Inverter Output Circuit conductor ampacity

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Smart $

Esteemed Member
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Here's a more interesting example, that shows why these subtle rules matter:

Inverter Imax = 38A
Conditions of use derate factor: 0.82
(1.25)(38A) = 47.5A
Choose 50A OCPD

Start with #8 THWN-2.

90 degree ampacity 55A, conditions of use derated at 0.82 (direct sunlight) to 45.1A
45.1A > 38A


75 degree ampacity 50A, required to equal 125% of 38A due to a 38A continuos load. 50A > 47.5A.

45.1A conductor ampacity > 45A
50A termination ampacity > 45A
Therefore it is protected by the 50A OCPD, since 45A is the previous standard OCPD rating.

The good question to ask is, is 45.1A greater than 45A "enough", in order to be protected by a 50A OCPD? And how would you know?
Please note the conductor ampacity must be the larger of 690.8(B)(1) or (B)(2)... which for a 38A output rating is:
1) 125%*38A=47.5A
2) 38A/0.82=46.3
47.5A > 46.3A

#8 THWN-2 is too small to be compliant in this scenario because its derated ampacity is less than required per 690.8(B).
 

Carultch

Senior Member
Location
Massachusetts
Please note the conductor ampacity must be the larger of 690.8(B)(1) or (B)(2)... which for a 38A output rating is:
1) 125%*38A=47.5A
2) 38A/0.82=46.3
47.5A > 46.3A

#8 THWN-2 is too small to be compliant in this scenario because its derated ampacity is less than required per 690.8(B).

I'm confused. Are you supposed to double-dip on the derate factors? Because that is what you are doing.

I always thought they meant the conductor ampacity directly from the table, needs to be greater than either Imax*1.25, or Imax/derate.

It would intuitively make sense to me, if Imax*1.25 had to be less than or equal to both derated conductor ampacity, and non-derated termination ampacity. But the NEC is specifically worded so that this isn't the case. The first part tells you Imax*1.25 needs to be less than or equal to conductor ampacity (and termination ampacity implied by 110.14(C)) before conditions of use derate factors. The second part tells you that 100% of Imax needs to be less than or equal to conductor ampacity after applying conditions of use derate factors.
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Inverter Imax = 38A
OCPD minimum (1.25)(38A) = 47.5A
Choose 50A OCPD


Conductor Ampacity
690.8(B)(1): (38A)(125%) = 47.5
690.8(B)(2): (38A) (55/50.1*) = 41.7A, *You didn't state what the adjustment and correction factors, only the result.
690.8(B): 47.5A is the larger of the two

Choose #8 THWN-2

Verify 240.4(B) compliance:
90°C ampacity 55A, conditions of use derated to 50.1A, no continuous factoring
• 50.1A > 47.5A
75°C ampacity 50A, 125% continuous, no derating
• 50A > 47.5A
50A > 45A, so the wire is protected by the OCPD (NOTE: conductor ampacity cannot exceed termination temperature limitation ampacity; OCPD is not rated for over 75°C operation)

The conditions of use derate I used was 0.91. It's for 2% ASHRAE maximum ambient temperature of 99 degrees F (typical to my location in Texas), not in direct sunlight on a roof (no adder), and 2 CCC's in conduit.

I only use 1.25 X Imax to place me on Table310(B)(16) for my first approximation of wire size and to determine OCPD; from there on I derate the conductors appropriately and compare the ampacities to Imax. The only difference between the AC and DC sides is that AC Imax is equal to the max output current of the inverter and the DC Imax is 1.25 X Isc. As long as both derated ampacities exceed Imax and the conditions of use 90 degree ampacity exceeds the next rating down from the OCPD I am using, the conductor size is adequate.
 
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Smart $

Esteemed Member
Location
Ohio
I'm confused. Are you supposed to double-dip on the derate factors? Because that is what you are doing.
...
I agree, and perhaps I did because I was trying to mix my method into yours, albeit originally ggunn's. Sometimes it's better if I just do the numbers from scratch myself, without mixing in other's methods.

Per 690.8(B)...
1) 125%*38A=47.5A
2) 38A/0.82=46.3
47.5A > 46.3A
47.5A is the required conductor ampacity.

All we have to do is compare this number to the Table values. And that permits a #8 THWN-2, but I must say this is a bit different than making the same type of determination under Chapter 2.

Does that make you happy...? :D
 

Carultch

Senior Member
Location
Massachusetts
I agree, and perhaps I did because I was trying to mix my method into yours, albeit originally ggunn's. Sometimes it's better if I just do the numbers from scratch myself, without mixing in other's methods.

Per 690.8(B)...47.5A is the required conductor ampacity.

All we have to do is compare this number to the Table values. And that permits a #8 THWN-2, but I must say this is a bit different than making the same type of determination under Chapter 2.

Does that make you happy...? :D

Thanks for confirming.

However, the reason why I even mentioned chapter 2 in the first place, is that there used to be (NEC2011) a requirement to have the OCPD protect the wire, where requried. This is absent in NEC2014 from Article 690, although it can be understood to be implied by Article 240. The first four chapters apply to everything.
 

Smart $

Esteemed Member
Location
Ohio
Thanks for confirming.

However, the reason why I even mentioned chapter 2 in the first place, is that there used to be (NEC2011) a requirement to have the OCPD protect the wire, where requried. This is absent in NEC2014 from Article 690, although it can be understood to be implied by Article 240. The first four chapters apply to everything.
That is correct. There is nothing in 690 to negate that.

Imax @ 38A, 50A OCPD, and #8 THWN-2 is a poor example of where there'd be an issue.
 
I understand it that both the derated 90C ampacity for conditions of use, and the non-derated 75C (or other if applicable) terminal ampacity that must exceed the previous size down OCPD. And per NEC rounding conventions, I believe it should exceed it by a half an amp greater than the previous size (I've wondered just how close it can be for a long time, and only recently learned this rounding convention).

The intent is that the OCPD is the "weak link" in the chain, so that it trips before the wire or terminations overheat.

In the case of the PV output circuit, do we take the sum of the source circuit series fuse ratings as the OCPD? Is there a need to have a fuse at the inverter end of the conductor?
 

Carultch

Senior Member
Location
Massachusetts
In the case of the PV output circuit, do we take the sum of the source circuit series fuse ratings as the OCPD? Is there a need to have a fuse at the inverter end of the conductor?

In the case when an overcurrent protection device is not needed, you don't even need to think about OCPDs governing the size of the wire. 1.56*total ISC (without derates) and 1.25*total ISC (with derates) are the only current figures that govern the ampacity. In such a case, it is 1.25*total Isc that is used as a working value for sizing the EGC from table 250.122 per 690.45 (2014 rule).

Examples are a single combiner feeding an inverter, or two identically sourced combiners feeding the same inverter, and "paralleled" without fuses or breakers.

When you do have an OCPD required in this position, is a large central inverter that parallels 3 or more combiner DC feeders on its input.
 
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GoldDigger

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Location
Placerville, CA, USA
Occupation
Retired PV System Designer
As of the 2014 NEC I think it is clear that you use the sum of the panel nominal output ratings with specified multipliers rather than the sum of the string fuse ratings to determine the combiner output.
 
In the case when an overcurrent protection device is not needed, you don't even need to think about OCPDs governing the size of the wire. 1.56*total ISC (without derates) and 1.25*total ISC (with derates) are the only current figures that govern the ampacity. In such a case, it is 1.25*total Isc that is used as a working value for sizing the EGC from table 250.122 per 690.45 (2014 rule).

Examples are a single combiner feeding an inverter, or two identically sourced combiners feeding the same inverter, and "paralleled" without fuses or breakers.

When you do have an OCPD required in this position, is a large central inverter that parallels 3 or more combiner DC feeders on its input.

Here's a case in a residential system where OCPD is required but I don't know if it applies to the PV output circuit. Using 2011 NEC:
PV output circuit in conduit combining three source circuits, 15A series fuses, Isc = 9.23A. Assuming #8 AWG Cu
690.8(B)(2)(a) 1.25 X 1.25 X 9.23A X 3 = 43.27A. 75° ampacity of #8 AWG = 50A -> #8 AWG sufficient
690.8(B)(2)(b) 1.25 X 9.23A X 3 = 34.61A. 90° ampacity of #8 AWG = 55A. Conditions of use factor for temperature = .71. Derated ampacity = 55A X .71 = 39.05A -> #8 AWG sufficient
690.8(B)(2)(c) Protected by OCPD after application of conditions of use: If the OCPD is considered the sum of the three 15A string fuses, 45A, then the derated ampacity of 39.05A < 45A, then the #8 AWG is not sufficient and #6 is required.

In a literal interpretation of 690.8(B)(2)(c) the output circuit is protected by the three 15A series fuses and #8 AWG would be required, although they are intended to protect the source circuits and modules. Do we apply it to sizing the output circuit conductors?
 
As of the 2014 NEC I think it is clear that you use the sum of the panel nominal output ratings with specified multipliers rather than the sum of the string fuse ratings to determine the combiner output.

I see in 2014 NEC 690.9(A) "Circuits, either ac or dc, connected to current-limited supplies (e.g. PV modules, ac output of utility-interactive inverters), and also connected to sources having significantly higher current availabiilty (e.g., parallel strings of modules, utility power), shall be protected at the source from overcurrent."

Is this saying that if only current-limited supplies, i.e. modules, are connected then the output circuit doesn't have to be sized to the source circuit OCPD? It wasn't clear to me, but it seems to imply it and it would be reasonable.

Then what do we do with designs under 2011 NEC?
 

Carultch

Senior Member
Location
Massachusetts
Here's a case in a residential system where OCPD is required but I don't know if it applies to the PV output circuit. Using 2011 NEC:
PV output circuit in conduit combining three source circuits, 15A series fuses, Isc = 9.23A. Assuming #8 AWG Cu
690.8(B)(2)(a) 1.25 X 1.25 X 9.23A X 3 = 43.27A. 75° ampacity of #8 AWG = 50A -> #8 AWG sufficient
690.8(B)(2)(b) 1.25 X 9.23A X 3 = 34.61A. 90° ampacity of #8 AWG = 55A. Conditions of use factor for temperature = .71. Derated ampacity = 55A X .71 = 39.05A -> #8 AWG sufficient
690.8(B)(2)(c) Protected by OCPD after application of conditions of use: If the OCPD is considered the sum of the three 15A string fuses, 45A, then the derated ampacity of 39.05A < 45A, then the #8 AWG is not sufficient and #6 is required.

In a literal interpretation of 690.8(B)(2)(c) the output circuit is protected by the three 15A series fuses and #8 AWG would be required, although they are intended to protect the source circuits and modules. Do we apply it to sizing the output circuit conductors?

The three string fuses are required for protecting the individual strings. Particularly, from two properly operating source circuits feeding a faulted source circuit. Not the combined DC feeder circuit, which doesn't require a master OCPD. In this instance, #8 is correct.
 
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GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I see in 2014 NEC 690.9(A) "Circuits, either ac or dc, connected to current-limited supplies (e.g. PV modules, ac output of utility-interactive inverters), and also connected to sources having significantly higher current availabiilty (e.g., parallel strings of modules, utility power), shall be protected at the source from overcurrent."

Is this saying that if only current-limited supplies, i.e. modules, are connected then the output circuit doesn't have to be sized to the source circuit OCPD? It wasn't clear to me, but it seems to imply it and it would be reasonable.

Then what do we do with designs under 2011 NEC?
Yes, it is specific to current limited power sources.
And under 2011 you may not have the explicit mention but IMHO the same guiding principle can be applled.
 
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