VOC and ISC

[mrsolar]

Correct me if I'm wrong, but I am planning on Installing 30 panels with these specs (Rated wattage 100w, Isc 1.65a, Voc 99v, Vmax 77v, Amp max 1.29a)

Am confused as to whether to plan the cabling on the Isc figure of 1.65a in case of short circuit and fire hazard, or do I just go by the 1.29a which each panel puts out? (It will be fused)

I am also confused as to which voltage to go by, 77v Vmax or the 99v VOC.

As an example 30 panels in a series connection of 3 panels in 10 banks of 77v would give me a voltage capacity of 231 volts, and if I used 1.29a this would give a total current capacity of 12.9a (Giving a total of 3000w)

How does Voc or Isc or even the IV curve help me at all?


Thanking you <3

[Sunking]

Wow I would help you but so far you have demonstrated no knowledge of any electrical principle. To start with you will not be connecting 30 panels in series, 5 at most. Sorry but you are a great danger to yourself and have no business doing anything electrical. 30 panels in series would be 3000 volts, enough to blow your arm off in a cloud of black smoke.

But here is my best tip for you. Go to the Northern Arizona Wind & Sun Forum. They will gladly help you burn your house down and sale you all the parts you need to do it.

[Naptown]

What exactly will these be used for Off grid? grid tie?

[mrsolar]

What exactly will these be used for Off grid? grid tie?

Grid tie,

To clarify what I meant in the above post I meant 3 panels wired in series to give 231 volts per set x 10 banks.

[mrsolar]

Grid tie,

To clarify what I meant in the above post I meant 3 panels wired in series to give 231 volts per set x 10 banks.

Also the cabling may have to be rated at Isc of 1.65a because the chances of short circuit do exist, eg if someone accidentally short circuited the wires during installation, or water build up, or equipment failure etc rather than 1.25a Imax.

Sorry I am new to this, please be kind.

[inetdog]

Also the cabling may have to be rated at Isc of 1.65a because the chances of short circuit do exist, eg if someone accidentally short circuited the wires during installation, or water build up, or equipment failure etc rather than 1.25a Imax.

Sorry I am new to this, please be kind.

The National Electrical Code requires that you use the Isc figure from the panels and also multiply that by 125% ( or is it 125% times another 125%??) because it could be a continuous current.
You should read the relevant sections in the NEC on Photovoltaic (PV) systems and then get an electrician to advise you.

[mrsolar]

The National Electrical Code requires that you use the Isc figure from the panels and also multiply that by 125% ( or is it 125% times another 125%??) because it could be a continuous current.
You should read the relevant sections in the NEC on Photovoltaic (PV) systems and then get an electrician to advise you.

Thanks inetdog for clearing that up for me. Will I have to do the same with Voc, use this figure instead of Vmax?

[Naptown]

The National Electrical Code requires that you use the Isc figure from the panels and also multiply that by 125% ( or is it 125% times another 125%??) because it could be a continuous current.
You should read the relevant sections in the NEC on Photovoltaic (PV) systems and then get an electrician to advise you.

it is 125% x 125% or 156%

[inetdog]

Thanks inetdog for clearing that up for me. Will I have to do the same with Voc, use this figure instead of Vmax?

The voltage that the insulation, switches, fuses, CBs and the inverter or charge controller will have to withstand will be the sum of the Voc voltages of the panels in the string as calculated for the lowest expected ambient temperature. (The voltage goes up as the panels get colder.)

You have to use Voc rather than Vmax because you can expect that the circuit will be open at some point. Note that Vmax or Vmp is not the maximum voltage, it is the voltage at which the maximum power is produced.

[mrsolar]

The voltage that the insulation, switches, fuses, CBs and the inverter or charge controller will have to withstand will be the sum of the Voc voltages of the panels in the string as calculated for the lowest expected ambient temperature. (The voltage goes up as the panels get colder.)

You have to use Voc rather than Vmax because you can expect that the circuit will be open at some point. Note that Vmax or Vmp is not the maximum voltage, it is the voltage at which the maximum power is produced.

It becomes an Open Circuit in such situations as for example the inverter is switched off, or there is a power cut or perhaps the wires become disconnected etc?

Thanks

[inetdog]

It becomes an Open Circuit in such situations as for example the inverter is switched off, or there is a power cut or perhaps the wires become disconnected etc?

Thanks

Yes, and you are welcome.

Note that a GTI can become switched off by having the mains AC fail or disconnect while still keeping the DC from the panels connected to the GTI.

[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

[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.

[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.

[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.