Lyndsay Cotton avatar image
Lyndsay Cotton asked

Permissible volt drop (in a DC bus)

Hello all,

I think I made an expensive mistake

Question 1.

What is the permissible volt drop on the DC side.

In my scenario I have 2 x Multiplus 2 48/5000 in parallel

Our wiring code says that a volt drop (AC) should not exceed 5%

I see that most PV calculators do not recommend more that 3% on DC

Now my dilemma: I am using Two Multiplus 48/5000 in parallel and connected to a common DC bus bar to five Pylontec US5000 batteries

If I use the Victron toolkit calculator @ say max 166A on the bus bar (8000/48) then I can use a 16mm2 DC cable. The DC cable to the inverters from the busbar is 1.7m long as this is under the 3% (closer to 1%)

The Victron rule of thumb (in wiring unlimited) says current/3 which means I should use 50mm2. The example is based on 0.26% volt drop. The Victron table also suggests at 50mm2

Now what? Does this imply that volt drop should not exceed 0.26%?

Although I have Already wired this system, does this imply that I have made an expensive mistake and that I should start again.

Question 2

Another community member also suggested in his post that due to the size of the battery bank (I am using five x Pylontech US5000) that I should take a second parallel cable from the last battery back to the battery Isolator (KETO 160A). This would have been, as he suggested, is to ensure that the batteries charge at the same rate. Would the BMS not take care of this? OR, is this due to the limitation of the actual current carrying capacity of the Pylontech battery cable links.

Question 3

Then, wiring unlimited also recommends that all the DC cables should he the same size, make and length - No problem with that. I am using Two Smartsolar 250/85 and again I determined that the safe DC cable size, also 1.7m long is 16mm2. I also wired as such. Now what to do? Can these remain at 16mm2 or must they increase to 25mm2 / 35mm2 or also 50mm2.

Taking this further, if then indeed the inverter DC cable must be 50mm2, do I then use a 95mm2 from the bus bar to the battery.

Question 4

It is stated that the negative should not be fused in a parallel system. Is this BEFORE or AFTER the bus bar, or not at all? I am using a KETO 160A fused battery disconnector.

Due to the lateness of this discovery - I would appreciate some expert advice as soon as possible. I think my whole problem stemmed from my understanding of permissible volt drop

Many thanks

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3 Answers
seb71 avatar image
seb71 answered ·

If you used 16mm2 wire between the DC busbars and the 5kV/48V inverters, then it is way too small.

You have recommended wire sections in the inverter manual.

For the solar chargers, if they will output at full power/current, then use the biggest wire they can fit: 35mm2 (between the solar chargers and the battery). 16mm2 for 85A is also undersized.

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Lyndsay Cotton avatar image
Lyndsay Cotton answered ·

Thanks, I appreciate the answer.

I was applying the tables for acceptable voltdrop according to our code and not according to Victron requirements. There is a significant difference between the two.

1 comment
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it's not only voltage drop, but also heating of the cables.

otherwise you could use a very thin wire if it's only a short one ;)

I'd advice you to have someone with more experience work with you to install and setup the system. It's a lot to take in on a first install, and you don't want to damage things.

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David avatar image
David answered ·

I have a single 48v multiplus II 5000VA and a single 45kwh lithium battery. I use 70mm cable as per the Victron manual. There is no way you can use 16mm cable with 166amp. Forget voltage loss, you have temperature to worry about.

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