Feeder Tap

[BradPV]

If I tap a feeder to supply the PV system does the feeder then become a tap as well? The feeder is tapped in the combo panel but then the feeder is >10' from the load center.

 

[jaggedben]

It's unclear in the code how the 10 or 25ft rules would apply. But you do need to protect the downstream part of the feeder, per 705.12.

 

[BradPV]

Yeah. I have the feeder protected on both ends but the AHJ "doesn't like" a line side tap so he is considering the feeder a tap. It is definitely not clear to me and I was hoping someone had run in to this and had an argument prepared....

 

[wwhitney]

A one line diagram would help clarify the situation, but I assume you have the following:

A main service panel, which qualifies on its own under (2017) 705.12(B)(2)(3) (that requirement must not be overlooked).. A breaker supplying a feeder to a subpanel. And a splice to that feeder of the output of a PV system. So the resulting spliced feeder just has 3 "ends". It has 3 segments, call them A from feeder breaker to splice, B from PV inverter to splice, and C from splice to subpanel.

Segment A can either carry grid current or PV current and needs to be rated for both cases. For interconnection without upsizing an existing feeder, the PV output (including 125% factor) has to be no greater than the feeder breaker, in which case segment A is protected in both case.

Segment B can also carry grid current (e.g. if it also supplies loads downstream from the grid, or if there's a fault), so segment B can be sized the same as segment A. Or if it can meet one of the tap rules in 240.21, that can be used; it still needs to be sized for the PV output.

Segment C can carry current from both the grid and the PV. For the grid only, it has to be at least the size of the feeder breaker (as it will be for an existing feeder). But since it now has multiple sources of supply, 705.12(B)(2)(1) gives you two options for protecting it: (a) size it for the sum of the supplies, or (b) have a main breaker in the subpanel to protect it from overload.

I think of 705.12(B)(2)(1)(b) as a sort of unlimited length tap rule for this limited situation, since the OCPD to prevent overload is downstream of the splice, which is the source of the second supply.

All of the above assumes an existing feeder that was full sized, i.e. not sized using the tap rules under 240.21. If interconnecting at a 240.21 tap, the situation gets more complicated; I've not thought out all the ramifications.

Cheers, Wayne

 

[BradPV]

Thank you wwhitney, this does fall under the 2020 code as it is in Georgia which is why I asked if the feeder is now considered a tap. I understand the AHJ can interpret the code but was struggling to understand their interpretation. Your scenario was correct with A being <10' and B <10' with C being >25'.

 

[wwhitney]

I haven't checked 2020 but my understanding is that the relevant rules are the same, just renumbered.

If you want a more exact answer, tell us all the breaker sizes, feeder wire sizes, and busbar ratings.

Cheers, Wayne

 

[jaggedben]

One could definitely argue that the Wayne's section C of the feeder is a tap conductor as that term is used in 240.21(B) if the rating of the conductor (rounded up to the next standard OCPD) is less than the combined rating of the PV breaker and the subpanel in the main. It's just that that is reading between the lines, not something that's explicitly addressed in 705 of 240. If the AHJ wants to be that way, the the location of your downstream panel breaker matters.

BTW I'm sure your feeder is rated high enough to meet the 25ft rule if it isn't too long for that.

 

[wwhitney]

One could definitely argue that the Wayne's section C of the feeder is a tap conductor as that term is used in 240.21(B) if the rating of the conductor (rounded up to the next standard OCPD) is less than the combined rating of the PV breaker and the subpanel in the main. It's just that that is reading between the lines, not something that's explicitly addressed in 705 of 240.

I would say that segment C is directly addressed by (2017) 705.12(B)(2)(1)(b), and since that allowance doesn't place a distance limit, the limitations of 240.21 don't apply. Chapter 7 is implicitly modifying 240.21 there.

Cheers, Wayne

 

[jaggedben]

I would say that segment C is directly addressed by (2017) 705.12(B)(2)(1)(b), and since that allowance doesn't place a distance limit, the limitations of 240.21 don't apply. Chapter 7 is implicitly modifying 240.21 there.

Cheers, Wayne

Although I agree, I'd probably settle out of court before taking that argument in front of a jury.

 

[wwhitney]

Although I agree, I'd probably settle out of court before taking that argument in front of a jury.

Nah, it's a slam dunk, as the next section specifically refers to 240.21(B). So it's clear that if the intention were to require segment C to comply with either 705.12(B)(2)(1)(a) or with 240.21, then 705.12(B)(2)(1) would say so, and subsection (b) wouldn't be there at all. Subsection (b) only makes sense as a replacement for the usual 240.21 requirements.

Cheers, Wayne

 

[jaggedben]

Nah, it's a slam dunk, as the next section specifically refers to 240.21(B). So it's clear that if the intention were to require segment C to comply with either 705.12(B)(2)(1)(a) or with 240.21, then 705.12(B)(2)(1) would say so, and subsection (b) wouldn't be there at all. Subsection (b) only makes sense as a replacement for the usual 240.21 requirements.

Cheers, Wayne

I don't follow your logic at all. Under section (a) the conductor doesn't meet the definition of a tap conductor in 240 anyhow, so clearly 240.21(b) wouldn't apply. But if you choose to comply with section (b), 705 leaves the location of the OCPD vague. I've seen the question of where that OCPD is supposed to go get asked on the forum a number of times, and I've seen Mike Holt and his team briefly debate it in a video. Going back to 240.21(B) to answer that question is as valid as any other approach under the code, since as a general principle Chapters 1-4 remain in effect unless clearly modified by later chapters. There's no clear modification of that in 705. In fact, your comment about the next section seems backwards to me; that section is so badly worded and vague that it could be read just about any way one wants, e.g. to apply to our 'Section C' in question, if an AHJ were motivated to do so. Indeed, perhaps the general rejoinder is that nothing is a 'slam dunk' with certain AHJs when there's any room for interpretation in the code.

 

[Carultch]

A one line diagram would help clarify the situation, but I assume you have the following:

A main service panel, which qualifies on its own under (2017) 705.12(B)(2)(3) (that requirement must not be overlooked).. A breaker supplying a feeder to a subpanel. And a splice to that feeder of the output of a PV system. So the resulting spliced feeder just has 3 "ends". It has 3 segments, call them A from feeder breaker to splice, B from PV inverter to splice, and C from splice to subpanel.

Segment A can either carry grid current or PV current and needs to be rated for both cases. For interconnection without upsizing an existing feeder, the PV output (including 125% factor) has to be no greater than the feeder breaker, in which case segment A is protected in both case.

Segment B can also carry grid current (e.g. if it also supplies loads downstream from the grid, or if there's a fault), so segment B can be sized the same as segment A. Or if it can meet one of the tap rules in 240.21, that can be used; it still needs to be sized for the PV output.

Segment C can carry current from both the grid and the PV. For the grid only, it has to be at least the size of the feeder breaker (as it will be for an existing feeder). But since it now has multiple sources of supply, 705.12(B)(2)(1) gives you two options for protecting it: (a) size it for the sum of the supplies, or (b) have a main breaker in the subpanel to protect it from overload.

I think of 705.12(B)(2)(1)(b) as a sort of unlimited length tap rule for this limited situation, since the OCPD to prevent overload is downstream of the splice, which is the source of the second supply.

All of the above assumes an existing feeder that was full sized, i.e. not sized using the tap rules under 240.21. If interconnecting at a 240.21 tap, the situation gets more complicated; I've not thought out all the ramifications.

Cheers, Wayne

Excellent explanation.

I'd like to provide the following illustration to your description, and give some numbers that would show a compliant example, and add some further points to this kind of situation.


In my example, you can see that the main panel has its own requirement to follow a busbar protection rule, and the 120% rule is the one that complies, since (125%*48A+200A) <= 120%*225A. No similar rule needs to apply to the subpanel in this example, because we aren't interconnecting where we can feed its bus with more than the rating of its bus. You might also see this situation, in a case where it isn't a main panel upstream of the tap point, but rather a meter/main combo device with the service's main breaker, that then feeds the main panel in the position where the subpanel is in the above sketch.

Disregarding fault conditions, wire segment A could carry 100A max, wire segment B could carry a maximum of 60A (after the 125% safety factor on 48A), and wire segment C could carry a maximum of 100A. There is an OCPD in some form or another, that limits the load current on each of these wire segments to not exceed these values.

Wire segments A and C would classify as feeders, and be built with 100A of ampacity. Wire segment B would either be built as a 240.21(B) feeder tap with 60A of ampacity (most likely). Alternatively or it could be built with the full 100A of ampacity, if you'd like to promote it from a tap to a feeder so tap rules no longer apply. Provided of course, that you have 100A terminations in the disconnect on the other end.

A tap is a section of conductor protected in excess of its ampacity (by the feeder breaker), that eventually will be protected at its ampacity from overload. The feeder breaker protects it from a short circuit fault that will be far in excess of 100A. The 60A fuse protects it from overloads between 60A and 100A.

It is essential that the subpanel either a) have a main breaker, b) have a branch breaker acting as a main, or c) have an externalized device taking the place of a main breaker. Otherwise, a main-lug-only subpanel in this example, could draw up to 148A. Without the PV, the subpanel depends on the 100A breaker in the main panel to protect it from the unlikely event that all its loads draw more than 100A at once.

 

[jaggedben]

...

Disregarding fault conditions, ...

But aren't the tap rules all about fault conditions? I mean, if fault conditions didn't matter then OCPDs could be anywhere in any circuit, and tap length wouldn't matter a whit, but 240 doesn't allow that.

I think if the AHJ thinks that C (and for that matter, A) is a tap you really have to go outside what the code says and point out that the available fault current from the inverters is waaaay lower than the utility. Unfortunately the code kind of takes the opposite position with 705.12(B)(2), which says the inverter must be considered the same as the utility for 240.21(B) calcs. That section should probably be changed, but I don't know exactly how.

 

[Carultch]

But aren't the tap rules all about fault conditions? I mean, if fault conditions didn't matter then OCPDs could be anywhere in any circuit, and tap length wouldn't matter a whit, but 240 doesn't allow that.

I think if the AHJ thinks that C (and for that matter, A) is a tap you really have to go outside what the code says and point out that the available fault current from the inverters is waaaay lower than the utility. Unfortunately the code kind of takes the opposite position with 705.12(B)(2), which says the inverter must be considered the same as the utility for 240.21(B) calcs. That section should probably be changed, but I don't know exactly how.

To rephrase that, segment C is what you say either needs to be compliant with a 240.21(B) tap rule based on 160A in this example, or be built with 160A of conductor. Is this correct?

If segment C is less than 10 ft or 25 ft, or if it is outside underground, all that would matter is that you are not depending on a next-size-up-rule or an 83% residential service conductor (feeder extension) rule, from what it would be by default. But if segment C is too long to be a tap, then it would need to be 160A. Given that 160A is not a standard size, this would mean that 240.4(B) could not govern its sizing. 240.4(B) could only apply if they add up to a standard size.

This seems like it is anticipating a very rare kind of fault, that is between 100A and 160A, such that it is in the blindspot of both the 60A fuse and 100A breaker combined. Given a grid-interactive inverter, it will only operate when the grid is present anyway, and the grid will feed a fault with a lot more than 160A.

 

[wwhitney]

I don't follow your logic at all.

I guess my logic boils down to this: for the alternative possible intention of 705.12(B)(2)(1)(b) you posit, it could just read "(b) Compliance with 240.21(B) in accordance with 705.12(B)(2)(2)." The actual language is so far from that, reprising certain conditions and failing to reference 240.21(B), that it is providing a substitute for 240.21(B) for this situation.

Also, FWIW, 240.21(B) refers in the various subsections to "the rating of the overcurrent device protecting the feeder conductors." Absent 705.12(B)(2)(2), it wouldn't be clear how to apply that to segment C, in Carultch's example, should it be the 100A OCPD or the 60A OCPD? So 705.12(B)(2)(2) is serving the important functions of saying "no, it has to be the sum of the two" (well, when the OCPD is exactly 125% of the inverter output current).

But I see your point now and retract the "slam dunk" comment.

(and for that matter, A).

Sorry, how can Segment A possibly be considered a tap in any rational fashion? If its source of supply is the feeder breaker, it complies with 240.21(A). If its source of supply is the PV breaker, it still complies with 240.21(A). And both can't be true at once, contrary to what one misinformed AHJ may have once asserted to one of the members here.

Cheers, Wayne

 

[BradPV]

Thank you all for the discussion, it seems this AHJ is just not allowing a load side tap regardless of any argument....

 

[jaggedben]

To rephrase that, segment C is what you say either needs to be compliant with a 240.21(B) tap rule based on 160A in this example, or be built with 160A of conductor. Is this correct?

I'm saying that if Brad's AHJ interprets the code that way, I don't see a slam dunk refutation.

If segment C is less than 10 ft or 25 ft, or if it is outside underground, all that would matter is that you are not depending on a next-size-up-rule or an 83% residential service conductor (feeder extension) rule, from what it would be by default. But if segment C is too long to be a tap, then it would need to be 160A. Given that 160A is not a standard size, this would mean that 240.4(B) could not govern its sizing. 240.4(B) could only apply if they add up to a standard size.

This seems like it is anticipating a very rare kind of fault, that is between 100A and 160A, such that it is in the blindspot of both the 60A fuse and 100A breaker combined. Given a grid-interactive inverter, it will only operate when the grid is present anyway, and the grid will feed a fault with a lot more than 160A.

Yes, I know, and I agree. But the code seems to be written as if these rare kinds of faults matter. I mean, suppose a 20ft tap is installed such that it complies with the 10ft rule but not the 25ft rule. How rare is the fault that doesn't trip the feeder breaker because of that? Probably more rare than the one you describe.

 

[jaggedben]

Also, FWIW, 240.21(B) refers in the various subsections to "the rating of the overcurrent device protecting the feeder conductors." Absent 705.12(B)(2)(2), it wouldn't be clear how to apply that to segment C, in Carultch's example, should it be the 100A OCPD or the 60A OCPD?

705.12(B)(2)(3) says that the answer is 100A plus 125% of inverter output. (This makes no sense scientifically, but the code says it clearly enough.) And this applies to 'all taps'. But it doesn't make clear whether and how A, B, and C are determined to be 'taps' or not in the first place. Is that just based on just the 100A utility side feeder breaker or upon the combination of the sources somehow?

Consider also that if there's a fault, A, B and C are all the same in terms of how much fault current could feed the fault. So if it makes sense to consider 'B' a tap, why not A and C?

Sorry, how can Segment A possibly be considered a tap in any rational fashion? If its source of supply is the feeder breaker, it complies with 240.21(A). If its source of supply is the PV breaker, it still complies with 240.21(A). And both can't be true at once,

They can in a fault. Which seems to be what 240.21(B) is concerned with, since (as I said above) if operating current was all that mattered then distances like 10 and 25ft wouldn't matter at all. Mind you, I doesn't appear to me that 240.21(B) is based on any kind of science or real-world study. I don't dispute with you whatsoever that the overcurrent device on 'C' could be anywhere on that segment and it wouldn't likely create any meaningful difference in safety.

 

[wwhitney]

705.12(B)(2)(3) says that the answer is 100A plus 125% of inverter output.

I lost you here, it's 2017 705.12(B)(2)(2) or 2020 705.12(B)(2) that tells you how to interpret 240.21(B)'s "the rating of the overcurrent device" language when there are multiple sources of supply.

Consider also that if there's a fault, A, B and C are all the same in terms of how much fault current could feed the fault. So if it makes sense to consider 'B' a tap, why not A and C?

tl;dr Just skip to the last sentence of the body.

I think the fault current discussion may be a (potentially informative) tangent. It seems to me that what this boils down to is how to interpret the first sentence of 240.21 when there are multiple supplies: "Overcurrent protection . . . shall be located at the point where the conductors receive their supply except as specified . . ."

Circuits can be divided up into segments where each segment has connections at just its two ends. [And I'll assume the one line diagram is a tree (a graph with no loops); parallel conductors can be considered a single segment.]. When there's just one supply in the system, then one end of any segment is the supplyward end, and you can follow the graph upstream to the first OCPD. That OCPD is protecting the segment and determines if the above 240.21 requirement is met.

How should this be applied when there are multiple sources of supply? Article 705 is supposed to tell us, but it's not very explicit. For the case of two supplies in the system, some segments will still only have one source of supply at one end (i.e. following that end upstream will hit an OCPD before hitting a segment with multiple sources of supply), no problem. Otherwise, a segment can either have (1) two sources of supply, at opposite ends, or (2) two sources of supply, both connected to one end.

For case (1), such as segments A and B, I'm going to posit that the only plausible option is to apply 240.21 to each source of supply separately. I can't think of any other procedure or way to interpret things; I'm curious to know if there's some option I'm overlooking. So iegment A always complies with the basic 240.21 requirement and we don't have to look at 240.21(B). Segment B complies with respect to the PV source but may not with respect to the utility source, so for the latter 240.21(B) may be required. [Whether 705.12(B)(2)(2) applies to segment B in this case is an interesting question. I guess by the letter of 705.12(B) it does, but is there a fault condition that causes a conductor segment to simultaneously carry current from both sources?]

So that brings us to case (2), such as segment C. Before we look at 705, a few possible ways of applying 240.21 come to mind. We could say "there needs to be an OCPD at the end with two sources of supply to comply with the basic 240.21 requirement; otherwise, look at 240.21(B)." Or we could say "obviously we had better add the OCPDs upstream towards each source of supply, and treat the segment as if it had an OCPD of the resulting size at the end; then apply 240.21". Or maybe there's some other way I'm not thinking of?

Now we look at article 705, and it provides guidance on how to interpret 240.21 for case (2). 705.12(B)(2)(1)(a) directs us to the second interpretation, rather than the first interpretation. Great. 705.12(B)(2)(1)(b) is presented as an alternative allowance, where OCPD distance is unspecified and therefore unlimited. So my argument boils down to "since 705.12(B)(2)(1)(b) is presented as an alternative to 705.12(B)(2)(1)(a), which tells us how to interpret 240.21, it is itself an alternative to 240.21."

Cheers, Wayne

PS If the above is the correct intention, I totally agree that 705.12(B)(2)(1)(b) could be a lot clearer, or that 240.21 should have an explicit subsection referencing 705.12(B)(2)(1).