Paralleling high volt low amp panels

bbbuddy · October 2010

I have 10 Dupont DA100-A2 panels, Imp 1.34, Vmp 75, VOC 100, ISC 1.66.

Outback FM60 controller.

We have concreted in two 6 inch pipes, and two 8 inch pipes (because we already had them) and we plan to mount 4 panels on top of each pipe (Eventually. The four pipe allow for expansion to 16 panels, and we only had to pour concrete once)

The pipes are 12 feet apart in a square.

We plan to mount a combiner box on one of the pipes.

I have been reading about the wiring, and getting confused. Some posts say no more than 2 panels in parallel before going to a breaker, some posts say a breaker is needed for more than 3 panels, etc.

Because these are high voltage low amp panels, they all need to be paralleled before the FM60. No series wiring.

So my question is, can I parallel 4 panels per pipe, then a run of 1 + and 1- to a breaker at the combiner box, or do I need to parallel no more than 2 panels on each pipe mount, then run 2+ and 2- to 2 breakers at the combiner box?

I can't size a combiner box until I figure this out...plus don't know whether to buy multibranch "y" and "t" MC4 connectors or just use split bolts..

thanks!

BB. · October 2010

Re: Paralleling high volt low amp panels

If I understand your question correctly...

If you have 1 or 2 parallel strings of solar panels, you do not need series fusing/breakers (i.e., combiner box + breakers).

If you have 3 or more parallel strings of solar panels, EACH STRING needs a series fuse/breaker.

If you, for example have two parallel panels on one mount, and two parallel panels on a second mount--Each panel/string needs a series protection device (you cannot combine pair A on the first mount and add one series fuse on that output and combine with Pair B/C/etc.).

Basically, with only two parallel strings, any short circuit (in panel wiring or solar panel)--The maximum current is Isc flow anywhere in array/wiring is 1x ISC.

With 3 or more parallel strings, the Isc current flow in any wire/panel can be (N-1)xIsc (i.e., the other arrays can back feed the sort and add up Isc > short circuit max rating).

-Bill

bbbuddy · October 2010

Re: Paralleling high volt low amp panels

BB. wrote: »

If I understand your question correctly...

If you have 1 or 2 parallel strings of solar panels, you do not need series fusing/breakers (i.e., combiner box + breakers).

If you have 3 or more parallel strings of solar panels, EACH STRING needs a series fuse/breaker.

If you, for example have two parallel panels on one mount, and two parallel panels on a second mount--Each panel/string needs a series protection device (you cannot combine pair A on the first mount and add one series fuse on that output and combine with Pair B/C/etc.).

Basically, with only two parallel strings, any short circuit (in panel wiring or solar panel)--The maximum current is Isc flow anywhere in array/wiring is 1x ISC.

With 3 or more parallel strings, the Isc current flow in any wire/panel can be (N-1)xIsc (i.e., the other arrays can back feed the sort and add up Isc > short circuit max rating).

-Bill

So I guess what I'm missing is, what constitutes a string?

None of the panels can/will be wired in series, but are you saying it doesn't matter how many panels are wired in parallel?
Can I wire all 4 panels on a top mount in parallel, then run the + and - to the combiner box, to one breaker, and do that for all four panels on each top mount? Then I would only need 4 breakers for 16 panels on the four top mounts??

And if so, the breakers would be 4 x 1.34(IMP) x 1.56, then round up? 10 amp breakers?

BB. · October 2010

Re: Paralleling high volt low amp panels

Parallel is parallel... whether it is 4 high voltage panels connected in parallel or 3 panels in series (a string) then 4 of those strings connected in parallel (still four fuses/breakers in each case).

So, the normal setup for 3+ parallel stings (or single panels) connected in parallel should look like this (2 panels in series x 4 parallel strings--note, the fusing remains the same if only one panel instead of two in series):

[FONT=Fixedsys]                      fuses/breakers
[/FONT][FONT=Fixedsys]+----[panel+]--[panel+]----o^o----+
+----[panel+]--[panel+]----o^o----+
+----[panel+]--[panel+]----o^o----+
+----[panel+]--[panel+]----o^o----+----[positive to charger]
|
+--------------------------------------[negative to charger][/FONT]

Now, I have not looked closely at the Dupont high Vmp panels before. I searched around for the specifications for the series protection fuse and did not find any listed (data sheet here).

The Imp=1.3 amps and Isc=1.55 amps...

Normally, I would guess that Ifuse would be around 2xIsc or ~3.0 amps (or closest standard fuse/breaker).

Normally, we are working with higher current panels that have 12-14 AWG wiring and Ifuse ~ 10-15 amps.

So--the question is what is the gauge of wire for this panel and it ETL rated maximum short circuit current...

If the panel has 12 awg wiring and is rated for 15 amp fuse--then you could parallel:

15 amps / ~1.55 amps = ~9

You could parallel upwards of 5-9 strings in parallel without a fuse/combiner box. If--however--the panel is truly rated for a ~3 amp fuse, then the statement above (more than two panels/strings in parallel, each string requires a fuse) is true.

You should contact your retailer and get the series protection fuse rating from them/Dupont.

If they are unable to supply you with a fuse rating (should be very unlikely), the conservative action would be a ~3 amp fuse per string as in the drawing).

-Bill

bbbuddy · October 2010

Re: Paralleling high volt low amp panels

Bill the wire size on the panels is 10 awg. No idea of the IFuse. I guess this probably means I CAN parallel at least 4 before the combiner box...

I will try to contact sunelec.com, and/or Dupont about the IFuse.

So if I get an 8 breaker box, I could parallel up to 32 panels, and I doubt I ever will get that high...but you never know.

BB. · October 2010

Re: Paralleling high volt low amp panels

The power of positive thinking!:D

-Bill

bbbuddy · October 2010

Re: Paralleling high volt low amp panels

Ok, I found out from dealer that the fuse rating is 8.

So...if I parallel 4 panels into one 8 amp breaker, I would be slightly over...

4 x 1.34 Imp = 5.36, but aren't you supposed to then multiply by 1.56 for added protection?

Which then puts it at 4 x 1.34 Imp x 1.56 = 8.36

So would you use an 8 amp breaker or go up to a 10 amp?

BB. · October 2010

Re: Paralleling high volt low amp panels

Remember it is all power sources into the panel.

8 amp fuse plus 3x1.56 equals roughly 12.5 amps into a sorted panel...

Much more than the 8 amp rating (unless you only have 4 panels total).

-Bill

bbbuddy · October 2010

Re: Paralleling high volt low amp panels

Well Bill I'm just getting confused by your replies.
Each time I read them I get more confused.

4 panels that put out 1.34 amps, paralleled into an 8 amp breaker, how does that add up to 12.5 amps?

BTW, I would try to get an answer locally, but our local solar co just says to split bolt all the panels together and run it straight into the controller, who needs breakers. And I know that can't be good....

BB. · October 2010

Re: Paralleling high volt low amp panels

You have 10 panels to install--Correct?

a) 4 panel and 1x 8 amp breaker
b) 4 panel and 1x 8 amp breaker
c) 2 panel and 1x 8 amp breaker

All 3x 8 amp breakers tied together and feed a single charge controller.

You get a short in one panel at a) somewhere... Three panels feed shorted panel 3xIsc.

Plus b) and c) feed back through a)'s 8 amp breaker (combined input to charge controller)...

Now you have 8 amp (breaker) + 3xIsc from other panels on string a) feeding into the one shorted panel in a)

Is this going to cause a fire? Don't know... However, you do exceed the 8 amp maximum series fuse rating by combining 4x panels with only one fuse/breaker that can be back feed from other panels feeding the common controller.

If you put blocking diodes on each fuse/breaker output, so there could be no back feed---that would "solve" the common fusing back feed problem. (note that one diode still would not meet code--it would take two in series--diodes frequently fail shorted--plus you have more losses from the diode voltage drops).

I would guess that if you connect two panels in parallel behind one 4 amp fuse/breaker--You would meet the intent behind the ETL 8 amp maximum short circuit current limit...

-Bill

PS: I guess I should be using Isc=1.66 amps for the panel max current (the earlier reply was typed on my phone and I missed the Isc rating).

bbbuddy · October 2010

Re: Paralleling high volt low amp panels

BB. wrote: »

You have 10 panels to install--Correct?

a) 4 panel and 1x 8 amp breaker
b) 4 panel and 1x 8 amp breaker
c) 2 panel and 1x 8 amp breaker

All 3x 8 amp breakers tied together and feed a single charge controller.
Yes, this is what I thought we could do, but wanted to know how to determine how many can go on one breaker...

You get a short in one panel at a) somewhere... Three panels feed shorted panel 3xIsc.

Plus b) and c) feed back through a)'s 8 amp breaker (combined input to charge controller)...

Now you have 8 amp (breaker) + 3xIsc from other panels on string a) feeding into the one shorted panel in a)

Is this going to cause a fire? Don't know... However, you do exceed the 8 amp maximum series fuse rating by combining 4x panels with only one fuse/breaker that can be back feed from other panels feeding the common controller.

If you put blocking diodes on each fuse/breaker output, so there could be no back feed---that would "solve" the common fusing back feed problem. (note that one diode still would not meet code--it would take two in series--diodes frequently fail shorted--plus you have more losses from the diode voltage drops).

I would guess that if you connect two panels in parallel behind one 4 amp fuse/breaker--You would meet the intent behind the ETL 8 amp maximum short circuit current limit...

ok, how would the panels connected two to a breaker not backfeed the same way? Don't the amps backfeeding remain the same amount?

Most importantly is: how are multiple panels ordinarily paralleled? What am I missing here, it seems like wanting this answer has gotten way over my head technically ...

all I really am taking from it in total is that you recommend not paralleling more than two panels into each breaker.

-Bill

PS: I guess I should be using Isc=1.66 amps for the panel max current (the earlier reply was typed on my phone and I missed the Isc rating).

Ok, I was incorrect in using the Imp amps then...so my numbers would be 2 panels x 1.66 x 1.56 = 5.179 so the breakers should be 6 amps?

Thanks Bill, I guess you can see why I was getting frustrated reading other posts about wiring the panels, none of them I could find dealt with just paralleling multiple panels, no series connections..

BB. · October 2010

Re: Paralleling high volt low amp panels

The way I do the over current analysis (when I used to design power systems for computers)--I would imagine a short in the worst possible location (say, pick the positive wire leading into a single panel).

Now, look at the sources of current that will flow through the short.

First, there is the current from the attached panel (basically Isc). Then there is the current from any panels sharing the common circuit (wire)... That was 1 or 3 (2 or 4 total panels). And past the circuit breaker (8 amp from vendor) and any attached panels/power sources that can supply current backwards through the 8 amp breaker.

So, I have Isc from the panel coming in one side from the short. And I have 1 (or 3)x Isc coming from local panels, and I have Ibreaker from any other common arrays beyond the 8 amp breaker.

So, if I wanted to reduce the number of circuit breakers, I would look at:

Isc=1.66 amps
Series Fuse = 8 amps

So, any combination of Isc+Ifuse (from any other solar arrays) that would be <=8amps might look like:

8amp max - 0xIscx1.25 = 8amp - 0x1.66ax1.25 = 5.9 amp fuse maximum (1 panel)
8amp max - 1xIscx1.25 = 8amp - 1x1.66ax1.25 = 5.9 amp fuse maximum (2 panels)
8amp max - 2xIscx1.25 = 8amp - 2x1.66ax1.25 = 4.2 amp fuse maximum (3 panels)

And number of panels * Isc * 1.25*1.25:

1x Isc x 1.56 = 1x 1.66 x 1.56 = 2.6 amp fuse minimum
2x Isc x 1.56 = 2x 1.66 x 1.56 = 5.2 amp fuse minimum
3x Isc x 1.56 = 3x 1.66 x 1.56 = 7.8amp fuse minimum

So, from the above back of the envelope calculations--It would appear that you could parallel two panels safely (i.e., to code) and functionally if you could fine a fuse/breaker rated between 5.2 amps and 5.9 amps (perhaps round to 5 - 6 amps--code does allow a bit of rounding to next standard sizes--if I recall correctly).

Normally, the correct way is to simply place one fuse/breaker per panel string (a series string is one or more panels all connected in series) when you have three or more parallel strings of panels connected together.

Remember that combining panels in series only affects the voltage, not the current flow---so, for a fuse/breaker/wiring point of view, it does not matter how many panels are in series (1 panel, 10 panels, or 20 panels)--there is only the 1x Isc/Imp current flow.

If you look at my ASCII drawing--There is one fuse/breaker per series string, (in my drawing, there are two panels in series, but it could be 1, 5, 10 etc.) and the outputs of all breakers (4 in this case) are then connected all together and sent back to the charge controller.

The rule of two parallel strings maximum for no breaker/fuse assumes that there are only two parallel strings total. If you have other power sources 9say other arrays behind their own breakers, then they can back feed a short into the first array (one or two or etc. panels). That is why I would not normally tell somebody to put two panels behind a 6-8 amp breaker, and then tie it to other parallel strings in the same way.

In your case, 2*Isc*1.25 (note NEC safety factor) is the minimum fuse rating that you should use so that you can avoid false trips.

The panel "system" is designed and constructed to support up to an 8 amp fuse without overheating/failing in such a way as to cause an unacceptable risk of fire/injury.

Sorry, am I making any sense at all?

-Bill

bbbuddy · October 2010

Re: Paralleling high volt low amp panels

Are you making sense? Probably, to someone who is an engineer. To me? no.

But my husband has read this several times and believes what you really are saying is one breaker per panel, because they are all paralleled. Am I right?

BB. · October 2010

Re: Paralleling high volt low amp panels

Yes. That is the standard way of doing it.

-Bill

bbbuddy · October 2010

Re: Paralleling high volt low amp panels

BB. wrote: »

Yes. That is the standard way of doing it.

-Bill

whew, that's all I needed to know....one breaker per panel.

thanks.

Paralleling high volt low amp panels

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