[RE-wrenches] wire sizing question

Sat Jan 3 08:16:03 PST 2015

Hi:

This link takes you to a site in Romania so you (likely) wont be able to 
read the text, but  it does have a lot of good pictures.

This installer has been using XW since about 2007 and has a tons of 
experience.  I like what he does with the DC wiring/busbars.  The breakers 
are in the box below the inverters and charge controllers.

http://customer.lpelectric.ro/doc/sibiu2/sibiu2_28kw_fotovoltaic.html

JARMO
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From:
drake.chamberlin at redwoodalliance.org
To:
"RE-wrenches" <re-wrenches at lists.re-wrenches.org>, 
Date:
01/02/2015 05:18 PM
Subject:
Re: [RE-wrenches] wire sizing question
Sent by:
"RE-wrenches" <re-wrenches-bounces at lists.re-wrenches.org>

4/0 copper is good for a 24V, 4000 W inverter. What is being used for
multiple inverters?

I've paralleled 4/0 into outback boxes using 2, 2" PVC conduits. It gets
pretty messy trying to put 4 sets of 4/0s into enclosures and battery
boxes (although I have worked on such systems). Does anyone have clean
solutions for these systems?

Thanks,

Drake

> Some other things:  If you keep the conduit less than 24" long, which is
> recommended anyway for main battery connections, the conduit fill
> requirements don't apply. (see exception: 310.15(B)3a2)
> Using table 310.15(B)16 yeilds 230 amps for 4/0 at 75 C.  Then 240.6
> lists 250 amp as the next standard size of OCPD which is allowed to be
> used by 240.4(B)  So you can just justify 4/0.
> Further, many manufacturers (like Midnite) use the open air ampacity
> table: 310.15(B)17, since the cable is not "in conduit" and 4/0 cable at
> 75 C is rated to 360 amps.
> The Free Air interpretation is not completely substantiated in the code
> from my read of it though.  Maybe someone more knowledgable could cite
> the chapter and verse that defines when the free air rating can and
> cannot be used.
> Also remember those breakers are 100% duty cycle rated, and that the
> inverter usually can't run any where close to 250 amps continuously.
> Bottomline is we've been using 4/0 with 250 amp breakers for almost 20
> years, and I've never seen or heard of a problem.
>
> R.Ray Walters
> CTO, Solarray, Inc
> Nabcep Certified PV Installer,
> Licensed Master Electrician
> Solar Design Engineer
> 303 505-8760
>
> On 1/1/2015 7:23 PM, jay peltz wrote:
>> Happy new year all.
>>
>> I have a question on wire sizing for the following.
>>
>> 250 amp breaker, in a UL enclosure ( say midnite or outback etc)
>> conduit from battery box to enclosure
>>
>> I'm wondering how folks size this for ampacity not voltage drop.
>>
>> thanks
>>
>> jay
>>
>> peltz power
>>
>>
>>
>>
>>
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