Nec 690.8

[erickench]

There are two things about this NEC section that puzzles me. First, NEC 690.8(A)(1) states:

The maximum current shall be the sum of parallel module rated short-circuit currents multiplied by 125 percent.

Since when is maximum current higher than short circuit current? I would've thought that the nameplate current is the maximum current and the short circuit current would be higher.

Second, NEC 690.8(B)(1) states the following:

The circuit conductors and overcurrent devices shall be sized to carry not less than 125 percent of the maximum currents as calculated in 690.8(A).

If we were to assume that the maximum current is 125 percent of the so called short-circuit current(or nameplate current) then we would be applying the 125% factor twice in performing the calculation. Here's what I mean:

short-circuit current multiplied by 125% and then multiplied again by 125% to obtain the conductor ampacity and OCPD rating.

Has anyone ever seen the nameplate of a PV module? Does the nameplate designate it as a short-circuit or maximum current? Anyone?

 

[iwire]

You understand it fine, the NEC requires us to add 25% to the calcultions twice.

PV modules can put out more than the lable under some conditions.

The labeling on a PV mod is 'STC' (Standard Test Conditions) The conditions in the real world can exceed the 'standard test conditions'.

For instance if you have a snow storm, followed by a sunny day the irradiance that the PV modules are exposed to can exceed the test conditions.

Or if the PV mods are cold they put out more.

Standard test conditions (STC): The testing conditions to measure photovoltaic cells or modules nominal output power. Irradiance level is 1 000 W/m?, with the reference air mass 1,5 solar spectral irradiance distribution and cell or module junction temperature of 25?C.

 

[erickench]

I still see a problem in that the NEC states that maximum current can be higher than short-circuit current. If I were to submit a proposal switching them around it would read like this:

The short circuit shall be the sum of parallel module rated maximum currents mutliplied by 125 percent.

The 125% factor would not be applied twice in the calculation. I understand what iwire is saying that under certain conditions the current may exceed the STC current. But 1.25 squared comes to 1.5625! That's more than a 50% increase in current! It just seems to me that somebody got it backwards when this rule was written into the NEC.

 

[erickench]

It seems to me the maximum current would be the rated current and would be written on the nameplate as STC current. I could insert an informational note to explain that.

 

[iwire]

I still see a problem in that the NEC states that maximum current can be higher than short-circuit current. If I were to submit a proposal switching them around it would read like this:

I have never seen a PV module tag with a 'max current rating'

There will be a short circuit current rating and a current rating at the "Maximum Power Point'

Maximum power point (MPP) maximum power point I-V characteristic of a solar cell, showing maximum power point. Credit: University of Southampton The point on the current-voltage (I-V) curve of a solar module under illumination, where the product of current and voltage is maximum (Pmax, measured in watts). The points on the I and V scales which describe this curve point are named Imp (current at maximum power) and Vmp (voltage at maximum power.)

But 1.25 squared comes to 1.5625! That's more than a 50% increase in current!

Yes it is, it is also exactly what is intended and required.

It just seems to me that somebody got it backwards when this rule was written into the NEC.

And it just seems to me you have made that determination without any facts, just your feeling.

Here are some links that will bring you to a ton of infor about PV and the NEC.


College of Engineering Southwest Technology Development Institute

College of Engineering Southwest Technology Development Institute Codes & Standards

Photovoltaic Power Systems and the National Electrical Code: Suggested Practices

The North American Board of Certified Energy Practitioners (NABCEP)

You will keep seeing the name John Wiles, he has written a lot of 690.

 

[erickench]

Well that answers the question. The short circuit value is written on the nameplate. But I still see a problem in that we are sizing the conductor and the ampacity to be 51% higher than the short circuit value. 25% I can see but 50% sounds to be a little high.

 

[K8MHZ]

Well that answers the question. The short circuit value is written on the nameplate. But I still see a problem in that we are sizing the conductor and the ampacity to be 51% higher than the short circuit value. 25% I can see but 50% sounds to be a little high.

Starting with the test conditions, note that one of them is 1000 w / meter squared of irradiance. That is what direct sunlight delivers. That does not account for what reflected light, particularly that of snow, can add to that 1000 w / meter squared parameter. Cold weather makes PV cells far more efficient. The test parameter is 25 C. At minus 10 C, the output will be considerably more than in test conditions. Air Mass is 1.5. A cell at higher altitudes and lower latitudes will have a lower AM, which increases output by adding even more to the 1000 w / m squared value. If conditions were such that all of these parameters were met, say on a mountain with snow on it, 50 percent can be expected to be added to the output.

Environments that approximate the above best case scenario exceed the rated STC output less than a snow covered mountain, but still can easily climb well over the STC rated output.

Let's say a Yugo was being tested by the factory for a top speed rating. In central Europe, STC would likely be an approximation of an average day. The test would be done on a fairly well groomed dirt road with as few potholes as possible on as flat a land as could be found and done during a light rain storm. Flat out, the Yugo goes 78 1/2 mph.

Now lets change the environment from the test parameters to what a Yugo may see in some parts of the US.

Smooth asphalt, speed jumps to 86 mph. Not having to veer for oxcarts gets us a cool 89 mph.

Down a big hill we are pushing 95 mph.

No rain decreases drag and we get up to 96 mph.

Turning off the wipers and shutting off the lights puts us 114 mph as we make the bottom of the hill.

That's a 46 percent increase in top speed over the factory's STC conditions. Sizing up to the next speed rated tires would require a 120 mph tire. That rating is for a new tire and must be derated for not only condition at expected end of service life, but temperature can also affect top speed reliability.

Fortunately, the life expectancy of a Yugo is less than that of a tire mounted on one by about 80 percent. More so if you drive them. That means that new 60,000 mile tires will get get 48,000 miles of wear. A 135 mph rated tire of that quality will still be above the 120 mph rating required when the Yugo gives up it's ghost and gets pushed to the back of the property by the owners wife and is made into a deer blind. So, you can see how sizing critical components must take consideration many possible deviations from STC performed certification and adjust for such.

New installers that wonder about the validity of the 156 rule do so with at least a modicum common sense and logic. Then ambient temp rears it's ugly head and is teamed up with distance above roof. But, common sense and logic are gained by experience. PV is so new, few have what a construction worker would call experience. Common sense? It could happen.

I did some calcs on how much I would have to up size an array conductor under my area's conditions. It was a remarkable increase in size. My calcs took me from #3 to 250mcm. Putting Z rated low profile 200 mph rated tires on a Yugo would be cheaper.