use the opposite end of bus rule... as the feed thru conductors are a bus extension or tap.

What if the PV breaker were in the next to end spot and a 200A load breaker feeding an MLO subpanel were at the end of a non feedthrough bus? That's a 120% rule violation for sure. What's the difference between that and the PV breaker in the last spot and a 200A (main and bus) subpanel on the feed through lugs? The loading on the bus is the same, it seems to me.

 

[Smart $]

What if the PV breaker were in the next to end spot and a 200A load breaker feeding an MLO subpanel were at the end of a non feedthrough bus? That's a 120% rule violation for sure. What's the difference between that and the PV breaker in the last spot and a 200A (main and bus) subpanel on the feed through lugs?

Ummm... my first reply was agreeing with you. From my perspective, this question is moot... but to answer in kind, there is essentially no difference.

 

[jaggedben]

A design I am reviewing has a PV interconnection through a 70A breaker in a 225A (bus rating) MDP with a 200A main breaker. The designer has the PV breaker at the opposite end of the bus from the main, so it would be OK by the 120% rule except that the MDP is a feedthrough design with a 200A panel with a 200A main on the feed through lugs. I am inclined to direct him to move the subpanel to a 200A load breaker in the MDP. What say you?

In my opinion that main breaker satisfies your 705.12(D)(2)(1)(b) requirement, and the design is okay. If the panel connected to the feedthrough lugs were MLO, I'd agree with your suggested change, but the way you described it I don't think it's necessary.

I do not subscribe to this notion that feed through conductors are an extension of a panelboard busbar. Different testing and listing requirements. I don't believe the location of the extra overcurrent device is critical because we're talking about a continuous load issue and not a fault issue.

 

[Smart $]

...
I do not subscribe to this notion that feed through conductors are an extension of a panelboard busbar. Different testing and listing requirements. I don't believe the location of the extra overcurrent device is critical because we're talking about a continuous load issue and not a fault issue.

Well the conductors are an extension, just not of the bus type. That's just a matter of physics and has nothing to do with listing requirements.

The concept of the opposite end of bus rule is so that neither source current is combined on the bus, i.e. loads are always placed on the bus between the sources... the one exception being a breaker(s) directly across from the last space(s) on either right or left. With feed thru option being used, all the loads are not between the two source

 

[jaggedben]

If you look at the Article 100 definitions, one can make a solid contention that panelboard busbars are feeders. And I'm happy to say that the feed-through conductor is an extension of the feeder which begins in the panelboard. But that's not the same as saying it's an extension of the panelboard busbar in the sense that it must follow rules for busbars, or that it relocates the end of the busbar.

705.12(D) in the 2014 code is clear about which rules apply to feeders in general and which apply specifically to panelboard busbars.

I believe the opposite end rule has as much to do with suppositions about heat dissipation in the busbar as it does about combining currents. That's why it's so conservative in using 120% instead of 200%.

 

[Smart $]

If you look at the Article 100 definitions, one can make a solid contention that panelboard busbars are feeders. And I'm happy to say that the feed-through conductor is an extension of the feeder which begins in the panelboard. But that's not the same as saying it's an extension of the panelboard busbar in the sense that it must follow rules for busbars, or that it relocates the end of the busbar.

705.12(D) in the 2014 code is clear about which rules apply to feeders in general and which apply specifically to panelboard busbars.

I believe the opposite end rule has as much to do with suppositions about heat dissipation in the busbar as it does about combining currents. That's why it's so conservative in using 120% instead of 200%.

But I'm referring to the last 2-3" of busbar between the last breaker stabs and the feed through lugs. When you have a load on the subfeed panel that last little short piece of the bus will carry current potentially from both sources.

 

[jaggedben]

But I'm referring to the last 2-3" of busbar between the last breaker stabs and the feed through lugs. When you have a load on the subfeed panel that last little short piece of the bus will carry current potentially from both sources.

And that's why 705.12(D)(2)(1)(b) requires a load side OCPD to protection that portion of the feeder.

I'll grant there's two points that are open to interpretation.
One point is whether the the location of the OCPD matters because of what article 240 says. My argument would be that this is a Chapter 7 'Special Condition', and 705 is not specific about the location. An alternative interpretation is to call the feed through conductors a tap and make sure they are sized according to 240.21(B), which they surely are.
A second point is whether the last breaker stab is not the 'opposite end' because the feed through lugs are on the other side of it. That seems to be what you're arguing. I admit I don't really have a counter argument except to say that I think 705.12(D)(2)(1)(b) speaks to the situation better than 705.12(D)(2)(3)(b).

In any case, I don't see anything unsafe about the situation, otherwise I would have a different interpretation.

Don't get me wrong, I had a project with feed through lugs where I moved the PV location to the opposite end of the second panel to avoid having to discuss these issues with the AHJ. Of course we're also on 2011 code here and I didn't have that main breaker in the sub.

 

[pv_n00b]

A design I am reviewing has a PV interconnection through a 70A breaker in a 225A (bus rating) MDP with a 200A main breaker. The designer has the PV breaker at the opposite end of the bus from the main, so it would be OK by the 120% rule except that the MDP is a feedthrough design with a 200A panel with a 200A main on the feed through lugs. I am inclined to direct him to move the subpanel to a 200A load breaker in the MDP. What say you?

As long as the sub-panel has a main breaker the 120% rule ends at the line side of that main breaker since the breaker protects the sub-panel from both the main feed and the PV back feed overloading the bus. There are some purest who will say, "But what if someone taps the feed through feeders?" but in my mind that is like asking what would happen if someone moved the PV back feed breaker and installed a new load breaker. It's possible but would violate the NEC and should be caught at that time, not planned for when the PV system is installed.

 

[ggunn]

As long as the sub-panel has a main breaker the 120% rule ends at the line side of that main breaker since the breaker protects the sub-panel from both the main feed and the PV back feed overloading the bus. There are some purest who will say, "But what if someone taps the feed through feeders?" but in my mind that is like asking what would happen if someone moved the PV back feed breaker and installed a new load breaker. It's possible but would violate the NEC and should be caught at that time, not planned for when the PV system is installed.

The 120% rule applies in every panel all the way back to the service. If, for example, you have a 200A MDP with a 200A main, and it feeds a 225A subpanel with a 200A main breaker, you could put 70A of PV into the subpanel under the 120% rule but you'd be in violation in the MDP.

 

[Smart $]

The 120% rule applies in every panel all the way back to the service. If, for example, you have a 200A MDP with a 200A main, and it feeds a 225A subpanel with a 200A main breaker, you could put 70A of PV into the subpanel under the 120% rule but you'd be in violation in the MDP.

I agree with your assessment, but I always hated the lack of logic behind it.

If you have a 225AF/200AT subpanel with load, the only way any of the PV 70A is getting back to the MDP is if the local load is drawing less than the PV system is putting out... which means the subpanel is not drawing any utility power for which it must be designed to handle. For example, let's say local load is a calculated 150A. The MDP has to be rated to handle that 150A load because it has to handle it when the PV system is dark. When PV is all lit up and power is getting back to the MDP, it is not handling any of the 150A design load at that instance. Seems like there should be some difference math involved... :happyyes:

 

[ggunn]

I agree with your assessment, but I always hated the lack of logic behind it.

If you have a 225AF/200AT subpanel with load, the only way any of the PV 70A is getting back to the MDP is if the local load is drawing less than the PV system is putting out... which means the subpanel is not drawing any utility power for which it must be designed to handle. For example, let's say local load is a calculated 150A. The MDP has to be rated to handle that 150A load because it has to handle it when the PV system is dark. When PV is all lit up and power is getting back to the MDP, it is not handling any of the 150A design load at that instance. Seems like there should be some difference math involved... :happyyes:

Much as we love arguing about what the code should say, and what it really means when it says something, and what it isn't saying when it says something, this is pretty straightforward. When a panel is fed by an inverter, it matters not if the panel is fed directly by the inverter or if the path is through another panel.

 

[pv_n00b]

The 120% rule applies in every panel all the way back to the service. If, for example, you have a 200A MDP with a 200A main, and it feeds a 225A subpanel with a 200A main breaker, you could put 70A of PV into the subpanel under the 120% rule but you'd be in violation in the MDP.

True, but what I was saying is that if you have a feed through main panel, the sub-panel has a main breaker, and you back feed the main panel you don't have to look at anything going on in the sub-panel. It's protected from the PV back feed and the main supply by its main breaker.