HeatSpring.com Advanced PV Course Q and A

Q:

Sean,

I think 690.8 is a little confusing because I usually think of the "Conditions of use" derates applying to the conductor ampacity, and the Maximum Circuit Current applies to source circuit output. For garden-variety strings, am I understanding the application of 690.8(B)(1) and 690.8(B)(2) correctly?

For 690.8(B)(1),

Minimum Conductor Ampacity

= [Module Isc] * 156%

(Roof temperatures 50C or less AND 3 or less Conductors)

For 690.8(B)(2),

Minimum Conductor Ampacity

= [Module Isc] * 125% / [Temp Derate Factor]

The roof temperature cutoff is based on 310.15(B)(2)(a)/THWN-2 where at 50C, the derate changes from 0.82 to 0.76 (The inverse of 0.8 being 125%).

Thanks

A:

You think 690.8(A)(1) and 690.8(B)(1) are only a little confusing? How about a lot! In fact, they should put an informational note there that says:

Informational note: This is really confusing, so if you think you get it right away, try again.

There is already an informational note in 690.8(A)(1) that refers to 690.8(B)(1) before you even read 690.8(B)(1).

Let’s just read the whole thing here:

690.8 **Circuit Sizing and Current**

(A) **Calculation of Maximum Circuit Current.** The maximum circuit current for the specific circuit shall be calculated in accordance with 690.8(A)(1) through (A)(5)

(1) **Photovoltaic Source Circuit Currents. **The Maximum current shall be the sum of the parallel module rated short-circuit currents multiplied by 125%

Informational Note: When the requirements of 690.8(A)(1) and 690.8(B)(1) are both applied, the resulting multiplication factor is 156%.

(B) **Conductor Ampacity**. PV System currents shall be considered to be continuous. Circuit conductors shall sized to carry not less than the larger of 690.8(B)(1) or (2)

(1) One hundred and twenty-five percent of the maximum currents calculated in 690.8(A) before the application of adjustment and correction factors.

(2) The maximum currents calculated in 690.8(A) after the application of adjustment and correction factors.

Sean’s comments: You have to remember here that the people who write the code are super smart dedicated engineers who contemplate this stuff 24/7. The thing is that most people applying the Code are normal people that speak English. My duty here is to be an interpreter and to interpret Code writer language to English, while making fun of everyone in the process.

Ok, now let’s tear this stuff up one step at a time…

Code:

690.8 **Circuit Sizing and Current**

(A) **Calculation of Maximum Circuit Current.** The maximum circuit current for the specific circuit shall be calculated in accordance with 690.8(A)(1) through (A)(5)

(1) **Photovoltaic Source Circuit Currents. **The Maximum current shall be the sum of the parallel module rated short-circuit currents multiplied by 125%

Sean:

690.8(A)(1) is telling us that the definition of “maximum circuit current” is Isc x 1.25.

Here are some of the confusing things here:

1-Why don’t they just say multiplied by 1.25 instead of multiplied by 125%? Who invented the percent system should have never been born. They have made people make mistakes moving the decimal point 2 places for generations. Probably many people have died as a result of improper movement of the decimal point.

2-Why is maximum circuit current based on a short circuit? Hopefully we never have a short circuit in our system ever. They do this because Imp (maximum power current) is very close to Isc (short circuit current) and they are being safe, because reflections and increased irradiance can lead to greater than module testing STC solar power (peak sun irradiance). Since Isc is about 7% more than Imp, maybe they should make it based on Imp x 1.32 or Imp x 1.33. Anyway, this is what we have and our definition of current as far as wire sizing goes for PV source circuits is Isc x 1.25. This is the wire sizing equivalent of what current comes out of the device. For instance maximum circuit current of the inverter is just the inverter output current. PV is unpredictable as the brightness of a sunny day next to a parabolic reflective glacier on a crisp sunny clear day where snow blind is more than an Ozzy song from Jr. High School.

3-This part is weird:

“**Photovoltaic Source Circuit Currents. **The Maximum current shall be the sum of the parallel module rated short-circuit currents”

The sum of parallel module what? Isn’t a string a series thing? I would just advise you to ignore the parallel thing, but there are some weird unusual things that most solar installers have never seen and will probably never ever see, such as a parallel source circuit. Unicorns are more common. Unicorns are the main installers of parallel source circuits.

So, have we torn up 690.8(A)(1) enough? Just to clarify:

Isc x 1.25 is the definition of current for wire sizing purposes.

Now on to 690.8(B)(2)…

Code:

(B) **Conductor Ampacity**. PV System currents shall be considered to be continuous. Circuit conductors shall sized to carry not less than the larger of 690.8(B)(1) or (2)

(1) One hundred and twenty-five percent of the maximum currents calculated in 690.8(A) before the application of adjustment and correction factors.

(2) The maximum currents calculated in 690.8(A) after the application of adjustment and correction factors.

Sean:

That is nice that PV circuits are considered to be continuous and the NEC definition of continuous is 3 hours. Too bad our solar systems are not really continuous and go 24/7, and then we would make a lot more energy. Space Based Solar Power (SBSP) is continuous and cool.

690.8(B) is just telling us that we have 2 ways of looking at wire sizing here and we pick the biggest wire. In other words, we do not stack 690.8(B)(1) and 690.8(B)(2), we look at them both independently and then pick the biggest wire.

690.8(B)(1) is going to be Isc x 1.25 x 1.25 or we can just say Isc x 1.56.

Sean’s informational note: When we do this step, we will be looking at the ampacity table of 310.15(B)(16) or 310.15(B)(17) in the column of the temperature rating of the conductor or the terminals, whichever is less and usually the conductor for a PV system has a temperature rating of 90C and the terminal has a temperature rating of 75C and we use the 75C column instead of the 90C column in the 690.8(B)(1) step.

690.8(B)(2) is going to be Isc x 1.25 and then the 310.15(B) conditions of use tables, such as:

Table 310.15(B)(2)(A) Ambient Temperature Correction Factors Based on 30C

Table 310.15(B)(3)(a) Adjustment Factors for More Than Three Current-Carrying Conductors

Table 310.15(B)(3)(c) Ambient Temperature Adjustment for Raceways or Cables Exposed to Sunlight on or Above Rooftops

The calculations in your question are right and if your cable or raceway is less than 36 inches above the roof in sunlight, then you need to add a temperature adder from Table 310.15(B)(3)(c)

That was fun.

Thanks,

Sean White

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