Max Backfeed on a Line-Side Tap

[innavent]

Apologies to those who have (i'm sure) continually answered these line-side tap questions. I've scoured the forums, but haven't found a situation exactly like mine, and have looked through 690.64 and still am unclear as to the requirements.

My situation. I have a client that i'm working on a permit set that has a split service with a 400A rated main. One split has a 150A breaker (to a 150A sub) and the other, a 200A breaker (to a 200A sub). He wants to backfeed 6 x SB3000HF's which are max output rated at 12.5A (x 1.25 x 6 = 93.75A) . The first question is that his original designer shows the inverters combining into a 200A AC panelboard. I think a 125A panel (MLO) should be ok -- correct? He's going to have visible fused utility disconnect. Secondly, he's concerned that a line-side tap is going to be an issue. I think it should be more than fine, but I cannot find guidance as to how many amps can actually be back-fed on a line-side tap.I understand the 20% rule, but what's (where can I find clearly) the capacity rule for a line-side tap?? Assuming we're OK capacity wise, where can I find guidance as to where the tap lives (i.e. tap lives in the AC disconnect??). This would assume there's room to tap as well??

Any help is greatly appreciated!!!

 

[jaggedben]

The first question is that his original designer shows the inverters combining into a 200A AC panelboard. I think a 125A panel (MLO) should be ok -- correct?

No, not correct. The subpanel has to be rated for the sum of the OCPDs from both the utility side and the solar. Hence 100A from each side, hence the designer is correct. Some AHJs might let this slide, but the code does not allow it. (What if someone puts a load in that subpanel? That's how the argument goes...) It is subject to the 20 percent rule so you could use anything down to a 160A panel but that's probably moot given that one doesn't find panels avialable between 200A and 150A.

He's going to have visible fused utility disconnect. Secondly, he's concerned that a line-side tap is going to be an issue. I think it should be more than fine, but I cannot find guidance as to how many amps can actually be back-fed on a line-side tap. I understand the 20% rule, but what's (where can I find clearly) the capacity rule for a line-side tap??

It can be up to the rating of the service. This was added in the 2011 NEC, it's not in the 2008. It's 705.12(A)

Assuming we're OK capacity wise, where can I find guidance as to where the tap lives (i.e. tap lives in the AC disconnect??). This would assume there's room to tap as well??

Not quite sure what you are asking here. Whatever tap hardware you use must be rated for the 100A of solar and you must not violate the listing of any equipment in which you are doing the tap. How you do it depends on the details of the existing installation.

 

[innavent]

Thanks for the feedback. A couple of questions.

No, not correct. The subpanel has to be rated for the sum of the OCPDs from both the utility side and the solar. Hence 100A from each side, hence the designer is correct. Some AHJs might let this slide, but the code does not allow it. (What if someone puts a load in that subpanel? That's how the argument goes...) It is subject to the 20 percent rule so you could use anything down to a 160A panel but that's probably moot given that one doesn't find panels avialable between 200A and 150A.

So if I understand correctly, the panel is rated based on the sum of the breakers on both sides, and the amount of PV that can be backfed is still subject to the 20% rule. Understood about the load comment. Thanks for the clarification.

It can be up to the rating of the service. This was added in the 2011 NEC, it's not in the 2008. It's 705.12(A)

So if the County is going by NEC 2008 -- how do I determine the max back feed with a tap? Is it still technically the same subject to utility AHJ approval?

Not quite sure what you are asking here. Whatever tap hardware you use must be rated for the 100A of solar and you must not violate the listing of any equipment in which you are doing the tap. How you do it depends on the details of the existing installation.

Are their rules/guidelines in regards to where you tap (other than voiding the UL listing) that I can refer to? For example, can I use an Ilsco Tap and design the system to go from the load side of the meter to the tap (inside the utility disconnect) and back to the main?

Thank you so much!

 

[ggunn]

No, not correct. The subpanel has to be rated for the sum of the OCPDs from both the utility side and the solar. Hence 100A from each side, hence the designer is correct. Some AHJs might let this slide, but the code does not allow it. (What if someone puts a load in that subpanel? That's how the argument goes...) It is subject to the 20 percent rule so you could use anything down to a 160A panel but that's probably moot given that one doesn't find panels avialable between 200A and 150A.

FWIW, (we have had this discussion before) I routinely use AC panels as AC combiners, and I use panels rated for the sum total of the inverters' output X 1.25. I have placed systems designed this way in several authorities' jurisdictions and none have questioned it. Some have been main lug panels and some have had main breakers. It may be that residential systems are looked at differently; I only deal with commercial systems.

 

[jaggedben]

So if the County is going by NEC 2008 -- how do I determine the max back feed with a tap? Is it still technically the same subject to utility AHJ approval?

Common sense says don't overload the service. Other than that, I'd say the AHJ can't put a limit on it. You can always point to the 2011 code to secure approval, since there's nothing in the 2008 that contradicts that part.

Are their rules/guidelines in regards to where you tap (other than voiding the UL listing) that I can refer to? For example, can I use an Ilsco Tap and design the system to go from the load side of the meter to the tap (inside the utility disconnect) and back to the main?

There really are no rules outside manufacturers instructions and listings. What you wrote seems a little confused, especially the part where you say 'back to the main', not sure what you mean by that. What you could probably do is tap the existing service entrance conductors between the meter and the existing service disconnecting means, and bring the new conductors to the fused disco for the solar.

BTW, the NEC doesn't consider this a 'tap'. It's considered to be creating another set of service entrance conductors. Following Art 230 would probably be a good idea.

FWIW, (we have had this discussion before) I routinely use AC panels as AC combiners, and I use panels rated for the sum total of the inverters' output X 1.25. I have placed systems designed this way in several authorities' jurisdictions and none have questioned it. Some have been main lug panels and some have had main breakers. It may be that residential systems are looked at differently; I only deal with commercial systems.

As I said, some AHJs may let it slide. I agree that it's very unlikely to lead to a dangerous situation, especially if there is no space to add loads to the AC combiner.

 

[innavent]

Thanks everyone, I really appreciate the feedback. We decided a line-side tap was the right way to go...

 

[Smart $]

No, not correct. The subpanel has to be rated for the sum of the OCPDs from both the utility side and the solar. Hence 100A from each side, hence the designer is correct. Some AHJs might let this slide, but the code does not allow it. (What if someone puts a load in that subpanel? That's how the argument goes...) It is subject to the 20 percent rule so you could use anything down to a 160A panel but that's probably moot given that one doesn't find panels avialable between 200A and 150A.

...

I know we have butted heads on this before, but a 125A MLO panel will work as a combiner, line-side connection panel (yes, I'd use a service-rated panel simply for grounding convenience). At 12.5A output rating per inverter times 125% means a 2P20A CB for each inverter output and six times 20A is 120A. Meets six disconnect rule and no more can be added because of the six disconnect rule. Line side connection is not required to meet the 120% rule, and this is nowhere close to exceeding the rating of a 400A service.

 

[ggunn]

I know we have butted heads on this before, but a 125A MLO panel will work as a combiner, line-side connection panel (yes, I'd use a service-rated panel simply for grounding convenience). At 12.5A output rating per inverter times 125% means a 2P20A CB for each inverter output and six times 20A is 120A. Meets six disconnect rule and no more can be added because of the six disconnect rule. Line side connection is not required to meet the 120% rule, and this is nowhere close to exceeding the rating of a 400A service.

You can put more than six inverter outputs on breakers in a combiner panel if there is a disconnecting means on their combined output.

 

[jaggedben]

I know we have butted heads on this before, but a 125A MLO panel will work as a combiner, line-side connection panel (yes, I'd use a service-rated panel simply for grounding convenience). At 12.5A output rating per inverter times 125% means a 2P20A CB for each inverter output and six times 20A is 120A. Meets six disconnect rule and no more can be added because of the six disconnect rule. Line side connection is not required to meet the 120% rule, and this is nowhere close to exceeding the rating of a 400A service.

I agree with you... if the OP can leave out the fused utility disconnect he specified. If that is not acceptable to the utility then the MLO panel becomes a load side connection. Perhaps that's stupid, but it's what this code cycle says. If you want to do otherwise I'd say you'd better know the AHJ will accept it.

 

[Smart $]

I agree with you... if the OP can leave out the fused utility disconnect he specified. If that is not acceptable to the utility then the MLO panel becomes a load side connection. Perhaps that's stupid, but it's what this code cycle says. If you want to do otherwise I'd say you'd better know the AHJ will accept it.

I assumed he was going to tap on the line side of the fused disconnect. Otherwise, you are correct.

 

[Smart $]

You can put more than six inverter outputs on breakers in a combiner panel if there is a disconnecting means on their combined output.

True... but then you have the situation that the combined output side of the breakers (panel busbar; feeder too if combiner panel is MLO) is on the load side of the service disconnecting means and subject to the 120% rule. That is, you can only have one disconnect/breaker between inverter output and line-side tap for it not to fall under the 120% rule. The alternative would be to use multiple MLO panels and line-side taps to meet the six-disconnect rule per tap.

 

[ggunn]

True... but then you have the situation that the combined output side of the breakers (panel busbar; feeder too if combiner panel is MLO) is on the load side of the service disconnecting means and subject to the 120% rule. That is, you can only have one disconnect/breaker between inverter output and line-side tap for it not to fall under the 120% rule. The alternative would be to use multiple MLO panels and line-side taps to meet the six-disconnect rule per tap.

"...you can only have one disconnect/breaker between inverter output and line-side tap for it not to fall under the 120% rule." Can you point me to that language in the Code? I've never seen it. I know of many AHJ approved systems in the field which have AC combiners with an AC disconnect on the output where the combiner is rated for the summed inverter output only. One of them has 42 inverters.

 

[Smart $]

"...you can only have one disconnect/breaker between inverter output and line-side tap for it not to fall under the 120% rule." Can you point me to that language in the Code? I've never seen it. I know of many AHJ approved systems in the field which have AC combiners with an AC disconnect on the output where the combiner is rated for the summed inverter output only. One of them has 42 inverters.

There is no specific language stating this. I thought about saying many AHJ errantly allow the combiner to be rated for the summed inverter output only... but I thought you might see the wisdom (?) in what I said without pointing it out. I have no problem with doing so on the DC side of the inverter. But on the AC side, busbars and feeders between individual inverter breakers and main AC disconnect are supplied by two power sources, utility and inverters... which is the reason for and where the 120% rule is applied. The main AC beaker serves as a service disconnecting means, and therefore any feeder or busbar on the PV side of it serves only load side connections. When you only have one disconnecting breaker between utility and individual inverter, equipment-wise only the breaker is subjected to both sources.

 

[Smart $]

"...you can only have one disconnect/breaker between inverter output and line-side tap for it not to fall under the 120% rule." Can you point me to that language in the Code? I've never seen it. I know of many AHJ approved systems in the field which have AC combiners with an AC disconnect on the output where the combiner is rated for the summed inverter output only. One of them has 42 inverters.

PS: I am not personally against the concept. But as the Code is currently worded, doing so is in violation.

 

[ggunn]

There is no specific language stating this. I thought about saying many AHJ errantly allow the combiner to be rated for the summed inverter output only... but I thought you might see the wisdom (?) in what I said without pointing it out. I have no problem with doing so on the DC side of the inverter. But on the AC side, busbars and feeders between individual inverter breakers and main AC disconnect are supplied by two power sources, utility and inverters... which is the reason for and where the 120% rule is applied. The main AC beaker serves as a service disconnecting means, and therefore any feeder or busbar on the PV side of it serves only load side connections. When you only have one disconnecting breaker between utility and individual inverter, equipment-wise only the breaker is subjected to both sources.

But the current can flow only one direction - from the inverters outward through the disconnect. In the case of a fault in the panel on the inverter side of the disconnect, the inverters will shut down and the fused disco will protect the busbars. GT inverters cannot feed a short. I cannot think of a situation where the busbars could be overloaded; can you? I have no problem with doing what is necessary to build a safe system, but ponying up for a 1200A AC combiner panel to feed in 240A of PV seems ludicrous.

 

[jaggedben]

... I cannot think of a situation where the busbars could be overloaded; can you? ...

To repeat, the scenario the CMP is concerned about (according to John Wiles) is that someone could add loads to the combiner panel at a later time.

I agree the current code is too conservative. The 2014 code may make things easier on this particular point.

 

[ggunn]

To repeat, the scenario the CMP is concerned about (according to John Wiles) is that someone could add loads to the combiner panel at a later time.

I agree the current code is too conservative. The 2014 code may make things easier on this particular point.

Well, I wasn't asking you.

I'm not too worried about that when it's a 480V panel on a rooftop with no power needs anywhere around it. If I were installing a residential system with easily accessible panelboards I would do it differently.

 

[Smart $]

Well, I wasn't asking you.

Even so, I cannot offer anything better

I'm not too worried about that when it's a 480V panel on a rooftop with no power needs anywhere around it. If I were installing a residential system with easily accessible panelboards I would do it differently.

I agree... conditions of the install should be considered. Unfortunately current Code wording does not permit such consideration.

 

[ggunn]

Even so, I cannot offer anything better


I agree... conditions of the install should be considered. Unfortunately current Code wording does not permit such consideration.

Even if that were unambiguously true, I know of many systems installed this way, some of them in jurisdictions of authorities locally renowned for being sticklers on code issues.

 

[Smart $]

Even if that were unambiguously true, I know of many systems installed this way, some of them in jurisdictions of authorities locally renowned for being sticklers on code issues.

That doesn't mean those installations are truly compliant. It just means those AHJ are either unaware of the violation or simply choose to ignore it... take your pick... but I certainly would not take any action to make them aware either.