Hi Garrison,
In my opinion you only need to size the conductors to the overcurrent
protection. Bill Brooks addressed a similar question of mine a few months
ago - I've attached the email here. If you do a node analysis (ie see what
would happen under both normal operation and fault conditions at any given
point) you'll see that the current at any given point along the wire path
will never exceed the breaker ratings on either side (or the higher of the
two if they're different). It is a different story from what the 120% rule
is addressing - in that case the current feeding into a busbar has the
potential of being supplied by both the main breaker and the solar breaker
thus potentially exceeding the bus capacity.
I've used insulation piercing connectors from Burndy and Ilso with good
results. Ilsco is called KUP-L-TAP and part number IPC-4/0-2/0 is a common
one. You'll need to find the part number that best matches your wire size.
It's a bit of a separate issue, but I've found that many jurisdictions are
very critical of supply side connections and can start to ask questions
about whether the whole setup is listed for the purpose. It is nearly
impossible to get the manufacturer to list the entire service and line tap
in my experience. You might want to look into that if you think your
jurisdiction might have similar concerns.
Good luck!
Best,
August
August Goers
Luminalt Energy Corporation
1320 Potrero Avenue
San Francisco, CA 94110
m: 415.559.1525
o: 415.641.4000
aug...@luminalt.com
*From:* re-wrenches-boun...@lists.re-wrenches.org [mailto:
re-wrenches-boun...@lists.re-wrenches.org] *On Behalf Of *Garrison Riegel
*Sent:* Wednesday, February 27, 2013 1:15 PM
*To:* 'RE-wrenches'
*Subject:* [RE-wrenches] Conductor Sizing for Supply Side Connection
Wrenches,
On an upcoming job we have a Xantrex XW4548 interconnecting with a 200A
service, and I’d very much appreciate some advice if y’all are willing.
Since the inverter output OCPD is required to be 60A we are looking at a
supply side connection, and I have two questions:
1. Before the point of interconnection the inverter output circuit
will first pass through the XW Power Distribution Panel and inverter
bypass, which is essentially a 60A breaker. The service disconnect for the
supply side connection will also be fused at 60A. Does the 120% rule apply
to the conductor between these two OCPDs? Or since this is a supply side
connection in accordance with 690.64(A), 690.64(B)(2) will not apply? [this
AHJ is on the 2008 NEC]
2. Can anyone recommend a reliable insulation piercing tap
connector? Or would you recommend something else entirely for a
residential supply side connection?
Thanks in advance,
Garrison
847-677-0950
Solar Service Inc.
--- Begin Message ---
Andy, August, and Eric,
There will likely be significant changes in the 2014 NEC to clarify the
situation you are discussing. Fault current has very little to do with
this issue.
The key distinction was used in my proposal to the 2014 NEC that removed
the statement "and conductor" in 705.12(D) since conductors are treated
very differently in the NEC. We in 690 are the ones that got this messed
up. The issue with conductors are taps. With two sources feeding a tap,
the sum of the feeder breakers would have to be taken into account in
sizing the tap. This does NOT mean that the tap is a full size conductor.
The tap rule determines the size and the new proposal simply requires you
to use both the feeder breaker and the PV breaker in sizing the tap. This
assumes that both breakers are feeding the tap in the event of fault on
the tap and that there would be no problem clearing that fault. If fault
current was used as an argument for oversizing (it is wrong), it only has
relevance in the tap scenario. A fault in a feeder with no taps does not
allow the sum of the currents to flow anywhere but where the fault is-the
rest of the conductor is undamaged in a fault.
As was pointed out, in a fault, the PV inverter will shut down in a few
cycles leaving no contribution from the inverter anyway. Don't even bother
thinking about high impedance faults-the NEC does little to deal with
these types of faults other than to require ground fault protectors on all
services 1000A and up.
Sizing a conductor for the sum of two breakers on opposite ends of a
feeder seems to be what the code says, but it is totally ABSURD from a
technical point of view. John's articles were merely pointing out that the
code language seems to be telling us to do this, regardless of whether it
makes technical sense. The 2014 NEC will do away with this craziness.
Bill.
From: re-wrenches-boun...@lists.re-wrenches.org
[mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Andrew
Truitt
Sent: Monday, May 07, 2012 11:24 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] 120% Rule - applying to multiple load centers
etc
August / Eric - My understanding of this requirement is that all equipment
in the circuit must be rated to handle the maximum fault current that
could flow at a given point. So, if your power source / overcurrent
protection scheme is: 50A (inverter 1) + 50A (inverter 2) + 100A (utility
grid via fused disconnect) then 200A is the max fault current at any point
in that circuit (including conductors and switches) and should be used in
your 120% rule calculation (as J.W. does). This can definitely present
issues, especially when attempting to interconnect at existing subpanels
with feeders that were not sized with the future addition of PV in mind,
but I think one intent of this article is to ensure that if there were a
fault in those feeders, and the PV inverters continued to operate
(unlikely), that the conductor could handle the sum of the fault currents
(PV + utility).
For a brighter energy future,
Andrew Truitt
NABCEP Certified PV InstallerT (ID# 032407-66)
Principal
Truitt Renewable Energy Consulting
<tel:%28202%29%20486-7507> (202) 486-7507
<http://www.linkedin.com/pub/andrew-truitt/8/622/713>
http://www.linkedin.com/pub/andrew-truitt/8/622/713
"Don't get me wrong: I love nuclear energy! It's just that I prefer fusion
to fission. And it just so happens that there's an enormous fusion reactor
safely banked a few million miles from us. It delivers more than we could
ever use in just about 8 minutes. And it's wireless!"
~William McDonough
On Mon, May 7, 2012 at 10:55 AM, August Goers <aug...@luminalt.com> wrote:
Hi Wrenches,
Please see question below forwarded from one of my fellow engineers.
Thanks, August
From: Eric Schoonbaert [mailto:e...@luminalt.com]
Sent: Monday, May 07, 2012 9:38 AM
To: August Goers
Subject: 120% Rule
The 120% rule, and how it is applied to panel board bus size has been
widely discussed. There is one part of the rule (2008 and 2011 NEC quoted
below) that gets much less attention and is the subject of this email.
That is, how and when is the rule applied to a conductor? The heading and
text both clearly say bus or conductor rating [emphasis added].
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