[RE-wrenches] 30 amp disconnect/40 amp breaker
Mark Frye
markf at berkeleysolar.com
Fri Aug 23 10:58:21 EDT 2019
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:blake at sunlightandpower.com
> <mailto:blake at sunlightandpower.com>]
> *Sent:* Thursday, August 22, 2019 12:38 PM
> *To:* william at millersolar.com <mailto:william at millersolar.com>;
> 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
> <william at millersolar.com <mailto:william at millersolar.com>> 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:re-wrenches-bounces at lists.re-wrenches.org
> <mailto:re-wrenches-bounces at lists.re-wrenches.org>] *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 <greg at remotepowerinc.com
> <mailto:greg at remotepowerinc.com>> 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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>
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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>,
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> website <https://sunlightandpower.com/>.
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