120 volt AC sub panel

Bigwooo
Bigwooo Solar Expert Posts: 62 ✭✭
in PV Installers Forum - NEC, Wiring, Installation #1
I'm wiring in a new sub-panel for my (new) main house. It's supplied with 120 volt power from two 2000 watt (parallel) outback inverters with a 60 amp breaker in the main distribution panel that's located in my utility room (seperate building from house).

The input to the sub-panel for the house is 120 volt AC and I'm using #2 wire. In the sub-panel, the two hot sides are jumpered together with #2 wire inside the box in order to use breakers on both sides. Can jumpering the two sides together cause some problems I have not forseen?

Note: I've entertained switching to 240 volts, but due to space considerations adding the additonal outback equipment needed for 240, and trying to avoid a bunch of re-wiring (including digging up the existing underground wiring and adding a third wire) , I'd like to try and stay with 120 volts.

Also, I'm in Mexico, so there is not code or inspectors.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,617 admin
    #2
    Re: 120 volt AC sub panel

    The issues, that I can see, revolve around the 120 VAC wiring.

    First, if you have, for example, a 200 amp rated box. Your total current at 120 VAC still needs to be 200 amps maximum. This respects the maximum ratings of both the 200 amp bus bars AND the 200 amp Neutral/Ground bus limits.

    Second, with 120/240 VAC, you can (safely) wire loads with a Black/Red/White (plus ground) cable from the panel to the load(s). The common white (neutral) wire will carry the maximum rated current of either the black or red wire. But when 240 VAC is use, the black and red being 180 degrees apart, they "subtract" from each other with respect to the white wire (i.e., the 10 amps black - 5 amps red = 5 amps in white wire). If this was done with your 120 VAC box, the black and red phases "add", so the white wire would get up to 2x the current (i.e., 10 amps black + 5 amps red = 15 amps in white). So--Each 120 VAC circuit needs its own white/return wire.

    When you change to a 120/240 VAC service, the above limitations change. The 200 amp panel handles 2x the maximum power (now 120 VAC to each bus bar), and the existing 120 VAC circuits are still fine (no white wire problem) and you can now run 3+1 romex (black/red/white/ground).

    If the 240 VAC is supplied by a generator or inverter (or you have A LOT of 120 VAC), you may need to re-balance your 120 VAC load evenly between the two bus bars to get the maximum 120 VAC leg below the genset/inverter max 120 VAC current per leg.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: 120 volt AC sub panel

    You've got two FX2012 inverters right? They can be stacked to provide 240 VAC without any additional equipment (assuming you have a MATE and HUB to take care of the programming, which you'd have to in order to get them to work parallel stacked).

    You probably do not need the full 4kW capacity of both inverters on either leg. If you did you could use a single autotransformer to balance the output. The wiring starts to get complicated here, especially with a 120 Volt generator for back-up.

    On the whole it's not much of a problem though, as so long as the feed from the inverters to the panel can take the full current capacity (about 34 Amps continuous) the panel should have no trouble (probably at least 60 Amp rating) and the individual lines from it back to the neutral bus bar will take care of themselves (each 15 - 20 Amp 'half' being about half the max current of the two inverters).

    We are not talking about anyplace near 200 Amps capacity here.
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  • Bigwooo
    Bigwooo Solar Expert Posts: 62 ✭✭
    #4
    Re: 120 volt AC sub panel

    Thank you both for the info. Yes I have two FX 2012 inverters. I always assumed that if I went to 240, I'd have to install the auto transformer. I'm not sure if using a power tool (drill, skilsaw), at the same time, for instance as when the fridge kicked on, would be too much of a surge without the transformer or not. Also, I have the feed wires underground, and they're running through an area I'd rather not dig up to add the third wire for 240.

    The feed from the inverters can handle more than the full load. I went one size up from what the charts recommended.

    As I'm self taught, and there's no one around here with any real knowledge about these systems, I occasionally run into something I didn't anticipate. Like when I got to the point of connecting the hot input to the sub-panel (after already burying the conduit and wires), "well...crap, there's two hot legs and I only have one hot wire!" I just assumed jumping the two hot sides together was OK, but I wanted to be sure.

    Here's a photo of how I connected the two sides.


    Attachment not found.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: 120 volt AC sub panel

    I wouldn't worry about it. The 2 AWG wiring and breaker buses can take far more current than the two FX's can possibly provide.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,617 admin
    #6
    Re: 120 volt AC sub panel
    Cariboocoot wrote: »
    We are not talking about anyplace near 200 Amps capacity here.

    Looks pretty close to a 200 amp panel there Marc. :p

    Anyway Bigwoo--A couple suggestions for next time... I would wrap the "phase tape" (white tape) a up much of the exposed wire--Make it more obvious.

    And, the Lightning Suppressors... I have mixed feelings about those. I would have suggested mounting those outside the electrical box so that if they "blow up", you do not have to replace your breakers and wiring from smoke damage. Also, you should be able to inspect them (swollen bodies after a surge event) without having to pull the main panel cover.

    On the other hand, you have a nice/clean install behind sheet rock. I would not like to install those in the wall space (next to wooden studs) that could be ignited by a failing surge suppressor.

    I don't know the right answer--We don't get much in the way of lightning in our area--So I really don't have any experience with these guys.

    One other question. The plastic tubing for your wire pulls... Not sure about that (beyond my experience) but you should look at properly terminating those in the box with appropriate fittings. I don't trust bundle ties (especially white/translucent ones) to do their jobs for decades to come (many seem to become brittle over time/exposure to light/uv/ozone, etc.). Those knockouts are usually pretty sharp.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: 120 volt AC sub panel

    Well if we're going to play "code demon" none of that would pass here, starting with the drywall around the box. :p

    Not sure the lightning arrestors are actually of any value since there's really no "antenna" to introduce it to the box (unlike with a utility hook-up where the line feed can bring it in). It would nail the charge controller and inverter first. Definitely have them on the PV array.

    Those neutral wires look very small. Could be an optical illusion (my eyesight isn't the best). If it's the white from 14 AWG it would be okay. Some of the 'small' black looks larger to me.

    I take it the green is 6 AWG ESG from other parts of the system?
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  • Bigwooo
    Bigwooo Solar Expert Posts: 62 ✭✭
    #8
    Re: 120 volt AC sub panel

    Thanks for all the additional input. I wish I had proper termination for the plastic tubing, but I just can't find any here in Mexico. You'd be amazed at what is "normal" wiring here. I didn't want to hold up construction waiting for a trip North to get them. I may, in the future, when I can get my hands on some, install them and do some drywall repair afterward. I'm sure it will be worth the effort.

    The orange tubing protects the wire, as here in Mexico, everything is run through concrete, block, around rebar and next to all sorts of sharp and abrasive objects.

    The box looks like it has a large load, but there are a lot of small circuits. It's much easier, and there's less risk of wire damage, to divide the wiring up into small service areas. The hot feeds are run through the footings, as floors and ceilings are poured toward the end of the build. Running wire laterally through block is very difficult and time consuming (plus the masons just don't watch out for the wire when they are working, lots of risk of damaged wires). Some guys do larger circuits and put the wire in the dirt, going cross country from wall to wall, but for me, there is too much risk of having a stake driven through the wire during construction, or a rock driven into it when compacting to do the floor pour.

    The neutral wires are all 12 and 14 gauge depending on the circuit, all the green #6 wires are from other parts of the system.

    My solar panels are on a different building, but since the wire run to the sub is underground, I thought it prudent to put in the arrestors. Figured a strike to ground might energize the wire into the sub.

    I thought about drilling a 1/2" hole in the panel cover so the arrestors could be outside the box, but having them sticking out like that, they might get bumped accidentally. Figured inside the box, although not perfect, might be the safest option available. Your comments made me think that it might be prudent to put the arrestors into an easily accessible metal box next to the sub. The wire is definitely long enough.

    Thanks again.
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  • northerner
    northerner Solar Expert Posts: 492 ✭✭✭✭✭✭
    #9
    Re: 120 volt AC sub panel

    Good idea to put in the arrestor on the ac side. I have read that some don't bother to protect the inverter from surges, and are susceptible to damage from nearby lightning strikes, etc... I plan to put mine on the outside of the ac connection panel near the inverters.

    The auto transformer is good to go with if you decide on split 120/240 vac. It provides load balancing and also allows more efficient operation of inverters, by allowing the slave to go to sleep if the load is less than 1000 watts. It still will provide split power to both legs from the master inverter, while the slave inverter is drawing minimal power.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,617 admin
    #10
    Re: 120 volt AC sub panel

    Years ago, our admin (Windsun) had said that most inverter lightning failures he has seen have been on the AC output--So, yes, surge protectors on the AC output are a good choice.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #11
    Re: 120 volt AC sub panel

    yes, the ac side is far more prone to lightning emp as there is far more wire for long distances to act like an antenna to pick it up. it is wise to put an spd there.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #12
    Re: 120 volt AC sub panel
    niel wrote: »
    yes, the ac side is far more prone to lightning emp as there is far more wire for long distances to act like an antenna to pick it up. it is wise to put an spd there.

    If there's a grid connection, then the input from the grid may be a bigger antenna than the output. Although the transformer should provide some sort of protection.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #13
    Re: 120 volt AC sub panel

    true as off grid will have far less ac wiring, but enough to still act as a very good antenna.

    how do you figure the transformer will protect anything? a high voltage spike into it will yield a high voltage spike out of it in concert with the turns ratio input to output. there's no regulation there.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #14
    Re: 120 volt AC sub panel
    niel wrote: »
    how do you figure the transformer will protect anything? a high voltage spike into it will yield a high voltage spike out of it in concert with the turns ratio input to output. there's no regulation there.

    Yes. It'll protect by reducing voltage. If I'm not mistaken, grid wire is 5kV. Transformer reduces it to 240V. That's about 20 times reduction. So, any spike will be reduced 20 times.

    Just crossed my mind: When a spike goes through the meter, do I have to pay for that?
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #15
    Re: 120 volt AC sub panel

    you didn't take that far enough for if the spike is 10,000v then it transforms to 480v. i don't see how you wouldn't pay for it as the meter doesn't know or distinguish where that power came from. the good news is that it's for only about a second or so so the wh won't amount to squat. the bad news is 480v is enough to fry some electronics.
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  • northerner
    northerner Solar Expert Posts: 492 ✭✭✭✭✭✭
    #16
    Re: 120 volt AC sub panel

    Another thing to watch for with grid power is an open neutral line. That can fry some electronics as well, and is not unusual. I just saw this with a house recently, but fortunately, for the home owner, There was a path through the water pipes to ground, likely making a path back through the neighbors house where there neutral is bonded to ground! We checked voltages, even with the furnace running and both hot legs to neutral were 120 v. There was current flowing through the copper water pipe to ground (about 5 amps at the time).
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #17
    Re: 120 volt AC sub panel
    niel wrote: »
    you didn't take that far enough for if the spike is 10,000v then it transforms to 480v. i don't see how you wouldn't pay for it as the meter doesn't know or distinguish where that power came from. the good news is that it's for only about a second or so so the wh won't amount to squat. the bad news is 480v is enough to fry some electronics.

    You're right. Need to disconnect from them ASAP!
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  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    #18
    Re: 120 volt AC sub panel
    NorthGuy wrote: »
    You're right. Need to disconnect from them ASAP!

    Nah. The meter only moves when the current on one conductor is going one way and the current on the other conductor is going the other. If the current is going the same way on BOTH legs at the same time, the meter doesn't move.

    Moral of the story -- make sure BOTH phase conductors get hit :D
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #19
    Re: 120 volt AC sub panel
    tallgirl wrote: »
    Nah. The meter only moves when the current on one conductor is going one way and the current on the other conductor is going the other. If the current is going the same way on BOTH legs at the same time, the meter doesn't move.

    Are you sure? How about the current that flows between one of the hot legs and ground/neutral? It doesn't affect the other hot conductor.
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  • northerner
    northerner Solar Expert Posts: 492 ✭✭✭✭✭✭
    #20
    Re: 120 volt AC sub panel
    NorthGuy wrote: »
    Are you sure? How about the current that flows between one of the hot legs and ground/neutral? It doesn't affect the other hot conductor.

    I tend to think it would depend on the load you're drawing at the time of the surge, as it will look for the path of least resistance. If your neighbor is drawing a heavy load, and you are drawing nothing, then a greater portion of the current will travel through the neighbors load, and none through yours.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #21
    Re: 120 volt AC sub panel
    NorthGuy wrote: »
    Are you sure? How about the current that flows between one of the hot legs and ground/neutral? It doesn't affect the other hot conductor.

    There are many ways to build an energy meter. For 120-120-->240 split phase one way to do it cheaply is to subtract the current in one hot let from the current in the other (and since they are in opposite directions, the magnitudes add) and measure the voltage to one or the other hot lead, assuming that the other is the opposite polarity at any instant.
    For normal loads and voltages, it works just fine and costs a bit less than one which measures both voltages. It does not measure the current in the neutral since it does not need to.
    For a spike which drives both hot lines in the same direction, it will measure zero current. Zero power.
    Another way to make a meter is to measure the sum of the currents (as above) and multiply the hot-to-hot voltage divided by two. This also measures normal loads just fine. In the case of a lightning strike which hits both hots equally, it will measure zero current and zero voltage for the spike. Zero power.

    And on a different point, if a spike hits both primary leads on the transformer (hot and hot or hot and neutral) equally, there will be no voltage induced in the secondary at all. It is only if one primary line is hit that there will be any effect on the secondary, and that will be reduced by the voltage ratio of the transformer.
    SMA SB 3000, old BP panels.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #22
    Re: 120 volt AC sub panel
    inetdog wrote: »
    There are many ways to build an energy meter. For 120-120-->240 split phase one way to do it cheaply is to subtract the current in one hot let from the current in the other (and since they are in opposite directions, the magnitudes add) and measure the voltage to one or the other hot lead, assuming that the other is the opposite polarity at any instant.
    For normal loads and voltages, it works just fine and costs a bit less than one which measures both voltages. It does not measure the current in the neutral since it does not need to.
    For a spike which drives both hot lines in the same direction, it will measure zero current. Zero power.

    There's a transformer over there. Hot legs are connected to the secondary winding, with ground/neutral connected to the middle.

    Regardless of the source of power on primary, it'll act the same way.

    If there's only one load, which is connected between L1 and L2 then the current will flow through both hot wires in opposite direction.

    If there's only one load, which is connected between L1 and N then the current will flow through only one hot wire (L1) and through the middle prong. The winding between L2 and N will not be affected. There will be no current on L2.

    The meter that will work as you said will not detect the difference between spike and grid power. It probably won't charge you for the whole spike because there must be limits on voltages and currents that it's capable of measuring.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #23
    Re: 120 volt AC sub panel

    I can't believe we're wasting space arguing about managing stray Voltage from lightning strikes and what will happen with transformers and power meters. :roll:

    Here it is a nut shell:
    One end of the possibility spectrum; nothing happens. The other end; things explode in a spectacular fashion. Inbetween are an infinite number of unpredictable possibilities.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #24
    Re: 120 volt AC sub panel
    Cariboocoot wrote: »
    I can't believe we're wasting space arguing about managing stray Voltage from lightning strikes and what will happen with transformers and power meters. :roll:

    Here it is a nut shell:
    One end of the possibility spectrum; nothing happens. The other end; things explode in a spectacular fashion. Inbetween are an infinite number of unpredictable possibilities.

    coot,
    that's ok, let them think it out. essentially you are correct as it all depends on many factors as to if damages occur, but my statement was that it could still happen with a transformer.

    i do have to correct myself though on the spike having put power through (in reference to being charged for it) as it will fully put power through only if in duration of a half cycle with the polarities matched up. when the ac power is reversed to the other polarity it will subtract some of the potential of the spike and it gets complicated here. overly oversimplifying a +480v spike on the negative 240vac portion of the cycle could be (using a transformed 480v output spike example) +480-240=+240v. the output could in my example go to varying dc at this point, but i only wanted to illustrate an ideal example as it is more complex than this. also keep in mind the meter itself may not respond to this spike considering its short duration and complex waveform possibilities. bottom line is still, don't worry about paying for the spike's power. hope i didn't confuse anybody.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #25
    Re: 120 volt AC sub panel

    Let's confuse 'em some more: lightning is high frequency.

    :p
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #26
    Re: 120 volt AC sub panel
    Cariboocoot wrote: »
    Let's confuse 'em some more: lightning is high frequency.

    :p

    you are only partially correct there as it is all frequencies and it has diminishing amplitude the higher in frequency you go. if it were only high frequency then you would not be able to see or hear it. the waveform could even resemble a square wave which is all frequencies combined. i wonder if anybody has ever tried to view the waveform on a scope in detail. this would be hard to do with the antenna pickup being frequency selective though.
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  • northerner
    northerner Solar Expert Posts: 492 ✭✭✭✭✭✭
    #27
    Re: 120 volt AC sub panel

    The other issue is that it's highly unlikely that a surge induced strike on a power line will be felt equally on two lines. It's interesting to think about that, but next to impossible in reality, I would think.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #28
    Re: 120 volt AC sub panel
    niel wrote: »
    this would be hard to do with the antenna pickup being frequency selective though.
    You just use a very short antenna (<1/4 wavelength of highest frequency of interest) and then compensate for the antenna efficiency when analyzing. As long as the antenna is short and non-resonant it will give you the volts/meter at all frequencies equally. You would have to match the impedance to the transmission line though.
    SMA SB 3000, old BP panels.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #29
    Re: 120 volt AC sub panel

    the highest frequency of interest would be the top end of the spectrum and goes beyond light and x-rays. if you roughly pick a frequency then the antenna would be super small and would correspond to frequencies in the lower to mid microwave area. this may present a problem in the pickup energy with such a small antenna.

    transmission lines tend to be somewhat frequency dependent too as the high end losses are severe. this makes putting the antenna at the scope ideal. not sure if the voltage would need amplified to correspond to the scope's needed input power either and the more in between the further the results get skewed.

    going up to say 1 gigahertz this would be about 2.95 inches relative to free space for 1/4 wavelength. this leaves out a great deal of the total spectrum as this is only into the uhf part of the spectrum. just how do you propose to make any antenna as being non-resonant?
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #30
    Re: 120 volt AC sub panel
    niel wrote: »
    just how do you propose to make any antenna as being non-resonant?

    I never said it would present a resistive source impedance. I just said non-resonant. And if you attach the antenna directly to the input terminals of your spectrum analyzer or whatever, you do not have to worry about SWR on any transmission line in between. Not practical (take your expensive equipment out in to your open field where you want your antenna, and then wait for lighting to strike), but the theory should be OK. :-)
    SMA SB 3000, old BP panels.
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