Breaker Sizing for Inverter Output

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andrew.tkelly

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Hi,

I have a brief question about breaker sizing in a PV system:

For example, a SolarEdge 10kW inverter has an output of 42A at 240V. Since the continuous output of the inverter is limited to 42A, could I use a 45A or 50A OCPD? Or do I still have to multiply it by a factor of 1.25, and use either a 55A or 60A breaker?

Thanks in advance.

-Andy
 
andrew.tkelly said:
Hi,

I have a brief question about breaker sizing in a PV system:

For example, a SolarEdge 10kW inverter has an output of 42A at 240V. Since the continuous output of the inverter is limited to 42A, could I use a 45A or 50A OCPD? Or do I still have to multiply it by a factor of 1.25, and use either a 55A or 60A breaker?

Thanks in advance.

-Andy
Click to expand...
For minimum OCPD you must multiply inverter nameplate current by 1.25 and select the next higher OCPD per 690.9(B) and 690.8(A)(3).
 
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ggunn said:
For minimum OCPD you must multiply inverter nameplate current by 1.25 and select the next higher OCPD per 690.9(B) and 690.8(A)(3).
Click to expand...

Thank you for your prompt answer, ggunn. As an addendum to this question, would you size the wire according to the max. continuous output current w/ the associated derate factors taken into consideration, or would the wire size have to be large enough to handle the amperage limit of the OCPD?

-Andy
 
andrew.tkelly said:
Thank you for your prompt answer, ggunn. As an addendum to this question, would you size the wire according to the max. continuous output current w/ the associated derate factors taken into consideration, or would the wire size have to be large enough to handle the amperage limit of the OCPD?

-Andy
Click to expand...

Both, actually, sort of, but it usually takes care of itself when you do your calculations.

The ampacity of the conductors must be greater than the nameplate maximum current for the inverter (Imax) both as (a) derated for conditions of use (ambient temperature, rooftop heat adder (but that one may be gone in the 2017 NEC), and conduit fill derates) and (b) derated for continuous use (80% of the 75 degree ampacity in most cases).

When you set your OCPD size as the next standard size up from 125% of Imax, the conductors as determined by the above calculations are nearly always adequately protected by the OCPD. To check it, though, compare the OCPD rating with the (a) conditions of use derated ampacity of your conductors. As long as Imax is less than 800A, the (a) ampacity must be greater than the next size down standard OCPD from the one you are using. The (b) ampacity derated for continuous use isn't a factor.

If Imax is 800A or greater, you must compare the actual OCPD rating (not the next one down) to ampacity (a) to determine if the conductors are protected.

It may sound complicated but it's really not.
 
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andrew.tkelly said:
Thank you for your prompt answer, ggunn. As an addendum to this question, would you size the wire according to the max. continuous output current w/ the associated derate factors taken into consideration, or would the wire size have to be large enough to handle the amperage limit of the OCPD?

-Andy
Click to expand...


There are four criteria that need to be met, for it to be sized correctly:
1. With the 125% factor, because solar is treated as a continuous load, the terminations must have enough ampacity for the current. Note that this usually is the 75C column of 310.15(B)(16). For 100A and less, you have a burden of proof that terminals & equipment on both ends be listed and labeled for 75C to use the 75C column instead of the 60C column, otherwise the 60C column is the default. Note that most equipment is, so this rule is usually academic instead of practical. For over 100A, the 75C column is the default.

2. Not including the 125% factor, but just going off the max continuous current itself, the derated wire ampacity must be enough for the current. This may include 310.15(B)(3)(a) bundling, and ambient/rooftop temperature corrections. Note that you usually start with the 90C rating, if you have 90C wire. This is usually the reason you would have a value to 90C wire, even if terminals are 60C or 75C.

3. The termination ampacity must be protected by any required OCPD. See 240.4(B). In many situations, you can round up to the next standard size.

4. Ditto of #3, for the derated wire ampacity.
 
ggunn said:
Yes, but derated for conditions of use only, not continuous use.
Click to expand...

So, given 42A of operating current on a 60A breaker, with #6 Cu wire, that ends up being derated to 48A for conditions of use, is this acceptable? Such a design would be contrary to what I said, because 48A doesn't round up to 60A. It would meet conditions 1, 2, & 3, but not 4. It is my understanding that both the terminal ampacity AND the derated conductor ampacity, have to exceed the previous OCPD, or meet the actual OCPD if greater than 800A.
 
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Carultch said:
So, given 42A of operating current on a 60A breaker, with #6 Cu wire, that ends up being derated to 48A for conditions of use, is this acceptable? Such a design would be contrary to what I said, because 48A doesn't round up to 60A. It would meet conditions 1, 2, & 3, but not 4. It is my understanding that both the terminal ampacity AND the derated conductor ampacity, have to exceed the previous OCPD, or meet the actual OCPD if greater than 800A.
Click to expand...

No, a 60A breaker does not protect #6 COU derated to 48A.

But here's an example:
50A of nameplate inverter current, max ambient temperature 97 degrees F, (2) #6 90 degree CCC's in conduit, not exposed to sunlight on a rooftop.

Min OCPD rating is (1.25)(50A) = 62.5A, "round up" to 70A
COU derated ampacity is (75A)(0.91)(1.0) = 68.3A
CU derated ampacity is (65A)(0.8) = 52A
Both ampacities are > 50A

The conductors are protected because the COU derated ampacity is > 60A. The CU derated ampacity is not a factor.
 
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ggunn said:
No, a 60A breaker does not protect #6 COU derated to 48A.

But here's an example:
50A of nameplate inverter current, max ambient temperature 97 degrees F, (2) #6 90 degree CCC's in conduit, not exposed to sunlight on a rooftop.

Min OCPD rating is (1.25)(50A) = 62.5A, "round up" to 70A
COU derated ampacity is (75A)(0.91)(1.0) = 68.3A
CU derated ampacity is (65A)(0.8) = 52A
Both ampacities are > 50A

The conductors are protected because the COU derated ampacity is > 60A. The CU derated ampacity is not a factor.
Click to expand...

Ok, that makes sense. I see that you've taken some algebraic liberty with your method of thinking about this problem, which initially lost me until I saw what you meant in more detail.
 
Carultch said:
Ok, that makes sense. I see that you've taken some algebraic liberty with your method of thinking about this problem, which initially lost me until I saw what you meant in more detail.
Click to expand...

Well, I use Imax (100% of inverter current or 125% of DC current from modules) as a starting point and compare derated ampacities to Imax rather than working it from the other direction and trying to remember whether to multiply by 1.25 or 1.56, if that's what you mean.
 
I have to say, personally I think that the redundancies between 690 and 705 with respect to inverter outputs could be edited out of the code, and I generally behave as if they have been. It's a lot simpler to just follow 705.60 and .65. Just take 125% of the inverter output and then follow Article 310.

I guess there may be those cases where invoking 690 let's you save a conductor size. But it doesn't make any sense. Those rules were clearly written for PV source and output circuits.
 
ggunn said:
No, a 60A breaker does not protect #6 COU derated to 48A.

But here's an example:
50A of nameplate inverter current, max ambient temperature 97 degrees F, (2) #6 90 degree CCC's in conduit, not exposed to sunlight on a rooftop.

Min OCPD rating is (1.25)(50A) = 62.5A, "round up" to 70A
COU derated ampacity is (75A)(0.91)(1.0) = 68.3A
CU derated ampacity is (65A)(0.8) = 52A
Both ampacities are > 50A

The conductors are protected because the COU derated ampacity is > 60A. The CU derated ampacity is not a factor.
Click to expand...

Thanks for the all the input, everyone. Regarding the quoted bit above..

Questions: What does COU and CU mean? I am guessing CU does not mean "copper" here. Also, why does it start off as 75A being derated in line #2, but then it's 65A being derated in line #3?

I'm on my way to understanding! -Andy
 
andrew.tkelly said:
Thanks for the all the input, everyone. Regarding the quoted bit above..

Questions: What does COU and CU mean? I am guessing CU does not mean "copper" here. Also, why does it start off as 75A being derated in line #2, but then it's 65A being derated in line #3?

I'm on my way to understanding! -Andy
Click to expand...

In this context, ggunn is using COU to mean conditions of use, and CU to mean continuous use.

75A is the 90C rating of #6 Cu wire.
65A is the 75C rating of #6 Cu wire.

Terminations are most often rated for 75C. There is a burden of proof to have products on both sides listed and marked 75C for 100A and less, which most products are. Even if you have 90C wire, you cannot take credit for that in the calculation for line#3. You can take credit for that as a starting point for the temperature correction and bundling adjustment calculations, but the final ampacity of the system cannot be more than the unmodified 75C rating. For continuous loads specifically, you need the 125% continuous use safety factor to apply to breaker sizing and termination ratings.
 
andrew.tkelly said:
Thanks for the all the input, everyone. Regarding the quoted bit above..

Questions: What does COU and CU mean? I am guessing CU does not mean "copper" here. Also, why does it start off as 75A being derated in line #2, but then it's 65A being derated in line #3?

I'm on my way to understanding! -Andy
Click to expand...

What Carultch said: CU = Continuous Use, COU = Conditions Of Use. Sorry for my untranslated shorthand. If you are using 90 degree #6, 75A is the 90 degree ampacity and 65A is the 75 degree ampacity. The first you use to protect the 90 degree wire insulation and the second you use to protect the 75 degree terminals, which most terminals are that we use in PV design.
 
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