690.8 or 125% Twice?

Status
Not open for further replies.
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Can anyone explain why 690.8 requires the conductors from a solar array to be sized at 156% of the short circuit current?
 
Last edited:
Pete Benoit said:
This is based on a 1989 revision to UL standard 1703 and allows for peak irradiance and colder temps
Click to expand...

Not sure I understand all that but it sure sounds like you know what your talking about. Thanks. :smile:


So I assume this means that the short circuit rating given on the panels label is not considered accurate?
 
iwire said:
Not sure I understand all that but it sure sounds like you know what your talking about.
Click to expand...
If you can't dazzle them with brilliance, baffle them with . . . well, you know. :cool:
 
Pete Benoit said:
From what I understand the panel can produce more than its 100% rating with cloud or snow enhancement.
Click to expand...
Then wouldn't that higher number be the 100% rating? :-?
 
LarryFine said:
Then wouldn't that higher number be the 100% rating? :-?
Click to expand...
Why are continuous loads calculated at 125%?
Why is single phase alternating current called that?

You see Little Timmy, a long time ago, someone way smarter than you and I will ever be made a decision...

the key to great bull hockey is to make it believable...:grin: :roll:

Your best bet is probably to just accept it and move on. Or, as someone once said, "Just shut up and put that freaking switch in."
 
LarryFine said:
Then wouldn't that higher number be the 100% rating? :-?
Click to expand...
The nameplate info is established using standard test conditions set by UL,the increased currents caused by solar irradiance,cold temps are considered by using the additional 1.25% This is similar to breakers being tested at 40c and having a lower or higher rating at a different temp
 
STC information etc.

STC information etc.

Pete gets the gold star here...

STC is ran at:
Irradiance level = 1,000 W/m?,
Air mass = 1.5 solar spectral irradiance distribution
Cell temperature = 25?C.

So the info on the panel shows the data taken at those parameters but things like indirect light (from white surfaces), colder temperatures, wind blowing across the panel (cooling it) etc. are not considered.

Don't forget that these conductors are also going to be on or by a very hot surface all day every day. :-x

Climates vary significantly and that is the reason the STC may not show the "actual" max power, SC current etc.

The 1.5625 (1.25*2) is in there to offset this.

At least that's my interpretation of it... I could always be completely wrong ;)
 
Well the job that prompted me to ask these questions is progressing.

If anyones interested here is a link to some video. It is a .mov file so you may need quicktime to view.

I recommend using a right click and save as. It downloads much faster that way.

Solar Project

You also get to see I look nothing like Eastwood. :smile:
 
Last edited:
iwire said:
Well the job that prompted me to ask these questions is progressing.

If anyones interested here is a link to some video. It is a .mov file so you may need quicktime to view.

Solar Project

You also get to see I look nothing like Eastwood. :smile:
Click to expand...

Thanks a lot Bob. You owe me a new computer screen now. It cracked and exploded when they cut to you in the video.



:D

Cool project. :cool:
 
Last edited:
iwire said:
Can anyone explain why 690.8 requires the conductors from a solar array to be sized at 156% of the short circuit current?
Click to expand...

This is my first post, so please be kind. :smile: Also, I'm not a certified electrician, I'm just doing a huge amount of research for solar PV installation because I'm getting a system for my home. I'm basically designing it myself, and have a real electrician overseeing my ideas so I don't get in too much trouble.

I think I can actually help answer that question.

The 156% is usually easiest as 1.25 * 1.25 (= 1.56), because 690.8(A)1 requires the "maximum current," which is then calculated with NEC Table 690.7 Voltage Correction Factors, the maximum "maximum current" being 125% of the rated short circuit current. But that is for -5 to -40 degree F. Where I live in Central Florida, we haven't seen -40 or even -5, so I could use the 114% factor for 31 to 14 degree F. (The record low here was 18 F in 1962)

And one other interesting note, it seems that for the NEC 2005 the factor was 113% (1.13) but it changed for the NEC 2008, which is more detailed regarding temperature. So now the temperature becomes 22 to 14 degree F as 114% (1.14). And for the NEC 2008 the new temperature for the 125% becomes -32 to -40F, temperatures which most of the U.S. won't ever see.
 
sub3, you seem to be conflating voltage and current adjustments.

NEC Table 690.7 deals with voltage as affected by temperature only. This is important for determining the DC input voltages on your inverter.

NEC 690.8 defines currents for a PV system at different points. For PV modules, continuous currents under foreseeable environmental conditions (e.g. snow reflectance, amplification of solar radiation from water vapor, etc.) could be higher than what the specs say. It's important to note that current varies very little with temperature. This is the reasoning behind the first 1.25 factor (690.8 (A)(1)). The 2005/8 handbook gives a short explanation for this 1.25 factor, too.

The second 1.25 factor (690.8 (B)(1)) is your standard safety factor when sizing conductors.

You must always use 1.56 (1.25*1.25) in your calculations for conductor ampacities and OCPDs for PV module source circuit currents.
 
Wow, that is interesting. I'm not going to debate the answer, because reading the title closely for Table 690.7 does only say for voltage.

However, there is a reason I have made this mistake then.

Quoting from a previous link in this thread: http://solar.wiseowlnetworks.us/Solar-Math.html

"Each group of panels is a 'PV Source Circuit'. My panels are wired in parallel (two per circuit); the math runs like this:
[NEC 690.8(A)(1)]

the sum of all the Isc numbers in the circuit, multiplied by the weather correction factor equals the 'PV Source Circuit Current'
3.65A + 3.65A = 7.3A times 1.17 = 8.541A
The PV Source Circuit Current is then multiplied "because it's solar" by 1.25. 8.249A times 1.25 = 10.67A. This is the maximum possible current that each circuit could possibly produce. If I selected only a 10 amp fuse for this circuit, there is a strong possibility I'd have to replace fuses pretty often in the winter time, especially on bright sunny days with lots of snow reflecting even more light onto the solar panels.
[NEC 690.8(B)(1)] "

So does that mean this person's math is incorrect? From NEC 690.8(A)(1) in the 2005 NEC book it only says "multiplied by 125 percent."
 
Status
Not open for further replies.
Top