[RE-wrenches] Supply Side Interconnections - Maximum Backfeed

Thu Mar 4 17:37:03 PST 2010

The serving transformer is also an issue. I've seen 800-amp service
conductors on a 25kVA transformer. Just because you have wires large enough
for a 400-amp service, does not mean you have a transformer capable of
delivering 400-amps continuous. That is why you have to work with the
serving utility with larger systems.

Bill.

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Scott
McCalmont
Sent: Thursday, March 04, 2010 3:30 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Supply Side Interconnections - Maximum Backfeed

Going all the way up to the rating of the service might not be such a good
idea. The utility may use smaller wires for the service drop than you would
expect based on the ampacity tables in the NEC. Considering the example of a
400 A service:

- PG&E (local utility) requires 350 kcmil Aluminum conductors.

- Table 310.17 rates 350 kcmil Aluminum for 395 amps before temperature
correction.

With load diversity the smaller wires will be fine, but the output of a PV
system is continuous. Running the output of a system sized at 400 amps
through the wires on a hot summer day could overload them.

So... it depends.

Scott

Scott McCalmont, P.E.

McCalmont Engineering

San Jose, CA

(408) 204-0318

On Thu, Mar 4, 2010 at 3:02 PM, Michael Kelly <mike at solarflair.com> wrote:

Kirpal,

That was always my interpretation as well.  So in the case of a 400A
service, I can backfeed 400A when utilizing a supply side connection.
Others have thoughts on this?

- Mike

--
Michael Kelly
Project Manager / Project Engineer
NABCEP Certified Solar PV Installer T

SolarFlair Energy, Inc.
11 Mayhew Street
Framingham, MA 01702
Direct Mobile: 617-899-9840
Main Phone: 508-293-4293
Main Fax: 508-293-4003
mike at solarflair.com

On Thu, Mar 4, 2010 at 4:16 PM, Kirpal Khalsa <solarworks at gmail.com> wrote:

Mike.....i may be wrong....but I think i am right....you are limited by the
wire size feeding the supply and/or  the transformer size which supplies the
service.....depending on the physical location of the tap you may be limited
by the enclosure and the enclosure ratings as well
would love to hear any other ideas...
Cheers,
-- 
Sunny Regards,
Kirpal Khalsa 

NABCEP Certified Solar PV Installer

Renewable Energy Systems
www.oregonsolarworks.com <http://www.oregonsolarworks.com/> 
541-218-0201 m
541-592-3958 o

On Thu, Mar 4, 2010 at 12:50 PM, Michael Kelly <mike at solarflair.com> wrote:

Hi All,

Next to grounding, I find the calculation of the maximum amount of backfeed
current allowed in a supply side connection to be the most vague issue in
Article 690.  Let's assume we have a 400A, 480/277 service in a commercial
building, a 400A main service disconnect, and the jurisdiction is under NEC
2008.  What is the maximum backfeed current allowed in a supply side
connection?  The only place the code points you to is 230.82(6), which is no
help, or to Article 705, which is no help either.  I know that in the draft
version of NEC 2011, there is an expansion of 705.12(A) which states "
Supply Side. An electric power production source shall be permitted to be
connected to the supply side of the service disconnecting means as permitted
in 230.82(6). The sum of the ratings of all overcurrent devices connected to
power production sources shall not exceed the rating of the service."  That
is a helpful sentence from the Code, but as it is in draft form, what do we
do under NEC 2008?  What have you used as a maximum backfeed current for
supply side connections?

Thanks,

Mike
--
Michael Kelly
Project Manager / Project Engineer
NABCEP Certified Solar PV Installer T

SolarFlair Energy, Inc.
11 Mayhew Street
Framingham, MA 01702
Direct Mobile: 617-899-9840
Main Phone: 508-293-4293
Main Fax: 508-293-4003
mike at solarflair.com

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