"Line side" tap on sub panel for PV installation possible?

[ggunn]

We have a customer who wants to install a substantial amount of PV on a ranch. The ranch is on a single meter with a 400A main feeding several subpanels and the PV inverter would be located at one of the subs. The question that has been posed to me is if we can tap into the line feeding the subpanel from the main because the PV will not fit into it on a backfed breaker under the 20% rule. I'm not at all sure what the ramifications of such a connection strategy would be. It would, for one thing, change the breaker feeding the sub into a backfed breaker; would the size need to change? What about wire size from the main to the sub?

It seems to me that the correct way to do this would be to upsize the subpanel and replace its main breaker with the current sub main breaker to make room for the PV, but I have been asked to investigate this approach.

 

[ggunn]

We have a customer who wants to install a substantial amount of PV on a ranch.

We have a customer who wants to install a substantial amount of PV on a ranch. The ranch is on a single meter with a 400A main feeding several subpanels and the PV inverter would be located at one of the subs...

I apologize for the self reply here, but I misunderstood the question.

What we have is a 400A fused disconnect feeding multiple smaller disconnects which in turn feed subpanels. The rancher wants to install possibly as much as 35kW of PV out by one of the subpanels, which is probably 200A. Four SB8000's require a total of 160A OCP. The question is if we can combine the output of the inverters in an aggregation panel and tap into the 200A rated supply line feeding the 200A subpanel.

Sorry for any confusion.

 

[jbelectric777]

I'm not 100% sure I understand the actual question but it sounds like you want to tap the PV power to a feeder. The answer is no, the main reason is when the 400A main is opened the system will still be energized by the PV. I dont have a codebook handy right now but thats a definite no no, if thats what you meant. Hope that helps.

 

[ggunn]

I'm not 100% sure I understand the actual question but it sounds like you want to tap the PV power to a feeder. The answer is no, the main reason is when the 400A main is opened the system will still be energized by the PV. I dont have a codebook handy right now but thats a definite no no, if thats what you meant. Hope that helps.

Thanks for reply, but the PV inverter(s) will shut down if the 400A main is opened per UL 1741.

 

[OTT2]

690.64 refers you to 705.12. The way I read 705.12(D), the answer is still no.


2010 NEC

 

[ggunn]

690.64 refers you to 705.12. The way I read 705.12(D), the answer is still no.


2010 NEC

First of all, thank you for taking the time to refer me to a specific article in the code. I really appreciate that.

However, I have gone through the article you reference, and I haven't found anything which would proscribe the connection I am proposing; could you elaborate? The only difference between a traditional line side tap and the approach I am investigating is that there would be two fusible disconnects in series between the tap point and the meter (the 400A service disconnect and the 200A disconnect which it feeds) instead of the single service disconnect. There are no busbars involved.

I.E.=> Meter------------400A disconnect----------200A disconnect----------TAP------------200A panel

Can you point me to the specific language that would be problematic for me?

Thanks.

 

[ggunn]

First of all, thank you for taking the time to refer me to a specific article in the code. I really appreciate that.

However, I have gone through the article you reference, and I haven't found anything which would proscribe the connection I am proposing; could you elaborate? The only difference between a traditional line side tap and the approach I am investigating is that there would be two fusible disconnects in series between the tap point and the meter (the 400A service disconnect and the 200A disconnect which it feeds) instead of the single service disconnect. There are no busbars involved.

I.E.=> Meter------------400A disconnect----------200A disconnect----------TAP------------200A panel

Can you point me to the specific language that would be problematic for me?

Thanks.

PS: I realize now that I may have titled this thread incorrectly. The 200A panel I have shown above is probably not correctly termed a subpanel, as it is fed only through disconnects, not from another panel. I apologize for any confusion that may have caused.

 

[Sierrasparky]

This type of tap is ok this is not really a line side tap. A line side tap is one before the main breaker at the service entrance. In your scenario you will still have the issue of max 20% rule of the feeder. The theory is you don't want to overload the wires or buss. For example if you have a 100 amp panel and you backfeed with 100 amps of PV. You have the potential of using 200 amps through that 100 amp panel. You will burn up the buss.

In your case one could argue that at the 400 amp service you could draw 400 amps through the main and 200 amps through the sub feeder. You still should max out at the 20% rule

 

[OTT2]

First of all, thank you for taking the time to refer me to a specific article in the code. I really appreciate that.

However, I have gone through the article you reference, and I haven't found anything which would proscribe the connection I am proposing; could you elaborate? The only difference between a traditional line side tap and the approach I am investigating is that there would be two fusible disconnects in series between the tap point and the meter (the 400A service disconnect and the 200A disconnect which it feeds) instead of the single service disconnect. There are no busbars involved.

I.E.=> Meter------------400A disconnect----------200A disconnect----------TAP------------200A panel

Can you point me to the specific language that would be problematic for me?

Thanks.

Where 705.12(D) says you are permitted to connect at any distribution equipment on the premises. I read this as a panelboard or switchboard, not a feeder.

Like sierrasparky said, at the point you tap that feeder the downstream current to the panelboard could potentially far exceed the rating of the feeder and the panelboard.

705.12(D) also says where the equipment is capable of supplying multiple branch circuits etc you must comply with (D)(1) through (D)(7). You would be required to connect to this equipment through a dedicated circuit breaker or fusible disconnecting means (D)(1).
Also, you shall not exceed 120% of the rating of the busbar or conductor you are supplying (D)(2).

 

[OTT2]

In my first post I stated 2010 NEC. Pardon me!! 2011 NEC

 

[ggunn]

This type of tap is ok this is not really a line side tap. A line side tap is one before the main breaker at the service entrance. In your scenario you will still have the issue of max 20% rule of the feeder. The theory is you don't want to overload the wires or buss. For example if you have a 100 amp panel and you backfeed with 100 amps of PV. You have the potential of using 200 amps through that 100 amp panel. You will burn up the buss.

In your case one could argue that at the 400 amp service you could draw 400 amps through the main and 200 amps through the sub feeder. You still should max out at the 20% rule

I am sorry if I appear dense, but I don't think we are communicating. The service is (if the person who reported it to me did so correctly) not through a 400A panel, it is through a 400A disconnect. The disconnect directly feeds a number (I do not know what the number is) of lesser disconnects, and the one in question is a 200A disconnect, which in turn feeds a 200A panel. There are no panels and therefore no busbars or loads between the proposed tap and the meter.

The wire is sized for the 200A feeder to the panel, and is protected at the panel by the panel's main breaker. The most the PV inverters can supply to the feeder is 160A (it is less than that by 20%, actually, but let that slide).

Does this crude "drawing" make sense?

Meter---400A disconnect (not a panel)---200A disconnect (not a panel)---TAP---200A panel

I do not see how the tap's 160A contribution to the feeder can ever push the current in the wire over 200A. No scenario I can imagine has more than 200A flowing through any part of the feeder.

Scenario: The panel is fully loaded at 200A and the PV is putting out 160A. The current in the feeder between the tap and the panel is 200A and the current in the feeder between the 400A disconnect and the tap is 200A-160A = 40A in the direction of the panel.

Scenario: The panel is half loaded at 100A and the PV is putting out 160A. The current in the feeder between the tap and the panel is 100A and the current in the feeder between the 400A disconnect and the tap is 100A-160A = (-)60A in the direction away from the panel.

Scenario: The panel loads are all off and the PV is putting out 160A. The current in the feeder between the tap and the panel is 0A and the current in the feeder between the 400A disconnect and the tap is 0A-160A = (-)160A in the direction away from the panel.

Scenario: The 400A disconnect is open. The PV shuts down and there is no current anywhere in the feeder.

I understand the 20% rule, but I don't see how it applies here any more than it does to a "normal" line side tap.

Aside: I do not wish to appear argumentative, but this is a real world situation and I need to get it right. If I am missing something I would sincerely appreciate your guidance.

 

[drive1968]

If I understand your layout correctly, here's the possible scenario where there could be an overload. The 200 amp panel is drawing 200 amps from the utility through the meter. The 200 amp panel could also be drawing an additional 160 amps from the PV tap. You would then have 360 amps placed on the bus of the 200 amp panel. Perhaps you could put a 200 amp main breaker on the panel to avoid that one scenario.

 

[drive1968]

I have one thing I wanted to add about UL 1741. You are correct that the inverters are designed to shut down whenever power is cut from the utility, but the inspectors in my area require a disconnect anyway. The PV disconnect must also be next to the main utility disconnect, per my local fire department rules. I'm not sure what your local rules would be, but I bet there must be a disconnect for the PV.

 

[mpd]

your installation is still load side of the service disconnect and must comply 690.64 (B) (2)

 

[Sierrasparky]

Sorry for the confusion about the single line diagram.
Can I ask why you have a 400a disco feeding only a 200amp panel.

 

[ggunn]

If I understand your layout correctly, here's the possible scenario where there could be an overload. The 200 amp panel is drawing 200 amps from the utility through the meter. The 200 amp panel could also be drawing an additional 160 amps from the PV tap. You would then have 360 amps placed on the bus of the 200 amp panel. Perhaps you could put a 200 amp main breaker on the panel to avoid that one scenario.

The 200A panel already has a 200A breaker. I believe I mentioned that.

 

[ggunn]

Sorry for the confusion about the single line diagram.
Can I ask why you have a 400a disco feeding only a 200amp panel.

The 400A disco feeds some other smaller discos in parallel. One of these is a 200A disco that feeds a 200A panel, and the tap we are proposing would go in between the 200A disco and the 200A panel. I only drew the branch we are dealing with.

 

[ggunn]

I have one thing I wanted to add about UL 1741. You are correct that the inverters are designed to shut down whenever power is cut from the utility, but the inspectors in my area require a disconnect anyway. The PV disconnect must also be next to the main utility disconnect, per my local fire department rules. I'm not sure what your local rules would be, but I bet there must be a disconnect for the PV.

Yes, there will be a disco on the PV between the inverter and the tap.

 

[ggunn]

your installation is still load side of the service disconnect and must comply 690.64 (B) (2)

And that indeed is the heart of the matter. How does that usually work with a line side tap? Normally, you have a service disconnect which feeds a main panel. Say it's a 200A panel. If the conductors feeding the panel are rated at 200A, then why isn't there the same issue with the 20% rule as there would be with a backfed breaker if you want to connect, say, 50A of PV? Maybe this is a question for John Wiles.