12 Vdc Distribution and Branch Circuit Schematic Review

[Dave0963]

I've just bought a1974 22' Tanzer with a poor electrical system (no fusing, lots of tape, wire nuts and abandoned wires). I've used the ABYC Tables for ampacity and done the calculations for Voltage Drop and Derating for bundled wires, given conductor length and temperature ratings of the insulation.

The kicker motor is a pull start, so there is no electrical for an engine and also there is no provision for shore power. I will add a solar charger at a later date tied directly to the Pos and Neg Buses.

A question I have not been able to resolve is regarding fusing. I believe the fuse size is a function of the conductor size, ie the fuse is intended to protect the conductor. Many sites suggest selecting the size of the fuse to protect devices. From an electrical perspective, I don't see a safety issue with using an "undersized" fuse. Can anyone provide insight on the matter or a snippet of the ABYC Electrical Code that addresses use of fuses.

I have attached a copy of my Schematic and Panel Directory, is it appropriate to ask for input on my design?

I've tried to keep my Branch circuit ampacities low (below 2 amps) where possible to decrease voltage drop. I believe 14 awg is required for the Anchor light and also for the VHF (transmit ampacity is large).

Thank you for any and all feedback.

 

[kappykaplan]

I believe the fuse size is a function of the conductor size, ie the fuse is intended to protect the conductor.

Yes. Welcome to the forum!

 

[dlochner]

The Tanzer 22 is a great little boat. I owned 2245 for a long time and sailed it on Lake Ontario. There are days I wish I had her again.

The wiring looks good. The one change I would consider is using a larger gauge wire from the batteries to fuses. While 10ga can easily handle the amps, the distance may cause some unwanted line loss.

Fusing is always done to protect the wire, which means the fuse should be smaller than the rated amp carrying ability of the wire. Many electronics come with an inline fuse and recommend a small 2 or 3 amp fuse. This fuse is to protect the device from any power surges on the line, which is a separate issue. The main battery fuse should be within 7" of the positive battery terminal unless it is covered in a flame retardant sheath, then you get 60" (IIRC).

Are the lengths you show one way lengths or round trip?

ABYC has a "recreational boater" membership that is free. It is time limited to about a week and gives you access to all the Standards. These are read only, however, with a little tech savvy they can be downloaded.

 

[jssailem]

Welcome to the forum.
Nice schematic and fuse plan. Yes as @kappykaplan indicated, the fuse is for the wiring.
Only thought I had was, where do you need to have the information on your batteries. I have mine right next to the switch I use to turn the batteries on. I check it before or when I power up the system. You indicated you are going “solar” makes sense now a days. You may want to look in to a Charge Controller. It helps maintain the best voltage for your batteries. Straight solar can swing from more than is recommended to nearly nothing.

 

[pgandw]

IF your circuit goes to a SINGLE device (load), you could get away with using a smaller fuse at the panel to protect both device and wire. I really dislike device in-line fuses "somewhere near" the device, especially on smaller boats where even boat yoga won't get you the access you need.

But the moment a given circuit has more than one device, you have to fuse the wire at the panel, not the device. Then separately fuse each device in the circuit.

The other problem with a single fuse at the panel is will you (or subsequent owner) remember that there is no device fuse when you go to add a 2nd device on the circuit, see the wire is adequate for a bigger fuse, and put the bigger fuse in?

Fred W
Stuart (ODay) Mariner 19 Sweet P

 

[dlochner]

But the moment a given circuit has more than one device, you have to fuse the wire at the panel, not the device. Then separately fuse each device in the circuit.

Circuit Interruption devices (fuses, breakers etc) always go near the source of power, which can be the battery, the panel, or bus. The only case where this is not true is a wire from a regulated power source and the wire can carry the maximum amount of current the source can produce. This is usually a solar panel, battery charger, or alternator. In these cases the fuse needs to be at the connection to the unregulated power source (battery).

On branch circuits where the wire size gets smaller, a CI needs to be installed at the point where the branch starts.

 

[Stu Jackson]

BlueSea is a very helpful company and has a great website.

https://d2pyqm2yd3fw2i.cloudfront.net/files/resources/reference/20010.pdf

More here: https://www.bluesea.com/support/reference

 

[Foswick]

Circuit Interruption devices (fuses, breakers etc) always go near the source of power, which can be the battery, the panel, or bus. The only case where this is not true is a wire from a regulated power source and the wire can carry the maximum amount of current the source can produce. This is usually a solar panel, battery charger, or alternator. In these cases the fuse needs to be at the connection to the unregulated power source (battery).

This is correct - no disagreement whatsoever, but I think a point of clarification might be important for people that might not quite understand circuit design, just yet. To be clear: the critical design consideration is that circuit protection goes just upstream of the conductor it is trying to protect and not putting it close to the source.

What Dave wrote about putting circuit protection near the power source is exactly how most marine wiring ends up laid out, since we’re usually protecting an entire branch circuit from the panel outward.

Strictly speaking, though, the fuse or breaker should go just upstream of whatever conductor it is trying to protect. If that is the entire branch circuit, then the protection goes close to the source because everything else is downwind - not because the guiding principle is to put protection near the source. (I'm hoping that distinction makes sense and doesn't just muddy the waters.)

When we are talking about entire branch circuits, the end result is the same - the protection goes near the source. But, for example, if you peel off a short lead to a single device on smaller wire, that little run becomes its own protected circuit and the inline fuse belongs right where that smaller conductor starts. In effect, that point becomes the source for that little circuit - making Dave's statement still technically correct but maybe misleading if a person only thinks of the "source" as the main source as in the battery or panel.

Just adding this in case it helps anyone picture why an inline device fuse (for example) may not sit at the "main" source but is still correctly placed. Dave covered this in another post.

 

[dlochner]

What Dave wrote about putting circuit protection near the power source is exactly how most marine wiring ends up laid out, since we’re usually protecting an entire branch circuit from the panel outward.

This is not what I said, the circuit protection goes near the unregulated source, i.e., the battery regardless of what is at the other end of the wire. With a regulated source, such as a solar panel, charger, or alternator no circuit protection is needed near that source because the wire should be sized to handle the max output. However, circuit protection on these wires is needed where it connects to an unregulated source, i.e., the battery. This is necessary because if there is a short in the regulated source the full power of the battery will go through the wire which is too small to handle the power.

 

[Foswick]

This is not what I said, [...]

Well, I thought we were in complete agreement, but now I'm confused by what you just said. A CPD only goes near the battery if it is meant to protect everything beyond that point.

That is, a CPD goes at the point where the protected conductor starts - regardless of where that is relative to the battery. When you are protecting an entire branch circuit, this is near the battery, but not all CPDs go near the battery. But, say, in the case of an inline fuse protecting a thin wire to a radio, this could be some distance from the battery and panel. You would not put that fuse right off the battery just out of principle. (I know that's not what you're saying, or at least I hope it isn't.)

This inline CPD is at the end of the protected conductor nearest the source, but not necessarily anywhere near the source.

I was only pointing this out, not to contradict what you were saying, but as a point of clarification for anyone that might wonder why a fuse might be far away from the apparent source, i.e. far away from the battery.

Are you using "circuit protection" to narrowly refer only to entire branch circuits? (i.e. everything downstream of the battery) What you are saying holds true in that narrow definition. Or, are you actually saying that all CPDs should be near the battery, regardless of how far downstream the protected conductor is?

If you are saying this then, yes, you CAN do it that way, but you don't have to. That is, you don't have to put a 2A fuse right off your distribution panel that feeds a 14 AWG just because you have a 2A branch somewhere down the line. You can put that CPD where that 2A conductor starts.

Now, there are simplicity reasons you might not want to do it that way. That is, you might not want to track down successive CPDs to find out where the fault is. You might prefer that the CPD fails at the distribution panel, just for simplicity. But, that doesn't buy you any protection.

Or, maybe you are saying something else? Like I said, I thought we were in agreement, but now I'm confused.

 

[dlochner]

If you are saying this then, yes, you CAN do it that way, but you don't have to. That is, you don't have to put a 2A fuse right off your distribution panel that feeds a 14 AWG just because you have a 2A branch somewhere down the line. You can put that CPD where that 2A conductor starts.

When there are branching circuits the main branch needs to be wired such that it can support the all the loads downstream and the CPD must be appropriately sized for the total load on the main branch. Each circuit that branches off the main branch needs its own CPD sized appropriate to the wire size and load. When a circuit branches off a main branch, the main branch becomes the source and the CPD should be at or close to the connection between the main branch and the off shoot.