Solar disconnect wiring

nastynasty Posts: 31Solar Expert ✭✭
Hello

I just got a Midnite Solar disconnect and am wondering if the way I am going to wire it looks ok. No shunt and no DC-GFP.
Also can I ground all components to the ground bus in the disconnect and to one ground rod? Or does the inverter and AC loads need a seperate ground? Thanks for any advice.

Comments

  • CariboocootCariboocoot Posts: 17,615Banned
    Re: Solar disconnect wiring

    We need a bit more info here.
    Is this off-grid or grid-tied?
    Grid-tied will require the DCGFI as per the new NEC regs. Even if it's off-grid, if it's subject to inspection, they might "go on about it".
    If the inverter is a true sine unit, then the AC ground rod and DC negative ground can be the same and the AC neutral bonded to ground. Should not be a problem with most equipment. Unfortunately there are always exceptions.

    A shunt is a fixed resistance used for connecting things like battery monitors. For the most part there's no point in having one anywhere if you don't need to have some sort of meter connected across it.

    Others will have additional perspective.
  • NEOHNEOH Posts: 74Solar Expert ✭✭✭✭
    Re: Solar disconnect wiring

    Are DC Circuit Breakers sensitive to the direction of currrent flow ?
    ie The Source (+ Volts) is always on one side and the Load always on the other side?
  • BB.BB. Posts: 24,676Super Moderators admin
    Re: Solar disconnect wiring

    As I understand, some are--The shape/orientation (This Side Up) of the internals helps quench the DC arc when the breaker trips/opens.

    For installations where you have two power sources--Such as a Charge Controller or inverter/charger connected to a battery bank, I would put the "+" side of the breaker to the source that has the highest current source capability--That is generally the battery bank.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Posts: 17,615Banned
    Re: Solar disconnect wiring

    Those that are should be labeled "LINE" and "LOAD" (or similar).
    In this case it is too much of a problem because the breakers won't be connected to a common (+) bus; the PV side (+) and battery side (+) must be separate.
  • BB.BB. Posts: 24,676Super Moderators admin
    Re: Solar disconnect wiring

    Hmmm... Marc is of course correct--The breakers are labeled Line and Load--But that brings up the next problem.

    Not all systems are positive ground--A few are positive ground (plus you have the possible ground inversion caused by the NEC DC GFI Recommendations).

    The polarity of the ARC is important here--not just which side is source and which side is load (at least in my humble opinion/guess).

    When I had Line/Load on various breakers, relays, and such--That was usually AC and they many times had accessory circuits (like a trip wire to the Big Red Button)--So it mattered which side had "power" and which side would be cut when the breaker was opened.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Posts: 17,615Banned
    Re: Solar disconnect wiring

    Er, most systems are negative ground I think. DC breakers are designed to work with same by default. But Bill's point is valid; if the system is "the other way 'round" the breaker (if direction-dependent; not all are) has to be "reversed" because of the purpose of the polarity as he said. It's about making sure there's a good, sudden, complete disconnect. AC breakers usually don't have this spec (but some do) because it's easy to interrupt alternating current since it essentially hits zero Volts sixty times per second. This is why there's such a big Voltage rating between AC and DC on switches/fuses/breakers design to handle both types of current.
  • BB.BB. Posts: 24,676Super Moderators admin
    Re: Solar disconnect wiring

    Thanks Marc,

    I had ground inversion on the mind.

    -Bill "what is left of it" B. :cry:
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • VicVic Posts: 2,461Solar Expert ✭✭✭✭
    Re: Solar disconnect wiring

    Some larger DC-rated breakers do have Line & Load marked, some Have a + on one terminal.

    From the Midnite Forum, this FAQ adds some deatil, escpcially the polarity of the breaker that connects the output of a Charge Controller (MPPT) to the battery bank. And that Magnetic-Hydraulic breakers need to be mounted with the long side vertical, and the up end, up.

    http://midnitesolar.com/smf_forum/index.php?topic=113.0

    Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • nastynasty Posts: 31Solar Expert ✭✭
    Re: Solar disconnect wiring

    Thanks for the replies. Vic thanks for the link that helps. This is an off grid system. All breakers have + side down including line side of the 125A inverter disconnect. I have a meter that comes off of charge controller. DC loads are lights and water pumps. Hope this clears some of the questions up.
  • NEOHNEOH Posts: 74Solar Expert ✭✭✭✭
    Re: Solar disconnect wiring

    caribocoot,
    " ... it's easy to interrupt alternating current since it essentially hits zero Volts sixty times per second ..."

    At my house, the AC crosses zero volts 120 times per second.


    Also,
    Does there need to be a fuse near/at the BAT + terminal?
    What protects those heavy gauge wires from shorting and starting a fire?
  • CariboocootCariboocoot Posts: 17,615Banned
    Re: Solar disconnect wiring
    NEOH wrote: »
    Also,
    Does there need to be a fuse near/at the BAT + terminal?
    What protects those heavy gauge wires from shorting and starting a fire?

    There most definitely should be some form of circuit protection on the battery wire, whether fuse or circuit breaker. It is also a good idea to have it as close to the battery as possible for just the reasons you mention. Blue Sea makes some nifty terminal-mount fuses that work well for this, and there are also breakers and standard "inverter" fuses that can be used. It's all about finding something that has a rating suitable for the current expected (based on wire size; the circuit protection must always be the "weakest link") and that fits in with your particular mounting limitations.
  • nastynasty Posts: 31Solar Expert ✭✭
    Re: Solar disconnect wiring

    So do I still need the fuse at battery even though everything goes through a breaker before battery?
  • BB.BB. Posts: 24,676Super Moderators admin
    Re: Solar disconnect wiring

    I do not believe in double fusing/fuse-breaker on the same wire...

    The issue is to keep the fuse/breaker as close to the battery as practical (some specs. are like 18 inches or so)... Basically, look at how things may fail decide if the + wire from the battery to the first battery breaker/fuse is short/safe from damage. If so--then your current breaker is probably "good enough".

    If, for example, the + wire can get cut by something falling on it, pinched in a metal door, cut by sharp sheet-metal, etc.--Then you may want to look at a fuse/breaker closer to the battery.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nastynasty Posts: 31Solar Expert ✭✭
    Re: Solar disconnect wiring

    Batteries are about 3' away from breaker. There is nothing that will damage cable. The fuses Cariboocoot mentions are not expensive, is there any reason not to put one at the battery? If not what size? My cables are 1/0, I think that is 150A. 100A fuse?
  • BB.BB. Posts: 24,676Super Moderators admin
    Re: Solar disconnect wiring

    A 150 Amp cable can use a 150 amp fuse and should carry a maximum of:
    • 150 amps * 0.80 NEC derating = 120 amps maximum continuous current

    You should have a couple extra fuses stored with the application...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nastynasty Posts: 31Solar Expert ✭✭
    Re: Solar disconnect wiring

    Thank you Bill.
  • SolaRevolutionSolaRevolution Posts: 407Solar Expert ✭✭✭✭✭✭
    Re: Solar disconnect wiring

    Here's a nit to pick.
    Vic wrote: »
    From the Midnite Forum, this FAQ adds some deatil, escpcially the polarity of the breaker that connects the output of a Charge Controller (MPPT) to the battery bank.
    http://midnitesolar.com/smf_forum/index.php?topic=113.0
    Vic

    I disagree with Halfcrazy (from Midnite) on one point:
    "In a PV combiner the + sign marked on the breaker connects to the PV positive output."

    Here is why:
    - "The + sign designates the highest potential should be conneced there."
    - "The main job of this output breaker is to trip when and if there is a catastrophic failure."
    - "Since these two terminals inside the charge controller are normally connected up to a very large battery bank, you have a direct short across the battery bank if the controller fails. During this condition, the controller is acting like a piece of wire."

    This says to me that the PV combiner breakers should be hooked up with the + marks towards the charge controller.( towards the inverter for batteryless) Because if one string in a multi-string installation has a fault it will receive current, backfed through the combiner circuit, with all available current from all of the other strings or by current comming from the battery via. a faulted charge controller.

    Alex Aragon
  • BB.BB. Posts: 24,676Super Moderators admin
    Re: Solar disconnect wiring

    I agree with Alex/SolaRevolution on that interpretation. The "excess" current source is the Solar Array common bus.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • VicVic Posts: 2,461Solar Expert ✭✭✭✭
    Re: Solar disconnect wiring

    Alex,

    To further pick a nit ...

    Quoting from the linked MN FAQ, "In a PV combiner the + sign marked on the breaker connects to the PV positive output. The same breaker when hooked up to the battery circuit (not in a PV combiner) hooks up a little different. The + sign hooks up to the battery plus".

    I read this to mean that in the case of the PV Combiner, the + breaker terminal is connected to the PV module's + wire. THE + on the PV module, NOT the CC's PV INPUT + terminal.

    So, IMHO, you and HC are in agreement. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • westbranchwestbranch Posts: 4,144Solar Expert ✭✭✭✭
    Re: Solar disconnect wiring

    Been mulling this interesting Q over and rereading both postings and a BF occurred when I read this part again...

    This is an easy one to determine in a PV combiner. Follow the current path through the combiner, into the PV input of a charge controller and out of the controller to an output breaker and then into the battery plus. You would think that the end of the output breaker connected to the controller would be at a higher potential than the battery plus. In normal operation this is true. The main job of this output breaker is to trip when and if there is a catastrophic failure.......During this condition, the controller is acting like a piece of wire. The battery positive terminal is the highest potential!

    It occurs to me, simply, that if one wants to Keep the Magic Smoke IN (think: in either the PVs or the Battery) the breaker + needs to be pointed towards the GREATEST CURRENT Potential... at whichever connecting point you are at, Combiner or Battery.

    the more I think about it the clearer it gets... but still a bit confusing
     
    CL 647 asleep  24V 900Ah C&D AGM
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  • BB.BB. Posts: 24,676Super Moderators admin
    Re: Solar disconnect wiring

    The greatest "Current Potential"... And what failures you are protecting against (down stream shorts overheating the wiring, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SolaRevolutionSolaRevolution Posts: 407Solar Expert ✭✭✭✭✭✭
    Re: Solar disconnect wiring
    Vic wrote: »
    Alex,

    To further pick a nit ...

    Quoting from the linked MN FAQ, "In a PV combiner the + sign marked on the breaker connects to the PV positive output. The same breaker when hooked up to the battery circuit (not in a PV combiner) hooks up a little different. The + sign hooks up to the battery plus".

    I read this to mean that in the case of the PV Combiner, the + breaker terminal is connected to the PV module's + wire. THE + on the PV module, NOT the CC's PV INPUT + terminal.
    So, IMHO, you and HC are in agreement. Vic

    Vic,

    I disagree with your statement that HC and I are in agreement.:roll:
    The (+) labled side of the breaker needs to be connected to the CC side of the combiner, not the PV module's positive output.
    See the diagram:
    Attachment not found.

    Alex
  • westbranchwestbranch Posts: 4,144Solar Expert ✭✭✭✭
    Re: Solar disconnect wiring
    Vic,

    The (+) labled side of the breaker needs to be connected to the CC side of the combiner, not the PV module's positive output.

    Alex

    Alex is this true where there is a battery vs a GT?

    As I see it, in a battery system, if/when there is a breaker at/near the battery this eliminates the need for the + to point towards the CC. that is, if the battery breaker trips the CC becomes a null current device, ie no potential as the path is broken and the potential is now the PVs.
    Eric
     
    CL 647 asleep  24V 900Ah C&D AGM
    CL29032 FW 2079/ 2073/ 2054 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,Omnicharge 3024,
    Linksys Wet54g WiFi Bridge, ASUS RTN10 router, Hughes1100 Sat Modem
     Eu3000i & 1000i Gens, 1680W & E-Panel/WBjr to come,
    West Chilcotin, BC, Canada



  • SolaRevolutionSolaRevolution Posts: 407Solar Expert ✭✭✭✭✭✭
    Re: Solar disconnect wiring
    westbranch wrote: »

    As I see it, in a battery system, if/when there is a breaker at/near the battery this eliminates the need for the + to point towards the CC. that is, if the battery breaker trips the CC becomes a null current device, ie no potential as the path is broken and the potential is now the PVs.
    Eric

    Well, it is possible that a fault in the combiner output circuit could receive fault current back-fed from the battery via the CC. This is quite possible with a PWM charge controller even while it is working properly. It is my understanding that an MPPT CC would not backfeed current unless there is a major malfunction in the CC. With the MPPT CC, the CC to battery wiring is going to have a higher current rating than the PV to CC wiring. The CC/battery breaker is too high of a current rating to properly protect the PV/CC wiring. A wire with an oversized breaker on it can do a lot of arcing without tripping the over-rated breaker. It does seem unlikely for a fault to occur in both the CC and the PV output circuit at the same time except for the case when one may cause the other.

    The more common and likely hazard is the failed PV module. Panels have failed for no apparent reasons and others have been broken by bullets, arrows, baseballs, tree branches, horses, elk, bicycles, idiots...
    The circuits in the PV modules usually have a current rating of +/- 15 amps. (some as low as 6 amps) There is no way for the output current of a string of PV modules to trip it's own overcurrent protection. The only way for the overcurrent protection to trip is by being back fed from other parallel strings or from a CC fault.

    Imagine if the PV module in the pictures below were in a system with 6 parallel strings. The fault current could be more than 3 times the fuse rating of the module. If it was a 100kw GT system it could be +50 times the fuse rating with a high volt DC arc. You probably would not use breakes in such a system but you may very well have multiple combiner devices with similar fault direction issues.

    Attachment not found.Attachment not found.

    Alex
  • westbranchwestbranch Posts: 4,144Solar Expert ✭✭✭✭
    Re: Solar disconnect wiring

    OK, so what I said needs to have an addition at the end

    " If/when The PVs become a current source and as this potential has nowhere to go, it can back-feed the parallel panels, so the + breaker side must point towards the PV disconnect. "
     
    CL 647 asleep  24V 900Ah C&D AGM
    CL29032 FW 2079/ 2073/ 2054 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,Omnicharge 3024,
    Linksys Wet54g WiFi Bridge, ASUS RTN10 router, Hughes1100 Sat Modem
     Eu3000i & 1000i Gens, 1680W & E-Panel/WBjr to come,
    West Chilcotin, BC, Canada



  • SolaRevolutionSolaRevolution Posts: 407Solar Expert ✭✭✭✭✭✭
    Re: Solar disconnect wiring

    I think we are on the same page, Eric.

    Check out this pic of a failed string. This could never happen unless this burned up string was receiving parallel current.
    http://www.curcap.com/wp-content/uploads/2011/12/Panelfire_2.jpg
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