Conductor Sizing for Single Phase Inverters on 3PH System

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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
JMHO the pre-2014 code requires you (for 120% rule and some other purposes) to sum the numeric values of the nearest OCPDs to the inverters, and as mentioned earlier, allows no leeway to do a vector addition. The sizes of the OCPDs are not vectors.

This is what we are discussing. So where does the 120% rule permit one to use the vector sum?

The OPs question wasn't about the 120% rule. Granted, he referred to conductor sizing, but he was really asking about the circuit current. Let's be definitive: in his example, it would be perfectly fine to use 50A (and thus 62.5A ampacity and 70A breaker), instead of 58A (80A breaker, etc.). No section of the code requires anything else.

If his AHJ then asks him to apply the 120% rule to the conductor and combiner, that's a new calculation, however you do it.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
This is what we are discussing. So where does the 120% rule permit one to use the vector sum?
The current IS the vector sum. The 120% rule does not specify how to calculate it.

Three 7000W single phase inverters on three phase 240V produces 21000W/240Vsqrt3 = 50.5A. 1.25 X 50.5A = 63.14A toward the 120% rule and gets a 70A 3 pole breaker.
 
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Smart $

Esteemed Member
Location
Ohio
The OPs question wasn't about the 120% rule. Granted, he referred to conductor sizing, but he was really asking about the circuit current. Let's be definitive: in his example, it would be perfectly fine to use 50A (and thus 62.5A ampacity and 70A breaker), instead of 58A (80A breaker, etc.). No section of the code requires anything else.

If his AHJ then asks him to apply the 120% rule to the conductor and combiner, that's a new calculation, however you do it.
Yes, let's be definitive. You are making an assumption. He asked if he combines the inverters in a combiner panel what would be the load on each phase. Answered as "a little over 50A"... 50.2A if rounded to 1 decimal.

From there, the title queries "conductor sizing". Interpreting this with the question on the phase (ergo line) load would indicate the load side of the initial inverter breaker in the AC combiner panel... thus we immediately get into the 120% rule if the AHJ interprets Code in this manner. Code is not concise enough to exclude a combiner panel from the 120% rule.
 

Smart $

Esteemed Member
Location
Ohio
The current IS the vector sum. The 120% rule does not specify how to calculate it.

Three 7000W single phase inverters on three phase 240V produces 21000W/240Vsqrt3 = 50.5A. 1.25 X 50.5A = 63.14A toward the 120% rule and gets a 70A 3 pole breaker.
I noted earlier that I errantly used OCPD when I meant feeder and busbar ratings.

So how do you vector sum panel main ocpd rating and the inverter output current at 125% for busbar? If we go the vector sum route they would "offset" rather than "accumulate" when there is zero local load, right?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Yes, let's be definitive. You are making an assumption. ...

Actually, I'm not, because I happen to know that the OP knows the difference between applying and not applying the 120% rule on top of the other question. But in any case, we can still be definitive about what the answer is for coming up with an OCPD on the utility side.
 

Smart $

Esteemed Member
Location
Ohio
Actually, I'm not, because I happen to know that the OP knows the difference between applying and not applying the 120% rule on top of the other question.
If you "know" it would be from context outside this thread. If limited to just this thread, then you are making an assumption... period

But in any case, we can still be definitive about what the answer is for coming up with an OCPD on the utility side.
Not less than 70A would be the prudent answer... but that's not exactly definitive because the rating chosen may affect bus and feeder ratings, dependent on interpretation.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
So how do you vector sum panel main ocpd rating and the inverter output current at 125% for busbar?

As I said, three 7000W single phase inverters on three phase 240V produces 21000W/240Vsqrt3 = 50.5A. 1.25 X 50.5A = 63.14A toward the 120% rule and gets a 70A 3 pole breaker.
 

Smart $

Esteemed Member
Location
Ohio
As I said, three 7000W single phase inverters on three phase 240V produces 21000W/240Vsqrt3 = 50.5A. 1.25 X 50.5A = 63.14A toward the 120% rule and gets a 70A 3 pole breaker.
That's one way to look at the panel breaker... but it still amounts to (70A + 73A) ÷ 120% = 119A minimum for busbar or feeder falling under the 120% rule. That's absurd for a busbar or feeder that will only carry 63.14A max under nominal conditions (AC combiner).
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
That's one way to look at the panel breaker... but it still amounts to (70A + 73A) ÷ 120% = 119A minimum for busbar or feeder falling under the 120% rule. That's absurd for a busbar or feeder that will only carry 63.14A max under nominal conditions (AC combiner).
Are we back around to considering the breaker in the main panel as contributing to the 120% rule in the AC combiner? No AHJ I have worked in has ever raised that issue, and two of them (Austin and San Antonio) are known for being persnickety about code issues.
 

Smart $

Esteemed Member
Location
Ohio
Are we back around to considering the breaker in the main panel as contributing to the 120% rule in the AC combiner? No AHJ I have worked in has ever raised that issue, and two of them (Austin and San Antonio) are known for being persnickety about code issues.
Code does not exclude that possibility. I don't work in the PV area, so have no idea what actual practice entails across the many AHJ's out there.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I realize you work in PV area, but in how many AHJ's out of all that are out there?
Are you asking me to prove a negative? Of course I cannot prove that no AHJ looks at the 120% rule for PV AC combiners that way. I have, OTOH, submitted commercial PV designs in several jurisdictions in Texas, California, Colorado, and Nevada, and not one of them has ever considered the breaker in the MDP as "feeding" the busbar in an AC combiner and made me size the combiner up to include it in the 120% rule calculation. Not one. It would be a stupid expenditure serving no useful purpose.
 

Smart $

Esteemed Member
Location
Ohio
Are you asking me to prove a negative? Of course I cannot prove that no AHJ looks at the 120% rule for PV AC combiners that way. I have, OTOH, submitted commercial PV designs in several jurisdictions in Texas, California, Colorado, and Nevada, and not one of them has ever considered the breaker in the MDP as "feeding" the busbar in an AC combiner and made me size the combiner up to include it in the 120% rule calculation. Not one. It would be a stupid expenditure serving no useful purpose.
What about the feeder between MDP and AC combiner?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
What about the feeder between MDP and AC combiner?
What about it? It is sized for the current it will carry from the inverters to the service and protected from fault current from the service by OCPD at the MDP. Occasionally the conductors have to be sized up in order for them to be adequately protected by the OCPD but I have never seen that to be more than one size up. We went through all this ad nauseum a year or so ago, didn't we?
 

Carultch

Senior Member
Location
Massachusetts
Not one. It would be a stupid expenditure serving no useful purpose.

If you really are challenged on it, you can also take advantage of the rule that states, "the sum of the breakers excluding the main supply shall not exceed the rating of the busbar". There is a rule somewhere in 705.12(D) that states this.
 

Smart $

Esteemed Member
Location
Ohio
What about it? It is sized for the current it will carry from the inverters to the service and protected from fault current from the service by OCPD at the MDP. Occasionally the conductors have to be sized up in order for them to be adequately protected by the OCPD but I have never seen that to be more than one size up. We went through all this ad nauseum a year or so ago, didn't we?
Well if you don't want to discuss it, fine with me. :bye:
 

Carultch

Senior Member
Location
Massachusetts
Code is not concise enough to exclude a combiner panel from the 120% rule.

The 120% rule is for a mixed use panelboard, that has both loads and PV sources connected to it, with the PV source opposite the main supply.

Since there is no possibility of ever sourcing an AC combining panelboard dedicated to the PV system with more than the sum total of all branch breaker ratings (according to all phase combination rules), there is no reason to apply the 120% rule to this application.
 

Smart $

Esteemed Member
Location
Ohio
The 120% rule is for a mixed use panelboard, that has both loads and PV sources connected to it, with the PV source opposite the main supply.

Since there is no possibility of ever sourcing an AC combining panelboard dedicated to the PV system with more than the sum total of all branch breaker ratings (according to all phase combination rules), there is no reason to apply the 120% rule to this application.
So the utility side isn't a source? UII's won't operate unless there is another source.

Where does Code state that it has to be a mixed-use panelboard?

Under 2014 edition, the PV source breaker at the opposite end is just the option which kicks in the 120% rule. You have two other options [short of engineering supervision] that use either the sum of the breaker ratings or the sum of 125% inverter ratings.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
So the utility side isn't a source? UII's won't operate unless there is another source.

Where does Code state that it has to be a mixed-use panelboard?

Under 2014 edition, the PV source breaker at the opposite end is just the option which kicks in the 120% rule. You have two other options [short of engineering supervision] that use either the sum of the breaker ratings or the sum of 125% inverter ratings.
So what is your point? That everyone designing PV and all the AHJ's approving their designs are wrong? Or do you just want to argue about something with which you have no experience?

I know lots of PV designers. I am one, as you know. Not a single one of us is designing AC combiners and the conductors between them and the point of interconnection such that all the breakers fed by inverters plus the OCPD at the interconnection are less than 120% of the combiner busbar and conductor ratings. Not a single AHJ that I know of has held any of our feet to the fire to make us do this. The crux of the biscuit as I understand it is that either the inverters can feed the panel in normal operation, or the service can in the event of a fault, but not both.

If an AHJ ever calls me on this, I will deal with it. Until then, this is the way it is done as far as I am concerned.
 
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