[RE-wrenches] wire sizing question

[Ray Walters]

We just always use 4/0 X flex cable with a 250 amp breaker, and call it 
good.  (2/0 with a 175 A breaker)
If you want to make life really hard, you can look at the trip curve of 
the breaker (Heinnemans have 3 different trip curves depending on the 
model I believe) and then do all the normal NEC sizing calcs (conduit 
fill, etc), end up buying 250 MCM that isn't flexible, break off a 
battery terminal trying to land the cable, then possibly damage a 
breaker trying to land it in the enclosure, not to mention figuring out 
how to get 250 MCM to land on those lugs, so you special order some type 
of reducing lug......
Actually, I just checked the MTW X flex we use, and its rated to 105 C, 
so that helps, although the temp rating of the terminals on the Midnite 
Breakers is only 75 C.
One other thing, the batteries usually aren't UL listed anyway, so at 
least part of the circuit is already falling into NEC grey area anyway.

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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