Midnite Disconnecting combiner fuse sizing /isolation question

new2PV

Quick question, the panels I want to spec have a Isc of 9.1 amps, and I want to use a midnite solar disconnecting combiner, should I go with 10 amp breakers for each string or 15 amp breakers. Also the CEC code states that one of the DC conductors must be grounded on systems over 48 volts or both wires have to be isolated. My( - )Lead will be grounded internally through the GFCI jumper in the Mppt 60-150 charge controller or the GFCI won't work. Do i have to isolate both DC lines at the disconnecting combiner?

Vic

Hi New2..,

Normally, PV string breakers would be sized to 1.56 times the Isc rating of the PVs. But, the common MidNite MNEPV breakers have a Continuous rating, so, the factor would be 1.25 times the Isc of your PVs.

However, it is customary to just use the Maximum Fuse rating of the PVs that you are going to use. Often, this is 15 Amps for many of the 60-cell PVs that are rated less than 300 watts.

This Max Fuse rating should be on the PV spec sheet, and also on the label on the back of the PV.

Using the Max Fuse rating could possibly eliminate nuisance tripping of the PV string breakers with reflections from snow, or perhaps even from water.

EDIT: Oh, and Schneider should have good wiring diagrams on wiring the PV Combiner into the rest of the system.

FWIW, Vic

new2PV

Thanks Vic, however I'm still confused on the local code for the disconnecting combiner. Do I need to disconnect both lines at the combiner or not? What's the nec ruling it should
Be similar to the Canadian code?

Vic

Hi New2..,

I am trying to duck your question.

Here is the manual for the MNPV6 Disconnecting Combiner -- we do not know which one that you are planning to use, AND, I have no familiarity with the Code in your country ... :
http://www.midnitesolar.com/pdfs/MNPV6Disco_manual.pdf

IMO, it is uncommon to need to disconnect each of the two PV conductors.

If you could consult the Manual for the exact Combiner that you plan to use, the diagrams should apply to your situation.

In the US, each Jurisdiction will provide its own requirements. We would contact the Authority that will inspect the system, and have them specify what is required of a system, and design and implement the system to meet those requirements. Sometimes, we might hire an Engineering Consultant, familiar with the local Codes for guidance.

Since I have no familiarity with the specific requirements of the CEC, and Local Codes, it would be irresponsible to pass judgment on just what is require.

I do not want to mislead anyone. For What Little It Might Be Worth. Good Luck Vic

Ethan Brush

new2PV wrote: »

Thanks Vic, however I'm still confused on the local code for the disconnecting combiner. Do I need to disconnect both lines at the combiner or not? What's the nec ruling it should
Be similar to the Canadian code?

Generally where a disconnecting means is required, it only disconnects the ungrounded conductor(s). Additionally, there is typically not a fuse in a grounded conductor. Does that answer your question?

new2PV

Ethan Brush wrote: »

Generally where a disconnecting means is required, it only disconnects the ungrounded conductor(s). Additionally, there is typically not a fuse in a grounded conductor. Does that answer your question?

Yes and no but the charge controller has a fuse for the ground fault, I;m not sure what will happen if it blows. THe mppt-60-150 has built in ground fault protection with a 1 amp fuse. If the system is grounded anywhere else but internally in the charge controller than the PVGFCI becomes ineffective. Here are some of the rules..Vic that is the correct combiner you posted about that I want to use.

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inetdog

Ethan Brush wrote: »

Generally where a disconnecting means is required, it only disconnects the ungrounded conductor(s). Additionally, there is typically not a fuse in a grounded conductor. Does that answer your question?

And in an ungrounded system, all of the conductors are ungrounded. So the disconnect would have to interrupt both. As would the overcurrent protection.
If the system is ungrounded there must be a ground fault detector, which usually ends up incorporated into the inverter.

vtmaps

new2PV wrote: »

Yes and no but the charge controller has a fuse for the ground fault, I;m not sure what will happen if it blows. THe mppt-60-150 has built in ground fault protection with a 1 amp fuse. If the system is grounded anywhere else but internally in the charge controller than the PVGFCI becomes ineffective.

When the fuse blows you have a completely ungrounded system. That's the problem with this type of GFCI. Forum moderator BB has written a 'white paper' on the subject. Many of the smartest folks have purposely defeated their GFCIs with a permanent bond, but none of them will advise you to do so because it violates code.

By the way, there have been at least two documented fires caused by the use of GFCIs. Those who write the code (especially John Wiles) apparently feel that the benefits of GFCI outweigh the danger.

I believe that Outback makes an external GFCI that has a 1 amp breaker ganged to two 63 amp breakers. When the 1 amp breaker trips it trips the two ganged breakers which can be used to disconnect both PV pos and PV neg.

Another problem with internal GFCI is that if you have several controllers the fault current is divided among several 1 amp fuses and the GFCI is then less sensitive to fault currents.

--vtMaps

BB.

If you want to see way to much writing on the subject of DC GFI:

http://midnitesolar.com/Forum/index.php?topic=142.0
http://forum.solar-electric.com/forum/solar-electric-power-wind-power-balance-of-system/solar-beginners-corner/9345-system-grounding?t=10650

Here is my white paper:

http://www.midnitesolar.com/pdfs/DC-GFP-Draft3-5.pdf

The DC GFI breaker/fuse issue should be getting long in the tooth... That 4 years later it is still a question/issue goes to show how slow the regulatory system is, and how difficult it is for mfg. to design systems that can address fundamental problems (arc fault caused fires in this case).

-Bill

new2PV

After reading your White paper I am shocked in the last 4 years nothing has been done about the potential hazards. I know its expensive but I may have to go with the morningstar GFPD-150, just so I feel safer..

BB.

Arc fault is pretty rare, especially if everything is designed and installed correctly.

DCGFI is a poor method to address the DC arc fault issue. And, DCGFI should never have been suggested as a solution.

MorningStar dc arc fault breaker is certainly a much better solution, but is it cost effective? That I do not know.

And my condolences for you reading my paper.

-Bill