Class T fuse source request [RE-wrenches]

[Ray Walters]

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Hi Allan;

I too have pondered these same issues, and would like some clarification 
from Outback. Left to my own devices, I've done just as you: double runs 
of 4/0 to feed 4 inverters. Then there is all that unprotected cable 
from the batteries till the breaker. In some ways the breaker only 
protects the inverter and its 2 ft of cable in any system. John Wiles 
said we needed additional protection for longer battery cable runs.... 
he didn't say what length or how to do it. He did say no T fuse at the 
battery terminal. I agree with that, because of the corrosion problem, 
but otherwise a fuse right at the terminal does protect that whole 
cable: battery to load center.
I think both the NEC and manufacturers should deal with this. On sealed 
batteries, I happily add a secondary fuse at the terminal.
As far as fuse size, I used to look at the trip curve of the fuse vs the 
surge current / time curve of the inverter. That's how I came up with 
300 amp fuses that I had to special order. I can email you the trip 
curves if you don't already have them.
 NEC is really conservative on cable ampacity. Just so you feel better: 
In electric cars, we run 250 amp controllers on #6 wire, and #4 carries 
up to 400 amps. (This isn't just me, take a peak at a golf cart 
sometime, or a welder...)
My point is that you my friend are way ahead of NEC on safety. Don't 
believe me? Hit a 4/0 cable with 400 amps for as long as you ever 
dreamed the inverters could , and monitor wire temperature. The #4 in 
our vehicle barely gets warm at 400 amps for over 10 minutes, 4/0 
probably won't even be measurable. My calculation shows 9 watts per ft 
power lost in a 4/0 cable at 400amps. That's just not a lot of heat for 
a situation that would not be considered even close to continuous.
The final question in my mind, is where is that fuse going to be 
located? In the load center, so that it protects a couple of feet to the 
breakers? at the battery?  A second enclosure adjacent to the battery 
box? IMHO, if your going to protect the cables, then you should also 
protect against a short in the cables before the breaker. Big batteries 
feeding a short circuit in double 4/0 cables could cause much more 
trouble than the slight overload you're considering from the inverters.
(and while we're worrying about saving the cables, what about the kid 
about to drop a wrench across the battery terminals themselves.....)
 
I'd love to hear what you decide to do.

Ray

 Allan Sindelar wrote:

>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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