[RE-wrenches] Calculation of Maximum Circuit Current

Peter Parrish peter.parrish at calsolareng.com
Fri Feb 11 11:44:56 PST 2011


Short answer: DC/PV side two factors of 1.25; AC side one factor.

Reasons: the common factor of 1.25 is due to the treatment of PV systems as
continuous sources of power, the second factor of 1.25 is for the "cloud
effect". It is entirely possible to get more than 1,000 W/m^2 on a PV module
if there are large cumulus clouds that don't block the sunlight but act as
pretty good reflectors of sunlight.

As a result you need to increase Isc (times the number of combined strings
if that is the case) by 1.56, and all computed AC currents by 1.25. Same
goes for and switches and OCPDs.

Then of course you need to compute the ampacity of the wire being used,
taking care to apply the temperature  correction and conduit fill correction
before comparing to the "maximum current" calculation.

- Peter

                                                     
Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
CA Lic. 854779, NABCEP Cert. 031806-26
peter.parrish at calsolareng.com  
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885

 

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of
wirewiz at gmail.com
Sent: Friday, February 11, 2011 11:03 AM
To: RE-wrenches at lists.re-wrenches.org
Subject: [RE-wrenches] Calculation of Maximum Circuit Current

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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