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Old 28-05-2017, 00:53   #1
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Simple DC System - Diagram - Will This Work?

This is a diagram of the simple, DC system I'm designing for my future boat.

Do you see any safety issues or other problems with this set up?



I realize that much of the wiring is oversized, but the cost/weight difference is trivial, so if there's no other argument against oversizing (?), it's worth the additional margin of safety IMO. Also, the boat will be steel, hence there's no bonding system. There's also no ground; it's a floating system, which I know is controversial. Finally, I excluded the ultimate power source (solar) because I haven't gotten to that yet, and the engine because there won't be one.

Thanks
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Old 28-05-2017, 03:58   #2
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Re: Simple DC System - Diagram - Will This Work?

The fuse for the SSB is much too small. A 150-watt marine SSB transceiver draws upwards of 30 amps peak on transmit.

Transceivers in the 100-watt power class -- like most ham sets -- draw about 20 amps peak.

Further, it is good practice to wire MF/HF transceivers directly to the house battery bank, or as nearly so as possible, NOT to go thru a switch panel or connections used by other devices. That's in order to minimize radio frequency interference (RFI) both ways: from the radio into other devices and from other devices into the radio.

AWG6 should be used for wiring runs up to about 20 feet, AWG4 for longer runs.

The frig fuse seems too small for most frigs. And they, too, should be wired directly to the batteries if possible.

You show what appears to be five 50AH 12V batteries wired in parallel. This creates a nightmare of interconnections. IMHO, it would be much better to limit the total connections, e.g., use two 6V golf cart batteries in series which would give you 12VDC at 230AH or so and only ONE interconnection instead of eight.

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Old 28-05-2017, 05:36   #3
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Re: Simple DC System - Diagram - Will This Work?

On a metal boat it is worth considering switching both the negative and positive supply to all circuits. This means double pole switches, or more commonly double pole circuit breakers.
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Old 28-05-2017, 10:35   #4
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Re: Simple DC System - Diagram - Will This Work?

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Originally Posted by btrayfors View Post
The fuse for the SSB is much too small. A 150-watt marine SSB transceiver draws upwards of 30 amps peak on transmit.

Transceivers in the 100-watt power class -- like most ham sets -- draw about 20 amps peak.

...

The frig fuse seems too small for most frigs.
For the SSB, I was considering the Icom IC-718. Just rechecked the specs and you're right, 20A. I think I may have confused max draw with average draw as I'd estimated it for my AH budget. Thanks for the correction.

For the fridge, it's an Engel 35L, which is supposed to draw 2.8A at max per the manufacturer. Do you think that's unrealistic? I understand that most fridges of other brands do draw considerably more on start-up.

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Further, it is good practice to wire MF/HF transceivers directly to the house battery bank, or as nearly so as possible, NOT to go thru a switch panel or connections used by other devices. That's in order to minimize radio frequency interference (RFI) both ways: from the radio into other devices and from other devices into the radio.
Thanks, I recall reading that somewhere and it slipped my mind.

Do you have any suggestion about a RF ground (complex topic it appears)?

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And [the fridge], too, should be wired directly to the batteries if possible.
Is that just because of the amp draw or another reason?

Quote:
You show what appears to be five 50AH 12V batteries wired in parallel. This creates a nightmare of interconnections. IMHO, it would be much better to limit the total connections, e.g., use two 6V golf cart batteries in series which would give you 12VDC at 230AH or so and only ONE interconnection instead of eight.
Would it be sensible to fuse battery-battery connections in case one shorts?

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Originally Posted by noelex 77 View Post
On a metal boat it is worth considering switching both the negative and positive supply to all circuits. This means double pole switches, or more commonly double pole circuit breakers.
Could you explain the rationale behind that?
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Old 28-05-2017, 11:37   #5
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Re: Simple DC System - Diagram - Will This Work?

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Could you explain the rationale behind that?
The main rationale is to reduce the chance of stray current corrosion.

Voltage drop along any wire that is conducting current means the voltage potential along any wire, including a negative wire, will vary. It only takes small voltage difference to introduce stray current corrosion and this is most rapid form of corrosion.

By switching off all negative wires for devices that are not powered you reduce the opportunity for voltage differences to be induced in the metal structure. With your current system the negative wires for all systems are always connected irrespective of whether the device is "off".

You have a very simple electrical system without many of the devices such as electric anchor winches and autopilots that tend to cause the most stray current problems. On the other hand this simple system means isolating both poles can be done without a great deal of difficulty and cost.
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Old 28-05-2017, 13:23   #6
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Re: Simple DC System - Diagram - Will This Work?

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Originally Posted by noelex 77 View Post
The main rationale is to reduce the chance of stray current corrosion.

Voltage drop along any wire that is conducting current means the voltage potential along any wire, including a negative wire, will vary. It only takes small voltage difference to introduce stray current corrosion and this is most rapid form of corrosion.

By switching off all negative wires for devices that are not powered you reduce the opportunity for voltage differences to be induced in the metal structure. With your current system the negative wires for all systems are always connected irrespective of whether the device is "off".

You have a very simple electrical system without many of the devices such as electric anchor winches and autopilots that tend to cause the most stray current problems. On the other hand this simple system means isolating both poles can be done without a great deal of difficulty and cost.
If I'm understanding correctly, the idea is that, if there's a ground fault in a circuit, you want that circuit disconnected from the rest of the system, at both + and - ends? But, absent a ground fault, it wouldn't matter?
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Old 28-05-2017, 15:06   #7
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Re: Simple DC System - Diagram - Will This Work?

It is worth wiring a bilge pump independantly, so it can be left on auto when away from the boat with tge isolator off.

Running the fridge direct to the battery is probably to prevent flickering and spikes when the compressor kicks in?

It may be worth spliting the lights into two separate circuits so one blown fuse doesn't result in total darkness. Also may be worth upsizing that system so it can double as a charging circuit for gadgets via usb in the future.

No port/stb nav lights at deck level?
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Old 28-05-2017, 18:05   #8
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Re: Simple DC System - Diagram - Will This Work?

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Originally Posted by KISS View Post
If I'm understanding correctly, the idea is that, if there's a ground fault in a circuit, you want that circuit disconnected from the rest of the system, at both + and - ends? But, absent a ground fault, it wouldn't matter?
Not quite correctly

In essence you don't want the steel hull to be carrying any current because when it does, it's resistance (just like any other conductor) causes a small (very small) voltage drop (PD) along it. This voltage contributes to stray current corrosion.

Another way to look at is with all the -ve circuits un-switched, then any load that has the -ve side connected to the hull, then these loads have multiple return paths back to the battery -ve.
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Old 28-05-2017, 18:10   #9
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Re: Simple DC System - Diagram - Will This Work?

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Originally Posted by Snowpetrel View Post
It is worth wiring a bilge pump independantly, so it can be left on auto when away from the boat with tge isolator off.

Running the fridge direct to the battery is probably to prevent flickering and spikes when the compressor kicks in?

It may be worth spliting the lights into two separate circuits so one blown fuse doesn't result in total darkness. Also may be worth upsizing that system so it can double as a charging circuit for gadgets via usb in the future.

No port/stb nav lights at deck level?
I might be pedantic (which I am ) but I always provide a separate fuse for each lamp in the port/stbd/stern lights and switch them via a 2 or 3 pole switch (as appropriate).
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Old 29-05-2017, 09:22   #10
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Re: Simple DC System - Diagram - Will This Work?

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Not quite correctly

In essence you don't want the steel hull to be carrying any current because when it does, it's resistance (just like any other conductor) causes a small (very small) voltage drop (PD) along it. This voltage contributes to stray current corrosion.

Another way to look at is with all the -ve circuits un-switched, then any load that has the -ve side connected to the hull, then these loads have multiple return paths back to the battery -ve.
Let's say every device is running except the plotter, which is switched off on the + side. If there's a ground fault on the plotter's return wire, current from the other devices can run from the bus bar, up the plotter's return wrire, into the hull, causing stray current corrosion. Whereas, if the plotter's return wire is switched off, the ground fault's totally isolated, and this can't happen. Is that the idea?
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Old 29-05-2017, 09:41   #11
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Re: Simple DC System - Diagram - Will This Work?

Define mast light? I don't see specifics for running lights, steaming light or Anchor lights on your panel. No engine means no need for a steaming light, but I'd add the circuit anyway. (You're not even going to have an outboard? What are you going to do in unfavorable wind conditions, especially around a moorage/dock?) No deck lights? Stereo/TV? VHF in addition to your SSB? NMEA network?

From what I understand (At least in the US), Mast head tri colors are allowed on 65' or less and can not be used if motoring, requiring deck level running lights. I gather that cabin side mounted running lights are prefered, with possibly a RED over GREEN sailing light at the mast head.
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Old 29-05-2017, 15:28   #12
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Re: Simple DC System - Diagram - Will This Work?

Yes, the SSB can draw 20amps (or more), but only when transmitting. So you need to handle that, but you'd only be drawing that from your daily budget some minutes per day.
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Old 29-05-2017, 20:15   #13
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Re: Simple DC System - Diagram - Will This Work?

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Define mast light? I don't see specifics for running lights, steaming light or Anchor lights on your panel. No engine means no need for a steaming light, but I'd add the circuit anyway. (You're not even going to have an outboard? What are you going to do in unfavorable wind conditions, especially around a moorage/dock?)
The same thing sailors have been doing since 2000 B.C.

Quote:
No deck lights? Stereo/TV? VHF in addition to your SSB? NMEA network?
The stereo and other non-essential items get plugged into the "utility outlet."

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From what I understand (At least in the US), Mast head tri colors are allowed on 65' or less and can not be used if motoring, requiring deck level running lights. I gather that cabin side mounted running lights are prefered, with possibly a RED over GREEN sailing light at the mast head.
AFAIK, a sailboat of ~35' LOA requires only a masthead and anchor light.
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Old 30-05-2017, 04:00   #14
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Re: Simple DC System - Diagram - Will This Work?

There is a convenience benefit to port/starboard lighting and utility circuits. I would also include full navigation light circuits under the assumption that their absence would turn off many prospects on resale and doing it now costs little. Many prospective buyers will want to install auxiliary power.
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Old 30-05-2017, 17:36   #15
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Re: Simple DC System - Diagram - Will This Work?

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Let's say every device is running except the plotter, which is switched off on the + side. If there's a ground fault on the plotter's return wire, current from the other devices can run from the bus bar, up the plotter's return wrire, into the hull, causing stray current corrosion. Whereas, if the plotter's return wire is switched off, the ground fault's totally isolated, and this can't happen. Is that the idea?
Leaving aside ground faults, there are still good reasons to switch both sides of the circuit even when the battery negative pole is floating (i.e. not connected to the hull).

A picture tells a thousand words so here is a simplified circuit just consisting of two loads where each load has its negative side connected to the hull. In case you are wondering, there are many such potential loads where the negative side of the load is connected in some way to the hull; thinks motors, radios etc.

First lets ignore the red components.
RA+, RA-, RB+, RB- are the resistive components of the conductors etc of each load and RH is the resistive component between the round connections between the two loads (i.e. the hull).

As all these resistive values are different in real life, it can be seen that when one load is switched on, current will flow in RH i.e. in the hull. This is part of "stray current corrosion".

By adding a pole of the switch in the negative side (indicated by the red switch pole), the problem is migrated; of course it exists when both loads are switched on.

Ground faults only exacerbate the situation and are also the reason it is good to provide circuit protection on both sides of the circuits when using a floating DC supply in metal hulls.

FWIW, connecting the battery negative terminal to the hull is the worse case.
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