Phase Imbalance & NEC 690.63

System
System Posts: 2,511 admin
in PV Installers Forum - NEC, Wiring, Installation #1
NEC 2008 690.63 (A) states:

<< Single-phase inverters for photovoltaic
systems and ac modules in interactive solar photovoltaic
systems shall not be connected to 3-phase power systems
unless the interconnected system is designed so that significant
unbalanced voltages cannot result.>>


My question is what exactly is meant by "unless the interconnected system is designed so that significant unbalanced voltages cannot result"?

I've been told by an inverter rep in a training seminar that phase imbalance of up to 6 kW is tolerated by most utilities.

Can anyone offer any guidance on this?

Who determines the definition of 'significant' -- the local electrical inspector or the utility?

I know that installing 3 single-phase inverters, 1 on each phase, is a common practice to avoid imbalance, but I have a commercial client who wants a small array (5kW). More for showing off to customers than serious load reduction and energy savings.

And they just don't make 5 kW 3 phase inverters, to my knowledge.

This my first post here, thanks in advance to responders.
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,637 admin
    #2
    Re: Phase Imbalance & NEC 690.63

    I believe that will be decided by the utility...

    There are many variations for 3 phase power drops to smaller companies... Many only install one transformer for the "heavy" 1 phase loads and a second smaller transformer to power the random 3 phase loads (Open Delta configuration).

    For a small 3 phase system, you could look at an Enphase (micro inverter) installation... However, many codes/regulations seem to require that if 1 phase of 3 phase power is lost--then the generators/sources of the other two phases have to be cut too...

    Personally, I think that this is probably a bit of overkill--especially for smaller solar installs (see open delta configurations).

    You utility with their information on the local connection for the commercial customer is the best approach to a solution.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • System2
    System2 Posts: 6,290 admin
    #3
    Re: Phase Imbalance & NEC 690.63

    Thanks Bill. I think I'll go bug the utility company about it.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #4
    Re: Phase Imbalance & NEC 690.63

    Relates to requirement specified in 690.63(B) for de-energizing when any of three phases become 'significantly' unbalance voltage-wise.

    This is typically +/- 12.5% but is up to local power company.

    I don't believe NEC would specify a number less it causes someone to point a finger at local power company to their supplied voltage being 'out of spec'.
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  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    #5
    Re: Phase Imbalance & NEC 690.63
    BB. wrote: »
    For a small 3 phase system, you could look at an Enphase (micro inverter) installation...
    Was going to suggest that, too. Then you can do a 3-phase install with as little as one panel.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #6
    Re: Phase Imbalance & NEC 690.63
    drees wrote: »
    Was going to suggest that, too. Then you can do a 3-phase install with as little as one panel.

    now that may prove difficult to do unless they have changed their microinverters specifically for 3 phase. you most likely would need one for each phase and from what i have gathered is that that is not legal to do as one leg may fault and the others are required to react to the fault which 3 separate inverters will not do. i do disagree with the need for this rule as inverters only follow the source and will disengage in the absence of the utility provided legs of 3 phase. if the utility is only properly providing on 2 of those legs (or worse) they penalize the solar installation while allowing the utility to violate the very same ruling only because they are the source of the problem. way to go whiles.
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  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    #7
    Re: Phase Imbalance & NEC 690.63
    niel wrote: »
    now that may prove difficult to do unless they have changed their microinverters specifically for 3 phase.
    All their micro-inverters are available in single-phase 240V and 3-phase 208V configurations with the same performance. You can run more inverters per-string with 3-phase, though, so large installs may be a bit less labor intensive.
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