Power post > busbar conversion (sizing)

[brianc]

I have a question on sizing a busbar to replace a power post (or other suggestions are welcome as well). This is all existing, I'm trying to clean things up a bit (not stack more than 4 things on the power post) and re-do some poorly done connections (2/0 bare wire exposed, no heat shrink, corrosion on lug, electrical tape and no heat shrink on others...so I'm assuming I'll find the same situation) so I can add some solar without doing something people will roll their eyes at in the future.

Diagram(s):
https://imgur.com/a/CrdCDG9

Numbers for the diagram in the imgur link:
1. Solar controller input to batteries (does not exist yet)
2. Heater input (fused appropriately)
3. Switch feed (1/0, not fused but it is part of the start circuit), ~15 ft round trip
4. Inverter/charger feed (2/0), 300 A Class T fuse, ~15 ft round trip
5. Battery to power post (1/0), not fused, ~1 foot run

Photos:
https://imgur.com/a/2cWRwb0

Currently, the house battery is connected to a power post, the rectangle with the circle in the diagram, and all un-switched house side connections happen there, except the heater which goes to the positive battery terminal. The negative side goes to a Victron Smart Shunt (500a), and then to a negative busbar. The power post is on the lowest part of the battery box (not great) and the negative busbar is at the highest point in the compartment, so I want to improve the location of the positive side as well.

Neg bus bar - 3/16" d * 3/4" w * 7" l - I do not know the amp rating and I don't see any markings, but based on comparable busbars, I'd say 300A. It's nowhere near the thickness of the 600's. Everything goes here, panel, inverter/charger (2000w, 200a "nominal"...not sure what that means), starter (1.4 kw, so 115 continuous, maybe ~350 inrush based on a Maine Sail post I saw somewhere), heater, on the un-switched negative side.

The other big draws (windlass, 90a breaker, and winch, 80a breaker) are off a positive busbar on the switched side, and the winch negative runs back to the negative busbar.

I want to replace that power post with a busbar for the un-switched positive side, but I don't want to re-do the negative side. With the negative busbar being probably 300, and all the same connections running through that, is there any reason to use something bigger for the positive side?

Personally, I think I should go with either a BlueSea 600a (4 stud), but will it be way overkill? If the starter wasn't included in the switch feed, the combo wouldn't be close to that, and I could go with something less. I'm generally not sure how to size a busbar with a starter in play. Also, I wanted to fuse the switch feed, but with the starter in play, and knowing it will inrush over 300a, should I leave it as-is?

Thoughts?

[smac999]

how are you running the 300a 2/0 inverter wiring from the 1/0 battery wiring? it would have to be 2/0 as well... or bigger if you have much of a panel load.


obviously you need a bus bar rated at over 300a. my standard one is the blue sea 2104. blue sea makes a 250a or 600a with nothing inbetween.

[Frankly]

One little wiring arrangement trick you can use is to locate the common feed/ return connection in the middle of the busbar. The idea that simultaneous current flows will divide. Doesn't make a 400 amp bar out of a 200 amp rated one but can increase the margin if you can accurately predict the loads.

You can also double up on the stud with the feed/ return with the biggest load (maybe inverter or thruster).


Frankly

[Bycrick]

It’s all about current flow and temperature rise in the bus bar. From memory, I think the Blue Sea 600a bus bar is rated at 40*c heat rise, with current flowing end-to-end. If you stack, for example, the battery and the inverter connections on the same stud, the current flow through the bus bar is zero. So, you keep the highest current connections together, towards the center of the bus bar. In general, if there’s a source and a load connection on the same stud, the source connection goes on the bottom. The idea is to minimize the current that has to flow through the busbar.

[brianc]

Originally Posted by smac999

how are you running the 300a 2/0 inverter wiring from the 1/0 battery wiring? it would have to be 2/0 as well... or bigger if you have much of a panel load.

Yes.

According to this table, this particular cable should be up to 285 amps (105 C insulation rating, in free air), and it's only a one foot run to the power post, but that is still below the 300 rating for the inverter fuse.

This setup does not match the OEM wiring diagram, which shows 2/0 for the whole run, so I think this might have been a modification by the previous owner.

Originally Posted by smac999

obviously you need a bus bar rated at over 300a. my standard one is the blue sea 2104. blue sea makes a 250a or 600a with nothing inbetween.

I should have done this already, I think I got a bit ahead of myself, but I'm going to document all the big loads on the various segments so I can add everything up more clearly. I will post that on this thread when I have it.

But I agree here, too, it's over 300 on that negative bus bar.

Based on some formulas I found online, that bus bar is probably 300-350 based on dimensions and metal type, so at least my guess was close.

Thanks!

[brianc]

Originally Posted by Bycrick

It’s all about current flow and temperature rise in the bus bar. From memory, I think the Blue Sea 600a bus bar is rated at 40*c heat rise, with current flowing end-to-end. If you stack, for example, the battery and the inverter connections on the same stud, the current flow through the bus bar is zero. So, you keep the highest current connections together, towards the center of the bus bar. In general, if there’s a source and a load connection on the same stud, the source connection goes on the bottom. The idea is to minimize the current that has to flow through the busbar.

Thanks @Bycrick and @Frankly - this makes sense. In the existing setup, everything is stacked on the power post for the positive side, and on the negative side it doesn't appear to be in any particular order or use any particular stacking to reduce flow through the bar. Order from right to left is battery, winch, inverter/charger, panel/starter, so not ordered in the most efficient way.

[Bycrick]

The "wiring order" for a power post is different. There, the highest current terminals go together the middle of the stack, with progressively smaller loads alternated top and bottom.

You already noticed that if you put the inverter and battery connections on the same post, the current through the bar is zero. The starter and windlass wires go next on either side.

[brianc]

Originally Posted by Bycrick

The "wiring order" for a power post is different. There, the highest current terminals go together the middle of the stack, with progressively smaller loads alternated top and bottom.

There are 3 terminals on the power post now. I'm starting to think switching to a power bar on the positive side for the sake of appearances may not be worth it. Max connections would be 5 if I keep it (adding the heater which is currently on the battery and adding the solar controller output). What do you think about that? If I'm not using it to add any fuses, etc, I'm not sure there's any advantage.

Originally Posted by Bycrick

The starter and windlass wires go next on either side.

Yep. I think I need to re-order the connections here or get a bigger busbar and reorder.

[Bycrick]

It’s nice to have a minimum of wires to disconnect when you have to work on something. A power post with 15 connections might work, but it’s a pain to work on. .

The rules really are simple. Keep high-current connections as close to each other as possible. Don’t organize the wires because they look pretty that way. Good luck.

[Manateeman]

We posted on buss bars before and we remember that there was information online about amp capacity and temp if you wanted to use copper bar. Sorry, just can’t find my own post but think it was from the copper industry. We bought our copper on eBay. Be careful drilling as it wants to grab the bit as you finish the hole.
We bought some BEP stuff only to find it was half the weight of Blue Seas who rate their bars end to end.
Now it’s Blue Sea Systems only. You get what you pay for. Same for hydraulic crimpers. Made in the USA.
The manatees