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AC Disconnect Sizing and Type for Line Tap (no backfeed)

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  • #16
    I had a solar electrician wire my solar stuff in 2017. I was a little bit surprised there is no breaker, fuse, or disconnect for the load side tap he installed. The only way to de-energize the underground wire from the meter to the inverters is to pull the meter. It was all permitted and passed inspection, but I don't know that the inspector checked that side very closely.

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    • #17
      Originally posted by reader2580 View Post
      I had a solar electrician wire my solar stuff in 2017. I was a little bit surprised there is no breaker, fuse, or disconnect for the load side tap he installed. The only way to de-energize the underground wire from the meter to the inverters is to pull the meter. It was all permitted and passed inspection, but I don't know that the inspector checked that side very closely.
      The inverter is required to be protected by a breaker. Are you sure there's no breaker? You can have up to 25' of line that is unprotected from over current. IIRC this is a 'feeder line' exemption.

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      • #18
        Originally posted by reader2580 View Post
        I had a solar electrician wire my solar stuff in 2017. I was a little bit surprised there is no breaker, fuse, or disconnect for the load side tap he installed. The only way to de-energize the underground wire from the meter to the inverters is to pull the meter. It was all permitted and passed inspection, but I don't know that the inspector checked that side very closely.
        A line side tap will require the wires to be rated as much as the bus bars are or they can be 1/3 less if the length is 10 feet or less. There is another 25 foot rule that I can't quite remember what the requirements are.

        The whole idea is to make sure the wires are protected by some type of over current device.

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        • #19
          Originally posted by SunEagle View Post

          A line side tap will require the wires to be rated as much as the bus bars are or they can be 1/3 less if the length is 10 feet or less. There is another 25 foot rule that I can't quite remember what the requirements are.

          The whole idea is to make sure the wires are protected by some type of over current device.
          10-Foot Feeder Tap Rule [240.21(B)(1)]
          Feeder tap conductors can be run not over 10 ft without overcurrent protection at the point they receive their supply, but they must be installed in accordance with the following requirements: Figure 1
          (1) The ampacity of the tap conductor is:
          1 Not less than the computed load in accordance with Article 220, and
          2 Not less than the rating of the device supplied by the tap conductors or the overcurrent protective device at the termination of the tap conductors.
          (2) The tap conductors do not extend beyond the equipment they supply.
          (3) The tap conductors are installed in a raceway if they leave the enclosure.
          (4) The tap conductors have an ampacity of no less than 10 percent of the ampacity of the overcurrent protection device from which the conductors are tapped.

          25-Foot Feeder Tap Rule [240.21(B)(2)
          Feeder tap conductors can be run not over 25 ft without overcurrent protection at the point they receive their supply, but they must be installed in accordance with the following requirements: Figure 2
          (1) The ampacity of the tap conductors is not less than 1/3 the ampacity of the overcurrent protection device protecting the feeder.
          (2) The tap conductors terminate in a single circuit breaker, or set of fuses having a rating no greater than the conductor ampacity as listed in Table 310.16.
          (3) The tap conductors are suitably protected from physical damage or are enclosed in a raceway.



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          • #20
            Originally posted by nwdiver View Post

            10-Foot Feeder Tap Rule [240.21(B)(1)]
            Feeder tap conductors can be run not over 10 ft without overcurrent protection at the point they receive their supply, but they must be installed in accordance with the following requirements: Figure 1
            (1) The ampacity of the tap conductor is:
            1 Not less than the computed load in accordance with Article 220, and
            2 Not less than the rating of the device supplied by the tap conductors or the overcurrent protective device at the termination of the tap conductors.
            (2) The tap conductors do not extend beyond the equipment they supply.
            (3) The tap conductors are installed in a raceway if they leave the enclosure.
            (4) The tap conductors have an ampacity of no less than 10 percent of the ampacity of the overcurrent protection device from which the conductors are tapped.

            25-Foot Feeder Tap Rule [240.21(B)(2)
            Feeder tap conductors can be run not over 25 ft without overcurrent protection at the point they receive their supply, but they must be installed in accordance with the following requirements: Figure 2
            (1) The ampacity of the tap conductors is not less than 1/3 the ampacity of the overcurrent protection device protecting the feeder.
            (2) The tap conductors terminate in a single circuit breaker, or set of fuses having a rating no greater than the conductor ampacity as listed in Table 310.16.
            (3) The tap conductors are suitably protected from physical damage or are enclosed in a raceway.


            Thanks. I got my 10ft and 25ft rule mixed up. Funny how I should remember that but I have put away all of my NEC books and am currently going by memory without having to use the code for years

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            • #21
              Originally posted by nwdiver View Post

              The inverter is required to be protected by a breaker. Are you sure there's no breaker? You can have up to 25' of line that is unprotected from over current. IIRC this is a 'feeder line' exemption.
              There is a breaker next to the inverters. The wire from the meter to the breaker next to the inverters has no way to de-energize without pulling the meter.

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              • #22
                Originally posted by reader2580 View Post

                There is a breaker next to the inverters. The wire from the meter to the breaker next to the inverters has no way to de-energize without pulling the meter.
                That's common with line side taps as the rules posted up thread outline. I've done a couple installs like that. It's treated in a similar way to your main panel. No way to de-energize the wire feeding your main breaker without pulling the meter either.

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                • #23
                  [QUOTE=nwdiver;n420427
                  That's common with line side taps as the rules posted up thread outline. I've done a couple installs like that. It's treated in a similar way to your main panel. No way to de-energize the wire feeding your main breaker without pulling the meter either.[/QUOTE]

                  The connection from my meter to my main load center is less than 10 feet. My solar tap runs 70 feet before the first breaker. My new solar tap is going to be almost 200 feet before the first breaker.

                  I am not going to worry about it so long as what I am doing meets code.

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                  • #24
                    Originally posted by reader2580 View Post

                    My new solar tap is going to be almost 200 feet before the first breaker.
                    That's a long way to run 240vac. Why not locate the inverter near that breaker and run DC?

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                    • #25
                      Originally posted by nwdiver View Post
                      That's a long way to run 240vac. Why not locate the inverter near that breaker and run DC?
                      The breaker is next to the inverter and the inverter is next to the panels. My understanding has always been that AC has less voltage drop than DC. Remember the whole Tesla vs Edison thing when electricity was still a new thing? Tesla won out with AC because AC go go long distances with less voltage drop.

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                      • #26
                        Originally posted by reader2580 View Post

                        The breaker is next to the inverter and the inverter is next to the panels. My understanding has always been that AC has less voltage drop than DC. Remember the whole Tesla vs Edison thing when electricity was still a new thing? Tesla won out with AC because AC go go long distances with less voltage drop.
                        Ah... yeah... that's not true. AC won over DC because before solid state electronics transformers were really the only way to step-up and step-down voltage and transformers only work with AC. It's voltage that's important not AC or DC. Now that we have a way to change voltage with DC, DC is actually the preferred way to transfer power. Look up HVDC transmission.

                        I believe 240v DC is actually even better than 240v AC since DC is able to transfer more power per volt since it doesn't alternate. Most PV system operate at >350v DC. ~350vdc is WAY better than 240vac.

                        With grid-tied solar you want your DC voltage as close to 480v as you can get it without going over 600v open circuit. Then keep your DC lines long and your AC lines short

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                        • #27
                          Originally posted by nwdiver View Post
                          With grid-tied solar you want your DC voltage as close to 480v as you can get it without going over 600v open circuit. Then keep your DC lines long and your AC lines short
                          This is going to completely screw with my plans then. I don't really have any place to mount two inverters near my meter. The east wall of my house will already be covered in electrical and gas stuff. I will have a gas meter, meter base, 200 amp disconnect switch, and utility required disconnect for my solar on that wall already. It is also the wall all guests walk by, but I could count on both hands how many guests I have had in six years use the front door.

                          Solaredge uses a constant DC voltage of approximately 350 volts. I'm not sure if I will have issues running three DC strings through a single conduit. I will absolutely have to go with conduit and not direct burial if I have to run multiple strings of DC.

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                          • #28
                            Originally posted by reader2580 View Post

                            The breaker is next to the inverter and the inverter is next to the panels. My understanding has always been that AC has less voltage drop than DC. Remember the whole Tesla vs Edison thing when electricity was still a new thing? Tesla won out with AC because AC go go long distances with less voltage drop.
                            There are reasons to locate inverters near the panels, my inverters drive a 600 foot
                            wire loop to the PoCo meter. But efficiency is not the reason, Tesla managed efficient
                            long distance transmission by sending power at very high voltages and low current, then
                            using a transformer to bring the voltage down near the consumer. A transformer does
                            not work on DC. Bruce Roe

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                            • #29
                              Originally posted by reader2580 View Post

                              This is going to completely screw with my plans then. I don't really have any place to mount two inverters near my meter. The east wall of my house will already be covered in electrical and gas stuff. I will have a gas meter, meter base, 200 amp disconnect switch, and utility required disconnect for my solar on that wall already. It is also the wall all guests walk by, but I could count on both hands how many guests I have had in six years use the front door.

                              Solaredge uses a constant DC voltage of approximately 350 volts. I'm not sure if I will have issues running three DC strings through a single conduit. I will absolutely have to go with conduit and not direct burial if I have to run multiple strings of DC.
                              UL4703 PV wire is rated for direct burial. What size PV system are you planning? I'm assuming it's a ground mount since it's ~200' away? I assisted a friend with installing a 27kW ground mount a couple years ago. He ran 16 #10 wires through ~2" conduit (though I would have done direct burial since he was using USE-2 wire). Not only is DC more efficient due to higher voltages but your conversion losses of DC=>AC are taken AFTER line losses not before. Also, with panel prices being so cheap oversizing arrays is more and more cost effective. One ground mount I installed is oversized by 50%. It saturates the inverters from 10am to 3pm, any DC lines losses that occur when the inverters are maxed out are free.

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                              • #30
                                Originally posted by nwdiver View Post

                                UL4703 PV wire is rated for direct burial. What size PV system are you planning? I'm assuming it's a ground mount since it's ~200' away? I assisted a friend with installing a 27kW ground mount a couple years ago. He ran 16 #10 wires through ~2" conduit (though I would have done direct burial since he was using USE-2 wire). Not only is DC more efficient due to higher voltages but your conversion losses of DC=>AC are taken AFTER line losses not before. Also, with panel prices being so cheap oversizing arrays is more and more cost effective. One ground mount I installed is oversized by 50%. It saturates the inverters from 10am to 3pm, any DC lines losses that occur when the inverters are maxed out are free.
                                I am relocating an existing solar system and also adding four more panels at the same time. I am changing to a single 11.4KW Solaredge inverter I got cheap since my two Solaredge inverters were maxed out. I want to keep one of the old inverters to add more panels at a later date. Garage is being torn down and replaced with much larger garage so panels have to come off roof and ground mount has to move.

                                Current config is combo of ground mount and panels on detached garage roof with two Solaredge inverters. AC runs 80 feet to house with about a ten foot run of DC to each string.

                                New config will be two ground mounts with a single inverter to start. Original plan was to run AC 200 feet to inverter with DC going about ten feet to panels. It sounds like I should mount the inverters close to meter so AC run is a lot shorter and DC run is much longer.

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