Commercial Line-Side Install Questions/Checking

For a commercial install sized of either ~70kW or ~105kW (final size still in debate), I will be forced to do a line-side connection simply because the main panel isn't large enough to handle the power produced. Assuming the ~70kW choice is picked, I would be using a Solectria PVI 60kW.

Some helpful folks on this forum suggested I read this link. Furthermore, the 2009 June/July issue of SolarPro magazine has a great good article on making a line-side connection.

After some discussion with the utility company, they're fine with that. However, instead of just being able to tie into a 2nd potential set of lugs on the utility meter, or instead of being able to tie into the busses on the supply-side of the main panel, the utility is requesting that I put a new panel between the main panel and the utility meter/disconnect (see attached picture).

Referencing the SolarPro article and NEC Code references in said article, I would like to ask if my calculations seem correct:

1) As mentioned, the PVI 60kW outputs 166A at 208V 3-phase. With the 125% continuous-load factor applied, I end up with 207.5A. Using 90-degree-C THWN-2 wire and referencing NEC ampacity tables, I'll have to use 3/0 wire (rated for 225A).

2) From the inverter, my 3/0 wire would normally have to enter a bladed AC Disconnect. However, since the utility is requiring that we have a panel in between the utility meter/disconnect and building main panel, won't the breakers in the new panel suffice as a disconnecting means? The attached picture was drawn with the assumption that the new panel breakers suffice as a disconnecting means.

3) Since the wires were sized to handle 207.5A, the breakers will also need to have the same correction factor applied to them. Using standard NEC breaker sizes in 240.6(A), I'll need a 225A breaker.

3) The main panel in the building is a 1200A rated panel. Using this plus the 225A breaker that I will need for the power coming from the inverter, will I need to have a panel rated to handle 1200A+225A=1425A? This is the part that confuses me the most and this is where I need the most help.

4) How do I size the grounding electrode conductor? Would I base it on the GEC that the utility uses around their meter/disconnect or would I base it on my system like I normally would?

I feel like I have some more questions but I can't think of any right now and I have to run. I'd appreciate your assistance and critique. Thanks!

Comments

  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    Re: Commercial Line-Side Install Questions/Checking

    Calcs 1, 2 and 3 appear to be correct.

    Your second #3 regarding panel sizing depends on the NEC code your local jurisdiction follows.

    If your building main panel is rated at 1200A and your 60 kW inverter requires a 225A breaker for protection, according to the 2008 NEC (assuming your local jurisdiction allows that) you should be able to simply add your 225A breaker to the main panel.

    In the 2005 NEC for commercial installs, you would need a panel rated at 1425A minimum as you calculated.

    I have no idea what common size panels/breakers are for commercial installs...
  • arcturusk1
    arcturusk1 Solar Expert Posts: 26
    Re: Commercial Line-Side Install Questions/Checking

    Thanks for the reply, drees. I had thought that I would be able to connect the inverter output to the 1200A building panel (after all, you can go 20% over the panel rating and 20% of 1200A is 240A), and I would do that, but I was aiming for a line-side connection to give myself wiggle room. This would be especially important if the system size increased or if the municipality wanted to put more panels in at a later date. Referencing the attached drawing, it would be far easier to throw up some roof panels + a smaller Solectria and link that into a new panel (that would already be fed by a large Solectria) in a line-side scenario than it would to put the large Solectria into the building main panel and then have zero wiggle room to fit the smaller addition in a load-side scenario.

    Thanks again for the calc check/sanity check, dress!

    Anyone else have any thoughts on the grounding electrode conductor?
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Commercial Line-Side Install Questions/Checking

    i'm confused here. why the need for the new panel if you can tie into the present main panel? any future solar may need the line side tap or new panel as they suggested at that point as you've little extra room on the present main panel. a new panel will be even larger as all of the solar plus the present main panel (would become subpanel) would need to be bused in that new panel at the new added rating and round up to the nearest commercial panel available. any expansion would be very expensive.
  • arcturusk1
    arcturusk1 Solar Expert Posts: 26
    Re: Commercial Line-Side Install Questions/Checking
    niel wrote: »
    i'm confused here. why the need for the new panel if you can tie into the present main panel? any future solar may need the line side tap at that point as you've little extra room on the present main panel.

    The need for the new panel is going to arise because the system will likely be larger than the 70kW I mentioned. It will probably be the 100kW system. That means going from the PVI 60kW to the PVI 95kW.
    >At 208V, that means the rated current goes from 166A to 261A.
    >Now, 166A * 1.25 = 207.5A, and the next breaker size up to handle that is 225A.
    >We could work that into the panel because 1200 * 20% = 240A. 225A < 240A, so we're good.
    >That falls apart if the rated current is suddenly 261A. Now I'm talking about 326.25A with the 1.25 safety factor. I violate the 20% rule in that case.

    Even if I go with the 70kW system and the PVI 60kW, and I pipe that into the building main panel, what if the municipality wants to put panels on the roof at a later date? Regardless of whether a I use a few smaller inverters (like the Solectria 15kW) or another large inverter (like the PVI 60kW again), I don't have that 20% wiggle room anymore. I'll be forced to do a line-side connection at that point.

    I guess my thinking is that 1) We're probably going to go with the larger system and 2) Even if we didn't, there may be future expansion. With those thoughts in mind, it seems like it would make more sense to do a line-side connection right now and free myself from any limitations imposed by the existing building main panel.

    Furthermore, to answer the part of your question about "why the new panel?": It's a utility requirement. Usually, in line-side connection scenarios, you can connect to the utility meter lugs if there are unused lugs. Or, you can connect to any busses in the building main panel before you hit the main breaker. I have attached two pdfs from SolarPro June/July 2009 to highlight connection locations. Unfortunately, the utility is requiring me to install a panel instead of connecting at one of the points in the images. Thus, the new panel will be the central connection point for the building main panel, the utility supply, and our PV system. I need to know how to size that panel. =\
  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    Re: Commercial Line-Side Install Questions/Checking
    arcturusk1 wrote: »
    I need to know how to size that panel. =\
    It needs to be big enough so that you never exceed the 20% backfeed 2008 NEC rule.

    Which means it needs to be at least 1200A - bigger if you want room for expansion.

    At least if you go bigger, you can keep the main breaker rated at 1200A which will free up more room for expansion.

    For example, let's say you have 1500A panel - conceivably you can backfeed 300A of PV while maintaining the 1500A main breaker. Or by reducing the main breaker to 1200A, you can now backfeed 600A of PV.

    That should give you enough room to double the amount of installed PV. If you think they might want to install more than that in the future, then you'd want to look at an even bigger panel.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Commercial Line-Side Install Questions/Checking

    ok, it's clearer now as i thought you were going with the smaller system and possible later expansion. with their requirement of a new panel you are looking at some very big $ to have a solar setup of this size with any later expandability because the bus in the new panel has to handle all taps in an additive fashion.

    that is some huge current. i think i'd like to see some pics of your system after it's done if you wouldn't mind posting some.:confused::D
  • arcturusk1
    arcturusk1 Solar Expert Posts: 26
    Re: Commercial Line-Side Install Questions/Checking
    drees wrote: »
    It needs to be big enough so that you never exceed the 20% backfeed 2008 NEC rule.

    Which means it needs to be at least 1200A - bigger if you want room for expansion.

    At least if you go bigger, you can keep the main breaker rated at 1200A which will free up more room for expansion.

    For example, let's say you have 1500A panel - conceivably you can backfeed 300A of PV while maintaining the 1500A main breaker. Or by reducing the main breaker to 1200A, you can now backfeed 600A of PV.

    That should give you enough room to double the amount of installed PV. If you think they might want to install more than that in the future, then you'd want to look at an even bigger panel.
    I totally didn't even think about derating the main breaker. That's definitely an option, thanks!
    niel wrote: »
    ok, it's clearer now as i thought you were going with the smaller system and possible later expansion. with their requirement of a new panel you are looking at some very big $ to have a solar setup of this size with any later expandability because the bus in the new panel has to handle all taps in an additive fashion.

    that is some huge current. i think i'd like to see some pics of your system after it's done if you wouldn't mind posting some.:confused::D
    Sure, pics would be doable. It may just take a bit of time since we're still quite early in the project! 8)

    I guess the one thing that I still don't quite understand, and pardon me for asking a dumb question here, but I always assumed that the rating of the panel strictly dictated the quantity and size of the breakers to go in. Thus, a 1000A panel could allow 10x 100A breakers or 5x 200A breakers or 2x 500A breakers, etc. I believe this is how the 2005 NEC mandated things, correct?

    With the 2008 NEC and the 20% rule, I'm a little thrown off. When landing a PV system into a breaker, am I only concerned about the 20% rule?
    >What if it's a 1000A panel and there's only a 100A breaker in there. Does the 20% rule even matter then? The current usage at max at that point would only be 100A of the 1000A possible. I should be allowed to land 900A of PV power in there at that point (or so I guess).
    >What if the panel is filled with 10x 100A breakers? Do I only have 20% to work with?
    >What if the panel has 2000A of breakers? The 20% rule still applies, right? That's the situation at this municipality. The main building panel contains something like 2500A worth of breakers but the panel itself is only rated for 1200A. I was told this is acceptable because it's not like every single breaker is pulling their full amperage. Something still seems fishy to me. :confused:

    I guess in essence: Why can't it be simple such that the number of breakers added up must be less than or equal to the rating of the panel?

    PS - As a sidenote, I reread the email from the utility and I realize I may not need to use an entirely new panel now! I was focusing too much on one paragraph and forgot the part where they just stated that the PV system has to be landed on a common bus. That's easy and allows connections at the utility meter (if it has an extra lug) or before the main breaker in the main building panel! Hurrah!
  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    Re: Commercial Line-Side Install Questions/Checking
    arcturusk1 wrote: »
    I guess the one thing that I still don't quite understand, and pardon me for asking a dumb question here, but I always assumed that the rating of the panel strictly dictated the quantity and size of the breakers to go in. Thus, a 1000A panel could allow 10x 100A breakers or 5x 200A breakers or 2x 500A breakers, etc. I believe this is how the 2005 NEC mandated things, correct?
    I'm not sure about commercial situations, but residential you run a load calculation to determine the minimum size of your main breaker and panel. In most cases you end up with significantly more load than your main breaker since you're very unlikely to ever max out more than a fraction of your load circuits. The goal here is to avoid nuisance tripping of the main breaker.
    arcturusk1 wrote: »
    With the 2008 NEC and the 20% rule, I'm a little thrown off. When landing a PV system into a breaker, am I only concerned about the 20% rule?
    >What if it's a 1000A panel and there's only a 100A breaker in there. Does the 20% rule even matter then? The current usage at max at that point would only be 100A of the 1000A possible. I should be allowed to land 900A of PV power in there at that point (or so I guess).
    >What if the panel is filled with 10x 100A breakers? Do I only have 20% to work with?

    Think of it this way - you have loads and you have feeds. The amount of loads you can hook up to your panel depend on whatever the NEC says about maximum loads for a commercial install.

    For feeds with PV, you are allowed to exceed the panel's rating by 20% with PV without downsizing the main breaker. The goal here is to avoid overloading any one point of the panel's busbar. Which is why if you are going to exceed 100% of the panel's rating when you add PV feeds, you must place those breakers on the opposite end of the bus bar of the main breaker (it's a good idea regardless).
    arcturusk1 wrote: »
    PS - As a sidenote, I reread the email from the utility and I realize I may not need to use an entirely new panel now! I was focusing too much on one paragraph and forgot the part where they just stated that the PV system has to be landed on a common bus. That's easy and allows connections at the utility meter (if it has an extra lug) or before the main breaker in the main building panel! Hurrah!
    Nice - so it sounds like they just want you to hook up a panel to combine all your PV generation feeds before your line-side tap. So it is a typical line-side tap.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Commercial Line-Side Install Questions/Checking
    arcturusk1 wrote: »

    I guess the one thing that I still don't quite understand, and pardon me for asking a dumb question here, but I always assumed that the rating of the panel strictly dictated the quantity and size of the breakers to go in. Thus, a 1000A panel could allow 10x 100A breakers or 5x 200A breakers or 2x 500A breakers, etc. I believe this is how the 2005 NEC mandated things, correct?

    With the 2008 NEC and the 20% rule, I'm a little thrown off. When landing a PV system into a breaker, am I only concerned about the 20% rule?

    The load breakers don't have anything to do with the 20% rule. Forget about them. There's the rating of the busbar and the value of the main breaker, which are usually the same. There's the value of the backfed breaker(s), which in toto cannot exceed 20% of the rating of the busbar if the busbar rating and the value of the main breaker are the same. If the value of the main breaker is less than the rating of the busbar, then you can put more backfed breaker in.

    The bottom line is that the sum of all breakers feeding the busbar cannot exceed 120% of the busbar rating and the load breakers are not part of the calculation. There's also a calculation that will tell you if the backfed breakers must be at the opposite end of the busbar from the main (it has to do with current density in the busbar) but most of us just put the backfed breaker(s) at the bottom and be done with it.
  • arcturusk1
    arcturusk1 Solar Expert Posts: 26
    Re: Commercial Line-Side Install Questions/Checking
    ggunn wrote: »
    The load breakers don't have anything to do with the 20% rule. Forget about them. There's the rating of the busbar and the value of the main breaker, which are usually the same. There's the value of the backfed breaker(s), which in toto cannot exceed 20% of the rating of the busbar if the busbar rating and the value of the main breaker are the same. If the value of the main breaker is less than the rating of the busbar, then you can put more backfed breaker in.

    The bottom line is that the sum of all breakers feeding the busbar cannot exceed 120% of the busbar rating and the load breakers are not part of the calculation. There's also a calculation that will tell you if the backfed breakers must be at the opposite end of the busbar from the main (it has to do with current density in the busbar) but most of us just put the backfed breaker(s) at the bottom and be done with it.

    I think I actually get it now! And I see how adding in the load breakers really made me confused. It seems the most important line is the following, with italics and bold on the most critical word:
    The bottom line is that the sum of all breakers feeding the busbar cannot exceed 120% of the busbar rating and the load breakers are not part of the calculation.

    Ok, so I can ignore load breakers because they're not supplying anything. I'm still a little miffed, though, because it's easy to exceed the 20% rule given a decent-sized PV system. Here's a few samples, assuming busbar rating = main breaker:
    200A rating means 40A solar breaker (200*.2), and a 40A breaker with the 1.25 safety factor means a max current of 32A.
    600A rating means 120A solar breaker (600*.2), and a 120A breaker with the 1.25 safety factor means a max current of 96A.
    1200A rating means 240A solar breaker (1200*.2), and a 240A breaker with the 1.25 safety factor means a max current of 192A.

    These are not very generous current amounts given our future installations. :cry: Of course, we can downsize the system, upgrade the existing equipment, derate the main breaker, or do a line-side connection. I'm leaning toward the last option, hence this thread. It just seems like a no-brainer since it frees up the installer from so many restrictions.

    Thanks so much for all of your help, folks.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Commercial Line-Side Install Questions/Checking

    Yep... The deratings make things that much worse.

    One thing to watch out for--Sometimes people put a 4kW inverter with ~3kW of solar panels... The problem is that there can be a wide range of power output with a 3kW array.

    You are almost forced to breaker a 4kW inverter as if it has 4+kW of solar panels.

    If you have a limited branch circuit--You are better installing 3.5 kW of solar panels on a 3kW inverter because its output is guaranteed to never exceed the 3kW rating.

    With 3+kW of solar panels, you have to treat them (if I understand code) as 1.25x1.25=1.56x to account for variable sun/conditions on an over-sized inverter (say 5kW for this example).
    • 3kW * 1.56 = 4.69 kW assumed NEC branch circuit design limits
    Vs a 3kW inverter:
    • 3kW * 1.25 = 3.75kW branch circuit rating
    (of course, these should be in amps--just trying to show the 1.25 NEC issues on panel vs inverter ratings).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Commercial Line-Side Install Questions/Checking
    BB. wrote: »
    Yep... The deratings make things that much worse.

    One thing to watch out for--Sometimes people put a 4kW inverter with ~3kW of solar panels... The problem is that there can be a wide range of power output with a 3kW array.

    You are almost forced to breaker a 4kW inverter as if it has 4+kW of solar panels.

    If you have a limited branch circuit--You are better installing 3.5 kW of solar panels on a 3kW inverter because its output is guaranteed to never exceed the 3kW rating.

    With 3+kW of solar panels, you have to treat them (if I understand code) as 1.25x1.25=1.56x to account for variable sun/conditions on an over-sized inverter (say 5kW for this example).
    • 3kW * 1.56 = 4.69 kW assumed NEC branch circuit design limits
    Vs a 3kW inverter:
    • 3kW * 1.25 = 3.75kW branch circuit rating
    (of course, these should be in amps--just trying to show the 1.25 NEC issues on panel vs inverter ratings).

    -Bill

    The code is pretty straightforward about this. The breaker on an inverter output is the next standard size up from 1.25 X the rated output of the inverter, irrespective of how much PV you have going into it. A 3kW inverter is not going to put out 4kW of power if you have 4kW of PV attached to it, it's going to clip its output to 3kW. I suspect it will throttle back the PV with its MPPT circuitry if conditions warrant it.
  • rick
    rick Administrators Posts: 134 admin
    Re: Commercial Line-Side Install Questions/Checking
    ggunn wrote: »
    The code is pretty straightforward about this. The breaker on an inverter output is the next standard size up from 1.25 X the rated output of the inverter, irrespective of how much PV you have going into it. A 3kW inverter is not going to put out 4kW of power if you have 4kW of PV attached to it, it's going to clip its output to 3kW. I suspect it will throttle back the PV with its MPPT circuitry if conditions warrant it.

    We've got a Fronius IG4500 installed on our building. On very cold sunny days (we get lots of those in Flagstaff) our solar array puts out more than 4,500 watts. The Fronius does not clip the extra power. I've looked at the display on some days and it will be putting out about 5,000 watts. I'm not sure if that's the exception. Some brands of inverters may indeed clip the extra power, but not Fronius.
    Website administrator for Northern Arizona Wind & Sun
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Commercial Line-Side Install Questions/Checking

    It may also have to do with the input voltage range... The IG 4500 LV appears to have:
    • 183 – 229 Volts
    • 21.6 A
    Math wise:
    • 183v * 21.6 amps = 3,953 watts low line
    • 229v * 21.6 amps = 4,946 watts high line
    • 4,500 watts / 21.6 a = 208.3 volts nominal line
    High line/low line available current*voltage was always a limiting factor in designing/spec.ing power supplies for some of the computer systems I designed when looking for worst case conditions.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Commercial Line-Side Install Questions/Checking
    rick wrote: »
    We've got a Fronius IG4500 installed on our building. On very cold sunny days (we get lots of those in Flagstaff) our solar array puts out more than 4,500 watts. The Fronius does not clip the extra power. I've looked at the display on some days and it will be putting out about 5,000 watts. I'm not sure if that's the exception. Some brands of inverters may indeed clip the extra power, but not Fronius.
    Interesting. Their spec sheet says maximum output power = 4.5kW, maximum output current = 21.6A @208VAC. If it were putting out 5000W @208VAC, that would be 24A. It seems to me that that would be a violation of the UL standard.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Commercial Line-Side Install Questions/Checking

    See above, there is a 183-229 volt spread in the acceptable AC line Voltage... 4,500 watts is at 208 volt nominal rating. More or less, it is really the current that limits output... Higher voltage times same current, means higher energy transfer.

    As far as I know, does not violate UL Listing (assuming has been tested for min/max ranges).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • arcturusk1
    arcturusk1 Solar Expert Posts: 26
    Re: Commercial Line-Side Install Questions/Checking

    I'd like to chime in though: Shouldn't current inversely follow voltage? I'm going to use the Fronius IG Plus 11.4-1 here as an example. The spec sheet states that the max output power is 11400W.

    Now, let's assume it's a 240V service. At the proper 240V, max continuous current is 47.5A. 240V*47.5A comes out to 11400W. BB points out that the output voltage can vary and he's absolutely right. However, why are we assuming that the output current doesn't fluctuate as well?

    If for some reason the grid voltage shot up to 264V (the top of the Fronius' range) then wouldn't the current just be 11400W/264V=43.18A?

    If the grid voltage plummeted to 211V (the bottom of the Fronius' range) then you'd think the current would be 11400W/211V=54.03A. However, the max continuous current is supposed to be 47.5A. Maybe the sticking point is the word continuous? I don't think the inverter should put out 54A.

    Given great conditions that force the inverter to operate around its extremes, I feel that the inverter does its damnedest to output 11400W, with the caveat that current not exceed 47.5A. I mean, folks base wire sizing around that "max continuous output current". If that current exceeds that, then we shouldn't be seeing 47.5A, we should be seeing the absolute max the current can hit if variables line up just right so that we size wire appropriately! Am I missing anything here?