Wiring/ Stacking Two XW's

ligwydligwyd Registered Users Posts: 186 ✭✭
Hi there,

Hope all is with with everyone here. Just in the process of building another complete off-grid system. Simple question: When stacking two XW's, what gauge copper wire should be used for the output of both inverters?

Each XW comes standard with a 60 amp breaker, so I originally thought, 120 amp capable wire after the staking kit bus bar. However after talking with an engineer, He said breaker and wire size is based on continuous max load. Also, He said that 100 amp capable wire is all you'd need on the output of two stacked XW inverters.

I calculated max continuous output of each inverter is 6800 watts X 2 (two inverters) = 13600 watts divided by 240 V = approx 60 amps?
He also suggested #2 copper wire on the output of the two inverters which is good for 115 amps.

I looked up #3 copper and I think its good for approx. 100 amps. Although both inverters will rarely be pushed to the limit, if ever, I want to ensure I use the appropriate size of wire for this stack.

Thanks for any insight and or suggestion anyone may offer.
Much appreciated, as always.

John

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,814 admin
    When paralleling power supply outputs... The output wiring of each supply should go to its own circuit breaker (wiring and breaker rated for output of 1x power supply. The two breakers in parallel at the main panel now power the main bus at 2x rated power. And you have breakers and wiring that are rated for their respective loads...

    More or less, this is just like a solar array combiner box... Multiple smaller AWG outputs combining to one larger output (or in your case, the main bus bars in the panel). Of course, if you have a larger load or want an actual panel/sub-panel elsewhere--Then you would have 120 Amp wiring and breakers from the "combiner panel" to your actual point of use.

    Could you get "into trouble" with code... Possibly. In this case, you don't have one pair of breakers like utility power coming into your home--You have two parallel power supply circuits. I would expect that you need to placard the main panel to the effect that there are two input power sources and both must be turned off to shutoff power to the main panel and loads.

    This all assumes that the inverters are properly designed and configured to operate in parallel output mode.

    By the way--What about backup genset/AC power? Do you have/need a genset? Will you want/need to bypass inverters (during servicing/equipment failures, etc.)? You might want to invest in lockouts that allow you to lock one or more circuit breakers off during servicing/running in inverter+genset bypass mode (i.e., you lockout the breakers for the inverters during servicing. Lockout the genset breaker during "normal operation").

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ligwydligwyd Registered Users Posts: 186 ✭✭
    edited September 11 #3
    Hi Bill and thanks for your feedback.

    Yes I do have a generator. 13kW Isuzu Diesel. I talked to my local inspector he said he'd be OK with sizing bus bar AC output cable to match max continuous use load of both Inverters combined.

    MAX load of each XW 8500 watts for 30min X2 = 17000 watts divided by 240V = 70.8 Amps.

    I then went up from here and calculated MAX surge load of each XW of 12000Watts for 1 min X2 = 24000 watts for 1 min MAX before tripping on overload (which is highly probably that it never will be pushed that hard)

    So 24000watts dived by 240V = 100 Amps MAX. So may use #2 AL NMD90 for the output wire which is rated for 100Amps

    Yes the two 60 amp breakers could carry a load of 120 amps for a couple hours before tripping but that will never happen as far as I can see.

    The Inspector said he would consider it a "rapid shut down" of a sort, if the equip design is to trip if over loaded. Just wanted labeled placards placed indicating so.

    Also just going to post what Schneider said in reply to my question I posted here also. They of course will give you "the right" answer, which in this case is overkill. They said to take 120 Amps X 1.25 to get 150Amps and size cable to that.!!! Seem a little much.

    Maybe the two 60amp breakers could potentially pass full current of 120 amps for a couple hours if the Gen AC is passing power through, as well as the XW's Inverting battery power in Gen support mode???

    Anyway, here what Schneider said for you review and interest.and do really appreciate the dialogue an feedback Bill.


    "in reference to case 81524, thanks for reaching Schneider Electric Solar.

    Please confirm this with your local AHJ, as we cannot officially advise on what the local inspector will want. As an installer, it is your sole responsibility to install the system per local building codes and regulations.

    About your inquiry:
    Per NEC, the wire should be rated for 125% of the circuit breaker protecting the wire.

    Assuming you are using the provided 60 amp breakers in the PDP x2, that’s 120 amps of overcurrent protection. 125% of 120 amps is 150 amp rated wire. Looks like a #1 AWG THHN Copper wire in conduit, or #3 AWG THHN in free air would meet this requirement."

  • BB.BB. Super Moderators, Administrators Posts: 32,814 admin
    There is the NEC 80% or 1.25x derating for wiring and breakers when doing "continous current" circuits such as gym lighting and battery charging with can have "max current" flowing for hours at a time...

    Breakers (and fuses) in North America are generally rated to blow at 100% or more of rated load (may take minutes to hours) and not trip at 80% or less of rated load.

    Generally the output of your inverters/random AC (and DC) loads are not going to be at 100% of branch circuit/breaker ratings for hours at a time--Generally for a few seconds or minutes at a time at most.

    However, charging batteries is one of those cases where I really like teh the 80% or 1.25x rule... If you have an AC or DC current bath that will be supplying 60 Amps for battery charging (AC input to charger, or DC output to battery bank--Just example of math):
    • 60 Amps * 1.25 NEC derating = 75 Amps derated circuit--Round up to 80 Amp (or next standard breaker+wiring rating)
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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