Too many solar panels?

[heirloom hamlet]

Initially I avoided fuses by going with 1 or 2 strings. Multiple strings to a larger inverter now and I put a bunch of surplus touch-safe DIN holders on rails in a surplus box. Wasn't too much money. I was pulling wire in conduit anyway so this was convenient.

I can see that MC cable all the way from panel to inverter would be convenient place for the in-line fuse holders.

I plan to use MC4 holders for a couple strings that will be paralleled at the end of one wire run.

Note that MC4 fuse holders are not touch-safe, so you should disconnect the MC4 connectors at both ends before opening.

Neither the MC4 holders nor the touch-safe holders I used are for make/break while hot, so a disconnect switch is needed too. I have heavy duty (600 VDC) knife switches, but the newer inverters have rotary switch attached.

Depending on voltage you can also use Square-D QO with 2-pole breakers (one opens positive, the other opens negative PV wire). Square D says OK up to 125 VDC.

Wait, you can use a standard AC breaker box and breaker for incoming DC power from your array? How exactly would this be wired up or assembled, I didn't completely follow?

Also, you use the word string...what exactly do you mean by that?

 

[Hedges]

Wait, you can use a standard AC breaker box and breaker for incoming DC power from your array? How exactly would this be wired up or assembled, I didn't completely follow?

Also, you use the word string...what exactly do you mean by that?

One panel is a panel. Two or more panels in series is called a "string". One of my inverters had 12 panels in series, another 12 panels in series, and those two strings of 12 panels each in parallel. We refer to that as 12S2P. It was used with a 2500W grid-tie inverter that allowed 600 Voc maximum.

The Square-D QO branch circuit breakers are UL listed for up to 50 VDC. The manufacturer also says they are "Square-D Certified" for up to 125VDC when both positive and negative leads are disconnected by the two poles of a breaker (15A through 70A breakers). (each pole only has to withstand 62.5V) Not quite the same as UL listed, but good enough for non-government work.

So that would mean a lower voltage PV system up to 125 Voc could be wired using a main lug (no main breaker) QO panel and branch breakers.

I looked that up after seeing a video by Midnight solar which said that the Square-D QO breakers were only UL listed up to 50V, therefore couldn't be used for a 48V battery system which charges to over 50V.

A couple forum links with manufacturer links:

AC Breakers for DC voltage...

Can I use an AC breaker for shut-off only purposes between PV panels and charge controller? Between the breaker and charge controller would be DC protection. :unsure: Thank you, Dan.

diysolarforum.com

MC4 Connectors for 8 AWG Wire Connecting to MC4 for 10 AWG Wire

Hi Everybody, I am finally continuing to finish my set up and I am following (almost) Will's blueprint here: https://www.mobile-solarpower.com/simplified-400-watt-fewer-wires-and-alternator-charging.html BUT, I will be using 8 AWG wires coming from the SCC to the Y-branch connector. So here...

diysolarforum.com

So you could get a 125A load center, use several 15A or 20A 2-pole breakers for strings of panels not exceeding 125 Voc, and a 70A 2-pole breaker (or the lugs if no breaker needed) to connect charge controller. (the 100A and smaller load centers have aluminum bus, but 125A and larger are copper, which I prefer)

The 15 through 30A breakers have 4kA interrupt rating, and 35 through 70A have 10kA interrupt rating. That should be considered if connected to a battery; if the battery can source higher short-circuit current there ought to be a suitably rated device in its path.

 

[heirloom hamlet]

One panel is a panel. Two or more panels in series is called a "string". One of my inverters had 12 panels in series, another 12 panels in series, and those two strings of 12 panels each in parallel. We refer to that as 12S2P. It was used with a 2500W grid-tie inverter that allowed 600 Voc maximum.

The Square-D QO branch circuit breakers are UL listed for up to 50 VDC. The manufacturer also says they are "Square-D Certified" for up to 125VDC when both positive and negative leads are disconnected by the two poles of a breaker (15A through 70A breakers). (each pole only has to withstand 62.5V) Not quite the same as UL listed, but good enough for non-government work.

So that would mean a lower voltage PV system up to 125 Voc could be wired using a main lug (no main breaker) QO panel and branch breakers.

I looked that up after seeing a video by Midnight solar which said that the Square-D QO breakers were only UL listed up to 50V, therefore couldn't be used for a 48V battery system which charges to over 50V.

A couple forum links with manufacturer links:

AC Breakers for DC voltage...

Can I use an AC breaker for shut-off only purposes between PV panels and charge controller? Between the breaker and charge controller would be DC protection. :unsure: Thank you, Dan.

diysolarforum.com

MC4 Connectors for 8 AWG Wire Connecting to MC4 for 10 AWG Wire

Hi Everybody, I am finally continuing to finish my set up and I am following (almost) Will's blueprint here: https://www.mobile-solarpower.com/simplified-400-watt-fewer-wires-and-alternator-charging.html BUT, I will be using 8 AWG wires coming from the SCC to the Y-branch connector. So here...

diysolarforum.com

So you could get a 125A load center, use several 15A or 20A 2-pole breakers for strings of panels not exceeding 125 Voc, and a 70A 2-pole breaker (or the lugs if no breaker needed) to connect charge controller. (the 100A and smaller load centers have aluminum bus, but 125A and larger are copper, which I prefer)

The 15 through 30A breakers have 4kA interrupt rating, and 35 through 70A have 10kA interrupt rating. That should be considered if connected to a battery; if the battery can source higher short-circuit current there ought to be a suitably rated device in its path.

This is helpful, thank you.

But if I have 2S4P you are saying you think I need a separate inline DC fuse on each series...4 total? If I was 3S3P, then 3 total?

Can the wire sizing vs your potential voltage/amperage be your "fuse" if done right?
I feel like I've seen you mention something like this in the past. Example:
12awg is used right out of the panels.
10awg is used to put in 2S (75v/8.27a) or 3S (113v/8.27a).
8awg us used after branch connector en route to the SCC for 4P (75v/33a) or 3P (113v/25a), respectively.
With these potential outputs and 8awg wire used to travel 60ft or less to the SCC, would the wire sizing alone suffice...no breaker?

 

[Fred S]

- You would not need to go from #12 to #10 for the series panels - only the voltage is increasing, not the amperage. Even after combining two strings in parallel for a potential of 17a, #12 is still fine, for the short runs in an array. At the point all 4 strings have come together in parallel, yes #8 is good, even for the total of 60' you mentioned.
- If you mean making sure the wire size (#8 here) is large enough and so not use a breaker or a fuse - no, for safety reasons you still need one. You also need to be able to disconnect the array for service or troubleshooting, at the array, so a breaker is convenient to serve both the safety and disconnect purposes. E.g. I have a fuse at the array, and also a disconnect at the SCC for convenience.

Here is a 150v DC breaker for less than $20. They are available in multiple ratings:

MidNite Solar Photovoltaic DC Circuit Breaker - 30 Amp, Model# MNEPV30 - Solar Panels - Amazon.com

MidNite Solar Photovoltaic DC Circuit Breaker - 30 Amp, Model# MNEPV30 - Solar Panels - Amazon.com

www.amazon.com

 

[Fred S]

Depending on voltage you can also use Square-D QO with 2-pole breakers (one opens positive, the other opens negative PV wire). Square D says OK up to 125 VDC.

Good info, thanks!

 

[Hedges]

In some cases no fuse is required if the wire is way oversize for the maximum possible current. That is more than just the Isc rating of a panel or two panels in parallel, because PV panels can exceed their ratings under some conditions, also additional safety margin. (And by the way, most fuses or breakers can carry 120% to 150% of rated current for an extended time depending on conditions.)

NEC rules for fuses, disconnects, and grounding have changed over the years and I'm not 100% up on that, have read various articles but not located the wording in the latest NEC rules.

Inverters used to be transformer isolated. Some grid-tie inverters today use non-isolated PV arrays, and I've seen some hybrid inverters like that too.

When I put in SWR2500U inverters around 2005, they had an AC breaker of 15A and either by that or internal design they could never back-feed the array with excessive current. The negative PV wire was effectively grounded. The positive wire did not require a fuse for either one or two PV strings, so long as the wire ampacity was well in excess of what the one or two strings could produce (e.g. 12 AWG for one, 7.5A string or 10 AWG for two, 7.5A strings.) If two, 7.5A strings totaling 15A were wired to the inverter with 12 AWG, that was too close to the maximum fuse allowed for that wire, 20A, so a 20A fuse would have been required. Can't find the exact requirement but something like wire sized for 150% or maybe 125% squared = 156% of Isc to go without fuses.

Later transformerless inverters have PV positive and PV negative wires floating centered around earth potential - there is an ohmic path fro each to the AC grid. For 120/240V split phase, the PV leads sit at a DC potential, but for two legs of 208Y three-phase, they both carry an AC signal.

Some years ago transformerless required fuses on both positive and negative PV wires. That is what I have for my grid-tie inverters of the era. Now I read that 2017 NEC only requires fuses on one side (e.g. Positive) but does require a disconnect for both positive and negative, as in a drawing in the following link. I don't think I agree with that because I can see where a short to ground would carry current, and some of the wires (individual wires to negative side of first PV panel in each string) would be too small for the breaker on AC side of inverter.

Changes to Article 690 in NEC 2017

National Electrical Code (NEC) 2017 will bring some big changes to the world of PV design and permitting in 2018.

www.cedgreentech.com

When you have just two strings of PV it wasn't required (at least originally) to fuse them individually because in case of of a short it would only carry current from one string through the wire to the short. Each string would send current through wire on opposite sides of the short. After combining, wire back to the inverter had to be big enough for both. With 3 or more strings, you do fuse each because two or more strings would dump current into the shorted wire in another string. Each PV panel has a label giving maximum fuse for fusing individual strings.

 

[Hedges]

Here is a 150v DC breaker for less than $20. They are available in multiple ratings:

MidNite Solar Photovoltaic DC Circuit Breaker - 30 Amp, Model# MNEPV30 - Solar Panels - Amazon.com

MidNite Solar Photovoltaic DC Circuit Breaker - 30 Amp, Model# MNEPV30 - Solar Panels - Amazon.com

www.amazon.com

Midnight has breakers with a variety of voltage ratings, and some 2-pole breakers that handle twice the total voltage a single one would by interrupting both ends of the PV connection.
(some systems have longer strings, higher voltage, and some charge controllers accept up to 250 Voc)

One of my early setups had something like 6 ganged 15A 100V breakers segmenting a 600 Voc string into six, 100V sub-strings. That also served to reduce the voltage I was working with when connecting wires to screw terminals on the panels - not using MC connectors. Now with MC I just plug panels together for 600 Voc or less strings and wire that to a switch or touch-safe fuse holders. I unplug MC connectors at both ends of the string (and check with DMM) before putting a screwdriver on the fuse holder terminals.

 

[Fred S]

One day maybe I'll play with 600Voc strings, ha! Hmm, probably not!

I didn't fuse each of my 2-panel strings, so there is only a fuse after they are combined. That's because they are on ground-mounts with nothing combustible around, are only at 70Voc, and I'm cheap.

As far as operating a circuit with no fuse because there's no potential to exceed the wire rating -- in the case of a SCC that could fail to a short, I still would recommend a fuse or breaker because that occurrence could produce a lot of heat.

It is true that a similar situation exists in many of our homes/apts, where we have items plugged in that could short or overheat and start a fire, and yet not draw enough current to trip a breaker. Luckily this is very rare.

 

[Hedges]

As far as operating a circuit with no fuse because there's no potential to exceed the wire rating -- in the case of a SCC that could fail to a short, I still would recommend a fuse or breaker because that occurrence could produce a lot of heat.

A fuse is good but I don't think it will help in that particular situation.

If the SCC shorts PV positive to PV negative, almost no heat would be generated in either the SCC or the wires, because almost no voltage drop across the resistance. The wires will be carrying Isc rather than Imp so about 10% higher current, 20% more heating. All the PV panels and PV cells with be at approximately zero volts because all will be generating Isc, which occurs at V = zero. No voltage across each cell so no electrons leaking back through the diode that is the PV cell, but current flowing through doped semiconductor probably turns the electricity back into heat. (If open circuit, all current generated leaks back through diode that is the PV cell, same amount of heat maybe a different aging mechanism.)

There is no fuse you can use in the PV side of the charge controller that will open reliably in the case of a short but not open (nuisance trip) in normal operation. Cold day full sun (or more than one sun due to reflected light) and MPPT operation is going to be more current than Isc shorted wires on a hot day.

What the fuse will protect against is backfeed from battery due to short inside charge controller.

Where there could still be a problem is a high resistance short or high series resistance. A failed transistor can be higher resistance than a good transistor operating in saturation, for instance. With 3 kW of PV array, up to 3 kW can be dissipated within bad components in the charge controller, a corroded wire connection, or an abraded wire. GFIC is meant to detect arcing in the wiring and interrupt current with no more than 300W being dissipated in the fault.

 

[Fred S]

Yes, the last paragraph is the situation of concern.

For the 3rd paragraph, I don't quite follow but I am probably misunderstanding. In what circumstance would a controller draw more amperage than Isc from an array? Or are you referring to the controller's output amperage? My 25a array fuse never nuisance-blows, but I assume it would blow if a dead short occurs in sunlight (Isc=34a), regardless of the voltage approaching zero. I guess I could give it a try. If so, then my controller being "undersized" is an example of where a PV fuse would indeed be protective, but again I may be misunderstanding.

 

[Hedges]

A PV panel can deliver more than Isc. Isc is what it produces under standard test conditions (panel at room temperature and flashed with one standard sun, which is a particular spectrum and intensity. Measure fast before the panel starts to heat up.) Cold day, direct sunlight on panel, additional light diffused from nearby clouds and reflected off clouds means more than one standard sun.

Your fuses probably won't blow at 100% of rated current because it takes time for the fuse to heat up, etc. but sizing the fuse and wire 25% larger than 125% of Isc (1.56 x Isc) would make that more of a sure thing.

The idea being that a fuse or breaker should never open except in the case of a fault. And as I said earlier, Isc of a panel is too close to Imp to ensure it does open in case of a short. For instance, my SunPower SPR-327NE specs are:

Rated Current Impp 5.98 A
Short Circuit Current Isc 6.46 A

A thermal fuse or breaker can't discriminate between those because it is affected by ambient temperature. Magnetic, maybe. If those figures didn't change with temperature and illumination (which they do), with them being only 8% apart I'd have to design a relatively precision current sense and comparison circuit to actively open a switch. Their variation is so great that Isc in winter will often be less than Impp in summer. Or vice-versa, depending on orientation.

 

[brandonfox]

I am wondering what happens if you have too many solar panels hooked to your system.

Here is my specific scenario:

Using the Growatt SPF 3000TL . I bought it at this store and really think that it's a quite telable shop of solar panels. What happens if I put 10 250W 24V panels out for the charging the system. Since everything will be 24V, things will be in parallel. While I know these are used and will produce less than 100%, I would rather plan for maximum output for safety (and 200W for actual production value).

First, Amps. 10 panels at 100% production would be over 80A input. Will the Growatt just not accept the extra, should it be made?

Next is Volts. Since the Growatt is limited to 2kW input, what happens if more than 2kW is being generated?

I can order less than 10 of these panels. Since shipping is the same for 1 or 10, I figure I might as well get 10 of them. Or should I just get fewer?

I really do appreciate the help of this forum.

I recommend you to stay within the range of the Battery Specifications . Batteries have a maximum safe rate of charge. Depending on the battery chemistry, the maximum C rate of charge is generally between .1 to 1 C.

 

[DASH]

Let's see 10X250 /24 =104 amps. Hope you have plenty of money to shell out for very large electrical cables, good size fuses, and willing to accept the increase fire risk, not to mention spending way more then what You would normally need to for a Solar Charge Controller. Probably a bad idea, unless You have a need to have an unusual setup.