Class T fuse source request [RE-wrenches]

[Allan Sindelar]

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

----- Original Message ----- 
From: "Christopher Freitas --- OutBack Power" <cfreitas at outbackpower.com>
>
> There are some assumptions in this "rule" - that the DC cables, when
> used with a breaker, will be in a raceway/conduit - while the fuses
> would be used with DC cables in free air.  If I remember correctly both
> are rounded up to the next standard overcurrent device rating as well -
> as allowed by the NEC.
>
> Since it is not a big deal to reset a large DC breaker - it can be sized
> closer to the actual current of the inverter.  With the class-T fues
> they are expensive - so giving them plenty of margin will keep them from
> blowing.
>
> Christopher Freitas
> OutBack Power Systems, Inc.
> cfreitas @ outbackpower.com
> www.outbackpower.com
> Arlington WA USA
> Tel 360 435 6030
>


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