[RE-wrenches] sub-panel supply-side tap

[Jason Szumlanski]

First, let me say I am no NEC 240 expert...

Remember that 690 will supersede 240 if the requirements differ. I don't
see any way to make this work unless the conductor for the feeder circuit
is sized to 233A (280/1.2), which is certainly possible. That's assuming
the combined size of the breakers immediately connected to the inverters
adds up to 80A. If not, upgrading this feeder conductor may do the trick.

The bus in the main panel would also need to be a minimum of 233A, which
seems likely if the subpanel is 200A. Since the subpanel is protected by a
200A main breaker (I'm assuming), that bus is not at issue here.

I'm guessing in this case that 80A is the total rated ampacity of the
system, but that this comes from multiple inverter output circuits that may
be protected by, for example, two 50A breakers. If my guess is correct,
then 100A would need to be considered, and the conductor ampacity
requirement goes up to 250A.

As other have said, this is a load-side connection, and the requirements
are difficult to meet in this scenario. However, the feeder conductor
appears to be the main issue in this case.

Jason Szumlanski
Fafco Solar



On Wed, Jun 6, 2012 at 11:08 AM, Hans Frederickson <hans at fredelectric.com>wrote:

> Kirk,
> You're basically doing a feeder tap, which you'll find the rules for in NEC
> 240.21(B). The difference from the intent of this code is that your tap  is
> working in reverse. It will supply current to the feeder rather than
> receive
> current from it. If your subpanel doesn't have a main breaker appropriately
> sized to protect the feeder wiring, then technically your 80A from PV and
> 200A from the service could add to 280A and overload the feeder wiring
> between the tap and the subpanel, as well as the subpanel bussing.  If I
> was
> the AHJ, I would require a main breaker in the subpanel in addition to the
> conditions specified in 240.21(B). This is definitely one to check on with
> the AHJ ahead of time.
>
> Regards,
> -Hans
>
>
> From: re-wrenches-bounces at lists.re-wrenches.org
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Kirk
> Herander
> Sent: Tuesday, June 05, 2012 3:44 PM
> To: RE-wrenches
> Subject: [RE-wrenches] sub-panel supply-side tap
>
> Hello,
>
> Ideally I'd like to make a supply-side tap on a feeder to a sub-panel. And
> I
> mean tapping the feeder line-side of the sub-panels 200 amp breaker. This
> is
> due to the limitations of the sub-panels 200 amp buss. My PV system max
> output is 80 amps and therefore requires a 100 amp breaker. A no-go with
> the
> 200 amp buss. The 20% allowance won't work and I cannot downsize the 200
> amp
> breaker to make it work due to panel loads. I would have to upgrade the
> panel to 400 amps. FYI this is 208/120 3 phase.
>
> I have done supply-side connections before but only on a buildings main
> service. Do all the same rules apply? What might be different I should take
> into consideration? Thanks.
>
> Kirk Herander
> VT Solar, LLC
> dba Vermont Solar Engineering
> NABCEPTM Certified installer Charter Member
> NYSERDA-eligible Installer
> VT RE Incentive Program Partner
> 802.863.1202
>
>
> _______________________________________________
> List sponsored by Home Power magazine
>
> List Address: RE-wrenches at lists.re-wrenches.org
>
> Options & settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
>
> List-Archive:
> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
>
> List rules & etiquette:
> www.re-wrenches.org/etiquette.htm
>
> Check out participant bios:
> www.members.re-wrenches.org
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20120606/4627a6de/attachment-0004.html>