Newbie simple (yea right) question for 3-phase

[GoldDigger]

If I am interpreting correctly what you say, in my example the contribution to the 120% rule in the MDP would be 270A (six 45A breakers in the AC combiner) even though the backfed breaker in the MDP would only be 80A. That makes no sense to me whatsoever.

Not quite, since only four of the six breakers are connected to any one line conductor or bus. You do get the basic idea though, and I agree it does not make sense. But we are dealing with the NEC here!

 

[jaggedben]

The methods of calculation are already conventional. It's just that previous requirements make the 2014 use of the word sum ambiguous.

Here's what I currently have submitted:


Comments?

EDIT: Note the "?" before a "3" is a square root character in the actual substantiation.

I really think that it should not be an amendment or exception to any rule regarding interconnection in 705.12(D), and should be defined prior to the 125% requirement. Again, I'd rather it be part of the definition of "inverter output circuit current" in 705.60(A)(2). Something like:

"Where multiple single phase inverters are connected to polyphase systems, the combined inverter output circuit current shall be permitted to be calculated per phase using the vector sum of the inverters' rated output."

An informational note might be in order.

 

[Smart $]

I really think that it should not be an amendment or exception to any rule regarding interconnection in 705.12(D), and should be defined prior to the 125% requirement. Again, I'd rather it be part of the definition of "inverter output circuit current" in 705.60(A)(2). Something like:

"Where multiple single phase inverters are connected to polyphase systems, the combined inverter output circuit current shall be permitted to be calculated per phase using the vector sum of the inverters' rated output."

An informational note might be in order.

I can go with the general concept of your post, but I don't believe it would be appropriate in 705.60(A)(2). It refers to a single inverter and appropriately so. We're concerned with the combined output of two or more inverters with an out-of-phase relationship. What about within 705.60(B)? ...And an informational note in 705.12?

 

[ggunn]

Not quite, since only four of the six breakers are connected to any one line conductor or bus. You do get the basic idea though, and I agree it does not make sense. But we are dealing with the NEC here!

FWIW, I have done several systems in different jurisdictions with single phase inverters aggregated into three phase systems and have never been challenged on this point.

Just to be clear, we are talking only about the 120% rule in the MDP, not OCPD and conductor sizing, right?

 

[GoldDigger]

FWIW, I have done several systems in different jurisdictions with single phase inverters aggregated into three phase systems and have never been challenged on this point.

Just to be clear, we are talking only about the 120% rule in the MDP, not OCPD and conductor sizing, right?

Right. The wording of the 120% rule specifically is not tuned to other than single phase situations and may even be shaky in the case of 120/240 three wire.
Do I add up all of the breaker amps in the case of 120V inverters (hypothetical), or just the ones attached to each bus?

 

[Smart $]

...

Just to be clear, we are talking only about the 120% rule in the MDP, not OCPD and conductor sizing, right?

It affects OCPD rating and conductor sizing anywhere on the load side of inverter OCPD.

Also, 2014 modified the general statement of 705.12(D)(1) to:

Dedicated Overcurrent and Disconnect. The sourceinterconnection of one or more inverters installed in one
system shall be made at a dedicated circuit breaker or fusible
disconnecting means.

Highlighted text indicates the changes from 2011. Note the singular reference to circuit breaker or fusible disconnect. It could be interpreted as you can't use 3 breakers in a distribution panel to connect 3 inverters. You'd have to combine the 3 inverter's output prior to connection. How do we determine whether multiple inverters are multiple systems or just one???

 

[Smart $]

... What about within 705.60(B)? ...

First draft:

(B) Ampacity and Overcurrent Device Ratings. Inverter system currents shall be considered to be continuous. The circuit conductors and overcurrent devices shall be sized to carry not less than 125 percent of the maximum currents as calculated in 705.60(A). Where multiple single phase inverters are connected to polyphase systems, calculation of combined currents shall be permitted using 125 percent of inverter output circuit current and methods appropriate for the polyphase system. The rating or setting of overcurrent devices shall be permitted in accordance with 240.4(B)
and (C).

I left out the reference to vector sum as it is not really defined in the NEC. Also there are quite likely many non-engineering persons that have no idea how a vector sum differs from an arithmetic sum, let alone how to perform a vector sum calculation. The only place vector is used in the entire NEC is the informational note following the definition of Neutral Point.

 

[GoldDigger]

It affects OCPD rating and conductor sizing anywhere on the load side of inverter OCPD.

Also, 2014 modified the general statement of 705.12(D)(1) to:

Highlighted text indicates the changes from 2011. Note the singular reference to circuit breaker or fusible disconnect. It could be interpreted as you can't use 3 breakers in a distribution panel to connect 3 inverters. You'd have to combine the 3 inverter's output prior to connection. How do we determine whether multiple inverters are multiple systems or just one???

I just see this as permitting multiple inverters per OCPD rather than requiring it. It assumes that the inverter listing allows the higher maximum OCPD and does not call for a dedicated circuit. I see it as just adding flexibility, not creating more restrictions.
If the ampacity determination had still been based on the first OCPD, as before 2014, then you might be able to get a better match to standard size by combining.

 

[GoldDigger]

First draft:

​

I left out the reference to vector sum as it is not really defined in the NEC. Also there are quite likely many non-engineering persons that have no idea how a vector sum differs from an arithmetic sum, let alone how to perform a vector sum calculation. The only place vector is used in the entire NEC is the informational note following the definition of Neutral Point.

I like it on first impression. If anything comes up later I will let you know.

 

[Smart $]

I just see this as permitting multiple inverters per OCPD rather than requiring it. It assumes that the inverter listing allows the higher maximum OCPD and does not call for a dedicated circuit. I see it as just adding flexibility, not creating more restrictions.
If the ampacity determination had still been based on the first OCPD, as before 2014, then you might be able to get a better match to standard size by combining.

Perhaps... I'd have to look up the reason for the change... but here's the 2011 text:

(1) Dedicated Overcurrent and Disconnect. Each source
interconnection shall be made at a dedicated circuit breaker
or fusible disconnecting means.

Seems the flexibility you refer to was already built into the requirement. As I see it, the change has to be imposing some restriction that was not already in the previous edition. I realize inverters are not limited to PV systems, and I'm not familiar with other non-PV applications, but AFAIK, other than AC modules, I believe a dedicated OCPD is required for each inverter.

 

[jaggedben]

I can go with the general concept of your post, but I don't believe it would be appropriate in 705.60(A)(2). It refers to a single inverter and appropriately so. We're concerned with the combined output of two or more inverters with an out-of-phase relationship. What about within 705.60(B)? ...And an informational note in 705.12?

I don't agree with that. "Inverter output circuit current" should be read as 'the current of the inverter output circuit'. 'Inverter output circuit' is a term defined consistently in more than one place in the NEC as "the conductors between the utility-interactive inverter and the service equipment..." (my emphasis). Thus the output for combined inverters is just as much an 'inverter output circuit' as the branch circuits to individual inverters.

Since 705.12(D) uses the language "125% of the inverter output circuit current", the combined circuit current should be addressed where that term is defined in 705.60(A)(2). That way it the change will properly affect all the other sections that refer back to it, whether dealing with interconnection rules or overcurrent devices. That is, unless anyone thinks there's a safety reason that shouldn't be so.

 

[ggunn]

It affects OCPD rating and conductor sizing anywhere on the load side of inverter OCPD.

Affects how? Can you tie it to the example of three 7kW inverters I have been using?

 

[Smart $]

I don't agree with that. "Inverter output circuit current" should be read as 'the current of the inverter output circuit'. 'Inverter output circuit' is a term defined consistently in more than one place in the NEC as "the conductors between the utility-interactive inverter and the service equipment..." (my emphasis). Thus the output for combined inverters is just as much an 'inverter output circuit' as the branch circuits to individual inverters.

Since 705.12(D) uses the language "125% of the inverter output circuit current", the combined circuit current should be addressed where that term is defined in 705.60(A)(2). That way it the change will properly affect all the other sections that refer back to it, whether dealing with interconnection rules or overcurrent devices. That is, unless anyone thinks there's a safety reason that shouldn't be so.

I can see your point to a degree... yet what we are discussing actually affects "Ampacity and Overcurrent Device Ratings."... the title of 705.60(B). It does not affect what is currently stated in 705.60(A)(2).

And while we're discussing what and where changes should go, can you think of any place in other Article sections where the same permission should be included? Article 690, obviously, but utility-interactive inverters are not limited to just PV systems (e.g. Article 692 and 694, perhaps).

 

[Smart $]

I can see your point to a degree...

Upon further consideration, perhaps the following:

690.8(A)(3)
694.12(A)(2)
705.60(A)(2)

APPEND:

Where multiple single phase inverters are connected as or to a multiphase or polyphase system, the inverter continuous output current rating of combined currents (i.e. the vector sum) shall be permitted to be determined using a calculation method appropriate for the system.​

Article 692 covering fuel cells doesn't appear to have any implications... so I thought I'd leave well enough alone

Apparently Code uses both multiphase and polyphase throughout its text, so I used both to cover the possibilities.

I also put vector sum back in because I foresee the potential for confusion with 705.12(D)(2) using sum only. The alternative would be to remove the sum of occurrences in 705.12(D)(2). I'm on the fence on this, so appreciate comments...

 

[Smart $]

Affects how? Can you tie it to the example of three 7kW inverters I have been using?

Let's see. You have either...

a) 7000W ? 208V ? 125% ? 2 = 84.1A, or
b) 7000W ? 208V ? 125% ? 1.732 = 72.9A

...contribution to feeder or bus. Part "a" is uses the arithmetic sum. Part "b" uses the vector sum.

 

[ggunn]

Let's see. You have either...

a) 7000W ? 208V ? 125% ? 2 = 84.1A, or
b) 7000W ? 208V ? 125% ? 1.732 = 72.9A

...contribution to feeder or bus. Part "a" is uses the arithmetic sum. Part "b" uses the vector sum.

What about the three phase resultant? I am concerned with conductors and OCPD on the run to the service from the AC combiner.

 

[GoldDigger]

The three phase resultant is exactly the second result, 72.9A.
That would be appropriate for feeder and OCPD determination, and should be the same number you get using the power formula.
There is no question, IMHO, about how the NEC says to do that calculation in the case of actual loads.
But for sources, the specific language seems to rule out doing it the same way. If 690 had you add the power and then determine the current, it would give your result. But it seems to require "adding" the currents instead without saying just how that should be done.

 

[Smart $]

What about the three phase resultant? I am concerned with conductors and OCPD on the run to the service from the AC combiner.

Are they covered by 705.12(D)(2)?

If you are considering using this under 2014, I'd run it by the AHJ prior to installation.

 

[jaggedben]

I can see your point to a degree... yet what we are discussing actually affects "Ampacity and Overcurrent Device Ratings."... the title of 705.60(B). It does not affect what is currently stated in 705.60(A)(2).

Well, I think it affects anything which uses the term 'inverter output circuit current', which is to say that it affects both 705.12(D) and 705.60(B) and perhaps some other stuff.

And while we're discussing what and where changes should go, can you think of any place in other Article sections where the same permission should be included? Article 690, obviously, but utility-interactive inverters are not limited to just PV systems (e.g. Article 692 and 694, perhaps).

Well, there's more to comment on here than I feel I have time for. :roll:

To try be succinct, I feel the code ought to have a lot of extensive rationalization on this. Anything that involves multiple utility interactive inverters ought to be in 705 or refer back to it. (702 has been really ignored in this respect, in my opinion.) Combining of inverter sources to power stand-alone systems or optional standby systems could be in 702 or 705 or perhaps an entirely new article. Articles like 690, 692, and 694 should focus primarily on the power-source side of inverters and mostly refer to other appropriate articles when it comes to the AC output. Etc. etc.

It would/will probably take many many years of code revisions to get to that point. But that's my overall opinion.

 

[jaggedben]

Upon further consideration, perhaps the following:

It's a start. See comments in previous post.

I also put vector sum back in because I foresee the potential for confusion with 705.12(D)(2) using sum only. The alternative would be to remove the sum of occurrences in 705.12(D)(2). I'm on the fence on this, so appreciate comments...

To me, this part seems appropriate for a short Informational Note.