Re: [RE-wrenches] 30 amp disconnect/40 amp breaker

Fri, 23 Aug 2019 07:58:33 -0700 

Folks,
Thread points up the ever present weakness of the Code to deal with PV. In terms of specifying OCP, the Code completely fails to parse out overload protection from short circuit protection and always looks at it from the point of view of the grid. To Blake's credit he did bring in the issue of current interrupt capacity. 


Point is that a 40A breaker between the gird and (grid-tied) inverter 
will never see a condition where the the inverter is drawing any current 
of any significant magnitude for any significant period of time. The 
only time it will draw current from the grid is if some element within 
it is in short-circuit mode. In this case it really doesn't matter what 
the rating of the breaker is, 30 A, 40 A, of 100 A, it is going to trip 
very quickly when it sees a short circuit condition. All the over-sizing 
stuff in the code really only addresses high load/over load conditions 
which are not relevant to the situation we are talking about.



Another way of saying this, which would be more sensible is: yes, 
evaluate the circuit in both directions, but only consider the load to 
be continuous in the direction from the inverter to the grid.


Mark

On 8/22/2019 2:29 PM, William Miller wrote:


Blake:

Thank you for the reference from Schneider.


I think the point being missed here is that the ampacity rating of 
each device needs to be evaluated in light of the power flow in either 
direction.  If a 40 Amp breaker is supplied between the utility and 
the disconnect in question, in case of a fault, it could supply more 
current from the grid than a 30 Amp disconnect is rated to handle.   
This is particularly relevant considering the grid is not nearly as 
much a current limited source as an inverter.


Sincerely,

William Miller

Miller Solar

17395 Oak Road, Atascadero, CA 93422

805-438-5600

www.millersolar.com <http://www.millersolar.com/>

CA Lic. 773985

Quote of the month:  “As they age, batteries transition from energy

storage devices to energy consuming devices.“ W. Miller


*From:*Blake Gleason [mailto:[email protected] 
<mailto:[email protected]>]

*Sent:* Thursday, August 22, 2019 12:38 PM

*To:* [email protected] <mailto:[email protected]>; 
RE-wrenches

*Subject:* Re: [RE-wrenches] 30 amp disconnect/40 amp breaker

Hi William,


Yes, the bulletin I attached was just an example of a 100% rated 
disconnect (which happened to be DC since that's the one I had 
handy!).  My understanding is that most of the NON-fused Square D 
switches are rated for 100%.



See this FAQ from the manufacturer: 
https://www.schneider-electric.us/en/faqs/FA237949/, quoted below:


    *Resolution:**
    *All General Duty, Heavy Duty or Double Throw Safety Switches
    maximum current rating, either 80% or 100% of the nameplate
    rating, depends on if the switch is Fusible or NON-Fusible.

    oFusible switches are rated to carry up to 80% of the amp rating
    of the fuses installed in the switch

    oNon-fusible (unfused) switches are rated to carry 100% of the
    switch nameplate ampere rating

 Regarding this point:

    If, hypothetically, you could find a 30A disconnect that was
    continuous duty rated, if it is fed by a 40 Amp breaker, as I
    believe would be required, then the 30 Amp disconnect is still not
    suitable.  You can not feed a 30 Amp rated device with more than
    30 Amps.


I believe the original poster was asking about a 6000W inverter with 
25A max output.  So in that case you could use a 100% rated 
non-fusible disconnect, because the max current is only 25A.  (The 
required 40A breaker is sized larger because commonly available 
breakers are only 80% rated, not 100%.  But the max current is still 
only 25A.)  See 690.8(A) for calculation of maximum circuit current, 
specifically section (3) for inverters: "The maximum current shall be 
the inverter continuous output current rating."  (Not "the inverter 
continuous output current multiplied by 1.25.")


Best,

Blake


On Wed, Aug 21, 2019 at 10:59 PM William Miller 
<[email protected] <mailto:[email protected]>> wrote:


    Blake:

    Thanks for sharing the data sheet.  The document applies to DC PV
    systems, however, and I believe the discussion pertains to the AC
    inverter output. Therefore the bulletin you attached does not
    apply here.

    (Note also that this data sheet indicates you need to use two
    poles for any application, ungrounded or grounded.  I believe that
    requirement was usurped later on for Square D Heavy Duty switches,
    but again, that is for a different application.)

    I looked again at Square D general and heavy duty safety switches
    rated at 30 Amps and I could not find any reference that these
    switches are continuous duty rated.  I still believe that for the
    application described, the 125% rating has to be applied.

    If, hypothetically, you could find a 30A disconnect that was
    continuous duty rated, if it is fed by a 40 Amp breaker, as I
    believe would be required, then the 30 Amp disconnect is still not
    suitable. You can not feed a 30 Amp rated device with more than 30
    Amps.

    Bottom line, a 30 Amp disconnect will not meet code on the
    described project.

    William Miller

    Miller Solar

    17395 Oak Road, Atascadero, CA 93422

    805-438-5600

    www.millersolar.com <http://www.millersolar.com/>

    CA Lic. 773985

    Quote of the month:  “As they age, batteries transition from energy

    storage devices to energy consuming devices.“ W. Miller

    *From:*RE-wrenches
    [mailto:[email protected]
    <mailto:[email protected]>] *On Behalf Of
    *Blake Gleason
    *Sent:* Wednesday, August 21, 2019 4:20 PM
    *To:* RE-wrenches
    *Subject:* Re: [RE-wrenches] 30 amp disconnect/40 amp breaker

    Typically a fused disconnect can only handle 80% of it's nominal
    current on a continuous basis.  Unfused disconnects are often 100%
    rated.  For example, see attached Square D bulletin #3110DB0401R0410.

    So, a 100% rated (unfused) 30A disconnect (at least, from Square
    D) should be fine here, as long as your OCPD is provided elsewhere
    (eg, the breaker you mentioned).

    NEC 690.8(B)(1)(Exception) explicitly allows a 100%-rated device
    to be used without multiplying the inverter output current by 125%.

    On Wed, Aug 21, 2019 at 3:22 PM Greg <[email protected]
    <mailto:[email protected]>> wrote:

        Fellow Wrenches,  I'm having a discussion with a utility
        engineer regarding the following situation:

        Installing a 6kW inverter that has a full rated output of 25
        amps.  80% of 30 is 24. That means to me I need to go up to a
        40 amp breaker.  I'm running #8 (continuous load 25 x 1.25 etc.)
        The engineer believes I should use a 60 amp disco since it's
        on a 40 amp breaker.  My thinking is I can use a 30 amp disco
        since it can not produce more than 25 amps ever.

        Do any of you have a reference that shows what I'm doing is
        okay? Not okay?

        Thanks again,
        Greg Egan
        Remote Power Inc.

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*Blake Gleason, PE***| Employee-Owner |

*Director of Innovation & Technical Excellence*


510.845.2997 x128


Follow us onFacebook 
<https://www.facebook.com/pages/Sun-Light-Power/119675658118322?ref=hl>, 
LinkedIn <https://www.linkedin.com/company/sun-light-and-power> or our 
website <https://sunlightandpower.com/>.



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