AC disconnect sizing for dual voltage inverter

Jcpen68
Jcpen68 Registered Users Posts: 5 ✭
in Solar Beginners Corner #1
I am in the midst of an install and I am questioning the sizing of the AC disconnect for the inverter.

Inverter is an SMA SB20000TL, 277/480V, 3-phase. AC disconnect is 30A.

Inverter output is rated at 24A
Inverter Maximum permissible fusing is at 50A.

Our utili connection is 277V.

If we calc the short circuit current for the inverter at 277V, I get 90.25A
If we calc the short circuit current for the inverter at 480V, I get 52.08A

Both values are above the permissible fusing of 50A.

Note that the inverter has a listed output of 24A, which applied to the 1.25 s.f., gives us 30A, which I am sure is why the 30A disconnect was chosen and installed.

Please advise.

Note, another reason for the question is driven by a recent ding from the inspecting agency on our tagging on other projects. In the past, we would simply list on the tag the rated output current for the inverter (from the inverter nameplate), but the inspector wanted us to list the short circuit current for the inverter, which is not listed and needs to be calculated.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #2
    Re: AC disconnect sizing for dual voltage inverter

    Technically, there is no "short circuit output" from the inverter...

    There is an old requirement (10+ years ago?) where the inverter had to list how much back-feed current from the array would feed into the AC output if there was a short--And the answer was Zero Amps for any sane GT Inverter design. Here is Xantrex's response to the question from 2005:

    http://millersolar.com/MillerSolar/Resources/Xantrex/TechNote_GT3.0.pdf

    The inverters will shut down if the the AC line voltage is out of range (too high or too low) and/or the frequency is out of range (by 0.5 Hz or so).

    Then there is the maximum operational output current--Which is 24 Amps--And 24a*1.25NEC=30 amp rated branch circuit/breaker minimum.

    So, my answer (I think it is correct)... The unit is "LISTED" for 24 Amp maximum output current. It has anti-Islanding functions that meet UL-xyz specifications (cannot output into a short circuit). And there is no current available direct from solar array to shorted AC output of the Inverter.

    So--You treat the GT inverter just like any other rated "load" (other than the power flow is "backwards" due to it behaving like a current source--i.e., not a battery. Current sources cannot have a "short circuit current" that is much above operating current--Just like solar array Imp vs Isc).

    -Bill

    I guess you could list Isc of the solar array on the tag as DC Array Isc (short circuit current) = XX Amp DC. That would actually be (somewhat) useful. But again, then they should also "care" about Voc-cold of the array too (i.e., 21 VDC or 600 VDC, etc.). Which gets into a different wresting match on your system labels. But it is at least something that make sense and is useful.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #3
    Re: AC disconnect sizing for dual voltage inverter

    FWIW, although there are some variations with code cycle (year) the NEC generally wants to see the breakers sized according to the nominal GTI output current times 1.25 for unusual output conditions and times 1.25 for continuous current loading. Net result is nominal output times 1.56. That makes for a minimum breaker size of 40 in this case.
    And, yes, there is no such thing as "short circuit current" for a GTI.
    And for a single three phase inverter box, you can really expect the delivered power to be equal on all three line conductors.
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: AC disconnect sizing for dual voltage inverter

    Just so we're on the same page (and to elucidate for anyone else reading):

    'Y' configuration 3-phase power system. Voltage between any two legs is 480. Voltage between any leg and the 'center' (Neutral, grounded) is 277.

    Configuration of the inverter (which I couldn't find; must be model number wrong) is 3-phase output of 2kW from each leg to center @ 277 Volts 24 Amps (6648 Watts / 3 = 2216 Watts per leg, current at maximum on center connection). 24 Amps * 1.56 = 37.44, round up to 40 Amp breaker. This is below the 50 Amp maximum. The 30 Amp may be undersized, but no matter as it is above the maximum expected inverter output and below the amount the grid could feed if the inverter shorts.

    If this is a disconnect and not a breaker then the disconnect should be increased in capacity so that the breaker is the lowest current value of anything in the circuit but still within operating range.

    Once again it sounds like another inspector who doesn't understand what he is doing or is unable to communicate properly.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #5
    Re: AC disconnect sizing for dual voltage inverter
    Cariboocoot wrote: »
    Just so we're on the same page (and to elucidate for anyone else reading):

    'Y' configuration 3-phase power system. Voltage between any two legs is 480. Voltage between any leg and the 'center' (Neutral, grounded) is 277.

    Configuration of the inverter (which I couldn't find; must be model number wrong) is 3-phase output of 2kW from each leg to center @ 277 Volts 24 Amps (6648 Watts / 3 = 2216 Watts per leg, current at maximum on center connection). 24 Amps * 1.56 = 37.44, round up to 40 Amp breaker. This is below the 50 Amp maximum. The 30 Amp may be undersized, but no matter as it is above the maximum expected inverter output and below the amount the grid could feed if the inverter shorts.
    The 40A size is the NEC mandated minimum circuit ampacity, so the 30A breaker would be perfectly safe if used with large enough wire (capable of handling 37.44A), but would not pass NEC-based inspection
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: AC disconnect sizing for dual voltage inverter
    inetdog wrote: »
    The 40A size is the NEC mandated minimum circuit ampacity, so the 30A breaker would be perfectly safe if used with large enough wire (capable of handling 37.44A), but would not pass NEC-based inspection

    Yup. Tricky, isn't it?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #7
    Re: AC disconnect sizing for dual voltage inverter
    inetdog wrote: »
    FWIW, although there are some variations with code cycle (year) the NEC generally wants to see the breakers sized according to the nominal GTI output current times 1.25 for unusual output conditions and times 1.25 for continuous current loading. Net result is nominal output times 1.56. That makes for a minimum breaker size of 40 in this case.
    And, yes, there is no such thing as "short circuit current" for a GTI.
    And for a single three phase inverter box, you can really expect the delivered power to be equal on all three line conductors.

    I would argue it slightly differently...

    The 24 amps is the maximum LISTED rated output current regardless of nominal AC Output Current and solar array input. So the Unusual Output conditions do not apply for the "additional" derating.

    However, I will argue that it can apply in other conditions... For example, lets say the inverter is rated for 24 amps at 240 VAC or 5,760 Watts nominal output... We know we can up size the array by 1/0.77 easily for 7,481 Watt "nominal" array. And the inverter's output will not exceed 24 amps (at 212 VAC or 240 VAC or 264 VAC--Typical AC mains voltage range).

    But, many times, we get people that have a smaller array for some reason, and a "large" inverter.

    Say somebody wants to put on a 3 kWatt array... What size AC branch circuit could be used. I would argue that it would be:

    3,000 Watt array * 1.25 NEC "ususual conditions" * 1.25 NEC derating * 1/212 VAC line minimum = 22 Amps minimum Branch Circuit+Breaker rating...

    Because of all of the deratings--In most cases, the "worst case deratings" is "close enough" to LISTED AC inverter output requirements--It is not worth (as in this example) to install a 25 amp branch circuit when a 30 Amp branch circuit would handle the actual max rated output of the inverter.

    Similar issue for MPPT type solar charge controllers--Their output is "controlled" to their rated output... So the extra 1.25 derating is not needed on the output side.

    For PWM solar charge controllers--They do not have the native ability to control limit their output current to a "rated value"--So there, the extra 1.25x NEC unusual conditions ratings would make sense (sunny day, high altitude/snow/water/sand reflections/etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #8
    Re: AC disconnect sizing for dual voltage inverter

    The biggest reason for trying to use the smallest breaker possible on the output of a GTI is if your are covered by any NEC earlier than 2014 and so have to size the upstream wiring and apply the 120% rule based on the sum of the first breaker amps instead of the sum of the rated amps with multiplier. Even 2 amps can make the difference between the backfeed fitting on your existing service panel or not fitting.
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: AC disconnect sizing for dual voltage inverter
    inetdog wrote: »
    The biggest reason for trying to use the smallest breaker possible on the output of a GTI is if your are covered by any NEC earlier than 2014 and so have to size the upstream wiring and apply the 120% rule based on the sum of the first breaker amps instead of the sum of the rated amps with multiplier. Even 2 amps can make the difference between the backfeed fitting on your existing service panel or not fitting.

    You sound just like an electrical inspector.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #10
    Re: AC disconnect sizing for dual voltage inverter
    Cariboocoot wrote: »
    You sound just like an electrical inspector.
    Nope, just someone who has to deal with them.....
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: AC disconnect sizing for dual voltage inverter
    inetdog wrote: »
    Nope, just someone who has to deal with them.....

    Have you tried using a ... [don't think we want to go there. -Bill B]? :p
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