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Old 08-12-2017, 04:04   #1
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Electrical question.....

Quick hypothetical question about power surges.

Scenario:

2/0 cable running off of the positive terminal of the battery. Leads to the battery switch. Then 2/0 cable running from the switch to a positive bus bar. Cable bolts to the end of the bus bar. Then the same bolt/terminal of the bus bar run a 2/0 cable to the starter on the engine. The bus bar has other leads for various loads. One being the power feed to the distribution panel. These leads would be smaller gauge wire........say AWG 10. The engine starter has a "rolling amp" rating of 100 amps. So there is a momentary jump above 100 amps then a continuous 100 for a short period.

Questions:

What happens to the other leads on the bus bar when the engine is turned over?

If the other leads share the same bus bar do they experience a jump in amperage as well?

The 2/0 cable from the bus to the starter shares the same terminal on the bus. Does the electricity flow the path of least resistance and seek only the load (starter) that is being asked for?

In the end the dc panel would be protected with a series of circuit breakers. But it is not a good scenario to have all of the breakers open when you hit the starter. Any advice and insight would be greatly appreciated.

Tx
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Old 08-12-2017, 04:32   #2
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Re: Electrical question.....

Current doesn't flow through a device forcefully. The device draws the current it needs from the source. So to answer your question, no, the other devices/smaller gauge wires do not have the 100+ amps running them. But rather have the potential of the 100 amps to draw from if they device/scenario requires that much current.

Hope this helps and is clear

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Old 08-12-2017, 04:41   #3
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Re: Electrical question.....

Great!!

So if I read you correctly there is no jeopardy with the various loads sharing the same bus bar?
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Old 08-12-2017, 05:01   #4
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Re: Electrical question.....

There will likely be considerable voltage drop, then surge on the other terminals of the busbar. The starter can draw a LOT of current while it starts up and before it drops back to 100A.

IMHO, it would be better to isolate the starting circuit from the rest of the 12VDC devices.

Bill
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Old 08-12-2017, 05:06   #5
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Re: Electrical question.....

As far as voltage drop and spikes to accessory devices. If using the same battery, there is no advantage to running separate starter cable to battery instead of bus bar. You may still need a fuse between the battery and bus bar.
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Old 08-12-2017, 05:10   #6
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Re: Electrical question.....

Quote:
Originally Posted by btrayfors View Post
There will likely be considerable voltage drop, then surge on the other terminals of the busbar. The starter can draw a LOT of current while it starts up and before it drops back to 100A.

IMHO, it would be better to isolate the starting circuit from the rest of the 12VDC devices.

Bill
Thanks Bill.

So would it be better to step back to the batt switch?

The bus and the starter circuit would share the same terminal at the battery switch........

S
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Old 08-12-2017, 05:47   #7
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Re: Electrical question.....

There will always be some disturbance when cranking the engine starter, but usually is not a big problem.
Ask yourself what will happen if the number 10 wire shorts to negative somewhere downstream. This needs to be fused where it comes off of the buss bar.
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Old 08-12-2017, 06:00   #8
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Re: Electrical question.....

Flee's advice is sound; it doesn't matter if you have smaller gauge wire on the same circuit, those wires only come into play if the load at the end requires current and it will only take as much current as it needs no matter how much is available. (The smaller loads still need to be fed via a breaker to protect the wire).

Your set-up is therefore sound...although I would adjust it slightly. The first thing off your battery should be a fuse. Although it's good to have a battery switch to turn off all loads, the trouble with having a switch there is the incoming load which comes from one of your charging sources, i.e the alternator. If the alternator pushes a load with no where to go, it will likely permanently damage it. So, put a fuse where your switch is now and put the switch between the bus-bar and the starter. That way, you can isolate the engine starter when you are not needing it.

This way, your charging source can go directly to your bus bar and back to the battery via the larger cable.
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Old 08-12-2017, 06:16   #9
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Re: Electrical question.....

Quote:
Originally Posted by Privilege View Post
Flee's advice is sound; it doesn't matter if you have smaller gauge wire on the same circuit, those wires only come into play if the load at the end requires current and it will only take as much current as it needs no matter how much is available. (The smaller loads still need to be fed via a breaker to protect the wire).

You're set-up is therefore sound...although I would adjust it slightly. The first thing off your battery should be a fuse. Although it's good to have a battery switch to turn off all loads, the trouble with having a switch there is the incoming load which comes from one of your charging sources, i.e the alternator. If the alternator pushes a load with no where to go, it will likely permanently damage it. So, put a fuse where your switch is now and put the switch between the bus-bar and the starter. That way, you can isolate the engine starter when you are not needing it.

This way, your charging source can go directly to your bus bar and back to the battery via the larger cable.

Very good info.........tx.

I intended to use a 250 amp fuse just off of the battery. Then I was going to fuse the smaller gauge wires off of the bus as well.

I like your idea of moving the battery switch. My intention was to run the positive lead off of the alternator directly to the battery so that the alternator never sees an inadvertent power interruption. Better to move the batt switch. It will be a much shorter run to the bus...........I like it.

But if I do that won't the bus remain hot? On just a quick thought I am not sure that is a big deal. Have to think about that.

Thanks again.

S
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Old 08-12-2017, 06:35   #10
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Re: Electrical question.....

Quote:
Originally Posted by w32honu View Post

I intended to use a 250 amp fuse just off of the battery. Then I was going to fuse the smaller gauge wires off of the bus as well. S
Great. Your fuse only needs to be bigger than your maximum load consumption. If the smaller gauge wire runs for a short distance to your breaker panel, it doesn't need a separate fuse, the main breaker on the panel is the fuse.

S[/QUOTE]
But if I do that won't the bus remain hot? On just a quick thought I am not sure that is a big deal. Have to think about that. S[/QUOTE]

It's fine for the bus to 'remain hot'. It's just a fused extension of your battery terminals and only has current running through it if something is happening.
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Old 08-12-2017, 07:05   #11
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Re: Electrical question.....

Quote:
Originally Posted by Privilege View Post
Great. Your fuse only needs to be bigger than your maximum load consumption. If the smaller gauge wire runs for a short distance to your breaker panel, it doesn't need a separate fuse, the main breaker on the panel is the fuse.

S
But if I do that won't the bus remain hot? On just a quick thought I am not sure that is a big deal. Have to think about that. S[/QUOTE]

It's fine for the bus to 'remain hot'. It's just a fused extension of your battery terminals and only has current running through it if something is happening.[/QUOTE]

All good points....thanks.

I am a bit reluctant to leave that bus hot tho. In this scenario if the bus remained hot the engine could be started even with the batt switch off.

I probably should have mentioned that this scenario is just a single battery set up. Another way to set up would be to isolate with a separate starting battery and avoid the bus bar all together.
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Old 08-12-2017, 07:06   #12
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Re: Electrical question.....

Options........
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Old 08-12-2017, 08:06   #13
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Re: Electrical question.....

Your battery feeds the bus bar and then from the bus bar to the starter motor. Between the bus bar and the starter is a battery switch which you only turn on when you want to use the engine. Your engine can't be started unless that switch is on.
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Old 08-12-2017, 08:16   #14
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Re: Electrical question.....

Ragarding the battery bank fusing, here is the best advice available anywhere from our guru contributor Maine Sail:

Battery Bank Fusing Photo Gallery by Compass Marine How To at pbase.com
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Old 08-12-2017, 08:20   #15
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Re: Electrical question.....

Quote:
Originally Posted by w32honu View Post
Options........
No required.

OEM 1-2-B Switch Wiring History Alternator/Batteries & "The Basic" 1-2-B Switch BEST Wiring Diagrams

Basic Battery Wiring Diagrams This is a very good basic primer for boat system wiring: Basic Battery Wiring Diagrams

This is another very good basic primer for boat system wiring: The 1-2-B Switch by Maine Sail (brings together a lot of what this subject is all about)
http://forums.catalina.sailboatowner...d.php?t=137615

This is a newer primer for boat system wiring design with a thorough digram: Building a Good Foundation (October 2016)
http://forums.sailboatowners.com/ind.../#post-1332240

The Short Version of the 1-2-B Switch Stuff: Electrical Systems 101 This is a link to the Electrical Systems 101 Topic, reply #2

What are ACRs, Combiners & Echo Chargers? (by Maine Sail) [scroll to the top]
http://forums.catalina.sailboatowner...d.php?p=742417 and Battery isolator / voltage regulator / batteries
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