Bus Bar Design

[stephen.m]

Hi,

When using more than 6 batteries (EG4 280AH indoor), EG4 recommends the use of an external bus bar.
Following their specs, it doesn't look like fuse or DC breakers are required, is that correct?

Since I can't really locate a bus bar for my setup, I'm considering getting them made, started thinking about it and realized I had some questions.

Assuming 12 of those batteries feeding 3x Flexboss21, each Flexboss21 capping at 250A max realistically the max theoretical current in the bus bar would be 750A. Practically, it would much less than that as the various lugs would be strategically placed. For instance if the bus bar is 16" long, the first inverter's lug would be at 4", second at 8" and third at 12". and in between each inverter lugs, you'd have 3 pairs of lugs for the batteries (all assuming the wires between batteries and bus bar are all the same length). So really, the max current in the bus bar should never even reach 500A.

But I digress, assuming a Max current of X, you can use cross sectional data to assess acceptable values. but what about the length? how does the symmetry (like I was suggesting above) play into the bus bar design and is it important? From all I've read, the emphasis really is on the length of the cables (bus bar to battery) more than the bus bar itself.

 

[BarracudaBob]

There are online calculators for busbar capacity

Bus Bar Size Calculator

Current carrying capacity and budget as under size busbar can cause heating and damage in busbar while over size busbar can affect the cost of project. By using BUSBAR Size Calculator we can prevent these issues by predicting them in the first place.

www.allumiax.com

I like this chart for basic wire gauge, current, and distance:

 

[SplitUniverse]

When using more than 6 batteries (EG4 280AH indoor), EG4 recommends the use of an external bus bar.
Following their specs, it doesn't look like fuse or DC breakers are required, is that correct?

Unless they provide an actual diagram with specs I would still use breakers / fuses.

Google is your friend, there are a few places online that provide a calculator for busbar sizing, in fact I remember seeing a chart attached to a message on this board somewhere, it's a couple of months old post.

In an ideal world you want to keep the "roundtrip wire length" the same for all batteries, the easiest way to do that is to keep the inverters at one end of the bar and the batteries towards the other, especially when you don't know which inverter will / charge what amount of energy. Or you can play "combination games", it's just too early in the day for me to that, didn't have my coffee yet.

 

[stephen.m]

thank you.

considering the EG4 280AH indoor has a built in bus bar capable of combining 3 batteries for a single Flexboss21 inverter, would it be reasonable to leverage this internal bus bar, come out of it with 2 pairs of 2/0 into a bus bar which then connect to the 3x FB21?

the challenge with 12 of those 280AH's is the space they can linearly and that they can't be stacked, meaning it increases the distances between the batteries and as such forcing to use a lot of long cables. I'm still considering doing a 24" deep pallet rack setup + hoist to put 6 batteries on the ground and 6x on a second level (talked about it in my other threads), but even then, it will still mean a lot of long (and thick) cables.

So if it was possible to leverage this internal bus bar, I would go from 12 pairs to only 4 pairs and bus bars that wouldn't be as long.

If I was to do that, should I still go with 2-pole 200A breakers on those 4 legs going into the bus bar?

 

[stephen.m]

So I'm at the point where I need to design the bus bar + DC breakers and fit it all in an enclosure.
Since the FB's have their own battery breakers, I don't think it is needed or required to add additional breakers between the inverters and the bus bar, correct?

now when it comes down to the battery breakers, plan is to use the battery bus bars for each "trio" of batteries and connect them to a bus bar. I plan on getting each trio of batteries connected to the bus bar through a 2 pole 200A breaker (since each trio will use 2 pairs of 2/0 wires, the 2 positive will go through the breakers, while the 2 negatives will go straight to the negative bus bar). If that's the case the maximum current each individual battery will ever see will be 200A/3, or 66A, although practically considering I'm doing 6x trio's (18 batteries total) and 3 inverters rated for 250A max each, that's more like 750A / 18, or <42A.


side question that's been bugging me for a while. EG4 and their "integrated bus bar", you see they recommend this in many different configurations but the one that you see the most, is that "trio" one: where the center battery has 2 pairs of conductors going to the inverter and on each side of that center battery there is another battery connected with a single pair of conductor to the center battery (which is used as integrated bus bar). how does the center battery not get more current than the 2 side ones? Obviously the 2 batteries on the side are connected to the bus bar which a longer resistant than the center one. Does EG4 provide any data to show that's acceptable?