Breaker sizing

WxboyWxboy Solar Expert Posts: 70 ✭✭✭✭
I'm about to upgrade my system a bit and I have some questions about circuit breaker sizing.

First I'm going to add a second 255 watt panel going into my Classic 150. I currently have a 15 amp breaker between the panel and the controller. I assume I should bump that up to a 30 amp breaker but want to make sure that is correct. I do also have a breaker between the Classic and the batteries but that doesn't need to be upgraded(100 amp).

Then I have a second controller charging the same battery bank but I have no breakers yet on that side so I'm looking to add one or two. I'm assuming I probably don't need a breaker or fuse between the panel(125 watt 12 volt nominal, 7 amp) and the Sunsaver 10 amp pwm charge controller but I want to confirm that as well. Then I want to add a breaker between the charger and the battery bank. Can I use the 15 amp breaker that I'll be taking out of service from above?

Thanks for the help. Wiring is 8 awg for the system with the 125 watt panel and 10 amp controller.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Breaker sizing

    First question: how many panels of what Imp are feeding in to that Classic 150? The wiring and whole array breaker needs to be sized for the total. One 255 Watt panel with an Imp of 8.5 plus a second of the same would indeed exceed the current rating for the 15 Amp breaker. There's some difference of opinion regarding sizing these whole array breakers (which mainly act only as disconnects as the array output can never exceed Isc) but yes it should be 25 or 30 Amps. Think ahead too; if you want to add a third or forth panel (the Classic could take something like 1200 Watts on a 12 Volt system) you might as well make sure now wire & breaker is able to handle it.

    You are correct that one panel on the Sunsaver does not need a fuse/breaker. The output, however, should have a fuse on it (10 Amp will do as it's not being driven to the max).
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Breaker sizing

    you could put the 2nd identical 255w pv in series with the first and not worry about the pv to cc breaker. the breaker from the cc to the batteries is a bit high, but would serve well as a disconnect.
  • WxboyWxboy Solar Expert Posts: 70 ✭✭✭✭
    Re: Breaker sizing

    niel, thanks for bringing me back to my senses. I do plan to put the 2 panels in series so the voltage would double but the amps would stay the same. Well at least I'm good there and then I can pick up a 10 amp breaker for the Sunsaver.

    Cariboocoot, the Isc on the 255 is 8.66amps. So if I was going to set up the panels in parallel I would need to upgrade the 15a breaker but now that niel set me straight I don't need to.

    Since the Classic is rated up to 96amps output I picked up the 100 amp breaker and figured that would cover me no matter how much I expand. Right now it's just the one panel and soon it will be two. I can only expand up to 4 of the 255 watt panels(series parallel setup) with my 12 volt system so once I get above that I'll have to go with a higher system voltage.

    Thank you both.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Breaker sizing

    There is a small downside to putting the panels in series: with the higher array Voltage the controller efficiency will drop a bit. Using ~60 Volt array on a 12 Volt system will lose a few percent in the controller. But this probably isn't a problem here.
  • RichardCRichardC Registered Users Posts: 1
    ProWatt SW2000 - is the neutral at 0V ?

    I see you have the Xantrex Prowatt SW2000, I've been thinking about getting one of those but am trying
    to figure out whether it produces "real" 120V ac on the live wire with 0V on the neutral, or else
    does 60V ac on live and 60v ac on neutral in antiphase, like some cheap inverters. Do you have an answer
    for that ?

    Thanks
    Richard Cownie
  • BB.BB. Super Moderators, Administrators Posts: 31,765 admin
    Re: ProWatt SW2000 - is the neutral at 0V ?

    The Xantrex product Manual says nothing about if the AC Neutral is grounded to chassis or not... However, the unit does have a GFI outlet and they talk about AC ground faults--Which can only occur if the AC output "Neutral" is tied to chassis ground (or other ground point).

    With the inverter off, you can use an Ohm meter to check the Wide Blade of the AC outlet and see if it is ZERO Ohms to chassis ground screw (should also be zero ohms from outlet ground plug too).

    With the inverter turned on, you can use a volt meter and measure the wide and narrow blade to chassis ground voltage (or use one of those three light AC testers). If you see near zero volts on the wide blade and near 120 VAC on the narrow blade (plus the ohm testing showing the wide blade tied to chassis), then it should be AC Neutral bonded to chassis ground.

    Of course, you have to tie the Chassis Ground of the inverter to something (vehicle ground, AC "master" ground rod for the cabin, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: ProWatt SW2000 - is the neutral at 0V ?
    BB. wrote: »
    With the inverter off, you can use an Ohm meter to check the Wide Blade of the AC outlet and see if it is ZERO Ohms to chassis ground screw (should also be zero ohms from outlet ground plug too).

    With the inverter turned on, you can use a volt meter and measure the wide and narrow blade to chassis ground voltage (or use one of those three light AC testers). If you see near zero volts on the wide blade and near 120 VAC on the narrow blade (plus the ohm testing showing the wide blade tied to chassis), then it should be AC Neutral bonded to chassis ground.

    Of course, you have to tie the Chassis Ground of the inverter to something (vehicle ground, AC "master" ground rod for the cabin, etc.).

    -Bill

    There is a chance, with an MSW inverter mainly, that there will be no connectivity between the chassis ground and the "neutral", wide-blade wire, except through the battery bank connection. If you are using an ungrounded battery bank, there may not be any connectivity between neutral and chassis ground, but there may be between neutral and one terminal of the battery bank. This is the situation you want to avoid or be careful of.
    For an inverter which isolates the inverter output from the battery input you may still see no connectivity between neutral and ground unless the inverter is actually connected to your house wiring at the time. And yet you would find that the neutral wire really is 0 volts for practical purposes. (You would not be advised to take ohmmeter readings unless you have disconnected the inverter or otherwise confirmed that there is no voltage present.)

    For those reasons, I prefer the test of actually measuring the voltage between hot, neutral and both chassis ground and battery bank with the inverter running instead. You should do this with the inverter disconnected from any load wiring.

    PS: Xantrex inverter models are fine in this respect, as are almost if not all pure sine wave inverters. :-)
    In the case of the SW2000, although it is not conclusive, the unqualified recommendation to connect the chassis ground of the unit to your DC ground is suggestive that the AC output is isolated.
    SMA SB 3000, old BP panels.
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