Help with breaker / fusing sizing

stereoman405stereoman405 Solar Expert Posts: 56 ✭✭✭✭
Just need some help figuring out what size breakers to put in the following locations:

- Combiner box at PV array.

- Just before charger controller.

- Just leaving charge controller, heading into batts.

My other locations, such as inverters, I'll just be using the manufacturer recommended wire size and fusing.

For specifics I'll have (4) REC 210's in (2) series strings put into parallel at my combiner, then hooked up to a 60A Xantrex controller connected to a 12v bank.

- REC 210 Specs: 28.1v Vmpp & 7.5A Impp.

My math led me to a 15A DC breaker for each 'series' at the combiner (56.2V @ 7.5A), and a 30A DC breaker for the 'IN' of the charge controller (56.2v @ 15A), and 60A for the 'OUT' of the charge controller. (I picked 60A because that is the biggest breaker offered through NAWS's store, and I figured the Xantrex can't output more than 60A anyways.)

Is there a general rule of thumb for sizing? I thought I've read that you just take the max current from the item and multiply by 1.5. That's basically what I did, then rounded up to get to the closest breaker that NAWS offers.

If you want to double check my inverter plans, I have an 1800w and 1000w Xantrex mod sine wave inverter. I won't be using them simultaneously, and I won't have a heavy load on either when running. I plan on:

- 1000w Inverter - 2 AWG - 110A fuse
- 1800w Inverter - 4/0 AWG - 200A fuse

Am I in the ball park with all this?

Comments

  • audredgeraudredger Solar Expert Posts: 272 ✭✭
    Re: Help with breaker / fusing sizing

    Something like a 10 amp breaker at the end of each series string .... 20 amp at the controller input .... 60 amp OK on controller output.
    Is there a general rule of thumb for sizing? I thought I've read that you just take the max current from the item and multiply by 1.5.
    As a "general rule" I read the data plate or the manual .... fuse / breaker size is generally stated!

    As to the wire size; how far are you running?
  • stereoman405stereoman405 Solar Expert Posts: 56 ✭✭✭✭
    Re: Help with breaker / fusing sizing

    From PVs to controller about 75'. I was thinking 8 AWG. Inverters will easily be within 3' - 4' of batts.

    I figured the fusing for the panels and the charge controllers would be up in the air since there is an almost infinite amount of combinations you could do. How would the manufacturer know if you were imputing 12v at 6A vs 90v @ 5A? I've been reading the manuals of the two controllers I'm torn between but haven't seen that info yet. I'll do some more digging. I just figured there was a formula you could use when trying to plan this stuff out since it varies sooooo much.
  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Re: Help with breaker / fusing sizing
    Just need some help figuring out what size breakers to put in the following locations:

    - Combiner box at PV array.
    Yes, using the Series Fuse Rating of the solar panel, if more than two parallel strings, then one fuse/breaker per string.
    - Just before charger controller.
    fuse, breaker, switch, or nothing--your choice. Lots of people like a switch/breaker to turn off power easily for service of the controller. Normally not required unless you have a whole bunch of parallel solar panels and their Isc>Panel Input Current rating...
    - Just leaving charge controller, heading into batts.
    Actually, the "dangerous current source" is the battery bank. Any wires connected to the battery bank should have their fuses/breakers located close to the battery bank.
    My other locations, such as inverters, I'll just be using the manufacturer recommended wire size and fusing.

    For specifics I'll have (4) REC 210's in (2) series strings put into parallel at my combiner, then hooked up to a 60A Xantrex controller connected to a 12v bank.
    If you only have one or two "parallel" panel strings--then you do not need a series protection fuse/breaker. If you plan on adding more parallel panels later, you can add a combiner box/breakers now.
    - REC 210 Specs: 28.1v Vmpp & 7.5A Impp.
    Check the specifications for the series fuse requirement--I would bet it will be 15 amps (or one standard size up).
    My math led me to a 15A DC breaker for each 'series' at the combiner (56.2V @ 7.5A), and a 30A DC breaker for the 'IN' of the charge controller (56.2v @ 15A), and 60A for the 'OUT' of the charge controller. (I picked 60A because that is the biggest breaker offered through NAWS's store, and I figured the Xantrex can't output more than 60A anyways.)
    Again, the 15a and 30a breakers are not required for your current installation plans. No problem if you want to add them now. Remember that the wiring needs to be sized to handle your maximum current (30 amps in this case).

    Remember that wiring and circuit breakers when "rated" for 60 amps is the maximum safe limit where fusing/breakers/etc. will begin to trip. For actual 100% rated current, you need the NEC 1.25 factor... So, a 60 amp circuit should never be more than:
    • 60 amps / 1.25 = 48 amps
    If you plan on running your 60 amp MPPT controller at 60 amps--then you would need fusing/breakers/wiring rated for:
    • 60 amps * 1.25 = 75 amps minimum (80 amps next standard rating)
    Is there a general rule of thumb for sizing? I thought I've read that you just take the max current from the item and multiply by 1.5. That's basically what I did, then rounded up to get to the closest breaker that NAWS offers.
    The NEC safety requirement is 1.25x the maximum current.

    For solar installations they want 1.25x1.25=1.56 as their margin of safety for the solar panel Isc current (I think NEC is out of their minds for some of this--but it does not hurt to have heavy wires).... Also, the whole safety multiplication issue really depends on PWM or MPPT type charge controllers. PWM cannot limit heavy currents and can burn out. MPPT does control current/power (switch mode power supply)--and is more rugged in this respect.
    If you want to double check my inverter plans, I have an 1800w and 1000w Xantrex mod sine wave inverter. I won't be using them simultaneously, and I won't have a heavy load on either when running. I plan on:

    - 1000w Inverter - 2 AWG - 110A fuse
    - 1800w Inverter - 4/0 AWG - 200A fuse

    Am I in the ball park with all this?
    Assuming 10.5 minimum operating voltage (low battery, voltage drop), 80% efficiency, and 1.25x wiring/breaker derating:
    • 1,000 watts * 1/10.5 volts * 1/0.80 * 1.25 = 149 amp minimum
    • 1,800 watts * 1/10.5 volts * 1/0.80 * 1.25 = 268 amp minimum
    So--the actual maximum current depends on you loads--if you never plan on using more than 500 watts on your 1kW inverter--you could step back on the wire/breaker sizes.

    Also, check the manuals--I am just using generic numbers to make the above calculations.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stereoman405stereoman405 Solar Expert Posts: 56 ✭✭✭✭
    Re: Help with breaker / fusing sizing

    Wow, thanks Bill, so to sum up, ASSUMING I'm NOT planning on adding any panels in the near future:

    - No need for combiner box / breakers where I need to parallel my two series together. (I'm guessing I could just use a Multibranch Connector?)

    - No need for a breaker at all pre-charge controller, except for 'shutoff convenience'.

    - Assuming I'll be pushing the MPPT to the full 60A, put an 80A fuse between it and the battery bank. Closer to the bank the better.

    - Inverters = OK to step back on fusing, just do the math first to make sure I'm not pushing it.

    - Forgot to mention my AC to DC backup charger. If I'm using a Trucharge 20, I should put a 25A fuse between it and the batts?

    I'm curious why I essentially don't need any fusing pre-charge controller. Do the panels themselves already have protection built in, or is the current so low with a smaller setup like this that its just a non issue? Unless of cource I'm misunderstanding here.
  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Re: Help with breaker / fusing sizing
    - Forgot to mention my AC to DC backup charger. If I'm using a Trucharge 20, I should put a 25A fuse between it and the batts?
    Yep.
    I'm curious why I essentially don't need any fusing pre-charge controller. Do the panels themselves already have protection built in, or is the current so low with a smaller setup like this that its just a non issue? Unless of course I'm misunderstanding here.
    Solar panels are naturally current limited--basically, at 1,000 Watts per sq.meter they output Isc maximum... A bit more light (reflection from snow, etc.) can increase current proportionally. Hot panels, a bit more current--Hence the use for 1.25*1.25 derating on the solar panels.

    PWM are like a switch--Closed, current flows. Switch Open, current stops. No ability to limit current to XX amps maximum.

    With MPPT controllers--however, they are both current and power limited by the electronics. So, you can put (at least some) MPPT controllers directly on a 24 volt lead acid battery bank and the controller will not take any more power than it needs/exceeds its specifications.

    Switched-mode power supply - Wikipedia,

    Sort of like a battery charger or any electronic power supply on a 120 VAC main... They are naturally current/energy limited unless something fails--The reason for Fuses/Breakers to protect the home's wiring.

    -Bill

    PS: Read about Buck Mode power supply here... Many/most MPPT charge controllers are some version of this type.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stereoman405stereoman405 Solar Expert Posts: 56 ✭✭✭✭
    Re: Help with breaker / fusing sizing

    Awesome news. You just cut down some of the cost and complexity I was looking at. Though I can already see the next hurdle....

    How do I transition from the MC4 multibranch connector to the 8 AWG I was planning on running the 75' to the MPPT?

    Or for the simplicity would it be worth the loss to just use one of these pre-made cables: http://store.solar-electric.com/100fomc4so2e.html Even though its 10 AWG?

    (I found this referenced in another thread that could be helpful for others with the same "fusing at the p[panels" question: http://www.nmsu.edu/~tdi/pdf-resources/CC125.pdf )
  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Re: Help with breaker / fusing sizing

    Or get the wind-sun_2095_21496163 Foot MC4 Extender Cable Male/Female


    Version... Just cut in 1/2 and splice to your home run.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stereoman405stereoman405 Solar Expert Posts: 56 ✭✭✭✭
    Re: Help with breaker / fusing sizing

    I thought of that, but I though I'd need a box to enclose the splice. What's the best way to splice without a box? Can I just "butt connect" with heat shrink tape to keep the water out? Or would soldering be better?


    Related - Would it be worth putting all 4 panels in a 116v series to keep the loss down, or would that be pretty inefficient for the MPPT to step from 116v down to 12v? Would I need fusing then with a 4 panel series?
  • bobdogbobdog Solar Expert Posts: 191 ✭✭
    Re: Help with breaker / fusing sizing
    I thought of that, but I though I'd need a box to enclose the splice. What's the best way to splice without a box? Can I just "butt connect" with heat shrink tape to keep the water out? Or would soldering be better?

    Just curious if there is an answer to his question?
  • audredgeraudredger Solar Expert Posts: 272 ✭✭
    Re: Help with breaker / fusing sizing

    I would put the splices in a box, get a "handi box", put two holes in the bottom. Run the wires down then up into the box, makes a drip loop. After you've made your connections, silicone the holes so you don't make a wasp home.
    Related - Would it be worth putting all 4 panels in a 116v series to keep the loss down, or would that be pretty inefficient for the MPPT to step from 116v down to 12v? Would I need fusing then with a 4 panel series?

    If your controller can handle the voltage and you won't go over voltage on cold days, yes you can do that. It will cut down your line losses. If you only have one series string, then you won't need to fuse. If, you have multiple series strings, then you need to fuse.
  • stereoman405stereoman405 Solar Expert Posts: 56 ✭✭✭✭
    Re: Help with breaker / fusing sizing
    audredger wrote: »
    I would put the splices in a box, get a "handi box", put two holes in the bottom. Run the wires down then up into the box, makes a drip loop. After you've made your connections, silicone the holes so you don't make a wasp home.

    How would you recommend making the actual splice? Just butt connect?
    audredger wrote: »
    If your controller can handle the voltage and you won't go over voltage on cold days, yes you can do that. It will cut down your line losses. If you only have one series string, then you won't need to fuse. If, you have multiple series strings, then you need to fuse.

    This is probably a separate topic, but would one big series be better then to two smaller strings put into parallel when light shading might be an issue?
  • audredgeraudredger Solar Expert Posts: 272 ✭✭
    Re: Help with breaker / fusing sizing
    How would you recommend making the actual splice? Just butt connect?
    Normally the wire to the charge controller is a larger gage than the #10 from the panels .... A split bolt works best in this condition.
    This is probably a separate topic, but would one big series be better then to two smaller strings put into parallel when light shading might be an issue?

    I don't know about the shading issue but, my high voltage string out performs my medium voltage string under low light conditions.
  • westbranchwestbranch Solar Expert Posts: 5,163 ✭✭✭✭
    Re: Help with breaker / fusing sizing

    It is important to know the Max voltage for your MPPT CC and then look at the Vmp since you are at high elevation.
    In winter you might/will exceed the Vmp with edge of cloud effects and the cold temps. You will need the temp correction factor, per degree, for your panels, and then calculate the adjustment for min temp. to see what the 'cold max V is...

    HTH
    Eric
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • stereoman405stereoman405 Solar Expert Posts: 56 ✭✭✭✭
    Re: Help with breaker / fusing sizing
    westbranch wrote: »
    It is important to know the Max voltage for your MPPT CC and then look at the Vmp since you are at high elevation.
    In winter you might/will exceed the Vmp with edge of cloud effects and the cold temps. You will need the temp correction factor, per degree, for your panels, and then calculate the adjustment for min temp. to see what the 'cold max V is...

    HTH
    Eric

    I just ordered an Outback FM 60 which claims a max input of 145v I believe (shooting from the hip here). My panel specs are here:

    http://www.wholesalesolar.com/pdf.folder/module%20pdf%20folder/REC_SCM_solar_panels_specs.pdf

    Vmp = 28.2 X 4 = 112.8V

    That should be enough of a cushion even on a sunny winter day right? How do I factor the temp correction just to be sure?
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Help with breaker / fusing sizing

    for the max to a cc you should be looking at the voc (voltage open circuit) and your pv is 36.3v. 4 in series is 4x36.3v=145.2v at 25 °C.
    this is from the fm60 pdf,
    PV Open Circuit Voltage (VOC) is 150 VDC absolute maximum coldest conditions / 145 VDC start-up and operating maximum

    pretty close. now the pv specs for temperature differences from 25 degrees c is as,
    Temperature Coefficient of Voc (mv/°C) is -104

    now to freezing that's 25 x .104v = 2.6v to be added to the original 145.2v for 147.8v. if you go below freezing then it'll add more and this is cutting it very close and would depend on your lowest recorded temperature. personally that's cutting it too close for me, but is technically ok if that max isn't possible to reach.
  • stereoman405stereoman405 Solar Expert Posts: 56 ✭✭✭✭
    Re: Help with breaker / fusing sizing
    niel wrote: »
    for the max to a cc you should be looking at the voc (voltage open circuit) and your pv is 36.3v. 4 in series is 4x36.3v=145.2v at 25 °C.
    this is from the fm60 pdf,
    PV Open Circuit Voltage (VOC) is 150 VDC absolute maximum coldest conditions / 145 VDC start-up and operating maximum

    pretty close. now the pv specs for temperature differences from 25 degrees c is as,
    Temperature Coefficient of Voc (mv/°C) is -104

    now to freezing that's 25 x .104v = 2.6v to be added to the original 145.2v for 147.8v. if you go below freezing then it'll add more and this is cutting it very close and would depend on your lowest recorded temperature. personally that's cutting it too close for me, but is technically ok if that max isn't possible to reach.

    Hmmmm, well I know for a fact that it does get below freezing regularly where the setup wil be at. Sounds like its best to not push it here. Not worth a $500 CC for 1%-2% wire efficiency gain......

    Thanks for the help. That could've been a costly learning experience.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Help with breaker / fusing sizing

    do know it is not required that all pvs go in series as that can be arranged as 2 in series paralleled with the other 2 in series unless you have need of a 48v output for a 48v battery bank, but you can go up to 3 in series with no problem.
  • stereoman405stereoman405 Solar Expert Posts: 56 ✭✭✭✭
    Re: Help with breaker / fusing sizing
    niel wrote: »
    do know it is not required that all pvs go in series as that can be arranged as 2 in series paralleled with the other 2 in series unless you have need of a 48v output for a 48v battery bank, but you can go up to 3 in series with no problem.

    Ya I had originally planned on (2) 2 panel series paralleled together. This just got me thinking I could do one large series and gain some wire efficiency and save a few $$ in multbranch connectors. I think I'll just stick to the original plan now.
  • solarvicsolarvic Solar Expert Posts: 1,053 ✭✭✭✭
    Re: Help with breaker / fusing sizing
    Ya I had originally planned on (2) 2 panel series paralleled together. This just got me thinking I could do one large series and gain some wire efficiency and save a few $$ in multbranch connectors. I think I'll just stick to the original plan now.
    Checked that out on outback string chart for my area temp. _26f I could use 2 strings of 3 for 24v system. Solarvic
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