[RE-wrenches] Supply Side Interconnections - Maximum Backfeed

[Scott McCalmont]

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 ™
>
> 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
>> 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 ™
>>>
>>> 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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