AC disconnect sizing?

heynow999
heynow999 Solar Expert Posts: 80 ✭✭✭✭
in PV Installers Forum - NEC, Wiring, Installation #1
I have a 2.5kw central inverter system and I am adding a 3.3kw Enphase system. I need an AC (fused) disconnect which was not installed when my 2.5kw system was originally installed. I will have a breaker panel with a 15 amp 2 pole and 20 amp 2 pole breakers. I then need the disconnect between the breaker panel and the meter.

The 2.5kw needs a 15amp disconnect and the 3.3kw needs a 20amp discconnect. Will I have 15A and 20A on each phase so I need a 35A fuse in the dissconnect? Or will it be 30A on one phase and 40A on the second phase?

Thanks
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #2
    Re: AC disconnect sizing?
    heynow999 wrote: »
    I have a 2.5kw central inverter system and I am adding a 3.3kw Enphase system. I need an AC (fused) disconnect which was not installed when my 2.5kw system was originally installed. I will have a breaker panel with a 15 amp 2 pole and 20 amp 2 pole breakers. I then need the disconnect between the breaker panel and the meter.

    The 2.5kw needs a 15amp disconnect and the 3.3kw needs a 20amp discconnect. Will I have 15A and 20A on each phase so I need a 35A fuse in the dissconnect? Or will it be 30A on one phase and 40A on the second phase?

    Thanks
    You've got a total of 5.8kW.
    (5.80)(1000)/240 = 24.2A
    (24.2)(1.25) = 30.2A
    Next size up fuse is 35A. Same size fuse on both phases.
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  • Jburgess
    Jburgess Solar Expert Posts: 130 ✭✭✭
    #3
    Re: AC disconnect sizing?

    Wouldn’t 220.5 (B) permit you to drop the fraction and use a 30 amp fused disconnect?

    220.5 Calculations.
    (B) Fractions of an Ampere. Calculations shall be permitted
    to be rounded to the nearest whole ampere, with decimal
    fractions smaller than 0.5 dropped.
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  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    #4
    Re: AC disconnect sizing?

    Yep, a single 30A disconnect should be OK - hard to say for sure without knowing exactly what the rating of your old inverter and how many Enphase inverters you have.

    Otherwise 60A disconnect blade switches are readily available, though they might be a bit more expensive.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #5
    Re: AC disconnect sizing?
    Jburgess wrote: »
    Wouldn’t 220.5 (B) permit you to drop the fraction and use a 30 amp fused disconnect?

    220.5 Calculations.
    (B) Fractions of an Ampere. Calculations shall be permitted
    to be rounded to the nearest whole ampere, with decimal
    fractions smaller than 0.5 dropped.

    Good catch.

    Now I have to go through all my electrical drawings and revise my OPD's.

    Thanks a lot. ;^)
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