PV WIRE AMPACITY

[Smart $]

It makes no difference whether you multiply current by 1.25 or multiply ampacity by 0.8; you get to the same place. It's a distinction without a difference.

All my electrical plans show compliance to code this way, and AHJ plan reviewers have never questioned it

But the requirements of 240.4 are based on conductor ampacity (MOC x 125%) not maximum output current (MOC) [<(ampacity x 0.8)].

 

[ggunn]

But the requirements of 240.4 are based on conductor ampacity (MOC x 125%) not maximum output current (MOC) [<(ampacity x 0.8)].

So what? It gets to the same correct answer. At any rate, back to my original point, which is that none of this, whatever you call it or however you get to the bottom line, is a consideration in determining if a conductor is protected by an OCPD. The conditions of use derated ampacity must be equal to or greater than the next size down device.

 

[Smart $]

So what? It gets to the same correct answer. At any rate, back to my original point, which is that none of this, whatever you call it or however you get to the bottom line, is a consideration in determining if a conductor is protected by an OCPD. The conditions of use derated ampacity must be equal to or greater than the next size down device.

There's no point in beating the dead horse. I disagree... period.

 

[ggunn]

There's no point in beating the dead horse. I disagree... period.

Thank you for the period.

 

[Zee]

so now I just want to verify how, at the end of above 90C calculations, how EXACTLY the 75C terminal rating calculation (comparison?) works.
With 2 real examples. One for DC and one for AC.
Can someone write down the MATH in simple math, not words?


DC: (same from above)
(CU) Optimizer Output: 15.00 x 1.25 = 18.75 A
(COU) Derated Conduct.: 40A (10AWG in 90C column) x 0.76 (ambient Temp on roof) x 0.80 (4 CCC in conduit) = 24.32 A
18.75 A < 24.32 A ===> Wire Sizes OK
What do i do at this point to ensure the terminal ratings are not exceeded?

AC: (7600 W inverter output circuit)
Wire Sizing After Inverter
Inverter Output: 32.00A x 1.25 = 40.00 A
Derated Conduct.: 55A (8AWG at 90C) x 0.91 (Ambient T) x 1.00 (2 CCC's in conduit only)= 50.05 A
40.00 A < 50.05 A Wire Sizes OK
What do i do at this point to ensure the terminal ratings are not exceeded? ( i know here it is fine, but in theory)

 

[Carultch]

so now I just want to verify how, at the end of above 90C calculations, how EXACTLY the 75C terminal rating calculation (comparison?) works.
With 2 real examples. One for DC and one for AC.
Can someone write down the MATH in simple math, not words?


DC: (same from above)
(CU) Optimizer Output: 15.00 x 1.25 = 18.75 A
(COU) Derated Conduct.: 40A (10AWG in 90C column) x 0.76 (ambient Temp on roof) x 0.80 (4 CCC in conduit) = 24.32 A
18.75 A < 24.32 A ===> Wire Sizes OK
What do i do at this point to ensure the terminal ratings are not exceeded?

AC: (7600 W inverter output circuit)
Wire Sizing After Inverter
Inverter Output: 32.00A x 1.25 = 40.00 A
Derated Conduct.: 55A (8AWG at 90C) x 0.91 (Ambient T) x 1.00 (2 CCC's in conduit only)= 50.05 A
40.00 A < 50.05 A Wire Sizes OK
What do i do at this point to ensure the terminal ratings are not exceeded? ( i know here it is fine, but in theory)

In this particular example, verify the following.

DC:
1. 18.75A (125% max continuous current) must be less than or equal to 35A (75C terminal rating of #10 Cu wire).
2. 15A (100% max continuous current) must be less than 24.32A (derated 90C wire ampacity of #10 Cu wire)

Is an OCPD required? If not, we are done. Otherwise...

If yes, we'll assume a 20A OCPD.
3. Verify that terminal ampacity of 35A > 15A, and not an exact match.
4. And that derated conductor ampacity of 24.32A > 15A, and not an exact match.
Note that 15A is the previous standard size OCPD.

AC:
1. 40A (125% max continuous current) must be less than or equal to 50A (75C terminal rating of #8 Cu wire).
2. 32A (100% max continuous current) must be less than or equal to 50.5A (derated 90C wire ampacity of #8 Cu wire).
3. Verify that terminal ampacity of 50A > 35A, and not an exact match.
4. Verify that derated conductor ampacity of 50.5A > 35A, and not an exact match.
Note that 35A is the previous standard size OCPD.


In this particular example, the OCPD is far from being a factor to driving wire size, since the other rules already take care of it.

 

[Smart $]

In this particular example...

This is the point of contention I've had with ggunn... and considering 690.8(B) I see why he does it like he does... and cannot fault him for that. If he's still reading this thread, I retract my 'period' and offer my apology for being errant and overly adamant.

The problem is in the way 690.8(B) is written, same as precursory requirements of 210.19(A)(1) and 215.2(A)(1). It goes back to the 2011 edition, and I have not revisited the issue in detail because I thought the 2014 and 2017, though a few words have changed, had the same result. They do not, and I believe it is a blunder on the CMPs' and TCC's part.

In summary, the 2011 edition requirements for these sections and including 110.14(C) was that the maximum circuit ampacity was the lesser of the 75°C table value or the derated insulation temperature rating (e.g. from the 90° column) for the conductor size. Note in the 2011, there is no (a) or (b) to 210.19(A)(1) or 215.2(A)(1), and also note specifically 690.8(B)(1)(b)...

(b) Terminal temperature limits shall be in accordance
with 110.3(B) and 110.14(C).

Enter 2014 and the ampacity requirements of 210.19(A)(1) and 215.2(A)(1) now state the greater of (a) or (b) and 690.8(B)(1) of 2011 was completely eliminated. So circuit ampacity is now the greater of the 75°C table value or the derated insulation temperature rating (e.g. from the 90° column) for the conductor size.

The logic of 2011 ampacity requirements correlated with the main statement of 110.14(C)...

(C) Temperature Limitations. The temperature rating associatedwith the ampacity of a conductor shall be selected
and coordinated so as not to exceed the lowest temperature
rating of any connected termination, conductor, or device.
Conductors with temperature ratings higher than specified
for terminations shall be permitted to be used for ampacity
adjustment, correction, or both.

Now if we go by my earlier statement about 210.19, 215.2, and 690.8 do not specify which column to use, they also do not specify how they coordinate with 110.14(C)... so we could (and should) conclude that 210.19(A)(1), 215.2(A)(1), and 690.8(B)(1) do not ovverride the general statement of 110.14(C) and cap the circuit ampacity at the terminal temperature limitation for the wire size used.

That's my story and I'm sticking to it.


(As an exercise, reevaluate based on this prenise.)

 

[Smart $]

Two more arguments in favor of my point...

110.14(C)(1)(a)(2)...

(2) Conductors with higher temperature ratings, provided the
ampacity of such conductors is determined based on the
60°C (140°F) ampacity of the conductor size used.

...and 110.14(C)(1)(b)(2)...

(2) Conductors with higher temperature ratings, provided
the ampacity of such conductors does not exceed the
75°C (167°F) ampacity of the conductor size used, or
up to their ampacity if the equipment is listed and identified
for use with such conductors

One issue which may come to bear is that nothing in 110.14(C) says the terminal temperature limitation shall be based on noncontinuous load plus 125% of the continuous load. On face value of the text, the value to use for comparison is the calculated load (or the ampacity determined for the wire size used must be considered protected by the OCPD?).

This is why I said how to properly implement this process is part of the 'unwritten' Code. I had to do extensive research in the past to formulate my premise.

 

[Smart $]

You guys might find this public input for 2020 interesting...

Public Input No. 1959-NFPA 70-2017 [ Section No. 690.8(B) ]
(B) Conductor Ampacity.
PV system currents shall be considered to be continuous. Circuit conductors shall be sized to carry
not less than the larger of
690.8(B)(1) or (B)(2) or where protected by a listed adjustable electronic overcurrent protective
device in accordance 690.9(B)(3), not less than the current in 690.8(B)(3).
(1) Before Application of Adjustment and Correction Factors.
One hundred twenty-five percent of the maximum currents calculated in 690.8(A) before the
application of adjustment and correction factors, in accordance with 110 .14(C).
Exception: Circuits containing an assembly, together with its overcurrent device(s), that is listed
for continuous operation at 100 percent of its rating shall be permitted to be used at 100 percent
of its rating.
(2) After Application of Adjustment and Correction Factors.
The maximum currents calculated in 690.8(A) after the application of adjustment and correction
factors, in accordance with 310 .15(B).
(3) Adjustable Electronic Overcurrent Protective Device.
The rating or setting of an adjustable electronic overcurrent protective device installed in
accordance with 240.6.
Statement of Problem and Substantiation for Public Input
Adding reference to 110.14(C) and 310.15(B) will help the user better apply the requirements.
Submitter Information Verification
Submitter Full Name: Mike Holt


Organization: Mike Holt Enterprises Inc
Street Address: City:
State: Zip:
Submittal Date:
Wed Aug 09 12:03:08 EDT 2017

The downside is he didn't catch the "larger".

 

[ggunn]

This is the point of contention I've had with ggunn... and considering 690.8(B) I see why he does it like he does... and cannot fault him for that. If he's still reading this thread, I retract my 'period' and offer my apology for being errant and overly adamant.

Thanks for that. I submit that in determining wire sizes for PV systems there is no difference between saying that 125% of the maximum inverter current must be equal to or less than the 75 degree ampacity of the conductor and saying that 80% of the 75 degree ampacity of the conductor must be equal to or greater than the maximum inverter current. I teach classes in PV design from time to time; I find that this approach is much more easily understood by my students and gets them to the same answer.

The product of my daily work is the design of PV systems and I am, of course, very concerned about building safe and compliant systems; if you can show me where the way I prefer to get to the bottom line can result in an unsafe or noncompliant design, please do so. Seriously. Otherwise we can agree to disagree and get on with our lives. Live long and prosper.

 

[Smart $]

Thanks for that. I submit that in determining wire sizes for PV systems there is no difference between saying that 125% of the maximum inverter current must be equal to or less than the 75 degree ampacity of the conductor and saying that 80% of the 75 degree ampacity of the conductor must be equal to or greater than the maximum inverter current. I teach classes in PV design from time to time; I find that this approach is much more easily understood by my students and gets them to the same answer.

The product of my daily work is the design of PV systems and I am, of course, very concerned about building safe and compliant systems; if you can show me where the way I prefer to get to the bottom line can result in an unsafe or noncompliant design, please do so. Seriously. Otherwise we can agree to disagree and get on with our lives. Live long and prosper.

The difference is the ampacity for which the conductors are considered protected by the OCPD.

The current biggest problem is that the respective articles for determining ampacity permit the larger of the two determinations. It formerly was that we had to comply with both... which amounted to the lesser of the two.

In many cases it won't make a difference, but I know there is just that one combination (or more) which it will. I would have to go through many iterations of the determination to figure out the particular combination(s). I'm not prepared to invest that much time to prove my point. :happyno:

 

[wwhitney]

In summary, the 2011 edition requirements for these sections and including 110.14(C) was that the maximum circuit ampacity was the lesser of the 75°C table value or the derated insulation temperature rating (e.g. from the 90° column) for the conductor size. [. . .]

Enter 2014 and the ampacity requirements of 210.19(A)(1) and 215.2(A)(1) now state the greater of (a) or (b) and 690.8(B)(1) of 2011 was completely eliminated. So circuit ampacity is now the greater of the 75°C table value or the derated insulation temperature rating (e.g. from the 90° column) for the conductor size.

I think you are misreading the 2014 language. 210.19(A)(1) says "Conductors shall be sized to carry not less than the larger of 210.19(A)(1)(a) or (b)." That means you calculate the minimum conductor size for (a), calculate the minimum conductor size for (b), and pick the larger of the two as your minimum size. For a fixed conductor size, that still means the ampacity is the lesser of (a) and (b).

Cheers, Wayne

 

[Smart $]

I think you are misreading the 2014 language. 210.19(A)(1) says "Conductors shall be sized to carry not less than the larger of 210.19(A)(1)(a) or (b)." That means you calculate the minimum conductor size for (a), calculate the minimum conductor size for (b), and pick the larger of the two as your minimum size. For a fixed conductor size, that still means the ampacity is the lesser of (a) and (b).

Cheers, Wayne

Perhaps you are correct. I still prefer the original writing back in 2011 where you just had to comply with both. No comparison involved. Nevertheless, must be having a bad week as far as interpretations go.

A problem with the requirements is still they do not say which column you must use. And there's no indication what the circuit ampacity actually is, the value that must be protected by the OCPD under 240.4.

There are several PI's for 2020 on this matteer. A guy from Eaton is actually proposing a table for 110.14(C) so the requirement is a lot less ambiguous than using Table 310.15(B)(16) for something other than what it's header states. Mike Holt has PI's in for 210.19 and 215.2, perhaps other, similar to the one I posted above for 690.8. I had proposed essentially the same previously and got rejected. We'll see how much clout is given when Mike makes the proposal. :blink: