Feeder conductors "becoming" tap conductors?

ohmti787

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Location
Orlando, FL
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Electrical Engineer
Soo, I designed this PV system with a load side tap after a 100A breaker at the meter main that's feeding a subpanel inside. Of course, a 100A breaker will be installed downstream at the subpanel. Basically we're tapping between two breakers, which should be totally fine as long as my PV output doesn't exceed the rating of these feeder conductors. However, I got this inspector telling me that the feeder conductors that I'm tapping into need to be 25' or less according to Article 240.21. For some reason he believes that once you tap a feeder conductor, the feeder conductor itself becomes a tap conductor as well. Has anyone heard such a thing before?
 

GoldDigger

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Placerville, CA, USA
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Retired PV System Designer
It might appear that conventional tap rules might apply IF the added PV conductors have a 100A or better ampacity and are protected by a 100A breaker at the PV end, but technically while the PV is operating the PV end of the wire is the source end of the wire and a breaker there would protect the PV connection.
The other common rule interpretation is to apply the 120% rule to the tapped feeder wire. As long as there are no other load connections between the 100A main and 100A subpanel conductors, the 120% rule should not really apply, but if its conditions are in fact met that might satisfy the inspector.
 

wwhitney

Senior Member
Location
Berkeley, CA
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Retired
The tap rules apply to the #6 AWG, since from the grid side they are protected at 100A.

The #3 AWG between the PV interconnection and the subpanel are subject to (2020) 705.12(B)(1), but with the 100A main breaker in the subpanel, 705.12(B)(1)(b) is satisfied. [I think of this as a sort of tap rule.]

How is the meter main complying with 705.12(B)(3)? What other breakers are in it, and what is the busbar rating?

Cheers, Wayne
 

pv_n00b

Senior Member
Location
CA, USA
The AHJ is incorrect. If there is an existing conductor going from A to B and we tap it in the middle then no part of that existing conductor magically becomes a tap conductor. It's never worked that way.
What does come into play though is that 705.12 requires that when a PV interconnection is created by tapping an existing feeder conductor the existing conductor load side from that tap has to be sized based on both the utility current and 125% of the PV inverter output or there has to be an OCPD on the load side of the interconnection to protect the existing conductor from being overloaded.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
... or there has to be an OCPD on the load side of the interconnection to protect the existing conductor from being overloaded.
And there is; it's the main breaker on the subpanel. What concerns me is the main panel. It's shown with a 200A main breaker; if the bus is also rated at 200A, 125% of the inverter output cannot exceed 40A, but the PV OCPD is shown as 50A.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
The AHJ is incorrect.
Agreed as far as the #3 AWG conductors being an issue is concerned

If there is an existing conductor going from A to B and we tap it in the middle then no part of that existing conductor magically becomes a tap conductor.
I don't think it's that simple, and I see where the inspector is coming from. The 240.2 definition of tap conductor:

Tap Conductor. A conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4.

So when you splice into the middle of a feeder with a second source of supply, for the feeder segment that receives both sources of supply at one end, it's not quite clear how to apply that definition. The logical and physics-based approach would be to add the OCPD ratings from the two sources of supply, even though the definition isn't explicit. If you interpret the definition via adding them, then the #3 AWG conductors from the splice to the 100A subpanel are, in fact, tap conductors.

Regardless, as (2020) 705.12(B)(1) explicitly addresses this situation, compliance with it is sufficient, and there's no need to deal with 240.21 with regards to that feeder segment. But because of the above, I would say 705.12(B)(1)(b) is a type of tap rule.

240.21 still applies to the #6 AWG conductors.

Cheers, Wayne
 

pv_n00b

Senior Member
Location
CA, USA
Agreed as far as the #3 AWG conductors being an issue is concerned


I don't think it's that simple, and I see where the inspector is coming from. The 240.2 definition of tap conductor:

Tap Conductor. A conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4.
The reason the feeder interconnection and tap information was put in 705.12 was to provide direction where 240 did not. Keep in mind that the 240 tap rules are focused on a source, a feeder, a tap, and a load. Put in multiple sources or change the load to a source and it becomes the square peg in the round hole problem. The feeder interconnection and feeder tap language in 705.12 are intended to round that square peg so it all fits together again. So 240 does not turn the existing conductor into a tap conductor, 705.12 tells us how the existing conductor on the load side of the tap has to be protected because not we have multiple sources feeding it.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
So 240 does not turn the existing conductor into a tap conductor,
I agree with basically all of the last post except this part. I would say that from the point of view of 240, the #3 AWG feeder segment under discussion is a tap conductor, but that 705.12 takes precedence over 240.21.

I.e. if for some reason (2020) 705.12(B)(1) and (2) were deleted, then we would need to apply 240.21(B) to the feeder segment.

And I think this distinction may be helpful in dealing with the inspector in the OP. That inspector is presumably either not aware of the rules of the 705.12(B), or for some reason doesn't accept that they take precedence over 240.21(B).

Cheers, Wayne
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
In principle I agree with pvnoob but I have played the part of Wayne as devil's advocate on this one before. I don't think it's a slam dunk. The inspectors reading of 240.21 makes sense if that's his starting point, and it's only if he accepts that 705 modifies it that you can win the argument. The criteria for when exactly the later chapters modify the earlier ones isn't spelled out whatsoever, so good luck.
 

pv_n00b

Senior Member
Location
CA, USA
Most AHJs have much more familiarity with 240 than with 705.12. As with many things PV we can run into inspectors who lean heavily on what they know and are hesitant when applying the less frequently used modifiers in chapters 6 and 7. It helps to have a very good understanding of what 240 is trying to do to protect tap conductors and how 705.12 modifies the scenarios in 240, and not just apply the rules like they are a checklist. That will help you discuss situations that are not specifically covered verbatim in 240.
 
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ohmti787

Member
Location
Orlando, FL
Occupation
Electrical Engineer
squared up with my boy today and sure enough, he did not budge an inch. he kept bringing up this video:
(minute 27 i'm assuming), although he couldn't refer to any article in the code that could make that point clear enough. he kept saying that the code "hasn't catch up" to this. if he's right though, why there is no verbiage on the either the 2014 or 2017 codes regarding this? i mean this was recorded around 2012. you would expect some wording about these feeders "becoming tap conductors". honestly, this doesn't make any sense to me. the definition of a tap conductor is pretty clear in article 240. also, does the location of the breaker downstream matters that much? I mean, it should do the exact same thing, whether is installed at the subpanel downstream, half way through, or right next to the tap, correct?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Yeah check out from about 29:50 to 31:59 during which Mike is mentioning 240.21(B) and they are having a disagreement. And Mike says they had a fight about it. And Bill Brooks who wrote the code section says at the end that the overcurrent device needs to be right near the tap on the load side, and Jason (I don't know who that is but I agree with him) is saying it's not necessary. They are basically having this exact discussion and it more or less ends with Jason saying 'I don't like it' and Mike saying 'Okay! We're movin' on!'. :p

It would be a tap under 240.21(B) because you have two sources that can feed the conductor in parallel, so if the conductor runs for some distance on the load side of the tap then it doesn't have overcurrent protection at its point of supply. It sort of makes sense to use 240.21(B) to settle this dispute about how far the load-side conductors can go before it needs the overcurrent device. (I mean, it makes sense from a letter-of-the-code point of view. From a physics and engineering point of view the overcurrent device can usually be anywhere.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
It would be a tap under 240.21(B) because you have two sources that can feed the conductor in parallel
I certainly agree physics-wise but that does require interpreting the phrase "has overcurrent protection" in the definition of tap conductor to mean "add up the ratings when there is more than one." So I don't think it's clear code-wise.

Regardless, if the intention was for 240.21(B) to apply, (2017) 705.12(B)(2)(1)(b) would not be written as it is. It would just say "the conductors comply with 240.21(B) as modified by 705.12(B)(2)(2)."

Cheers, Wayne
 
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