More Circuit Breaker Questions

n1st
n1st Solar Expert Posts: 77 ✭✭✭✭
I'm looking to put a circuit breaker panel on the output of an inverter. I'm thinking of using a Square D 70amp box with 2 30 amp breakers. I'd connect a run of #10 wires to the output of each breaker. The 2 runs would go about 30 feet and into a Square D transfer/generator box that has 2 60 amp main breakers with a mechanical lever that only alows 1 breaker to be on at a time. The output of the 60 amp breakers feeds up to 4 breakers - I'd use 20 amp breakers and connect to #12/2 wire out to outlets, etc. I don't think there's anything unsafe or non-code wiht this, but I'd like other opinions.

Second question is about a panel for a 125 amp DC breaker between batteries and inverter. The breaker mounts by 2 screws in the front and has studs on the back for wire connection. I'd like to fabricate a box for this and a 60amp DC breaker between the charge controller and battery feed. Can I just find a box and cover (plastic or metal) a bit larger than the breaker, and cut slots in the cover or box, drill holes for the mounting screws, and mount it? Are there clearance requirements around the studs that I need to be aware of? I'd suspect an inch between the studs and the metal box would be a good idea. I would also think that a plastic box wouldn't dissapate any heat, but I don't know if that could be an issue.

Thanks.

Comments

  • a0128958
    a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: More Circuit Breaker Questions

    I believe the inverter's minimum output circuit conductor ampacity has to be greater than or equal to the inverter's continuous output current rating times 1.25.

    For example, using an SMA SB7000 inverter as an example, the inverter's output circuit can be calculated from the maximum continous power rating at nominal AC voltage. The 125% increase over the maximum inverter output circuit current is to account for the standard listing of wire to 80% of maximum circuit current for continous duty.

    I.e., if the inverter's continuous output rating is 7000 Watts, and if the minimum inverter voltage is 211 Volts, then the maximum operating current is 7000 Watts / 211 Volts = 33 Amps. And the minimum inverter output circuit ampacity = 33 Amps * 1.25 = 41 Amps.

    I also believe an inverter's output circuit breakers should be sized at 125% of the maximum continuous output of the inverter.

    For example, using the same SMA SB7000 inverter, and this time looking up the maximum continous output specs (34 amps and a maximum allowable overcurrent protection of 50 amps). This results in a minimum allowable overcurrent of 42 amps (34 * 1.25 = 42 amps) and a maximum of 50 amps. The next highest standard circuit breaker size larger than 42 is 45 amps.

    Hope this helps.

    Best regards,

    Bill
  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: More Circuit Breaker Questions

    a0128958 , how it that installation going?
  • n1st
    n1st Solar Expert Posts: 77 ✭✭✭✭
    Re: More Circuit Breaker Questions

    I should clarify that the 2nd 60 amp breaker in transfer panel would be used for a generator,not the inverter. Both runs from the inverter would be going to just 1 of the 60 amp breakers.
  • n1st
    n1st Solar Expert Posts: 77 ✭✭✭✭
    Re: More Circuit Breaker Questions

    Thanks for your thoughts Bill. I've heard the same, and this leads me to ask... "Why?" I don't get this. Shouldn't the circuit breaker protect the wire? I have a guzzillion amps coming into my house from the grid. My 60 main panel breaker sure isn't 125% of electric company's continuous output rating.

    a0128958 wrote: »
    I believe the inverter's minimum output circuit conductor ampacity has to be greater than or equal to the inverter's continuous output current rating times 1.25.

    For example, using an SMA SB7000 inverter as an example, the inverter's output circuit can be calculated from the maximum continous power rating at nominal AC voltage. The 125% increase over the maximum inverter output circuit current is to account for the standard listing of wire to 80% of maximum circuit current for continous duty.

    I.e., if the inverter's continuous output rating is 7000 Watts, and if the minimum inverter voltage is 211 Volts, then the maximum operating current is 7000 Watts / 211 Volts = 33 Amps. And the minimum inverter output circuit ampacity = 33 Amps * 1.25 = 41 Amps.

    I also believe an inverter's output circuit breakers should be sized at 125% of the maximum continuous output of the inverter.

    For example, using the same SMA SB7000 inverter, and this time looking up the maximum continous output specs (34 amps and a maximum allowable overcurrent protection of 50 amps). This results in a minimum allowable overcurrent of 42 amps (34 * 1.25 = 42 amps) and a maximum of 50 amps. The next highest standard circuit breaker size larger than 42 is 45 amps.

    Hope this helps.

    Best regards,

    Bill
  • a0128958
    a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: More Circuit Breaker Questions
    a0128958 , how it that installation going?

    Data collection underway, with respect to shading, to affirm / further change the design, before proceeding further.

    Best regards,

    Bill
  • a0128958
    a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: More Circuit Breaker Questions
    n1st wrote: »
    ... "Why?"

    As the owner, if you're doing the electrical work, in most places, like mine, you get to do whatever you want to do.

    My comments are what you'd need to do if you want to be compliant with the NEC. Certainly if you want to do differently you can.

    My comments I think are also what you'd get (and have to pay for) if you hired a licensed person to do the work. The license requires him/her to do the work to NEC standards.

    Best regards,

    Bill
  • n1st
    n1st Solar Expert Posts: 77 ✭✭✭✭
    Re: More Circuit Breaker Questions
    a0128958 wrote: »
    As the owner, if you're doing the electrical work, in most places, like mine, you get to do whatever you want to do.

    My comments are what you'd need to do if you want to be compliant with the NEC. Certainly if you want to do differently you can.

    My comments I think are also what you'd get (and have to pay for) if you hired a licensed person to do the work. The license requires him/her to do the work to NEC standards.

    Best regards,

    Bill

    Sure, I agree. But I'd still like to understand why the codes/regs, etc. are different between the two situations.
  • vcallaway
    vcallaway Solar Expert Posts: 157 ✭✭✭✭✭
    Re: More Circuit Breaker Questions

    Sounds kinda like what I'm working on.

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    I had to stop because it was too darn cold. I'm also waiting on one more switch to arrive.

    The two black boxes in the middle are IOTA 30A automatic switches. DPDT relays. When the third arrives it will connect to the left leg of the panel.

    The panel is a 70A panel with one leg connected to the inverter side. The other leg will be generator only. This is a re-do from an earlier attempt that did not work very well.

    The inverter will get moved to the back of the house in the spring when I put up solar panels. I wanted a hard switch near the panel for safety. The box is just a simple plug that bridges the in/out lines. I found it at home depot where they had the hot tub wiring stuff. A deal for $3.50 on closeout.

    The yellow romex is going away and replaced with flex and sealed up.

    The theory of operation is that when mains are hot it passes through to the panel to designated circuits in the house. When the power goes out one leg has inverter backup that switches on immediately. When the generator comes on both legs get powered by it.

    I added a ground strip to the 70A panel for earth ground. When running on inverter many of them don't like earth and neutral tied together. The IOTA switches are a DPDT so both hot and neutral get switched out. Simple elegant solution.

    I am not an electrician. Any suggestions are welcome.