Class T fuse source request [RE-wrenches]

[Christopher Freitas --- OutBack Power]

Allan Sindelar also asked me to respond to the specifics of his question 
which he posted as well.  Here is my response:

I agree that using four 2/0 AWG cables to the battery bank is 
unnecessary from an engineering perspective - the two 4/0 AWG cables 
should be enough.  Part of the problem is that the NEC only gives us two 
choices - in a "raceway" (conduit) or "free-air" (which is not really 
defined). A conduits is also assumed to have three current carrying 
conductors in it - which isn't the case with DC systems generally.  

The cable run from a battery is usually very short - and only a short 
part of the run is in conduit (often less than 24 inches - which the NEC 
defines as a "nipple" - honestly!).  I think for such short runs the 
cables should be considered to be in free-air - not in a raceway.  The 
ampacity of 4/0 cable at 75C is 360 amps - more than enough for your 
system with dual 175 amp breakers.  

I addressed the issue of adding two class T fuses for each of the 
batteries in the previous email - I think it is worth considering but it 
is not essential. There are numerous ways the batteries could fail that 
the fuses would not handle either unfortunately. I would be a little 
nervous of putting fuses with lower amperage ratings in the system than 
the circuit breakers - its much nicer if the breakers trip when they do 
something really stupid like try to run an electric hot-tub...     

Christopher


> So please let me present the situation with more specifics...
> 
> This is an offgrid VFX3648 quad-stack with X-240 in Outback stacking
> configuration. Somehow when I designed the system I specified four GJ175
> breakers. In hindsight, I think I should have used CD125s for the 
> inverters.
> When installation time came I used two pair of identical-length 4/0 
> cables,
> rather than four pair of 2/0 as would be technically "correct" but also
> absurd. The cables pass from the PSDC into the battery enclosure in dual 
> 2"
> PVC conduits, with the four inverter breakers bussed together. Batteries 
> are
> two strings of large cells, and I am not using parallel interconnects. 
> The
> home has no 240 loads, and the AC service panel is 120/240 split-phase.
> 
> So the Class T fuses are an afterthought, as I anticipate potential 
> future
> issues. With two inverters per 4/0 cable, the GJ175 breakers allow 350A 
> on
> each cable, which exceeds its ampacity, so the 225 or 250A Class T fuse 
> on
> each B+ terminal limits ampacity to within that  of the cable in 
> conduit.
> Also, if a cell fails, or one string reaches the end of its life before 
> the
> other, I want to limit the current in the other cable pair.
> 
> While it's a quad stack, two of the inverters are included only to 
> increase
> the charge rate (to around C/12). At 45A DC per inverter while charging, 
> DC
> input current will max out at 180A DC. Inverting, steady-state max draw
> (which is unlikely to ever be achieved) is 85A per inverter pair or 
> 340A,
> which should be shared evenly between the two 48V battery strings. The
> theoretical maximum surge is 150A per inverter or 600A, which could in
> theory exceed two 225A or 250A fuses. All of these figures are taken 
> from
> the FX manual.
> 
> So that's where the fuse idea comes from. Each fuse limits the maximum
> current in each B+ cable to within its ampacity. The breakers trip in 
> the
> event of an inverter malfunction, but otherwise serve mainly as service
> disconnects.
> 
> Is this a reasonable approach? Chris, can you chime in here?
> 
> Thanks,
> Allan at Positive Energy

Christopher Freitas
OutBack Power Systems, Inc.
cfreitas @ outbackpower.com  
www.outbackpower.com
Arlington WA USA
Tel 360 435 6030  


- - - -
To send a message: RE-wrenches at topica.com

Archive of previous messages: http://lists.topica.com/lists/RE-wrenches/read

List rules & how to change your email address: www.mrsharkey.com/wrenches/etiquette.php

Check out participant bios: www.mrsharkey.com/wrenches/

Hosted by Home Power magazine

Moderator: michael.welch at homepower.com
--^----------------------------------------------------------------
This email was sent to: michael.welch at homepower.com

EASY UNSUBSCRIBE click here: http://topica.com/u/?bz8Qcs.bz9JC9.bWljaGFl
Or send an email to: RE-wrenches-unsubscribe at topica.com

For Topica's complete suite of email marketing solutions visit:
http://www.topica.com/?p=TEXFOOTER
--^----------------------------------------------------------------