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HeatSpring PV Course Blog


Bypass Diodes and Shading

Posted on 2 April, 2016 at 0:17 Comments comments (0)


Why are all three bypass diodes affected by shading on the short edge of the module? What about the wiring/arrangement of the diodes causes them to be affected this way?
Also, what does MPPT stand for?
Thank you,


MPPT stands for Maximum Power Point Tracking.
As we showed on the IV curve, there is a "sweet spot" where PV will work best with the optimal voltage and current combination for a given temperature and irradiance. With temperature and irradiance constantly changing, the MPPT will shift to optimize the PV production.
Before bypass diodes were used with PV, when there was shading, it would take out the entire string and heat up the shaded PV cell.
A normal PV module has 60 cells arranged in rows of 6 x 10 with 20 cells per bypass diode. When 1 or more cells in a group of 20 gets shaded, the current will go through the bypass diode rather than getting stuck at the shaded cell.
If the short row is shaded, then there will be cells from each group that are shaded, effectively bypassing the entire module.

Shading, bypass diodes, portrait and landscape

Posted on 30 March, 2016 at 18:32 Comments comments (0)


Hi Sean,
The layout of an array, landscape vs portrait was to install as many modules as possible in the available space, maximizing the size of the system. From your discussion regarding the bypass diode, I note that if there were any shading issues, orientation of the module, when positioned in landscape will have the least affect on the functionality of the bypass diode and thus module output.


That is correct for typical crystalline PV assuming the normal shadow pattern with the shadow coming from the lower part of the module as with inter-row shading as opposed to a shadow coming from the side. This way we will take one third of the module's production rather than all of it if the shadow was shading 1 row of cells.

Note: if the entire string is shaded with MPPT, then the PV source circuit would operate as if the entire string was in the shade, since shaded cells can operate at a substantial decrease in output.

Another note: with thin film PV, the cells are in stripes, so the PV will be oriented so that the stripes are north to south and the inter-row shadow will shade all of the cells the same and then shading is directly proportional to production (no series penalty if cells are all shaded the same).

Sean White

National Permitting Process

Posted on 21 March, 2016 at 20:36 Comments comments (77)


Hi Sean,
Big picture here, in efforts to help deploy more solar in the residential market, do you foresee a national permit standard ever being adopted? I know of a few states who have moved to a statewide standard, bringing down cost of permit and turnaround times.
Also appreciate your breaking NEC into digestible sections (less intimidating).
Thanks again


I think that it is part of our political culture. 50 states, 50 state governments and then many many more municipalities. Everyone on the ground would like having a normal permitting standard, but that standard would have to be individually adopted and I do not think that the AHJs are going to want to give up their power.
You could argue that the NEC is a way that we are unified, sort of like the constitution for electric codes.
The expedited permit process and the Sunshot Initiative are attempts to harmonize solar permitting and bring down the price of solar. I always try to do the expedited permit process with new AHJs and many of them have their own way of doing things.
You can find information and contribute to a database at
We could adopt the German way and just have a 4-page form for installers to fill out after they install the system.
I think I am going to propose an amendment to the US Constitution. The Right to Have Solar!
Sean White

Expansion Joints, Metal Raceways and PV

Posted on 16 March, 2016 at 21:27 Comments comments (176)


Hi Doc,
here in Indiana land we mostly use schedule 40 conduit and armored cable and flexi steel conduit I have not had the opportunity to couple EMT,and my question is on 300 7 (B) expansion fittings,are these special couplings ,can you explain more of this,or a picture paints a thousand words ,and you could add some words to explain more .
Thanks much really enjoying this stuff.


Here is a report that is about a famous PV fire on a rooftop in Bakersfield. Nobody was hurt and the building didn't burn down, but the fire led to a big push towards ungrounded PV systems, which would not have caught on fire in this case.

Here is the Fire Report:

The second to the last and the third to the last pages have images of expansion joints on EMT.

Here is a SolarPro article regarding the fire:

With grounded PV systems, there are leakage currents that leak from the equipment through the ground fault detection and interruption fuse (GFDI fuse). If the system is big, the leakage currents can go undetected and in the case of the fire, there was a second ground fault and a loop of current that arced. I recommend reading the SolarPro article. We also discuss this in the video lectures.

To answer your question about conduit. When PV circuits are going through a house, they must be in a metal raceway or MC cable up until the first readily accessible dc disconnect. Some people think that this means ac inverter output circuits (I don't). Most people keep it simple and put their PV circuits in EMT.

You can see in the fire report and the SolarPro article, that there are some good examples of EMT melting, so think what would have happened with plastic. It is ok to use schedule 40 PVC when it is not inside of a building and is common in Hawaii where there is a lot of corrosion on steel EMT.


Sean White

Great Review Make My Day!!

Posted on 11 March, 2016 at 23:33 Comments comments (1)

Submitted: 03/11/2016
Training: 40-Hour Advanced Solar PV Installer Training / Online / Anytime

How would you rate this course?
What did you like about the course?:
Learning the code and how it should be applied/interpreted through real life solar examples provided by Sean.
How effective was the instructor's communication during the course?:
What would you change about the course?:
I can't think of anything.

Thanks for the great review!!! That is what I work hard for!

Crimping MC4 Connectors without the tool

Posted on 10 March, 2016 at 23:26 Comments comments (172)
From Advanced PV Discussion Forum


Hey Sean,
This might not be specific to wire sizing (kind of is) but do you have a good way to crimp MC4 connectors? I am doing an array on my house and don't want to buy a crimping tool. They have pre-made MC4 wires and I was thinking about skipping the hassle and buying the pre-fabricated ones.


That is a common thing that people try to do. I remember a job where someone tried it and the system wouldn't turn on. They had to spend a day taking apart things and figuring out under which module the bad connection was. That is what happens with needle nose MC4 crimping.

Last week I taught a Solar PV class in the Philippines and the distributor ( that I partnered with for the class said that they have seen many people having bad connections for this same reason. They were telling me about all of the burnt MC4 connectors that they find because of arc faults. We would not have the same problems with arc faults in the US, because of the dc arc-fault requirements put into the 2011 NEC.

For the class last week, everyone had to crimp MC4s and have them tested to pass the class.

I recommend getting a crimper and if you want to get a cheap one, they are on Ebay in the $30 range.

Sean White

From Advanced PV Discussion Forum

The 10-foot Tap Rule, 240.21(B)(1) is as solid as a flimsy 2014 NEC Code Book

Posted on 10 March, 2016 at 22:18 Comments comments (287)
From Adv. PV Discussion Board


Hey Sean,
With the 10 foot tap rule, do you know where that came from? Do you see a lot of inspectors looking for this? It does not seem like a very solid rule, one of those that is more a of guideline.


The 10-foot Tap Rule, 240.21(B)(1) is as solid as a flimsy 2014 NEC Code Book.

We have had the tap rules for plain old feeder taps for as long as I have been getting electrocuted (death by shock). I do not see the Tap Rules in the 1897 NEC here:

The 2014 NEC decided to refer to the Tap Rules for interconnections in 705.12(D)(2)(2) in order to protect the tap conductors from the extra potential currents coming from the inverter connected to the feeder in addition to the original currents coming from the supply breaker.

Tap conductors are sized to protect conductors from fault currents rather than overcurrents. This means that a super skinny conductor can not be connected to a big feeder, because it needs to be large enough to clear a short circuit. In reality, the inverter due to its anti-island provisions will shut off and not have fault currents. It was a compromise put into the 2014 Code, so that we could connect to feeders and have a guideline. The reason that a feeder tap conductor has to be larger when the conductor is over 10 feet, is because there will be more resistance in the conductor itself and the conductor might heat up and catch fire before opening the overcurrent protection device. Longer wire means more resistance and lower resistance has a better chance at clearing the fault at the breaker.

Since the tap rules for solar were put in the Code in 2014 and most of the solar installed in the USA is still installed under the 2011 Code, most inspectors are not applying this rule. This is because half of the solar installed in the USA is installed in California, which will adopt the 2014 Code on 1/1/2017.

If you are looking to connect to a feeder using the 2011 NEC, you can apply the 120% Rule, which applies not only to busbars, but also conductors. With the 120% rule, you are typically more limited in what you can do.

Thanks for the good question!

Sean White

From Adv. PV Discussion Board

Bonding Neutral to Ground

Posted on 9 March, 2016 at 1:29 Comments comments (188)


Hi Doc,
assignment 23/36 you explain bonding of neutral to a sub panel,
question: is a subpanel specifically manufactured as thus ,or is it as its name applies,my thoughts are they are manufactured as thus,because of the raised neutral buss ,am I correct in assuming the latter?


You can typically physically bond neutral to ground in many different places, including disconnects, panelboards (main panels and sub panels) and just about anywhere if you are determined.
Usually what will happen at a house is the neutral will be bonded to ground by having a common busbar for the green/bare and the white wires.
You could use a main panel for a subpanel as long as you didn't bond neutral to ground there.
When a supply side connection is made, usually the separate system (PV system) has neutral bonded to ground separately. I believe this will be a requirement in the 2017 NEC and is good practice now. Many times a supply side connection is made via a fused dc disconnect. When you buy a dc disconnect, it will often have a green screw that you can use to bond the insulated neutral busbar to the box, making it the system grounding point where neutral is bonded to ground.
Sean White

Great Reviews Make My Day!

Posted on 7 March, 2016 at 21:23 Comments comments (0)

Training: 40-Hour Advanced Solar PV Installer Training / Online / Anytime
How would you rate this course?
What did you like about the course?:
Some of this stuff is very complicated, but Sean made it as clear as possible.
How effective was the instructor's communication during the course?:
Very clear.
What would you change about the course?:
If you enjoyed the course and are interested in providing a testimonial, please enter one here.:
This is a great way to satisfy the NABCEP Advanced hours, and is also great preparation for the NABCEP exam.

Thanks for making my day!!!

Roof Slope Calculation, 120% Rule breaker Sizing & Calculator on “smart” Phone

Posted on 5 March, 2016 at 10:08 Comments comments (95)
From HeatSpring Advanced PV Course Discussion Board

hi Doc ,really enjoying your humor and the way your talking points come through.
Two questions though:this pertains to assignment 16/36,
1)roof slope angle short cut,the inverse tan function is not on my calculator is there another long method of completing that function and 
2)the 120% rule the 50 amps *0.8 where did the 0.8 come from ?
appreciate all the help,ok forgive me one more thing,in the string sizing calculation when I click on the [1/x] function it displays
using your figure of 44.62 X 1/. I am using the calculator on samsung galaxy phone they call it a "smart phone".
Thanks Doc

Roof slope and calculator question:
You can figure out a roof slope with rise and run with a few buttons on the calculator.
If you have trig functions, all you have to figure out how to do is the inverse tangent function, which is sometimes very difficult to figure out. On my apple computer I figured I have to press shift and Tan to get the inverse tangent function. The inverse trig functions give you the angle and the regular trig functions gives you the ratio of sides of a triangle.
Roof slopes in the US are typically done with the rise:run ratio, such as 4:12 being a roof that has 4 units of rise to 12 units of run.
Here is how to do it on a calculator for a 4:12 roof :
4/12=    you get 0.333333
0.333333 inverse tangent    you get 18.4
That’s all there is to it, a 4:12 roof is an 18 degree slope.
Impress a roofer, they will worship you.
I bet your phone does the inverse function, you just have to play with it. My iPhone I have to turn it sideways to get the scientific calculator.
I just did a Google search and it says that you can turn your phone sideways for the scientific functions. Give it a shot and impress people. The word “trig” puts the fear into most people and if you can use trig, some people will think you are a prophet. You can move to Los Angeles and start your own cult.
120% rule 705.12(D)(2)(3)(c) question:
To size the maximum allowable circuit breaker using the 120% rule, forget about the cryptic code, here is the math:
Multiply Busbar x 1.2 to get 120% of busbar
Example: if Busbar is 100A then 100A x 1.2 = 120A
Subtract the main breaker from the busbar
Example: If main breaker is also 100A then 120A – 100A = 20A
In this case 20A would be the maximum breaker that could work for the 120% rule.
A breaker has to be 125% greater than the inverter current
This is the same as saying the maximum inverter current can be 80% of the rating of the breaker.
This is because 80% is the inverse of 125%
1/0.8 = 1.25
1/1.25 = 0.8
Example: the most current a 20A solar breaker can take is 16A
20A x 0.8 = 16A
20A/1.25 = 16A
And an interesting note:
For US residential 240V a 16 A inverter is 3.8kW, which is a very common inverter size, much more common than a 4kW inverter, because the calculation that we just did for a 100A main breaker and a 100A busbar is a very common situation.
Sean White x 1.2
I am the Busbar