Why does Victron recommend comically large diameter DC cables?

[grassybreakfast]

I have bought a Victron Multiplus II 48/5000 (5000kW), and now I am looking at accessories for it.

One thing I'm failing to understand is why they recommend 70mm2 DC cabling?

It charges at max 70A (battery charge current), and inverts at max 4000W max. 4000W, let's say with 10% inefficiency, is 4400W. With a 48V battery that's around 92A. Let's say 100A.

Cable current rating depends on insulation, etc, but 25mm2 (4 AWG) wires are often rated at 150A or more (some much more), with a resistance of ~0.8 ohms/km. For 1m, cabling, that's about 0.08V voltage drop at 100A, which is about 0.2% loss.

Now maybe 25mm2 is a bit marginal, but surely 35mm2 would be enough? Why would they recommend 70mm2 which seems comically large?

 

[Supervstech]

Undersized cables are adding resistance to the circuit. Which reduces the voltage to the inverter.

Ignore insulation ratings... size by voltage drop minimums

 

[sunshine_eggo]

I have bought a Victron Multiplus II 48/5000 (5000kW), and now I am looking at accessories for it.

One thing I'm failing to understand is why they recommend 70mm2 DC cabling?

It charges at max 70A (battery charge current), and inverts at max 4000W max. 4000W, let's say with 10% inefficiency, is 4400W. With a 48V battery that's around 92A. Let's say 100A.

Cable current rating depends on insulation, etc, but 25mm2 (4 AWG) wires are often rated at 150A or more (some much more), with a resistance of ~0.8 ohms/km. For 1m, cabling, that's about 0.08V voltage drop at 100A, which is about 0.2% loss.

Now maybe 25mm2 is a bit marginal, but surely 35mm2 would be enough? Why would they recommend 70mm2 which seems comically large?

Don't be confused by Victron's honesty. While victron makes the distinction of 5000VA vs. 4000W, almost all other inverters are rated incorrectly referencing Watts when it's really VA. Go look at the manual for the Magnum MS4024PAE - a 4000W inverter. When you look at the specifications, it's a 4000VA inverter.

The best way to think about it is for resistive loads, they can power 5000W. For inductive loads, they can power 4000W.

5000W/48V/.9 = 115A

60°C open air rating for 70mm2 is around 150A. I don't consider that comically large by any means. Furthermore, Victron specs things for robustness. They also consider DC ripple to be from the devil, as it can severely limit an inverter's performance, so they minimize the potential for DC ripple by specifying a heavier gage cable.

35mm^2 cable rating @ 60°C is only about 90A.

Lastly, Victrons can run for extended periods ABOVE rated power when DC ripple and temperatures allow for a stable output.

 

[Lt.Dan]

Same reason I use 4/0 on my Solark 12k that's only capable of 185a. Better to have too much cable and as little voltage drop as possible.

 

[Quattrohead]

Because they are Dutch and not Chinese and therefore more honest with what you actually need.

 

[grassybreakfast]

Undersized cables are adding resistance to the circuit. Which reduces the voltage to the inverter.

Ignore insulation ratings... size by voltage drop minimums

Thanks. I do understand that, and that's why I calculated losses in my post. In this case even 25mm2 cable have negligible losses for a 48V system.

Don't be confused by Victron's honesty. While victron makes the distinction of 5000VA vs. 4000W, almost all other inverters are rated incorrectly referencing Watts when it's really VA. Go look at the manual for the Magnum MS4024PAE - a 4000W inverter. When you look at the specifications, it's a 4000VA inverter.

The best way to think about it is for resistive loads, they can power 5000W. For inductive loads, they can power 4000W.

5000W/48V/.9 = 115A

60°C open air rating for 70mm2 is around 150A. I don't consider that comically large by any means. Furthermore, Victron specs things for robustness. They also consider DC ripple to be from the devil, as it can severely limit an inverter's performance, so they minimize the potential for DC ripple by specifying a heavier gage cable.

35mm^2 cable rating @ 60°C is only about 90A.

Lastly, Victrons can run for extended periods ABOVE rated power when DC ripple and temperatures allow for a stable output.

According to the datasheet it's actually 5000VA/4000W, so for inductive loads it's a bit more complicated, but the maximum real power is 4000W (for resistive loads). It can power inductive loads at 5000VA apparent power, as long as the real power is 4000W or less.

I am not sure why there's such a wide range of current ratings for the same sized cables.
For example, this table here: https://www.altecautomotive.co.uk/2...tts-rated-various-colours-available-375-p.asp

Shows 240A for 35mm2. While it doesn't say what temperature that's at, given it's PVC insulation rated to 70C, that must be the temperature at 240A?

 

[Q-Dog]

The data sheet list max power at 9000 and DC voltage range down to 38 volts. How many amps will it pull doing 9000 watts at 38 volts?

 

[sunshine_eggo]

Thanks. I do understand that, and that's why I calculated losses in my post. In this case even 25mm2 cable have negligible losses for a 48V system.

According to the datasheet it's actually 5000VA/4000W, so for inductive loads it's a bit more complicated, but the maximum real power is 4000W (for resistive loads). It can power inductive loads at 5000VA apparent power, as long as the real power is 4000W or less.

Do you understand power factor? If you do, you have not correctly interpreted the ratings based on the above. If you don't understand power factor, you should look into it.

I am not sure why there's such a wide range of current ratings for the same sized cables.
For example, this table here: https://www.altecautomotive.co.uk/2...tts-rated-various-colours-available-375-p.asp
Shows 240A for 35mm2. While it doesn't say what temperature that's at, given it's PVC insulation rated to 70C, that must be the temperature at 240A?

What does the "**" indicate?



I would hesitate to take a rating to heart if I didn't have all the information.

 

[Prince Of Darkness]

Skimping on cable size for inverters has a negative impact on large surge performance.

Luke

 

[grassybreakfast]

The data sheet list max power at 9000 and DC voltage range down to 38 volts. How many amps will it pull doing 9000 watts at 38 volts?

That is a good point. I assumed peak means at most a few seconds (maybe less), but I don't believe that's actually specified. That said, they do recommend a 200A fuse, so I don't think it will draw close to 200A for a significant amount of time.

Do you understand power factor? If you do, you have not correctly interpreted the ratings based on the above. If you don't understand power factor, you should look into it.



What does the "**" indicate?

View attachment 156769

I would hesitate to take a rating to heart if I didn't have all the information.

Well, it's been a few years and it's not my specialisation, but I did take 2 courses on power systems, so I would think I have at least a basic understanding, but I may be wrong. What is your interpretation? In my mind a 5000 VA rating (non-linear load with crest factor 3:1) and 4000W continuous output power pretty clearly describe what I understood the ratings to be? By convention "VA" is used for apparent power (real + reactive), and "W" is for real power. It also makes sense for apparent power rating to be higher than real power rating, because by definition apparent power is real power plus reactive power.

It's true that I don't know what the "**" mean, but this kind of rating for these wire sizes are not unusual. https://www.engineeringtoolbox.com/wire-gauges-d_419.html

 

[sunshine_eggo]

Right... to use your terminology....

VA (real + reactive) = 5000VA

If reactive is 0VA, then real power, W = VA; thus 5000W = 5000VA.

Again, go check other manufacturer's specifications. Most either don't bother with the distinction, or they're misleading with discrepancies between marketing and technical data.

 

[Brucey]

That is a good point. I assumed peak means at most a few seconds (maybe less), but I don't believe that's actually specified. That said, they do recommend a 200A fuse, so I don't think it will draw close to 200A for a significant amount of time.

From my understanding the low frequency Victron and Schneider inverters have significant surge capability. Good video linked in that post.

Anyone else here using multiplus 2 48/3000? Question about surge capacity.

I am currently using 2 Victron Multiplus II in a split phase configuration. I have started the workings of being able to back feed my main panel in a power out situation (temporary only). I'm a little over a year into this build and I am almost content with its current state and performance. At...

diysolarforum.com

Basically if the voltage levels are maintained (robust battery and cables) there is the potential to run at up to double the rated power for up to 2 minutes. (Voltage and ambient temperature being variables). If voltage drops it will shut off within half a second. They suggest designing for a 30% overload as a longer term safety factor (30 minutes), as the 2 minute capability is so variable and hard to guarantee.

 

[Quattrohead]

The Schneider especially has a huge surge capacity for long periods of time so I could see that burning up smaller cables.
Electrically cables can never be too big and cooler is always better.

 

[Tomthumb62]

From my understanding the low frequency Victron and Schneider inverters have significant surge capability.

Just got a Victron Phoenix 12/1200. It’s a 1200VA aka 1000W inverter. Surge is 2200W. I forget the tiers of surge, but it can surge for something like 30 seconds at a lower level and 2-3 minutes at a level lower than that.

I ran my Dyson vacuum off this inverter, which surged at 2400W (at least) and then 1850W after that. The inverter ran just fine I think for a minute or two before it started beeping. It continued to beep but also powered the vacuum.

 

[Brucey]

Just got a Victron Phoenix 12/1200. It’s a 1200VA aka 1000W inverter. Surge is 2200W. I forget the tiers of surge, but it can surge for something like 30 seconds at a lower level and 2-3 minutes at a level lower than that.

I ran my Dyson vacuum off this inverter, which surged at 2400W (at least) and then 1850W after that. The inverter ran just fine I think for a minute or two before it started beeping. It continued to beep but also powered the vacuum.

Nice! What battery are you using? I was looking at the chins 12V 200Ah plus with 200A bms for $649.

 

[Tomthumb62]

Nice! What battery are you using? I was looking at the chins 12V 200Ah plus with 200A bms for $649.

We’re using the Powerurus 200Ah 200A. Has Bluetooth even has a semi accurate SOC. Not as good as a real shunt but good enough. And good for tracking cell voltages and deltas. Paid $730 out the door, free shipping no tax and a free 10A charger. Has low temp protection. Will Prowse gave a very good tear down review and is currently his top pick for budget lithium on his website. Better build quality than chins or any the other low budget brands.

I’ve been in almost constant email communication with Powerurus and they’ve been excellent in answering all my questions.

 

[rhino]

Is the reason why you are hesitating getting the recommended size cost reasons? What will the distance from battery to inverter be?

 

[Brucey]

Sorry that posted three or four times for some reason.

Thanks for the headsup on the Powerurus. Been thinking of maybe using a multiplus II 2000 or 3000 VA for more available output and integrated charging and other functionality. Basically use it as a UPS in the living room, assuming there's minimal fan noise with say a 400W base load. And then add an additional 200Ah battery down the track to support the bigger multiplus. Haven't figured out possible solar yet.

 

[JRH]

If one simply runs the largest wire that a given device can accept one will be so much happier with the end result. Be it big boat wiring , solar wiring, concert sound system wiring ,car subwoofer wiring , whatever… everything runs smoother and cooler. You will have other problems to address in life but it won’t be because you skimped out on wire size… just double the size of whatever “they say” ..then forget about it.

 

[rhino]

just double the size of whatever “they say” ..then forget about it.

There is a limit. The Quattro 5kVA for instance can't take any larger then 2/0 because of the oval opening on bottom the cable has to go through.. There is however two studs so you can use two 2/0 cables.