[RE-wrenches] Calculation of Maximum Circuit Current

[Bill Brooks]

To add to Brian's well formulated response, the 2011 NEC is much more
detailed and accurate on this subject. Oversizing wire is not a big deal on
residential systems, but it is prohibitively expensive in megawatt jobs.

 

Bill.

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Brian
Mehalic
Sent: Friday, February 11, 2011 11:19 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] Calculation of Maximum Circuit Current

 

Maximum current is defined in 690.8(A) as Isc x 1.25.  

 

Minimum OCPD size is defined in 690.8(B) as maximum current x 1.25 (Isc x
1.56).  The maximum size, for PV source circuits, is the module maximum
series fuse size.

 

Required conductor ampacity is the greater of Isc x 1.56 or Isc x 1.25 with
all conditions of use factors (and the conductor must be protected by the
OCPD under conditions of use).

 

Brian

On Fri, Feb 11, 2011 at 11:03 AM, <wirewiz at gmail.com> wrote:

In studying for NABCEP certification I am a little confused about whether
you multiply the short circuit current by 1.25 once or twice (1.56) in order
to calculate maximum circuit current in determining wire size. Same for
overcurrent protection.

I have always done it twice (1.56) for both PV source circuit and PV output
circuit. Once for over irradiance and once for continuous use.

But a handout at a recent seminar that I went to totally confused me.

Can anyone spell it out very clearly?

Thank you.

Larry Liesner
Wirewiz
Westport, CT
Phone: 203-644-2404
Fax: 203-557-0556
wirewiz at gmail.com
www.wire-wiz.com



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-- 
========================
Brian Mehalic 
NABCEP Certified Solar PV InstallerT
PV Online Coordinator
Solar Energy International
Carbondale, CO 81623
http://www.solarenergy.org

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