Large systems & handling high amperages?

arcturusk1
arcturusk1 Solar Expert Posts: 26 ✭
in PV Installers Forum - NEC, Wiring, Installation #1
We're starting to take on larger commercial systems and when I do the design and drawings I'm finding that the large central inverters (often an easy choice) pump out some serious amps. For instance, look at a Solectria PVI 95kW. Scroll down to see the specs. At 208V, this thing has an output current of 261A. The conductors that carry this must have the 1.25 safety factor applied, so that's 326.25A. Using 90-deg-C THWN-2 wire and referencing NEC ampacity tables, I see that one has to use 350kcmil wire. Now, the Solectria install manual states that the PVI 95kW can handle up to a max of 750kcmil, but I haven't even seen that kind of wire in person, let alone figure out whether some of the downstream equipment (AC Disconnects, meters, breaker panels) can handle said wire. Are meters capable of accepting 350kcmil wire? What about breaker panels?

I also worry about the skin effect in some of the scenarios should I have to run the AC wiring any sizable distance.

How do you guys go about handling huge wire sizes and amperages? Do you shift from one large central inverter to multiple smaller inverters? Am I just worrying over nothing and it's not such a big deal to have such high currents and wires? I'm trying to look at this from a safety (high amps), efficiency (any impact on power generated), and financial perspective (cost of wire that thick, cost of components to handle the amps).
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
    Re: Large systems & handling high amperages?

    Meters will have a shunt to handle the current.

    And for your first big system install, you may need to hire a power engineer familiar with such systems and the local customs and codes (outside of my experience).

    High power/voltage systems are very interesting... Circuit breakers you have to manually pump to turn on, special suits for flash hazards, etc. High energy is a whole different world.

    -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 ✭✭✭✭
    #3
    Re: Large systems & handling high amperages?

    good questions, but this forum doesn't usually encounter systems that large as they are so few and specialized. here we focus on the smaller commercial installs and most residential installs as they are much more common.
    it isn't that your questions aren't allowed for they are allowed. if somebody can answer your questions then great for i'd be curious to know too, but i don't know if somebody can answer all of your questions or not and if so then consider yourself lucky.
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  • arcturusk1
    arcturusk1 Solar Expert Posts: 26 ✭
    #4
    Re: Large systems & handling high amperages?
    BB. wrote: »
    Meters will have a shunt to handle the current.

    And for your first big system install, you may need to hire a power engineer familiar with such systems and the local customs and codes (outside of my experience).

    High power/voltage systems are very interesting... Circuit breakers you have to manually pump to turn on, special suits for flash hazards, etc. High energy is a whole different world.

    -Bill
    Thanks for the shunt info.

    As for the big systems, well, my boss and owner of the company is NABCEP certified and has some Naval experience with high power applications. I took and passed the PV Entry Level NABCEP exam, so I'm certainly not fully qualified, but that combined with my EE degree and a ton of research that I'm doing on these topics are bringing me up to speed. I'm trying to pore over everything I find because I want the job done right and don't want to miss something critical. The stuff I do then gets reviewed by the big guy, so I at least have someone to check my work. Lastly, the physical work itself is ultimately done by an Electrician that we contract the work out to (we're still a small company and don't have an in-house Electrician). Giving them the scope of work and running through the install plan usually helps iron out any issues that may remain.

    It is indeed a different world, although thankfully everything is staying below 600V, so we're not even MV. The power, though, is still impressive. Sometimes even I struggle to understand just how much juice is being produced. It's hard to perceive sometimes!
    niel wrote: »
    good questions, but this forum doesn't usually encounter systems that large as they are so few and specialized. here we focus on the smaller commercial installs and most residential installs as they are much more common.
    it isn't that your questions aren't allowed for they are allowed. if somebody can answer your questions then great for i'd be curious to know too, but i don't know if somebody can answer all of your questions or not and if so then consider yourself lucky.
    Not exactly the answer I was looking for :p but it's good to know the limits of the forums. Hopefully as I find the answers I can disseminate that information to you folks here and maybe we can breach the arena of large commercial-sized applications. And, regardless of getting exact answers to my questions, your thoughts, speculations, assumptions, and/or Code knowledge are still very valuable.

    If anyone has any further thoughts on my original post, please chime in!
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  • rick
    rick Administrators Posts: 134 admin
    #5
    Re: Large systems & handling high amperages?
    arcturusk1 wrote: »
    We're starting to take on larger commercial systems and when I do the design and drawings I'm finding that the large central inverters (often an easy choice) pump out some serious amps. For instance, look at a Solectria PVI 95kW. Scroll down to see the specs. At 208V, this thing has an output current of 261A. The conductors that carry this must have the 1.25 safety factor applied, so that's 326.25A. Using 90-deg-C THWN-2 wire and referencing NEC ampacity tables, I see that one has to use 350kcmil wire. Now, the Solectria install manual states that the PVI 95kW can handle up to a max of 750kcmil, but I haven't even seen that kind of wire in person, let alone figure out whether some of the downstream equipment (AC Disconnects, meters, breaker panels) can handle said wire. Are meters capable of accepting 350kcmil wire? What about breaker panels?

    I also worry about the skin effect in some of the scenarios should I have to run the AC wiring any sizable distance.

    How do you guys go about handling huge wire sizes and amperages? Do you shift from one large central inverter to multiple smaller inverters? Am I just worrying over nothing and it's not such a big deal to have such high currents and wires? I'm trying to look at this from a safety (high amps), efficiency (any impact on power generated), and financial perspective (cost of wire that thick, cost of components to handle the amps).

    The largest wire I've used is 500 Kcmil and there's definitely overcurrent protection available that will accept conductors of that size. If you want to use circuit breakers, Siemens Power Distribution & Control makes some that accept up to 750 Kcmil. You also have the option of using fuse blocks which can also be found up to the size you're looking for. If you're planning on designing a very large solar system, you may want to consider paralleling multiple inverters instead of feeding all the power through one single inverter. If all the power is being utilized by a large single inverter and the inverter fails, then you will not be producing any power with the system until the unit is repaired. If you parallel multiple inverters and one of them fails, your other inverters will still produce power. This can be an important issue on large grid-tie systems because down time translates into a lot of lost $.
    Website administrator for Northern Arizona Wind & Sun
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  • arcturusk1
    arcturusk1 Solar Expert Posts: 26 ✭
    #6
    Re: Large systems & handling high amperages?
    rick wrote: »
    The largest wire I've used is 500 Kcmil and there's definitely overcurrent protection available that will accept conductors of that size. If you want to use circuit breakers, Siemens Power Distribution & Control makes some that accept up to 750 Kcmil. You also have the option of using fuse blocks which can also be found up to the size you're looking for. If you're planning on designing a very large solar system, you may want to consider paralleling multiple inverters instead of feeding all the power through one single inverter. If all the power is being utilized by a large single inverter and the inverter fails, then you will not be producing any power with the system until the unit is repaired. If you parallel multiple inverters and one of them fails, your other inverters will still produce power. This can be an important issue on large grid-tie systems because down time translates into a lot of lost $.

    Rick, thanks for the reply! Many good points there and it's good to know I won't be out in the cold when hunting for products to accept large wires sizes.
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  • russ
    russ Solar Expert Posts: 593 ✭✭
    #7
    Re: Large systems & handling high amperages?

    You are doing it the 'right way' - engineering means worrying about all the details, both large and small, early on and not having to say 'Oh shyt' later.

    This way maybe the impellers won't fall off later:blush: reference to another forum
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