[RE-wrenches] Conductor Sizing for Supply Side Connection
gary at icarussolarservices.com
gary at icarussolarservices.com
Thu Feb 28 08:33:16 PST 2013
Garrison, et al:
Another consideration when sizing the POCC OCPD (either load side or
supply side) is charging current when the grid power returns and the
battery bank is near the LBCO voltage.
Bulk charging current and AC pass-through current both must be
considered when sizing the POCC OCPD.
Regards,
Gary Willett
Icarus Solar
On Thu, 28 Feb 2013 10:04:51 -0600, Garrison Riegel wrote:
Great point Dave. It crossed my mind that it was technically
possible, but figured since I would be removing loads off the main
panel it was highly unlikely. Thanks for the code reference. I’ll
go load side, and be prepared for some discussion.
Thanks,
Garrison
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Dave
Click
Sent: Thursday, February 28, 2013 9:21 AM
To: re-wrenches at lists.re-wrenches.org
Subject: Re: [RE-wrenches] Conductor Sizing for Supply Side Connection
As a note, supply side connections have an extra complication with
battery backup systems. With a regular GT inverter you're just pushing
current into that interconnection point and you can work out easily
that you won't have any overcurrent issues unless you made a big
mistake and your inverter output exceeds the rating of your service
conductors. However, with a battery system you're also potentially
pulling current from that point. Before the battery inverter is
installed your main breaker protects your service conductors, but if
the inverter is pulling in 60A and the main breaker is also operating
near its capacity, you could have >240A running over 200A service
conductors with no breakers tripping. You would overwhelm the capacity
of the service [230.23(A)] and no breakers would trip. You could fix
this by replacing your service conductors back to the transformer (I'm
just saying that it's an option) or downsizing the main breaker, and at
that point you're probably better off just making it a load side
connection. I'd go load side and argue with the AHJ to use the 2011
update mentioned earlier.
Dave
On 2013/2/28 9:55, Garrison Riegel wrote:
Allen,
The benefit I see would be to allow for a code compliant load side
connection on a 200A panel with a 200A MB, where the AHJ is on the 2008
NEC or older and will not listen to your good logic. I don’t think
this would always be the best option, but if backup loads and inverter
output were less than 32A then a 40A OCPD in the main panel should be
fine? Since it sounds like the 60A breaker in the main panel is not a
safety issue, but a design consideration, I suppose I would just prefer
flexibility when possible.
That said, this AHJ is on the 2008, and the loads will be less than
30A, but based on this conversation I plan to go with a 60A and try to
convince the AHJ that it will be code compliant in their future!
Thanks,
Garrison
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Allan
Sindelar
Sent: Thursday, February 28, 2013 8:14 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] Conductor Sizing for Supply Side Connection
Garrison,
I don't know why it's not listed as 60A max, but my own internal logic
would ask why it should be. The only benefit I could see for using
smaller than a 60A breaker would be to allow use of #8 conductors
(allowed with a 40A or 50A breaker) instead of the #6 necessary with a
60A breaker. And of course, you could use a 40A breaker with #6
conductors, so theoretically it would be fine. I just fail to see any
benefit to doing so.
Allan
Allan Sindelar
Allan at positiveenergysolar.com
NABCEP Certified Photovoltaic Installer
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder and Chief Technology Officer
Positive Energy, Inc.
3209 Richards Lane (note new address)
Santa Fe, New Mexico 87507
505 424-1112
www.positiveenergysolar.com
On 2/28/2013 6:27 AM, Garrison Riegel wrote:
Thanks Allen for clarifying why a 60A is required. I was
wondering. The spec does list a surge current of 9000W so I thought
that may be the rationale, but even that would only require a 50A, and
since this surge occurs during ‘stand-alone mode’ it didn’t seem
to apply to the OCPD at the main panel. Your explanation makes more
sense, but I wonder why then they don’t list the AC input breaker
size as 60A max. If you have few backup loads, and are not on the
2011 NEC, a 40A could theoretically be fine?
Thanks all for your thoughts, much appreciated.
Garrison
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Allan
Sindelar
Sent: Wednesday, February 27, 2013 6:08 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Conductor Sizing for Supply Side Connection
August,
The 60A breaker is intended to allow grid power to pass through to the
loads in excess of the inverter's stand-alone output. The point of
705.12 (moved in the 2011 NEC from 690.64 (B)(2)) is to differentiate
between load pass-through current and sell current. The amount of
current fed into the grid is (4500/230 =) 19.56A, while the amount that
can be taken from the grid and passed through to the load is much
greater. If you were limited to a 40A breaker in order to maintain 120%
of a 200A main bus, you'd be prone to nuisance trips under large
cumulative loads.
Allan
Allan Sindelar
Allan at positiveenergysolar.com
NABCEP Certified Photovoltaic Installer
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder and Chief Technology Officer
Positive Energy, Inc.
3209 Richards Lane (note new address)
Santa Fe, New Mexico 87507
505 424-1112
www.positiveenergysolar.com
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