[RE-wrenches] wire sizing question

[Ray Walters]

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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