USE-2 and THHN: WIre Insulation Types

Source: NEC

RHW-2 in this table is listed, and it's the same as USE-2, which is an outdoor-insulated wire, recommended to connect to the panels by all professionals in the PV industry.  Here is a table which states the number of said wires inside of EMT conduit.  Solar Panels have USE-2 wires coming from their j-boxes, and the connectors are made to create water-tight seals onto that type of outdoor insulation.  

In this table, it's noted that 8 of such wires can fit inside of 1" conduit, meaning that you can have a total of 4 strings entering a 1" conduit, or else you will have to up-size to 1 1/4, which will allow you to have 13, or a maximum total of 6 strings.

USE-2 is "Underground Service Entrance" and you can buy it from nearly any source of PV supplies.  It's coated with a blood thinner which acts as a way to keep animals from eating it (it's essentially covered in animal poison) so be careful with eating after you have handled USE-2 wire.  It's also UV resistant, and because of all this, it's much thicker than THHN wire, which is why fewer wires will fit inside of conduit.  You can run this type of wire from the panels to the combiner box.  

Source:  NEC Table C.1

By comparison, THHN and THWN and THWN-2, all conductors rated to be indoors or inside conduit, can fit up to 16 of the same #10 AWG conductors in 1" conduit.  It's also less expensive than USE-2, making it the wire of choice for installers to have inside of conduit.  

Source:  NEC Table C1

Calculating Hold-Down Strength

In order to ensure that your PV array is not going to fly away, you need to effectively ensure that it's connected adequately to the structure it's installed on.

That's done by using a chart like this one, which is provided by the Standard Handbook For Mechanical Engineers:

Each screw, usually a lag that fastens into the support of a rafter inside of the roof, has a different diameter. Different types of wood have different resistance strengths.  Each amount of allowable withdrawal is determined by 1" of threading inside that particular kind of wood.  For example:

• A 3/8" lag through Southern Yellow Pine that is 2" long will have a withdrawal strength of 381x2, or 762 lbs.  
• A 1/4" lag through Douglas Fir that is 3" long will have a withdrawal strength of 232x3, or 696 lbs.  

Let's also say that the plywood that separates the roof surface from the rafters is 3/4" thick.  You would need to subtract that 3/4" from the length of the screw.  

So how do you estimate whether a system has enough support to keep from flying away?  Wind speeds with force that is greater than 75 lbs/ft will cause the glass to shatter, thus creating a limit to how much hold-down strength the array needs.  

You can take the area of your array, in square feet.  Multiply the 75 lbs/sq.ft by a safety factor of 4.  That will help you determine that the array will require 300 pounds per foot of hold-down strength.  

Let's say that there are 30 solar panels, each with an area of 15 sq. feet.  That's 450 square feet. 

• Determine the number of feet, attached with 3/8" screws 3" in length, that are required to hold the array in place to satisfy the requirements of hold-down strength.  The wood type is Southern Yellow Pine.  

450 x 75 = 33,750 lbs hold down strength required. 

Each screw is 3" at 3/8" diameter in yellow pine (381 x 3 = 1143 lbs per screw).  

Divide 33,750 by the 1143 per screw and you get 29.5, which is essentially 30. 

This particular example requires 30 footings in order to satisfy the requirements of hold-down strength.

Use this formula and these calculations for any array! 

Variables

You will find the example to use 75 lbs per square foot as the panel's weight load.  Find the information on panel specifications as listed by "load resistance."  Officially, you must also account for the depth of the plywood as not included by the amount of thread that is actually fastened into the rafters.

Counter-Ballast Systems
Instead of using the hold-down strength of screws, just calculate the weight of your counter-ballast (usually cinderblocks) and then determine how many cinderblocks you need, with the same formula.  (Square area of array, pounds per square foot of required hold-down strength determining number of required blocks).


References
Nabcep Study Guide
Handbook For Mechanical Engineers

System Design Checklist

You're preparing your home for a photovoltaic system, and you are doing research about how to go about that process.

There's a ton of information online about Solar Electricity, not all of it useful.  I hope that you find the information on this website to be an exception, because my intent really isn't to sell you anything.  I really just want to share the things that I've learned from building and designing systems.  I want you to feel confident that PV will work.

If you are interested in assessing your own home, and you think that you're able to take measurements, here's what you can do:

Collect Data

1. Determine your home's orientation, using a compass.  (south, or 180°, is optimal).  
2. Find the southernmost facing roof surface, and determine its pitch with an inclinometer.
3. Measure the area of that roof surface.  Be safe.  You can also measure the house perimeter.
4. Pick a type of solar panels, and use those dimensions to determine how many panels will fit

Additional Data:

• What is the busbar rating of your main service panel?  (100, 200, 250 are common). 
• What is the main breaker rating of your service panel? (The top breaker that shuts all power off)
• What is the spacing of rafters in your roof?  Is it a truss system?  (structural system)
• How much shading is in your environment? (a pathfinder might be useful).  

Estimate Power

Once you can determine how many panels will fit, you can then multiply the number of panels by their STC wattage to determine what the wattage of your system will be.

Panels x Peak Power = "Max Rating" (the laboratory rating of your system at maximum power).  

For example, if you can fit 30 x SPR-230 panels, that's technically 6900 watts, or a 6.9 KW system.

Next, take your 6.9 KW number and perform the following calculations:

• Subtract the percentage that will be shaded.  If no shade, skip this step.
  
• Subtract the percentage that derates the system by orientation and angle.
  
• Take 75% of whatever's left. 

Let's say that your 6.9 KW system has 10% shading, it faces Southeast, and the roof is 30°. 

Southeast at 30° is 96% efficient.

6.9 x .90 x .96 x .75 = 4.4 kw

The 75% factor is an industry-standard practice recommended by Sunpower and other agencies, to keep your estimates from being too high.  Typically, other factors such as dust and debris on the panels, as well as voltage drop in the wiring and inverter efficiency, will subtract from the production of the system.  By taking a conservative estimate, you allow yourself some room.

Your new rating is 4.4 KW

Next:  Factor Your Insolation

Depending on the part of the country you're in, this system will perform differently.  You can find this information from the National Renewable Energy Laboratory Red Book.  Most of the country is between 4 and 6 w/m².  In this case, let's say we are in an area that has a rating of 5 w/m².

4.4 kW multiplied by 5 w/m² is 22

That number essentially represents the average amount of KWH per day that you can expect from your system on an annual basis.


Of course, that's going to fluctuate depending on what time of year it is, but that's why NREL provides annual averages for irradiance, because it's the best way of estimating your system's potential, with all the varying factors of sunlight energy potential throughout the year.  Annually, you can expect to see:


22 kWh x 365 = 8,030 kWh.  Therefore you have estimated your home to be capable of producing 8,000 kWh, or 8 megawatt-hours, of power, annually.  At 22¢, that's $1766 in savings per year.

This formula is used standard by professionals in estimating the annual production of the systems that they sell to people across the US.  You can try it yourself if you'd like.  Remember to account for your own factors, as listed by the information that you gather about your home's orientation, angle of your roof, the area shading, your region's irradiation, plus the type and number of solar panels that you wish to install.

See more site surveys for more detailed information about estimating energy potential.

Reading Panel Specification Sheets

Every Solar Panel not made in Jim's basement is UL listed, meaning that it contains lots of data pertaining to how it performs in a laboratory.  This article is going to explain what all that stuff means.

Here is the spec sheet from one of my favorite solar panels, the Sunpower 230-watt panel (*Spr230).

Underneath "Electrical Data" you're going to find that it says "Measured at Standard Test Conditions (STC) irradiance of 1000/m2, air mass 1.5g, and cell temperature of 25°C."  It's basically the conditions under which all solar panels are tested in order to get unbiased information so that their performance can be compared to those of other manufacturers.

So let's get into what all of these numbers and abbreviations stand for, and how to understand them.



Pmax is the maximum wattage of the panel in the conditions described (STC).
+/- 5% means that it could be 218w, or it might be 241w, and that's called module mismatch since not all solar panels are created with exactly the same identical properties.

Vmp is the voltage at maximum power, while the panels is under load (connected to a system).
Imp is the current (I) at maximum power, also while the panel is connected.

Voc is the open circuit voltage of a panel.  That's when you connect each lead to a voltage tester.  It's a theoretical voltage which could occur if something bad happened to the system.  VOC is used to determine the maximum number of panels that you can wire in series.

Isc is the short circuit current of the panel.  It's the reading you get, under STC conditions, where the negative lead is plugged into the positive lead of the panel and a current reading is taken.  This helps determine the size of the fuses that you will use for your system later on, and also helps you determine the size of your DC Disconnect.

Maximum System Voltage is a rating determined by UL, and it has more to do with code requirements than anything else.  Remember Voc?  It can't be greater than 600v in the US.

Series Fuse Rating is the size of fuse that is required for the string.


Peak Power per Unit Area has more to do with the amount of space on the panel being consumed by solar cells than anything else.

CEC PTC is the California Energy Commission's rating of the solar panel's ability to produce energy.  Most of the stuff on this website is with the STC ratings, not PTC which is a different test, although many rebate programs will account for the amount of money that can be granted to a project based on the PTC.

Temperature Coefficients tell how the power changes under different temperature situations.  in this case, the voltage increases by 132.5 millivolts (or .132v) per °C.  This statistic is also used wiht the Voc to determine the maximum number of panels in series.

Inverter Review

I've mounted, wired, and commissioned probably a couple hundred inverters made by several manufacturers, and on top of that, I've also troubleshot a fair share.

With that experience, I can tell you what I think are the best inverters for your grid-tied installation.  This is my unbiased opinion, and what's factored in most is which ones seem to require the least attention once they're installed.

Most inverters have a lifetime expectancy of 10-15 years, provided they're not installed in a bad place or the system they're connected to is the right size.  None of the inverters that I've installed have been working for that long, so I can't tell you if any of these will exceed those expectations, but some inverters seem a bit more durable than the others, based on my troubleshooting records.


Here are my reviews of the 4 most common inverter manufacturers.
These are listed in order of least to most preferred.

Xantrex.
These things are kind of cheap and they break frequently.  Sunpower used to sell them as part of the system with their panels, but that didn't last, because they had to recall a whole bunch of them.


Solectria
These are made in the United States, which is nice.  But they do seem a little cheap, and their commerical inverters have a tendency to break (their cores go bad, it's a common problem).

Fronius
Also built in the USA, these guys are reliable and easy to swap out parts if something goes wrong so that you don't have to re-install the whole thing.

SMA
These are by far the most durable of all inverters available on the market, as far as I can tell.  I've never had to replace or repair one.  The solid-state ones are really heavy, and that's because their built-in transformers weigh a ton! (actually about 150 lbs).  That said, I typically recommend these before any of the other inverters because I trust them to last.

You should really do your own research about inverters, as well.  Links to all of the various manufacturers are listed above in their names.  Efficiency is just as important as durability, and with none in the field actually installed for the duration of their lifetime expectancies, it's really anyone's guess as to which will last the longest.  My guess is the SMA transformerless inverters.


See Also: 
Inverter Placement

EMT Conduit

DC Solar electricity inside of residences in America are required to be encased in EMT, which is metal conduit.  It becomes a challenge to weave conduit in and out and between many of the commonplace things you encounter in the average home.  A good electrician can work wonders with a piece of EMT, though there are a few simple rules to remember when dealing with it.

You can't exceed 360 degrees in bends, with any pipe run.  In order to "reset" the bends in your pipe run, an electrician can use a "pull point" which can be an "LB" or a J-box of some kind.  Fun stuff.

EMT is necessary inside of homes for solar, because in the event that there is a fire in your home during the day, the idea is that a fireman cutting through a wall with solar current running through it will not get electrocuted (nor you or anyone in the future who decides to do some renovations).

The NEC is designed to prevent fires, primarily, but it also keeps people from getting electrocuted, which is also quite useful.  Make sure that you have a licensed, experienced electrician on site, constructing your PV system's conduit run.

Sizing an Inverter To A Service Panel

Let's say that your home's existing electrical system contains a main service panel that has a busbar rating of 200 amps.  The busbar rating is listed on the door of the panel.  Most houses' service panels are either 100 or 200 amps, unless you have something old and funky, or massively huge.

Every house electrical system has a main breaker.  That breaker has a rating, whether it's 200 amps or 250 amps.  That is the point at which the load from your house will shut off the power for safety.

To determine the amount of backfed solar power your house can withstand, there's a really simple calculation you can perform, and it's extremely simple.

Take the rating of the busbar.  Multiply it by 120%.  Subtract the main breaker.

Busbar = 200 (x120% = 240) - Main Breaker (200) = total of 40 amps to backfeed onto.

let's say it's a 225 amp panel, and a 200 amp main breaker.
225 (x120% = 270) - Main Breaker (200) = total of 70 amps to backfeed onto.  

What does that mean?  
If you can backfeed 40 amps, then multiply the 40 amps by the house power coming in.  That means you can have 7,680 watts backfed into the house, because 40 amps at 240v AC is 9600w.  But then you must divide by 125% for overcurrent protection, leaving you with the 7680 watts, or at most a 7000-watt inverter that can tie into the existing home electrical system.


Can you lower the rating of your main breaker? 
Yeah, technically you can but it's not recommended.  It's better to upgrade your service panel.


What's a Line-Side Tap? 
Please don't ask that question.