Does the 120% rule count on panels up stream ?

[jayrush13]

I was wondering if the 120% rule (NEC 705.12 (D)2) count on Panels up stream

I have a situation where we installed 3 2.4Kw wind turbines on a 125 amp sub panel in a shop. It has an 80 amp feeding it and 3-20 amp breakers with the turbines on them so the panel there is within the 120% rule

where the Inspector had a problem was the main panel in the shop already had a single 2.4kw turbine attached and is feed with 100 amps on a 125 amp panel feed from the main house out of a 200amp panel

He says the Main panel in the shop needs to be big enough for the 4 wind turbines and the feed from the house

Is this correct or do I treat each panel separate and if He is correct I would have to go back and change the house panel out to

 

[ggunn]

I was wondering if the 120% rule (NEC 705.12 (D)2) count on Panels up stream

I have a situation where we installed 3 2.4Kw wind turbines on a 125 amp sub panel in a shop. It has an 80 amp feeding it and 3-20 amp breakers with the turbines on them so the panel there is within the 120% rule

where the Inspector had a problem was the main panel in the shop already had a single 2.4kw turbine attached and is feed with 100 amps on a 125 amp panel feed from the main house out of a 200amp panel

He says the Main panel in the shop needs to be big enough for the 4 wind turbines and the feed from the house

Is this correct or do I treat each panel separate and if He is correct I would have to go back and change the house panel out to

The 120% rule counts in all panels, and in the case of sources feeding through subpanels the rating of the first breaker that sees the output from the source is the number used in the main. In your example, the three 20A breakers in the sub total 60A, and that 60A is applied to the main irrespective of the rating on the backfed breaker through which the combined output of the three turbines in the sub enters the main. If the turbine that feeds the main directly also backfeeds a 20A breaker, then the total backfeed to the main is 80A. 80A is 20% of 400A, so if the main busbar and main breaker have the same rating, then it must be 400A.

EDIT: This assumes that the main supplies loads.

The subpanel must be rated at 300A if it has both load and backfed breakers, but if it is used just as a combiner (no load breakers), the sum of the backfed breakers just needs to be less than the rating of its busbar and its breaker if it has one.

Anyway, that's how I understand the code, and I am sure someone will correct me if I am mistaken. I am curious, though on one point. The inspector is OK with multiple backfed breakers in the main? That is often not the case.

 

[jaggedben]

I think you're saying you have the following setup:

125A Wind Subpanel, with three 20amp wind breakers; fed by 80A breaker in Shop Panel.
125A Shop Panel, feeding the Wind Subpanel, and also containing an additional 20A wind breaker, fed from Main Panel at house through 100A breaker.
200A Main Panel at house

If that's the setup, you're in a bunch of trouble.

The Wind Subpanel is fine, but...

Not only is the inspector correct that the Shop Panel needs to be sized to handle the 4 wind breakers, but so does the main panel at the house.

Your Shop Panel needs to be upgraded to a 150A panel; that's the 100A feeder plus 80A backfeed, divided by 120%.

You don't say the breaker size on the Main Panel, but assuming that it's 200A, then you have two options. First, you can do a calculation on the existing loads in the house, and see if you can downsize the main breaker to 150A. (Also make sure that you could do so without violating the panel listing.) If you can't do that, option two is to upgrade the panel to something above 233A, which is (200+80)/1.2. That is, you could use a 250A or larger panel with a 200A main breaker.

(I guess option three is that you could hope that the inspector doesn't notice the problem with the Main Panel, and that the panel never catches on fire?)

Finally, the code also says that the conductors feeding the back fed panels have to comply with the 120% rule. So possibly the 100A feeder to the Shop Panel also would need to be changed. This is actually kind of stupid, but it's what the code says. Hopefully your inspector doesn't notice or is reasonable enough to understand that the code is kind of stupid on that one.

The subpanel must be rated at 300A if it has both load and backfed breakers, but if it is used just as a combiner (no load breakers), the sum of the backfed breakers just needs to be less than the rating of its busbar and its breaker if it has one.

I don't agree. The code makes no exception for 'combiner only' panels. (I tend to think there should be an exception, but it isn't in the current code.)

 

[ggunn]

I don't agree. The code makes no exception for 'combiner only' panels. (I tend to think there should be an exception, but it isn't in the current code.)

Panelboards are used as AC combiners in the PV industry all the time. I'm working on a design for one at the moment which will combine the output of 30 SMA inverters at 277VAC phase to ground for a 480VAC interconnect.

I don't mean to imply that they are exempt from the 120% rule, only that if there is a main breaker and loads in the panel, then the sum of the main and the backfed breakers would be limited to 120% of the busbar rating. In a combiner panel the sum of the backfed breakers still cannot exceed 120% of the bar rating and the main breaker (if there is one - usually there isn't) isn't a factor because it is not feeding the busbar.

I don't put loads in a combiner panel.

 

[jaggedben]

In a combiner panel the sum of the backfed breakers still cannot exceed 120% of the bar rating and the main breaker (if there is one - usually there isn't) isn't a factor because it is not feeding the busbar.

In my opinion, the language in the code is clear:

2011 NEC 705.12(D)(2)

Bus or Conductor Rating. The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor.

Unless you have a supply side connection (and usually even then) there is a breaker somewhere feeding the combiner panel, so I'm not sure what you mean by 'usually there isn't' a main breaker. In my experience there always has been one. It does not matter if the breaker supplying the panel is in the panel or in some other panel, or if there are loads in the panel or not.

Does the code make sense? I would say not entirely. Should an AHJ be reasonable and allow an AC combiner panel to be sized according the size of only the backfed breakers, especially if the panel is full and/or permanently labeled as a solar combiner to which loads should not be added? I would say maybe they should. But proposed changes to the code along those lines have been rejected. (See link.) Do AHJ's ignore this rule routinely in the industry either because they don't understand it or because they do understand that in some situations it's overkill? I couldn't tell you they don't. But I think that what the code actually says is pretty clear.

See also : http://www.iaei.org/magazine/2011/0...-utility-interactive-pv-inverter-connections/
And discussion here: http://solarprofessional.com/forums/forum/messageview.cfm?catid=3&threadid=749&enterthread=y

 

[ggunn]

In my opinion, the language in the code is clear:

2011 NEC 705.12(D)(2)

Bus or Conductor Rating. The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor.

Unless you have a supply side connection (and usually even then) there is a breaker somewhere feeding the combiner panel, so I'm not sure what you mean by 'usually there isn't' a main breaker. In my experience there always has been one. It does not matter if the breaker supplying the panel is in the panel or in some other panel, or if there are loads in the panel or not.

Does the code make sense? I would say not entirely. Should an AHJ be reasonable and allow an AC combiner panel to be sized according the size of only the backfed breakers, especially if the panel is full and/or permanently labeled as a solar combiner to which loads should not be added? I would say maybe they should. But proposed changes to the code along those lines have been rejected. (See link.) Do AHJ's ignore this rule routinely in the industry either because they don't understand it or because they do understand that in some situations it's overkill? I couldn't tell you they don't. But I think that what the code actually says is pretty clear.

See also : http://www.iaei.org/magazine/2011/0...-utility-interactive-pv-inverter-connections/
And discussion here: http://solarprofessional.com/forums/forum/messageview.cfm?catid=3&threadid=749&enterthread=y

I agree that the code is clear. Since the main breaker in the panel, if there is one (I am using a main lug only panel in the design I am working on presently and the OCP is a separate fused AC disco to a line side tap) in an AC combiner panel does not feed the busbar, then it is not relevant to the 120% rule because only the backfed breakers can supply power to the bus. Otherwise one could only connect 80A of inverter output to a 400A panel.

 

[jaggedben]

I agree that the code is clear. Since the main breaker in the panel, if there is one (I am using a main lug only panel in the design I am working on presently and the OCP is a separate fused AC disco to a line side tap) in an AC combiner panel does not feed the busbar, then it is not relevant to the 120% rule because only the backfed breakers can supply power to the bus. Otherwise one could only connect 80A of inverter output to a 400A panel.

Well, if that interpretation flies with the AHJs you work with, then more power to you. Or perhaps I should say, to your busbars.

The vast majority of people I have talked to at work and online think that the overcurrent device on the 'line side' of a panel 'supplies power' to the conductor or busbar, regardless of whether there are loads installed in the panel.

John Wiles, who (I believe) wrote the wording, clearly doesn't agree with your interpretation. From the link I posted earlier...

A proposal was made for the 2011 NEC that would apply to end-fed conductors that have a restriction (marking) that they not be tapped for either loads or supplies. If this proposal were accepted — it was rejected — then the conductor would need an ampacity only as high as the highest rating of one of the connected supply overcurrent devices [in an example with only two overcurrent devices].

In previous Code cycles, labels and placards that say, “Add no loads” have been proposed. Those proposals have been rejected. Proposals for dedicated ac inverter combining panels with no spaces for loads have been proposed. They have been rejected. Covering empty breaker positions with metal guards have been proposed — rejected. Marking conduits, “PV output circuits, multiple source, do not tap”— rejected.

 

[ggunn]

Well, if that interpretation flies with the AHJs you work with, then more power to you. Or perhaps I should say, to your busbars.

The vast majority of people I have talked to at work and online think that the overcurrent device on the 'line side' of a panel 'supplies power' to the conductor or busbar, regardless of whether there are loads installed in the panel.

John Wiles, who (I believe) wrote the wording, clearly doesn't agree with your interpretation. From the link I posted earlier...

Irrespective of that, AC combiners are commonly used, and the way that I have usually seen them implemented is with a main lug only panel with a fused disco between the combiner and the service. But I am open to alternatives, so I put it to you for your input. How would you configure the systems I am working on, given these particulars which I do not have the option to change:

System 1:
42 SMA SB7000-277's to be combined into 3 phase 480VAC for interconnect.
The inverters have a maximum output current of 25.3A, so they each take a 35A breaker.
Total current @ 480VAC is 353.6A, so the OCPD requirement is 450A.

The current strategy is to combine the SB's output in a 60 space 3 phase 480V 600A main lug only combiner panel with the SB's connected phase to neutral on single pole 35A breakers and balanced on the three phases. The output of the combiner will feed a 600V 600A fused disco with 450A fuses.

System2:
24 SMA SB7000-277's and 6 SMA SB6000-277's to be likewise combined into 480VAC.
Inverter output currents are 25.3A (35A breaker) and 21.7A (30A breaker), respectively.
Total current = 245.4A @480VAC => 350A OCPD

Combining into a 42 space 3 phase 480V 400A main lug combiner panel, connected and balanced similarly as above, feeding a 600V 400A disco with 350A fuses.

How would you do it? Yes, I know, with a central inverter, but that is not an option for me as the SB's were agreed upon before I got assigned to the project and is a non-negotiable point. Believe me, I would change it if I could.

I apologize if all this comes across as a urination competition; that is not at all my intent. If you have a cost effective alternative that does not violate the 120% rule as you interpret it, I would very much like to see it. I will just add that I know of many installed systems in several jurisdictions implemented the way I have described that passed muster without issue with their respective A's HJ.

 

[jaggedben]

How would you do it?

Unless I were going to pursue a conversation with the AHJ to make sure the design would be approved, I would use an 800A panel for System1 and 600A panel for system2. And if that were too costly I would pursue the conversation with the AHJ, because it would be more costly to replace the panels later if he didn't like your interpretation of the 120% rule.

My point is simply that if the AHJ decides to interpret the code my way, you don't have a leg to stand on, because most people interpret it that way.

To be clear, as far as I see it, your designs are perfectly safe, especially if you take measures to prevent any loads from being added to the panels. And I think any AHJ ought to approve them, with whatever code variance they might find necessary to document. That's my two cents on that.

 

[ggunn]

Unless I were going to pursue a conversation with the AHJ to make sure the design would be approved, I would use an 800A panel for System1 and 600A panel for system2. And if that were too costly I would pursue the conversation with the AHJ, because it would be more costly to replace the panels later if he didn't like your interpretation of the 120% rule.

My point is simply that if the AHJ decides to interpret the code my way, you don't have a leg to stand on, because most people interpret it that way.

To be clear, as far as I see it, your designs are perfectly safe, especially if you take measures to prevent any loads from being added to the panels. And I think any AHJ ought to approve them, with whatever code variance they might find necessary to document. That's my two cents on that.

I appreciate your feedback. The drawings will be submitted to the AHJ for approval, so if it is an issue we will find out before hardware is in place. The panels will be on the rooftop next to the inverters, so there is no danger of anyone adding loads to them, especially seeing as they will be 480VAC 3 phase boxes. I really don't think it will be a problem. I hope not, anyway; one of the systems has a 400'+ run to the interconnect and if the busbars are an issue, the conductors will be as well. It's a whole lot of copper already.

Regardless, I appreciate you bringing it to my attention; in other situations it might be more of an issue and forewarned is forearmed.