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Advanced Photovoltaic Systems

AET 230-8204 Notes-Week 5

Our guest today is:

Chris Sommerfeld of SunPower Corp. 

You can learn more about Chris's company from their website:

Chris is a trainer at SunPower in Richmond and has trained yours truly. Chris is NABCEP PV Certified.

Also, today we will have the opportunity to have an unscheduled field trip to an Electric Vehicle rally, which will be in the parking lot outside of our back door. Here is their announcement:

Of interest...

Want to take a PV quiz?

Click for an online PV quiz

The quiz came from, where you should get lost for a while learning about sustainable energy.

Feed in Tariffs in PG&E territory?

That is what PG&E says, but the rates are so low that no one is doing it. Also, according to the PV Magazine definition, it is not a feed in Tariff, since the rates are not guaranteed for 10 or 20 years, like in Europe. Maybe that is why nobody has a Feed in Tariff PV system in PG&E territory. Here is what PG&E is calling a Feed in Tariff:

PG&E Feed-in-tariff information

According to PV Magazine, the only real Feed in Tariff in the US is the one just put into place in Gainesville Florida.

Here are some articles about the real 1st FIT in the US:

(each Gainesville gator has a different FIT article)

New digital issue of Solar Today just released:

Pacific Energy Classes in the next few weeks (free & online):

Mon, May 4 9:00 am - 3:00 pm

Multifamily Affordable Solar Housing (MASH) and Low Income Workshop San Francisco--PEC

Sat, May 9 10:00 am - 12:00 pm

Solar Power Basics for Residential Customers San Francisco--PEC

Mon, May 11 9:00 am - 3:30 pm

California Solar Initiative Workshop Stockton

You can enroll in these classes and more at:

There was a class on 4/23/09 by Bill Brooks through the PEC

and here is a link to the notes:

This weeks topic is System Electrical Integration, which you can read about in the book, Photovoltaic Systems in chapter 11.

Electrical Integration is quite a subject & even though it is only 1 Chapter in the book Photovoltaic Systems, it is vast & vastly important.

If you haven't already done so, now would be a good time to familiarize yourself with Article 690 of the NEC. I would recommend re-reading it once a month for a year and reading all of the other sections that 690 cite once in a while too.

Here are a couple of links to some important documents regarding the subject:

Let's get down to business and start with voltage drop.

Voltage Drop-

The confusing thing for people to understand is that you do not calculate voltage drop with voltage. You are usually given voltage, but you want to figure out how much it drops.

You start with the equation: V = I R

for current, you do not need the short circuit current, because voltage drop is not dangerous. In fact the NEC does not care too much about voltage drop, because is will not start a fire, it will decrease production.

You take your current (I) as your operating current, which you also call Imp on the module and if you have multiple strings of modules, you multiply the number of strings by the Imp of the module.

The R (resistance) you get from a table, which you can find

in your textbook, Photovoltaic Systems on page 285.

You can find this information in the 2008 NEC in Chapter 9 Table 8 on page 70-681. We are usually using uncoated copper wire and looking for ohm/kFT. If you are looking at the NEC table, you will probably want to go with stranded wire, which you can differentiate by looking at the quantity column. For number 10 AWG, you have 7 strands and the uncoated resistance is 1.24 ohm/kFT.

A kFT is a strange combination of metric and imperial units, which means a thousand feet. That would mean that 100 feet would be 0.1 kFT and 50 feet would be 0.05 kFT.

So you need to know the length of the circuit in kFT, but one trick to remember is that the circuit is a round trip for an electron from the p-n junction, to the inverter and back to the p-n junction, so you need to double the distance of the wire run and put that number in units of kFT.

If you multiply ohm/kFT by kFT, the kFTs cancel each other out and you get an answer in Resistance=R=ohm.

Multiply R x I and that is your voltage drop.

If you want to find out your voltage drop as a percentage of voltage, you take your voltage drop, divide it by your voltage and turn the fraction into a percentage.

In other words: (Voltage Drop / Voltage) x 100

You can see by this equation, that as you increase your voltage, you decrease your voltage drop percentage. That is why we like to have high voltage strings of PV, but you probably already knew that.

If your voltage drop percentage is above 2 or 3%, you might be saving a few dollars on copper, but you are doing your customer a disservice by throwing away a percentage of production.

Wire Sizing-

Here is a brief definition in the back of chapter 11 (p.316) of Photovoltaic Systems relating to wire sizing:

"Nominal conductor ampacity must be derated for ambient temperature and number of conductors in a conduit."

Ampacity definition: "Ampacity is the current that a conductor can carry continuously under the conditions of use without exceeding it's temperature rating."

Therefore, we need to use some NEC tables to derate for

  • temperature

  • number of conductors in conduit

Different types of wire have different temperature ratings, which is the temperature that the insulation can handle.

When the voltage is dropping, it is causing heat from the resistance which heats up the wire.

A 90C rated wire is a wire that can not get hotter than 90C. We denote 90C with a -2 after it, such as USE-2 wire.

We usually end up using USE-2 #10 AWG in free air for PV source circuits.

A PV source circuit is from the PV to the junction or combiner Box.

A PV output circuit is from the junction or combiner box to the inverter.

The PV output circuit is usually THWN-2 or THHN wire & has to be in conduit.

Here is a link to an organizer used by Bill Brooks to help with electrical integration:

Wire sizing is done with tables and by derating.

You can use the NEC, or there are other reference books that have the required tables such as:

Code Check Electrical


Limit to size of solar breaker in service panel

(120% rule):

The sum of the ampere ratings of breakers supplying power to a service panel busbar shall not exceed 120% of the busbar rating.

Solar Breaker + Main Breaker = 1.2 x Busbar rating

Usually the main breaker in the service panel is the same as the busbar rating. Sometimes we will switch to a smaller main breaker, so that we can fit more PV.

Here is a Wiki link that has a link to free access to NEC on the bottom of the Wiki page:

Reminder: Next week we will meet for class as usual, but after lunch we will meet at 2000 San Pablo Ave. in Oakland for a look at a 200 kW PV system on top of the Alameda County Social Services Building. Let's plan on meeting in front at 1pm Saturday, May 2.

Post class follow up... The quiz:

quiz week 5 answers.pdf (PDF — 54 KB)

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