Question regarding NEC 705.12(D)(2)

[Smart $]

I'm quoting my own post as all seem to have ignored it. There is no engineering reason for you to size the feeder conductors as you are being asked to do by the POCO. Yes, as I said in my post #8, I recognize the 2011 NEC would require it to be so but this has been addressed in the 2014. You said the AHJ approved this as drawn (maybe they see the obvious) but the POCO says no. If this a long run that will cost money, why don't have a discussion with the POCO and show them that the NEC now recognizes the obvious.

I just read 2014 edition's 705.12(D)(2). Granted the Code double speak may not have set in yet, but the way I currently read it, it either fails to cover MLO panel feeder ampacity... or it is included under the Taps subsection. Whichever the case, all three subsections (Feeders, Taps, Busbars) use the sum of inverter outputs at 125% and the primary OCPD rating. So I have to ask how you see the issue as addressed???

 

[rburnham]

I'm quoting my own post as all seem to have ignored it. There is no engineering reason for you to size the feeder conductors as you are being asked to do by the POCO. Yes, as I said in my post #8, I recognize the 2011 NEC would require it to be so but this has been addressed in the 2014. You said the AHJ approved this as drawn (maybe they see the obvious) but the POCO says no. If this a long run that will cost money, why don't have a discussion with the POCO and show them that the NEC now recognizes the obvious.

I agree with you completely, but the trick is getting the utility (SRP) to accept the new 2014 NEC code, which I'm sure isn't adopted yet. I went ahead and submitted it with 250 MCM wire and should find out tomorrow what they say. The 400A loadcenter will be as close to the main panel as possible, so the wire lengths should not exceed 5-6 feet, but still the bigger the wire, the more $$ and larger the conduit required, etc.

 

[texie]

I agree with you completely, but the trick is getting the utility (SRP) to accept the new 2014 NEC code, which I'm sure isn't adopted yet. I went ahead and submitted it with 250 MCM wire and should find out tomorrow what they say. The 400A loadcenter will be as close to the main panel as possible, so the wire lengths should not exceed 5-6 feet, but still the bigger the wire, the more $$ and larger the conduit required, etc.

It sounds like in your case, since it is so short a distance, that it may not be worth arguing. My point is just that it is pure lunacy, as it has no basis in reality of the current that is possible on the conductors. The bus bar is another story and I see you addressed that in your design.
Your situation has come up many times. In fact, I just had a similar installation recently (a little larger values), but the feeder under the 2011 was going to cost a fair amount of money due to the length and it was clearly of no value. I approved it the way you drew yours without question and so did the POCO. I think the 2014 has addressed this, but I have to admit that is very hard to understand.

 

[texie]

I just read 2014 edition's 705.12(D)(2). Granted the Code double speak may not have set in yet, but the way I currently read it, it either fails to cover MLO panel feeder ampacity... or it is included under the Taps subsection. Whichever the case, all three subsections (Feeders, Taps, Busbars) use the sum of inverter outputs at 125% and the primary OCPD rating. So I have to ask how you see the issue as addressed???

Take a careful look at the 2011 wording and compare it to the 2014. I agree that is a confusing issue, but the 2014 has addressed this.

 

[Zee]

The bus bar is another story and I see you addressed that in your design.

So, let me see if i got this right?
You are saying that the 2014 NEC addresses two cases w/o addressing the situation that involves both cases...........which is actually the case here and almost always the case.

Case #1 - you have PV backfed at the end of a conductor with no loads added or "addable"
Case #2 - PV at the end of a busbar.

In one case labelling req.s apply,
in the other, the much loved 120% rule (opposite feed then permitted at 120%).

So .....what we have here is a feeder conductor AND a busbar then the PV, so what applies?
It seems we can apply "the larger of the two" breaker req. to limit the feeder and the 120% rule SEPARATELY to the busbar.... - and we are golden?

 

[Smart $]

Take a careful look at the 2011 wording and compare it to the 2014. I agree that is a confusing issue, but the 2014 has addressed this.

I am very familiar with the wording in the 2011 edition. When I read the 2014 edition, I'm interpreting as best I can without being influenced by 2011 wording... and I don't see it as being addressed such that ampacity of a feeder to an MLO panel can be sized based solely on the feeder OCPD (or calculated load). If you see where it is addressed, please try to explain.

 

[ggunn]

<deleted>

 

[texie]

I am very familiar with the wording in the 2011 edition. When I read the 2014 edition, I'm interpreting as best I can without being influenced by 2011 wording... and I don't see it as being addressed such that ampacity of a feeder to an MLO panel can be sized based solely on the feeder OCPD (or calculated load). If you see where it is addressed, please try to explain.

2014 NEC 705.12(D)(2)(1)....... "Where the inverter output connection is made to a feeder at a location OTHER THAN the opposite end of the feeder primary source"................only then does (a) apply.

 

[Smart $]

2014 NEC 705.12(D)(2)(1)....... "Where the inverter output connection is made to a feeder at a location OTHER THAN the opposite end of the feeder primary source"................only then does (a) apply.

Okay that says other than at the opposite end...

How do you size the feeder when connection IS at the opposite end?

Wouldn't it then be according to 705.12(D)(2)(2) and/or (D)(2)(3)?

If we size to the former, the tap has to be 125%PV plus primary OCPD. Because the busbar is technically a feeder, unless the backfed PV breaker connects directly to the busbar, stab connections have to be rated at 125% PV plus primary OCPD. Take the OP as an example: 120A + 200A = 320A... so the stab has to be rated for a 320A breaker. I'm going to interject a question here: 2014 uses the term "inverter output circuit current"... is this any different than inverter OCPD rating as in the 2011?

If we size to the latter, and even though it is regarding busbars, the busbar is just an extension of the feeder. In the case of an MLO panel, doesn't the feeder need to be sized to the same ampacity required for the busbar?

 

[texie]

Okay that says other than at the opposite end...

How do you size the feeder when connection IS at the opposite end?
Primary source OCPD + 125% of inverter output current ( NEC 2014). Just like the OP wanted to do (EXCEPT he has to use inverter breaker value under 2011).

Wouldn't it then be according to 705.12(D)(2)(2) and/or (D)(2)(3)?
No and no.

If we size to the former, the tap has to be 125%PV plus primary OCPD. Because the busbar is technically a feeder, unless the backfed PV breaker connects directly to the busbar, stab connections have to be rated at 125% PV plus primary OCPD. Take the OP as an example: 120A + 200A = 320A... so the stab has to be rated for a 320A breaker. I'm going to interject a question here: 2014 uses the term "inverter output circuit current"... is this any different than inverter OCPD rating as in the 2011?
Good observation on "inverter output circuit current" in the 2014 NEC VS "first overcurrent device" in the 2011 NEC. This will in many cases make things smaller values. I think most will like this change.

If we size to the latter, and even though it is regarding busbars, the busbar is just an extension of the feeder. In the case of an MLO panel, doesn't the feeder need to be sized to the same ampacity required for the busbar?

Why does the feeder have to be sized the same as the MLO bus? The OP has a bigger bus to meet the 120% rule as he is feeding other circuits from the same panel.

 

[texie]

Smart$,
Sorry for messing up my last post. My responses are buried in your text. Hope you can follow.

 

[Smart $]

Primary source OCPD + 125% of inverter output current ( NEC 2014). Just like the OP wanted to do (EXCEPT he has to use inverter breaker value under 2011).

Perhaps a tad bit smaller, because inverter OCPD can be greater than 125%, but most just go up to the next larger standard size.

No and no.

What other option is there???

Good observation on "inverter output circuit current" in the 2014 NEC VS "first overcurrent device" in the 2011 NEC. This will in many cases make things smaller values. I think most will like this change.

See reply not immediately above. I agree most will like this, but there is no grandiose difference unless one used a exorbitantly oversized inverter OCPD.

Why does the feeder have to be sized the same as the MLO bus? The OP has a bigger bus to meet the 120% rule as he is feeding other circuits from the same panel.

As I said, the busbar on an MLO panel is a portion of the feeder. Its just a different type of conductor (busbar vs. wire). The feeder must be sized to the minimum required ampacity at any point in the feeder circuit (similar to conductors in a raceway subject to different ambient temperatures in the run). There is nothing that I see in 705.12 which distinguishes the minimum ampacity of the busbar as separate from the minimum ampacity of the wire part of the feeder. So the wire must equal or exceed the same minimum ampacity rating as that required of the busbar.

The advantage I do see is 705.12(D)(7) has been deleted. I take this to mean that we don't have to size all upstream conductors and busbars to the 120% rule.

 

[GoldDigger]

Perhaps a tad bit smaller, because inverter OCPD can be greater than 125%, but most just go up to the next larger standard size.


What other option is there???


See reply not immediately above. I agree most will like this, but there is no grandiose difference unless one used a exorbitantly oversized inverter OCPD.


As I said, the busbar on an MLO panel is a portion of the feeder. Its just a different type of conductor (busbar vs. wire). The feeder must be sized to the minimum required ampacity at any point in the feeder circuit (similar to conductors in a raceway subject to different ambient temperatures in the run). There is nothing that I see in 705.12 which distinguishes the minimum ampacity of the busbar as separate from the minimum ampacity of the wire part of the feeder. So the wire must equal or exceed the same minimum ampacity rating as that required of the busbar.

There is a qualitative difference, at least in typical situations, in that the feeder delivers current (or takes current) only from the single point of connection to the panel, while the busbar can have loads tapped off the middle in arbitrary combinations.
If you envision tapping loads off the feeder wires at some intermediate point, then you are in fact indistinguishable from the panel busbar situation.
But even under the current (2011) rules, some AHJs will (correctly or incorrectly) not apply the 120% rule to feeders which do not supply any loads between the upstream OCPD and the panel they serve.

 

[texie]

Perhaps a tad bit smaller, because inverter OCPD can be greater than 125%, but most just go up to the next larger standard size.


What other option is there???


See reply not immediately above. I agree most will like this, but there is no grandiose difference unless one used a exorbitantly oversized inverter OCPD.


As I said, the busbar on an MLO panel is a portion of the feeder. Its just a different type of conductor (busbar vs. wire). The feeder must be sized to the minimum required ampacity at any point in the feeder circuit (similar to conductors in a raceway subject to different ambient temperatures in the run). There is nothing that I see in 705.12 which distinguishes the minimum ampacity of the busbar as separate from the minimum ampacity of the wire part of the feeder. So the wire must equal or exceed the same minimum ampacity rating as that required of the busbar.

The advantage I do see is 705.12(D)(7) has been deleted. I take this to mean that we don't have to size all upstream conductors and busbars to the 120% rule.

I think the real advantage of the change to the inverter current instead of breaker will be seen in micro inverter systems.

 

[rburnham]

Just an update to everyone, and also a thanks again for the advice... I just got word from my POCO that they approved my updated design with the 250MCM wire. Whether it meets code at this point or not is irrelevant since both the AHJ and POCO have signed off on it. Feel free to continue discussing though as this is good information for others that may come across this in the future.

 

[Zee]

I think the real advantage of the change to the inverter current instead of breaker will be seen in micro inverter systems.

Excellent point. With micro's, sometimes you have 5 amps of inv. curent on a 20 amp breaker........big allowance if 5 Amps can be used, not 20 Amps.
Also i must add my pet peeve: a 4000 Watt inverter may be 16.7 Amps (also often rated at 20A) .......but needs a 30 A breaker! Big diff.

 

[Zee]

There is a qualitative difference, at least in typical situations, in that the feeder delivers current (or takes current) only from the single point of connection to the panel, while the busbar can have loads tapped off the middle in arbitrary combinations.
If you envision tapping loads off the feeder wires at some intermediate point, then you are in fact indistinguishable from the panel busbar situation.
But even under the current (2011) rules, some AHJs will (correctly or incorrectly) not apply the 120% rule to feeders which do not supply any loads between the upstream OCPD and the panel they serve.

Yes, agreed. Hence the labelling requirements for feeders to NOT TAP.