[RE-wrenches] DC wire sizing

Bill Brooks billbrooks7 at yahoo.com
Fri Apr 2 09:11:43 PDT 2010


Erika,

 

The most correct answer (which is a really funny thing to say) is to use
John Wiles' "5-step program"-that's what I call it. It is published in one
of the appendices of his latest "Suggested Practices" document. If you can
actually follow what he leads you through (that's why I call it the 5-Step
program), it will lead you to the correct answer.

 

My short answer, that can result in a larger than necessary conductor, is to
determine the required overcurrent protection device (OCPD) rating and then
size the conductor accordingly. The reason it may be conservative is that
the NEC allows you to round to the next larger standard OCPD in 240.4(B).
This is the way all conductors are sized in the electrical industry. I'm all
about simplicity and being a little conservative is always better than being
overly liberal (I'm not making a political statement here, but if the shoe
fits.).

 

Bill.

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Erika M.
Weliczko
Sent: Friday, April 02, 2010 6:27 AM
To: 'RE-wrenches'
Subject: [RE-wrenches] DC wire sizing

 

To my understanding the 156% on PV source and output circuits is related to
the ability of PV to deliver more than rated and be continuous.

Therefore, the wire has to be able to carry this current, so now the
temperature and fill corrections are applied to find the wire capable of the
156%.

 

I am in a debate where the question is why correct for temp and fill on 156%
of ISC and spend all that extra money when the normal operating is at Imp.
Or why correct the 156% but why not correct the Isc or Imp.

 

I am going to stick to the fact that the circuit has to carry the 156% under
all conditions.

Thoughts? 

 

Erika

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