Outback VFX3648 inverter breaker and wire sizing

photovoltaic

I am putting together a 48v system and am curious about breaker and wire sizing for VFX3648 inverter. 
The Outback manual says to use 175 amp breaker - and I have bought this using a Midnite Mini DC box.
The Outback manual says to use 1/0 wire .
My question is why is the breaker and wire over sized from the 90 amps maximum DC battery input which is also from the Outback manual ?
I have some 2/0 wire that I picked up years ago at hamfest for $15 for 33 feet of two conductor  - great deal !  Anyway I will cut some of that off to use from inverter to breaker and to the batteries. Am I correct in thinking it is okay to use the larger than needed wire gauge on the 175 amp breaker ?
There is no labeling on the insulation of this wire but I don't have to worry about code inspections of my system  - just making a reasonable safe system.
photo of wire attached .

Thanks,
Larry

Blackcherry04

It ( size ) won't be a issue at all, it's a little harder to make short radius turns. The larger size would reduce voltage sag some, but at 48v thats not usually a issue. Proper lugs and crimping is the biggest issue you'll have. If you can't crimp them with the proper tool, take them to someone that can.

vtmaps

photovoltaic said:

My question is why is the breaker and wire over sized from the 90 amps maximum DC battery input which is also from the Outback manual ?

Most important is that the breaker be sized to protect the wire.  The 1/0 wire that Outback recommends has an ampacity of 150 amps.  A smaller gauge wire could handle the Outback's current draw, but would have higher resistance.   Low resistance of the wire can be critical to your success in handling the startup surge of motors.

Breakers are usually rated at least at 125% of maximum current draw, so as not to trip in the event that there is a sustained maximum current draw.

By the way,  the "90 amps maximum DC battery input" in the Outback manual is a continuous rating.  Peak surges can far exceed 90 amps.

--vtMaps

zoneblue

I found that outback manual a triffle vague in places, seemed like the manual was written to cover their bases for a range of products amd applications. (eg I used a wire 4 sizes smaller than they said was needed). The solution for all wire size question boils down to:

1. the length of the cable, and, a trip to the voltage drop calculator. For an inverter you want the return trip voltage drop to be as small as you can, say 0.4V for 48V at peak load current. Note that this is less than the usual recomendation of 2%. (PV and load circuit wiring is another matter).

2. for sanity double check the ampacity checks out for the proposed wire, however doing 1) generally covers 2)

3. Lastly, as vt said select the breaker to protect the chosen wire. In practice that means rounding to either 125,175, or 250.

photovoltaic

Yes - I have one of the small hydraulic crimpers I got at Harbor Freight Tools for cheap - but it works really well and was worth every penny for crimping on lugs. Okay - on that they rate the wiring for the surge. I had to use 4/0 wire for my 24v system.

So my next question is - can I run the 120v output of 24v outback inverter into the same AC box as the 48v outback inverter or do they need to be setup synced with each other to do that. I thought I saw that syncing them was if you do a 240v system but two at 120v no sync ?
Larry

vtmaps

photovoltaic said:

So my next question is - can I run the 120v output of 24v outback inverter into the same AC box as the 48v outback inverter or do they need to be setup synced with each other to do that. I thought I saw that syncing them was if you do a 240v system but two at 120v no sync ?

Do NOT connect their outputs to each other.  There will be an extremely reproducible disaster. 

Outback inverters can be synced in parallel, but it requires some programming with their hub and mate system.  I don't know enough about it to say whether you can do that with a separate 24 and 48 volt system... I think they would at least need to be connected to the same hub and mate.

--vtMaps