Tap sub feeders

[jaggedben]

...

705.12(D)(2)(1) concerns feeders, where a PV system interconnection point is made at a feeder conductor. Note that this is not a tap, and the tap rules don't apply.

705.12(D)(2)(2) talks about how to treat a non-PV related tap in a feeder that also has a PV system interconnection (not considered at tap) made elsewhere on the feeder.

The following reference to 705.12(D)(2)(2) is from the Apr/May 2014 edition of SolarPro:

"Pre-exiting taps or taps added to a feeder with a PV supply source must be sized per 705.12(D)(2)(2) and 240.21(B), based on the sum of 125% of the inverter(s) output-circuit current plus the feeder OCPD rating. See Figure 3 for an example tap conductor calculation. Note that inverter output-circuit conductors, which must not have an ampacity less than the rating of the circuit OCPD, cannot meet the definition of a tap conductor."

...

Okay, well... I categorically disagree with the parts in red. I can get behind that 705.12(D)(2)(2) applies to all taps, whether they are for a load or a source, in the 2014 code. But I don't see anything, in your link or in the code or anywhere, which overrides the 240 definition of a tap. And the statement I highlighted from the solar pro article quote is... well, completely mistaken.

Let's take a practical example: if I want to tap a 1000amp feeder with a 20amp source and run the source conductors to another enclosure, it cannot be the case that the source conductors are required to be either only 20amp (unsafe) or 1000amp (overkill). They should be for example something like 100amp (or 102amp) based on 240.21B(1) for less than 10ft. That's exactly the sort situation the tap rules are in there to address.

Finally, see the new section in the 2017 code 705.65(C), which establishes that 240.21(B) rules do apply to power source taps, as one of two minimums. I dont really agree with the way they did it (putting it in a far away section where I suspect many people will not notice) but that was the CMP's way of addressing misconceptions over how to apply the tap rules to pwer source taps.

 

[PWDickerson]

A couple years ago, I took a PV 2014 NEC changes class with Bill Brooks and Sean White, and it was a real "Aha!" moment for me and many others in the room when they explained that 705.12(D)(2)(2) does not apply to the PV interconnection, but is included so that electricians knew how to re-calculate the ampacity of existing taps on a feeder circuit with a new PV interconnection made at the same feeder. But jaggedben makes a good point. I think I will see what others say. I may be in over my head.

 

[SolarPro]

And the statement I highlighted from the solar pro article quote is... well, completely mistaken.

How so?

• Tap conductor is defined as "a conductor that has OCP ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4."
• Inverter output circuits are sized according to 240.4(B) and (C) [705.60].
• Therefore, the inverter output circuit conductor cannot meet the definition of a tap conductor.

I don't see any problem with Bill Brooks' logic here, which was no doubt forged by fire at countless NFPA meetings.

 

[Carultch]

How so?

• Tap conductor is defined as "a conductor that has OCP ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4."
• Inverter output circuits are sized according to 240.4(B) and (C) [705.60].
• Therefore, the inverter output circuit conductor cannot meet the definition of a tap conductor.

I don't see any problem with Bill Brooks' logic here, which was no doubt forged by fire at countless NFPA meetings.

So in slide 8 of this slideshow, what is the section of conductor at the blue arrow between the PV interconnection point and the 200A downstream breaker called?
http://www.slideshare.net/solpowerpeople/interconnection-rule

And should it have 200A or 400A worth of ampacity, provided its length is small enough. Possibly short enough that it doesn't even leave the enclosure of one of the breakers.

Similarly, what is the section of conductor at the red arrow called, just below the 200A PV breaker? I call it a tap, and treat it as a tap rule conductor. Am I wrong? Suppose the PV interconnection breaker were 100A, and that circuit had only 100A worth of wiring, tapped onto the 200A circuit. Is that section of conductor still a tap, and should it be treated per the 240.21(B) tap rules?

 

[SolarPro]

So in slide 8 of this slideshow, what is the section of conductor at the blue arrow between the PV interconnection point and the 200A downstream breaker called?

There are a number of ways to build that interconnection, right? You could add a panelboard to enclose both the interactive inverter breaker and the load-side breaker that protects the downstream conductor and busbar. You could also use fused disconnects and have power distribution blocks inside a wireway.

The proper nomenclature depends on the equipment that you use and the distances involved. In some scenarios, you are connecting directly to the feeder. In other scenarios, you are interconnecting to the feeder via a tap conductor. The 2014 Code includes guidance for both situations.

 

[jaggedben]

How so?

• Tap conductor is defined as "a conductor that has OCP ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4."
• Inverter output circuits are sized according to 240.4(B) and (C) [705.60].
• Therefore, the inverter output circuit conductor cannot meet the definition of a tap conductor.

I don't see any problem with Bill Brooks' logic here, which was no doubt forged by fire at countless NFPA meetings.

I don't buy that the inverter is the 'point of of supply' with respect to the tap rule definition. You have a conductor that can be fed fault current that's less than the primary source OCPD and greater than the conductor rating. With respect to physics and safety, there's no interesting difference from an ordinary load tap. And don't tell me I'm supposed to know that the 'point of supply' is the inverter when there are two points of supply and the word 'supply' is used to imply both directions in various parts of the code.

I'll take your word for it that that Bill Brooks and the CMP may have believed that this is what they were approving as the rules. But I don't agree that it's what the code language states, explicitly or implicitly. Nor does it make sense with respect to physics and safety. Fortunately the safety issue with source taps is addressed in the the 2017 in 705.65(C) as mentioned above. So it seems that since the article was published they recognized there was a shortcoming with Bill's logic. (There were a few public comments on this item, including one of mine.)

In the 2017 we now have another logical inconsistency, albeit less serious. A source tap can be minimum sized in some cases to 240.21(B) based on the feeder OCPD. But a load tap on the same feeder must be sized based on a sum of feeder OCPD and inverter in the badly worded 705.(D)(2)(2). Since the fault current for all practical purposes is identical between the two cases, this makes no sense with respect to physics and safety. Either 240.21(B) is good enough without considering fault current from the current limited interactive source, or the interactive source needs to be considered for both cases (and with more grammatical language).

 

[SolarPro]

(Actually, Bill wrote our NEC 2017 article. The NEC 2014 article was by Rebekah Hren and Brian Mehalic.)

If you act now, you can fix whatever problems you have with 705.12 for the 2020 Code edition. Then you just have to wait six years for CA to adopt those changes! :slaphead:

 

[Carultch]

There are a number of ways to build that interconnection, right? You could add a panelboard to enclose both the interactive inverter breaker and the load-side breaker that protects the downstream conductor and busbar. You could also use fused disconnects and have power distribution blocks inside a wireway.

The proper nomenclature depends on the equipment that you use and the distances involved. In some scenarios, you are connecting directly to the feeder. In other scenarios, you are interconnecting to the feeder via a tap conductor. The 2014 Code includes guidance for both situations.

So in THAT specific situation, if there is nothing more than conductors connecting the OCPDs and the common point, what should each of the conductor sections be called, and what sizing algorithm should be used? Suppose the common point is either a Polaris type multitap device, or an insulation piercing connector.

 

[SolarPro]

So in THAT specific situation, if there is nothing more than conductors connecting the OCPDs and the common point, what should each of the conductor sections be called, and what sizing algorithm should be used? Suppose the common point is either a Polaris type multitap device, or an insulation piercing connector.

For a direct connection to a feeder, I would size the the conductor between the inverter output lugs and the polaris tap according to 125% of the inverter output circuit currents [705.60] and ensure that the downstream feeder was either oversized to support the connection [705.12(D)(2)(1)(a)] or protected by a properly rated OCPD [705.12(D)(2)(1)(a)]. I would then put that plan in front of our master electrician and our in-house or consulting electrical engineer for their reviews. Because my opinion is irrelevant.

If I needed to locate the fused disconnecting means for the parallel power production source at some distance from the feeder (say 25'), I would apply the appropriate tap rule to the sum the feeder OCPD and 125% of the inverter currents and use this to size the conductor between the inverter OCPD and the Polaris taps. I would then put that plan in front of our master electrician and our in-house or consulting electrical engineer for their reviews. Because my opinion is irrelevant.

That is my understanding of things. If this is incorrect, please set me straight.

 

[pv_n00b]

So what we have learned is that if you put 5 people in a room who feel they have a firm grasp of 705.12(D)(2) and ask them how to design a load side interconnection you will get 6 different answers. Great work NFPA. :huh:

I'm going with this feeder interconnection, I'm not sure though, is it a tap or not?

 

[SolarPro]

Wow! That's a cost-effective way to make a supply side connection. Lugs are for whimps. (I don't think you need OCP because 10 AWG just turns to smoke in the event of a fault.)

On the one hand, the CMP provided more detail and guidance, which generally a good thing. On the other hand, that's more detail to try to parse and argue about. All things considered, it seems like an improvement, if only because the revision process will scrub the goo out eventually.

To be fair, the CMPs do the best they can under challenging schedule constraints and committee structures. Then they hope for the best. Until the language goes out into the world, they have no idea how many ways their best intentions can get twisted around.

 

[electrofelon]

So what we have learned is that if you put 5 people in a room who feel they have a firm grasp of 705.12(D)(2) and ask them how to design a load side interconnection you will get 6 different answers. Great work NFPA. :huh:

I'm going with this feeder interconnection, I'm not sure though, is it a tap or not?

What A classic.....I mean they couldnt AT LEAST have used a non-bugle head screw???

 

[GoldDigger]

What A classic.....I mean they couldnt AT LEAST have used a non-bugle head screw???

It should also have been wrapped in tape!
If for no other reason than to keep it from startling people.

Sent from my XT1585 using Tapatalk

 

[ggunn]

So what we have learned is that if you put 5 people in a room who feel they have a firm grasp of 705.12(D)(2) and ask them how to design a load side interconnection you will get 6 different answers. Great work NFPA. :huh:

I'm going with this feeder interconnection, I'm not sure though, is it a tap or not?

That's ridiculous! Everyone knows you shouldn't wirenut a black wire to a red wire!

 

[SolarPro]

So in slide 8 of this slideshow, what is the section of conductor at the blue arrow between the PV interconnection point and the 200A downstream breaker called?

I may not be very smart, but I sure am slow:

You are basically going to splice and extend the feeder for that small section of conductor.

The alternative is to design it and build it according to the tap connection allowance, which should allow for a smaller conductor.

(If all else fails, just get a drywall screw.)

 

[Carultch]

I may not be very smart, but I sure am slow:

You are basically going to splice and extend the feeder for that small section of conductor.

The alternative is to design it and build it according to the tap connection allowance, which should allow for a smaller conductor.

(If all else fails, just get a drywall screw.)

What I'm trying to get at, is if I were to construct that slide in practice, do I need to involve 400A worth of wire (600 kcmil) just for 1 foot of wire length?

 

[jaggedben]

What I'm trying to get at, is if I were to construct that slide in practice, do I need to involve 400A worth of wire (600 kcmil) just for 1 foot of wire length?

I sure hope not. I've already mentioned the new 705.65(C) in the 2017 a couple times. Perhaps it's time to quote it.

Conductor Ampacity. Power source output circuit conductors that are connected to a feeder, if smaller than the feeder conductors, shall be sized to carry not less than the larger of the current as calculated in 705.60(B) or as calculated in accordance with 240.21(B) based on the over-current device protecting the feeder.

 

[SolarPro]

Not to my mind. You are extending the feeder to the PV system. As long as the feeder is adequate to carry the inverter output, bob's your uncle.

If you are connecting via a tap conductor, those rules apply. Depending on the distance, that calculation might look something like this:

Inverter tap ampacity ≥ (supply OCPD + (inverter output circuit current x 125%)) x 33%