Hello everyone, I wonder if anyone with more experience then myself can explain this one to me. Looking at the Multiplus II manual in the installation section, the DC fuse recommendations don’t seem to match the cable recommendations for the 24/3000/70, the other models seem fine though borderline at times.
As I understand it, a fuse is supposed to primarily protect a wire from point A to point B, but for that inverter the DC Fuse recommended is a 300A yet the cable recommendation for less then 5 meters is AWG 1/0. Yet following my cable manufacturer from Canada recommendations, welding cable 1/0 AWG (780 strands) only support 200 Amps at 30 degree Celsius. So from my point of view a 300A fuse is worthless ? I do realize that all of those units have a peak of 5500 Watt which on a 24 volt system would draw about 230 Amp. Or is it that perhaps European cable have higher quality / higher standards ? I’m a bit at a loss here.
Same thing for the 12/3000/120 and 24/5000/120. If the AWG 1/0 is rated for 200A having 2 of them on a 400A fuse seems quite unforgiving.
1/0 is not 1 AWG. 6 AWG, 4, 2, 1, 0, 1/0 AWG, 2/0, 4/0 etc. Maybe you know that but I am just clarifying in case you don’t. And your wording kind of sounds like you are confusing them. People of all levels are on this forum. Sorry if I am patronizing.
You are correct that the fuse is meant to protect the circuit but that is not the only consideration. What loads will the circuit be potentially carrying?
The Multiplus 24/3000 can surge up to 6000VA I think. That’s about 250 amps potentially. So the 300amp fuse makes sense. Also 1/0 for shorter runs will be good up to that surge rating for short periods of time. So makes sense to me.
The MP should be running at the most in the 120 amp range during normal operation so 1/0 makes sense there.
Jumping up to 2/0 will probably cost you 20.00. Just do that if your concerned. That’s what I would do without question or debate.
I realized I made a small mistake in my post, the cable recommendations I got are indeed for 1/0 AWG cables, not 1 AWG cables and it is not a matter of cost, but rather just curiosity as I’m trying to understand.
The 300 Amp fuse does make sense for the potential 250 Amps peaks of these units, but then the cable seems too small. According to my cable manufacturer even 4/0 AWG wire is only rated up to 260 Amps, which seems too little to support such peaks and such a fuse, so I don’t understand the cable recommendations Victron are giving in this scenario, especially for the 24/3000 which should technically use 1/0 AWG wires.
Th manual states that is the minimum recommended size. Not the size… So for the installers and diyers who like to save a buck or two that is the floor.
The answer to your question is really what are the conditions of the installation.
The wire size is rated for open air as well. I think it is in one of the notes; so in conduit it should be a few gauges up in size anyway.
Not just peak but peak duration needs to also be considered.
If its peak for 5 seconds then the cable would not have much of a chance to heat up. But if the overload can be sustained then yes it needs a bigger gauge.
But for general continuous use it should be ‘ok’. Not every one buying a 3kVa will use it at 3 often.
That being said, we also usually use a gauge or two higher than the min. So i do see what you are saying, i am more comfortable with that as well. Really you are the designer and installer the responsibility is on you. And you have to live with your choices.
The 300A fuse if the battery is sized right will blow with the amps from the battery side and it won’t care about wire gauge+ i have seen 6mm sustain 300A. It can do it. (Not recommending anything here its just an interesting observation we experienced in a demonstration). The idea is the gauge of wire doesn’t start to smoke before the fuse blows. (Which 6mm obviously does so obvious is not safe)
There are different ampacities for wire “in free air” vs the typical ampacities for wires in walls and conduits and such. The manuals for these solar/off-grid systems appear to assume free air wire ampacities from what I’ve seen. Check the code tables for wires in free air ampacities.
