VOC and ISC

[Naptown]

Hey Naptown and Sunking thanks again for your efforts. Just thought I'd bring to light the following, see picture attached where it is specified in the manual to add 125% x 2 to the Voc value. Or has it already been included in the final rating of 99v?

Could be because they are thin film they differ from the norm?

There are three multipliers you need to use
Voltage only you multiply the open circuit of the string by a factor which corresponds to the lowest recorded temperature where you are located
In my case it is 1.18 which gets me down to -4 F Google "NEC solar temperature correction" for the chart.

The other ones for example the string and output circuits (output from combiner to controller or inverter. Use the Isc and multiply by 156% or 1.56)
If you are using a combiner (which you are required to in your case take the ISC of a module (string ISC will be the same)
Multiply that by the number of strings to get the total output circuit amps. Multiply the total by 156%
125% x 125% = 156%

[mrsolar]

Corrected comments in red

Hey Naptown and Sunking thanks again for your efforts. Just thought I'd bring to light the following, see picture attached where it is specified in the manual to add 125% x 2 to the Voc value. Or has it already been included in the final rating of 99v?

Could be because they are thin film they differ from the norm?

[Naptown]

According to the NEC table I have selected the temperature co-efficient of 0 - 4 degrees celsius with a correction factor of 1.10.

So PV Voc is 99v
99 x 1.10 x 5 Modules = 544.5v This is maximum open circuit voltage
Isc 6 x sets of Collectors at 1.65a = 9.9a x 125% = 12.375a x 125% = 15.46a This is the calculation for wire size from combiner to the inverter.

Do these look good to you guys?

Corrected comments in red

[Sunking]

According to the NEC table I have selected the temperature co-efficient of 0 - 4 degrees celsius with a correction factor of 1.10.

So PV Voc is 99v
99 x 1.10 x 30 Modules = 3267v / 6 Strings = 544.5v x 125% = 680.625v x Another 125% as per NEC = 850.775v
Isc 6 x sets of Collectors at 1.65a = 9.9a x 125% = 12.375a x 125% = 15.46a

Do these look good to you guys?

No you cannot exceed 600 volts.

[mrsolar]

One last thing needs to be calculated.
That is the Voc at the coldest recorded temperature in your area.
There are 2 ways to do this.
1- use the tables in the NEC
2- Calculate the string by the temperature coefficient figure corresponding to your panels.
In either case you will need to know the coldest recorded temperature for where you live.
It probably will not be a problem but many times permitting will require this calculation.

According to the NEC table I have selected the temperature co-efficient of 0 - 4 degrees celsius with a correction factor of 1.10.

So PV Voc is 99v
99 x 1.10 x 30 Modules = 3267v / 6 Strings = 544.5v x 125% = 680.625v x Another 125% as per NEC = 850.775v
Isc 6 x sets of Collectors at 1.65a = 9.9a x 125% = 12.375a x 125% = 15.46a

Do these look good to you guys?

[Naptown]

hanks for the good advice, I won't skimp on the inverter. As recommended by this forum the 30 panels will be in 6 strings of 5 collectors, so the formula for maximum voltage and current protection of insulation, CB's, breakers etc

5 x Strings at Voc of 99v = 495v x 125% = 618.75v
6 x sets of Collectors at 1.65a = 9.9a x 125% = 12.375a

30 panels rated at 100w = 3kW

If this sounds ok by you then I will finalize installation as of now to the former specs and hope that everything goes right?

Pmax = 100w
Vmp = 77w
Imp = 1.29a
Voc = 99v
Isc = 1.65a

One last thing needs to be calculated.
That is the Voc at the coldest recorded temperature in your area.
There are 2 ways to do this.
1- use the tables in the NEC
2- Calculate the string by the temperature coefficient figure corresponding to your panels.
In either case you will need to know the coldest recorded temperature for where you live.
It probably will not be a problem but many times permitting will require this calculation.

[mrsolar]

hanks for the good advice, I won't skimp on the inverter. As recommended by this forum the 30 panels will be in 6 strings of 5 collectors, so the formula for maximum voltage and current protection of insulation, CB's, breakers etc

5 x Strings at Voc of 99v = 495v x 125% = 618.75v
6 x sets of Collectors at 1.65a = 9.9a x 125% = 12.375a

30 panels rated at 100w = 3kW

If this sounds ok by you then I will finalize installation as of now to the former specs and hope that everything goes right?

Pmax = 100w
Vmp = 77w
Imp = 1.29a
Voc = 99v
Isc = 1.65a

[russ]

Whatever you do it is a very bad idea to buy a "no name" Chinese inverter - you need something that will last which means a name brand with a good guarantee.

[inetdog]

Cheers again inetdog, sunking and naptown. This forum has bee gold to me but just one more question, you recommend 5 collectors times 6. Would I want to keep the voltage lower in say for example 15 strings and 2 collectors?

Because most inverters (and Im on a budget here so will be shopping on Alibaba for a cheap Chinese inverter that is AS777 compliant) anyway most inverters have a lower DC input range so if there is lower irradiance the shut off point will be beneficial to me in keeping the voltage down because I will collect more electricity in the evening?

In terms of wiring cost and resistance, the higher the voltage you use the better. And GTIs which have input voltages in the range of 300VDC and up will be the ones which are better made and more likely to be efficient and long lasting.
By designing your panel setup to accommodate the limitations of cheap inverters, you will be, IMHO, making a big mistake.

The open circuit voltage, Voc, from a panel will be almost constant from very low light to full sun, as will the Vmp operating point. What goes down is the available current. The only reason that the voltage might go down in the evening is if some, but not all of the panels in a string are shaded. In that case you should try to put all of the shaded panels together in the same string.

The amount of electricity you collect in the evening will be small compared to the time around noon, so you will get more total energy by reducing losses at noon than by keeping the inverter running a little longer at very low output.

There is some potential merit to having both south-east and south-west facing arrays connected to the same inverter to reduce cost and enhance production in the evening if you have different power rates for peak hours.

[Naptown]

Cheers again inetdog, sunking and naptown. This forum has bee gold to me but just one more question, you recommend 5 collectors times 6. Would I want to keep the voltage lower in say for example 15 strings and 2 collectors?

Because most inverters (and Im on a budget here so will be shopping on Alibaba for a cheap Chinese inverter that is AS777 compliant) anyway most inverters have a lower DC input range so if there is lower irradiance the shut off point will be beneficial to me in keeping the voltage down because I will collect more electricity in the evening?

You will need an inverter LISTED to UL1741 Or you will not be able to connect
I have no idea what AS777 is and compliant doesn't cut it as far as the AHJ is concerned. Must be listed by a NRTL (nationally recognized testing laboratory)

[mrsolar]

You use max watts for inverter calculation. The calculations you are using are for sizing of the conductors only.
In your case it may be better to run 6 strings of 5 collectors per string. and use a 3KW inverter.

Cheers again inetdog, sunking and naptown. This forum has bee gold to me but just one more question, you recommend 5 collectors times 6. Would I want to keep the voltage lower in say for example 15 strings and 2 collectors?

Because most inverters (and Im on a budget here so will be shopping on Alibaba for a cheap Chinese inverter that is AS777 compliant) anyway most inverters have a lower DC input range so if there is lower irradiance the shut off point will be beneficial to me in keeping the voltage down because I will collect more electricity in the evening?

[Naptown]

Great I finally understand the DC side of things, but coming to this understanding has suprised me in realizing that I will need a 5kw Inverter for a system that will only output 3kw, reason for this is the panels are Voc 99v and Isc 1.65 so If I were to wire them in 2 respective sets with a bank of 15 panels (30 panels in total) this would give me, per bank a total voltage 198v and amperage of 1.65a x 15 = 24.75a (P=196 x 24.75a which equals 4900.5w) Even without adding the 125% and another 125% this doesn't seem right in my books, if anyone would be kind enough to differ that would be super helpful?[ATTACH=CONFIG]3601[/ATTACH]

You use max watts for inverter calculation. The calculations you are using are for sizing of the conductors only.
In your case it may be better to run 6 strings of 5 collectors per string. and use a 3KW inverter.

[inetdog]

Even without adding the 125% and another 125% this doesn't seem right in my books, if anyone would be kind enough to differ that would be super helpful?

2011 NEC Section 690.8(A):

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

690.8(B): The output conductors shall be sized to carry no less than

One hundred and twenty-five percent of the maximum currents calculated in 690.8(A)....

A similar statement applies to the size of the circuit protective devices (fuses or circuit breakers.)

That is just the way the NEC works to provide what it considers a necessary safety factor in the installation.

[Sunking]

Without adding the 125% and another 125% this doesn't seem right in my books, if anyone would be kind enough to differ that would be super helpful?

Your book does not matter, it is the Code Book you have to comply with.

[mrsolar]

Great I finally understand the DC side of things, but coming to this understanding has suprised me in realizing that I will need a 5kw Inverter for a system that will only output 3kw, reason for this is the panels are Voc 99v and Isc 1.65 so If I were to wire them in 2 respective sets with a bank of 15 panels (30 panels in total) this would give me, per bank a total voltage 198v and amperage of 1.65a x 15 = 24.75a (P=196 x 24.75a which equals 4900.5w) Even without adding the 125% and another 125% this doesn't seem right in my books, if anyone would be kind enough to differ that would be super helpful?asf100specs.png