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Old 19-01-2019, 13:28   #1
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Cable sizing for battery switch, bus bars etc

Cable sizing for battery switch, bus bars etc

Doing a lot of rewiring, installing breakers, fuses, lithium batteries, MPPT’s, new charger etc. A lot of stuff needs to be replaced and added when you want to with lithium.

The main issue now is the sizing of the cables. There are great cabling guides online by e.g. BlueSea. But these guides are based on the consumer being directly attached to the batteries. In my case I will have main fuse, main circuit breaker, load fuses, bus bars etc.

For the cables running to the consumers I know what sizes to use. But how do you calculate the size of the cables before they break up into the individual cables running to the consumers?

Examples
Battery -> Main battery switch
Main Battery Switch -> Bus bar (positive)
Main Battery Switch -> Battery Protect
Battery -> Shunt
Shunt -> Bus bar (negative)
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Old 19-01-2019, 14:03   #2
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Re: Cable sizing for battery switch, bus bars etc

You size the main fuse/protection based on the maximum load you ever expect to draw (or charge if you charge over the same connection and with a big charge source)-(with some excess depending on loads, type of fuse, etc.). You then size the main cables based on that main fuse. If you select a 200A main fuse then the cables must be rated 200A or higher. The fuse protects the cables. In order for that protection to actually work the cables must be able to take the full output that the fuse will allow. You may have to go bigger if voltage drop becomes a problem (due to length, etc.) but that's fine, just have to check which gives you a bigger cable, fuse size or voltage drop.

Everything up to the distribution circuit protection should be able to handle the main fuse current. That includes cables, bus bars, interconnects, etc. You should only switch to a lower rated cable once the appropriate circuit protection is in place. Said another way, everything up to the positive side of your circuit breaker for the cabin lights should be able to handle 200A (if that is your main fuse size). Assuming your lights are on a 5A breaker, everything after that breaker may be sized for 5A.
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Old 20-01-2019, 10:40   #3
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Re: Cable sizing for battery switch, bus bars etc

This might help. From West Marine - Marine Wire Size and Ampacity. I'd post the ABYC standards, but they're copywritten, but they're certainly the true source.
https://www.westmarine.com/WestAdvis...e-And-Ampacity
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Old 20-01-2019, 10:50   #4
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Re: Cable sizing for battery switch, bus bars etc

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Originally Posted by PJHoffnet View Post
This might help. From West Marine - Marine Wire Size and Ampacity. I'd post the ABYC standards, but they're copywritten, but they're certainly the true source.
https://www.westmarine.com/WestAdvis...e-And-Ampacity
West Marine is unfortunately blocking traffic from Europe...
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Old 20-01-2019, 11:17   #5
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Re: Cable sizing for battery switch, bus bars etc

90% of the time your battery cables will have little or nothing to do with voltage drop. If you size them for 2-3% at maximum load (of say 100A or 200A) you will be fine. The vast majority of the time you'll be running a load far less than the maximum and any voltage drop in battery cables will be negligible. The only time it will even become a factor is when you run a large load on the inverter or run the windlass. If you're using lead-acid batteries of any kind the voltage sag of the batteries under these loads will be far more significant. During the times you are running these large loads your light (as in drawing little current) loads will see a reduced voltage, but if you size the battery cables for 2-3% it won't be due to the battery cables.

The difficulty in this is actually breaking out the wallet to pay for cables at 2-3% at maximum load. You'll likely find that for a 200A maximum you'll need cables that are rated for 400-800A just to keep the voltage drop down, even though they are short.
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Old 20-01-2019, 11:34   #6
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Re: Cable sizing for battery switch, bus bars etc

Thanks for your replies.

I understand the fundamentals of no cable less than the fuse it is connected on. I will be using one combined battery bank for both start and house. To protect the lithium batteries I have a 500A T-Fuse. It's not possible to size the cables based on the 500A fuse. I never expect to be anywhere near 500A and just have this fuse to protect the batteries.

The reason for a 500A fuse to be able to handle the start motor running at the same time as I have other loads (short period of time). Even if I have two windlasses, one electric winch etc. I will never run all these at the same time.

All the consumers and chargers are properly fused. I am more wondering how to size the cables from the battery to the busbars and fuses. I am using BEP marine's linkbars with the bus bars and fuses to avoid a large installation. All the guides I find are based on the cables going to the consumers/chargers. Not for cables from the batteries to the bus bars etc.

Would be happy if anyone could take a look on sizing of the following cables, the rest I know what sizes to use.

Battery positive (BatteryBrain) -> Main fuse
Main fuse -> Main switch
Main switch -> Link bar
Main switch -> BatteryProtect
BatteryProtect -> Linkbar
Battery negative -> Shunt
Shunt -> Negative bus bar

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Old 20-01-2019, 12:04   #7
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Re: Cable sizing for battery switch, bus bars etc

Done properly () all of the cables you list should be sized for 500A, which means ~185mm2 (depending on temperature rating of insulation, etc.). A short in any one of them could create a 500A current before the fuse at the battery opens. Again, done properly, if you don't want cables that big then the fuse at the battery should be smaller.

There is some precedent in some codes to size short cables to the ampacity of the load side protection, but in this case that doesn't help you any. On the load side you have 200 + 80 + 100 + 70 + 90 = 550A load side -> requires larger cables than the 500A fuse. On the charging side you might get away with this because your total protection there is 230A. However, this approach does not meet any of the protection requirements (of which I am aware) that apply to boats.
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Old 21-01-2019, 10:41   #8
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Re: Cable sizing for battery switch, bus bars etc

Even a #4 wire is rated for 100 amps with 3% voltage drop at 10 feet.
A tiny #10 wire carries 100 amps 10 feet with a 10% drop in voltage.

Most people seem to use 2/0 as standard.
I use 1/0 cables for everything in the short runs between batteries and bus bars. Doing the math, I thought 1/0 was two sizes over my needs and it is a little easier to handle than larger stuff. (I have a 900+ amp hour bank with a 2 foot run to the positive and negative bus bars.)

If your batteries are 6 or 8 feet from your main bus, you might want larger cable.
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Old 21-01-2019, 11:07   #9
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Re: Cable sizing for battery switch, bus bars etc

Might want to reconsider that 500 amp class T. You will need a class T to safely interrupt the huge possible fault current from the Li batteries but I start and have started my 3GM for years on a 250 amp class T. Also might consider removing the starting feed from the rest of the power system (ABYC legal but still not the best idea).

Also would suggest a separate return for the house system as your diagram shows a common conductor with the starter.

Once you get larger than #1/0 in cable size they become a PITA to work with.


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Old 21-01-2019, 16:24   #10
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Re: Cable sizing for battery switch, bus bars etc

I’m with Frankly on this. Down size the main fuse and *always* keep your starting circuit separated from the house. Think about the starting circuit for your automobile. When you turn the key everything on the dashboard (radio, fan, lights etc) go dark. Only the headlights stay on. Personally I never turn on my lights until after the engine is running.
Another idea would be to consider relocating the bussbar. Possibly double checking your diagram dimensions as well as I don’t understand why you have a 2m cable from start motor to the motor...
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Old 21-01-2019, 18:33   #11
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Re: Cable sizing for battery switch, bus bars etc

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Originally Posted by jonase View Post
West Marine is unfortunately blocking traffic from Europe...

You tried it with a VPN? See uVPN for example (free)
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Old 21-01-2019, 20:28   #12
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Re: Cable sizing for battery switch, bus bars etc

While your drawing is a good start, I very much agree with the guys above and would strongly suggest that you might have a vessel/vehicle electrical engineer review your drawing to provide edits. The following examples are provided as items to start with:

1. As mentioned above, it is fundamental to isolate the start and house sides.

2. The LiFePo under and over voltage protections should be shown as part of the BMS, so in one segmented blue symbol, not three as shown. btw -

Li-ion normally uses an internal solenoid or fuse to disconnect the +HV which would be in the red line above the blue battery symbol. Some disconnect both + and -

A good Li-ion BMS should have temp and volt monitoring as part of balancing for each cell and for disconnection of the battery, not just sensing a single point for V or one sensor for each.

3. Once you have separated the start and house loads, the house side normally has a parallel switched power bus and one always hot. These may be separated electrically behind what appears as one common C/B panel.

4. The cable lengths indicate fuses and breakers will be in different places. Try to group them physically.

5. The "Always On Bus Bar" needs to be just that, meant for absolutely essential loads and downstream of its' own fuse/breaker. This essential loads bus would not be be downstream of two switches and two fuses, as shown. You have it just going to the stereo (is it that essential? ). The stereo normally is downstream of the C/B panel.

6. It's good to have the switched loads disconnect during engine start to prevent V spike damage.

7. The fridge, heater, etc, would be downstream from the switched side of the C/B panel, not parallel to it as shown.

8. The wind generator is 6V @ 420W according to the C/B?

9. The alternator fuse is a bit small and likely to trip at full load. For example, right after running on battery power for a while and then starting the engine.

Again, keep at it.
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Old 22-01-2019, 01:00   #13
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Re: Cable sizing for battery switch, bus bars etc

Let’s keep the discussion on cable sizes only and maybe some fuses ;-)

The topic of using or not using lithium, using one or two battery banks, separate lithium cells or plug-in lithium batteries, brand of lithium cells to use, using BMS or not, type of BMS with passive or active cell balancing, type of battery protectors etc. in a different topic.

These are all interesting things to discuss but are for a different topic.
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Old 22-01-2019, 01:55   #14
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Re: Cable sizing for battery switch, bus bars etc

the wire between the battery and upper "link bar" is a cranking circuit, and is also less then 72". so you don't need that fuse to meet ABYC if the wire is protected. therefor the wire size does not need to be required to conduct 500a.

I would do that in 2/0 or 4/0. it's cheap that short.


and as mentinoed above. the alternator and charger fuses should be 150%. you can't put a 100a fuse on a 100a alternator.
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Old 22-01-2019, 01:59   #15
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Re: Cable sizing for battery switch, bus bars etc

Thanks a lot for all the comments, it’s pretty clear that I need to downsize my main fuse. It is not realistic that I run all the devices at the same time e.g. only one of the bow windlass, stern windlass, winch will run at the same time. The voltage drop I get while starting the engine or running a windlass is something that I can live with. But everything that is charging I don’t want any voltage drops on.

The negative bus bar is at the best place it can be right now. The batteries, fuses and positive bus bars are in fairly water protected battery box under one bunk. The negative bus bar are just outside this area. The circuit breakers are just outside the battery area for ease of access.

To simplify in the discussion I will update the with some corrections to lengths and what I now consider to use as cable sizes for the new cables I will run. I will also leave the BMS and associated parts out since this is the wrong topics for that discussion.

PS: The oversized circuit breaker for the MPPT is to be able to connect more MPPT’s to the same circuit breaker later. But the MPPT has a built in fuse as well. But I want to be able to have easy on/off switches for all the solar panel MPPT’s and the wind generator when I want to disable charging from these.

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