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Branch Circuit and Conduit

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  • Branch Circuit and Conduit

    I'm designing a PV system using 39 panels and 39 mirco inverters that will be divided into 3 branch circuits. I'm being told I will need to run three conduit runs from the roof junction box to the combiner box in the garage. One conduit for each circuit.
    After reading the NEC 690.4(B)1-4 and if I'm reading it correctly it can be in 1 larger conduit as long as all conductors are identified.
    Does anyone know the ruling on this? It doesn't make sense having to make 3 separate pipe runs.

  • #2
    You can use 1 conduit, but there is a catch. Since you are running more than 3 Current Carrying Conductors in a raceway, you will have to derate the conductors. In other words use larger conductors than you would with 3 raceways. NEC 310.15

    Just guessing you wil have to use 10 AWG 90 degree insulation vs 12 AWG. Getting right is your responsibility.
    Last edited by Sunking; 06-06-2018, 08:25 PM.
    MSEE, PE

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    • #3
      Originally posted by CessnaTPA View Post
      I'm designing a PV system using 39 panels and 39 mirco inverters that will be divided into 3 branch circuits. I'm being told I will need to run three conduit runs from the roof junction box to the combiner box in the garage. One conduit for each circuit.
      After reading the NEC 690.4(B)1-4 and if I'm reading it correctly it can be in 1 larger conduit as long as all conductors are identified.
      Does anyone know the ruling on this? It doesn't make sense having to make 3 separate pipe runs.
      I don't see why one conduit wouldn't work, derated as sunking said. I have nine 10 AWG conductors running 150 feet inside a single 1 1/4 inch IMC conduit. Four pairs of wires, one pair for each string I will have connected when it's done. (Right now, just two of the pairs are live.) Those are DC conductors, whereas you are dealing with 240V AC from the microinverters, but I don't think it makes any difference to the NEC as long as it's all AC inside one conduit.

      Do they really call the place where the 240V AC outputs of the microinverters all connect together a "combiner box"? I understood a combiner box to be where the DC current from multiple strings of PV panels gets paralleled and fed to a charge controller or inverter with one combined conductor pair. But I suppose it has to be called something.

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      • #4
        There is nothing in the NEC preventing you from running multiple AC branches in the same raceway. I'm doing it myself in my setup, 5 circuits inside a 2" raceway - each on its own OCPD. As already stated, do your fill calculations and derating calculations for both number of current carrying conductors and temperature. Don't forget that the maximum current for a 20 amp breaker must be 16 amps or less of micro inverter output. And don't forget to do your resistance calculations - most micros want total losses from resistance to be 2% or less to help prevent out of range tripping - depending on your run length it may cause you to bump to a larger wire. Note that this is 2% from the furthest micro to the service panel and don't forget to take into account resistance from any manufacturer supplied wiring harnesses connecting the micros. A common way to help mitigate manufacturer long string harnesses resistance is to center tap them.
        Last edited by tyab; 06-07-2018, 11:26 AM.

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        • #5
          Thanks guys for the input, I figured 1 conduit would work.
          I'm using solar permit software from solardesigntool.com and it automatically generated 3 conduits and doesn't allow me to change it. After talking to them they believed it was a requirement and will need to manually change it.
          Other than that glitch it appears to produce professional looking prints to summit for approval. Has anyone else used it or can suggest a better place?

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          • #6
            re fill requirement. Power Transmission vs Appliance Wiring. Remember, these are running power 100% of the daylight time, there is no cooling off as thermostats cycle or anything, I'd oversize the conduit if feasible.
            Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
            || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
            || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

            solar: http://tinyurl.com/LMR-Solar
            gen: http://tinyurl.com/LMR-Lister

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            • #7
              Originally posted by Mike90250 View Post
              re fill requirement. Power Transmission vs Appliance Wiring. Remember, these are running power 100% of the daylight time, there is no cooling off as thermostats cycle or anything, I'd oversize the conduit if feasible.
              Well, maybe for an hour or two around noon on a summer day with no clouds. When power output dips to 70% as it will a few hours before or after noon, power losses in the wire will be cut in half. Does that hour or two really present any more of an issue, with respect to the conservatively determined NEC ampacity limits, than a space heater that stays on for hours overnight?

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              • #8
                Originally posted by BackwoodsEE View Post

                Well, maybe for an hour or two around noon on a summer day with no clouds.
                Here with no clouds it runs at peak for 8 hours or more. Bruce Roe

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