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Old 19-06-2014, 17:44   #1
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Circuit, Wire, Fuse Size

I reckon for specific questions I have on my project I will post new threads so we can find them and focus.

It appears there are contra arguments to everything so...

I totally get that you fuse to protect the circuit. In deciding wire size for my project it appears that I can get away with 16g wire for many circuits, some require 14g and then of course bundled runs need a bigger size. Resistance is the enemy so I will size for 3% drops. I also find 16g to be "light" and insubstantial from a mechanical perspective.

For simplicity of sourcing and installation I am planning to run all circuits (with a couple of exceptions for bigger circuits) at 14g.

So in researching wire protection I get opinions on equipment supplier websites everywhere from 11 amps max to 25 amps!

Take the 2 charts as an example.

On the WM chart 10% a 14g wire of 10 feet can take 25 amps - Do I size for that in all cases (for 14g wire) or do I look at the 3% chart, find my 20 foot run and max fuse is 8 amps because max load is recommended at 8 amps?

My technical guess is that the wire is capable (heat load) of 25amp so regardless of recommended load to protect that wire from melting 25amp will work. But a 25 amp fuse is overkill for the load itself. The chart is really about vdrop for the run - not about fuse protection.

The second chart is interesting because it just recommends a fuse (MDL/AGC) in the 25 amp range. Another site just says 11.85 max.

BTW - My plan at present is to size for the circuit load and not exceeding the wire capability. Like most circuits will be 5amp fuses.

In summary I will wire 14g all over, max run is like 40 feet (round trip) and fuse for 5amp standard with exceptions.

Thoughts?
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Old 19-06-2014, 17:56   #2
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Re: Circuit, Wire, Fuse Size

I suggest that you're overthinking it.
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Old 19-06-2014, 18:23   #3
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Re: Circuit, Wire, Fuse Size

Quote:
Originally Posted by Sailmonkey View Post
I suggest that you're overthinking it.
For sure I am. But when people say "Fuse for the wire" and then you start reading wire charts the question arises for 14g.

The wire is capable of 25 amps. I've never seen a boat with a 25amp fuse for the VHF. So there is an exception to the rule or the rule is wrong.

The rule (of thumb) should be, "Fuse for the circuit load or the wire's max capability whichever is lower."

I am a 30-year licensed Aircraft mechanic and graduate engineer - I think I know what to do but a newbie to electricity could get really lost. My project is a practical application of all the opinions around here - I hope these side topics raise useful points for the lurkers (and me...)

I know what I am gonna do - 14g wire and predominantly 5amp fuses - unless the smart guys point out something I am missing.
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Old 19-06-2014, 18:34   #4
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Re: Circuit, Wire, Fuse Size

There's no harm in fusing lower than the circuit ampacity, so 5A fuse with 14 gauge wire should be just fine. If 5A fuse is small enough to protect the device, you won't need a separate fuse at the device to protect it. So I agree with your rule of thumb: fuse to the circuit or the device, whichever is lower.
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Old 19-06-2014, 18:39   #5
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Re: Circuit, Wire, Fuse Size

Leave your A&P ticket at the door. The sailboat world is all about voltage drop not the amps to make a wire smoke as it is in the aviation world. Unlike aircraft where weight is king, it's the bigger the better in a cruising sailboat.
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Old 19-06-2014, 20:07   #6
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Re: Circuit, Wire, Fuse Size

There are two separate concerns to be addressed:
  1. Ampacity of the wire so that it does not become dangerously hot, and
  2. Having the desired voltage delivered to the device at the end of the circuit.
You need to design to address the need. All wiring must meet the first item. Some circuits will need a critical minimum voltage that call for a bigger wire size than is required by the ampacity limit.

An example might be an electric bow thruster where you have a high amperage load for a short duration that requires little voltage drop to work properly. A long wire run becomes a problem, so you locate the battery bank close to the thruster motor because it will not tolerate a voltage drop of a long wire run. The charging system for it will likely tolerate a longer wire run due to lower current levels. The opposite example would be a bilge pump running warning light. With a very low ampacity, and a lamp that tolerates lower voltages, a long run of light wire will be acceptable.
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Old 19-06-2014, 21:04   #7
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Re: Circuit, Wire, Fuse Size

run big enough wire to handle the voltage drop.

fuse for the device at the end.
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Old 20-06-2014, 05:09   #8
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Re: Circuit, Wire, Fuse Size

Quote:
Originally Posted by smac999 View Post
run big enough wire to handle the voltage drop.

fuse for the device at the end.
When multiple devices (frequently electronics) are wired to a single breaker or fuse on the panel, then the breaker needs to be sized to the ampacity of smallest wire and smallerfuses are needed to protect each device. Electronics are usually sold with a device fuse in the power supply cable.
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Old 20-06-2014, 05:50   #9
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Re: Circuit, Wire, Fuse Size

Here's one of the better online wire calculators
Circuit Wizard - Blue Sea Systems
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Old 20-06-2014, 06:04   #10
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Re: Circuit, Wire, Fuse Size

Quote:
Originally Posted by Ex-Calif View Post
I know what I am gonna do - 14g wire and predominantly 5amp fuses - unless the smart guys point out something I am missing.
You're not missing anything.

If the device will never draw more than 3-4 amps then a 5 amp fuse is fine. For a device that will actually draw 5 amps I would probably go to a 7.5 amp fuse (unless the device manufacturer calls for a 5 amp fuse). Obviously, (and I say this for the benefit of others, because you clearly understand) if the circuit requires more then you go to bigger fuses up to the ampacity of the wire, at which point you need both bigger wire and bigger fuses.

Frankly, I think you understand all of this stuff better than you are willing to let yourself believe. Good luck.
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Old 20-06-2014, 06:17   #11
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Re: Circuit, Wire, Fuse Size

Quote:
Originally Posted by denverd0n View Post
You're not missing anything.

If the device will never draw more than 3-4 amps then a 5 amp fuse is fine. For a device that will actually draw 5 amps I would probably go to a 7.5 amp fuse (unless the device manufacturer calls for a 5 amp fuse). Obviously, (and I say this for the benefit of others, because you clearly understand) if the circuit requires more then you go to bigger fuses up to the ampacity of the wire, at which point you need both bigger wire and bigger fuses.

Frankly, I think you understand all of this stuff better than you are willing to let yourself believe. Good luck.
Quote:
Originally Posted by smac999 View Post
run big enough wire to handle the voltage drop.

fuse for the device at the end.
Quote:
Originally Posted by Ziggy View Post
There's no harm in fusing lower than the circuit ampacity, so 5A fuse with 14 gauge wire should be just fine. If 5A fuse is small enough to protect the device, you won't need a separate fuse at the device to protect it. So I agree with your rule of thumb: fuse to the circuit or the device, whichever is lower.
Quote:
Originally Posted by Ex-Calif View Post
For sure I am. But when people say "Fuse for the wire" and then you start reading wire charts the question arises for 14g.

The wire is capable of 25 amps. I've never seen a boat with a 25amp fuse for the VHF. So there is an exception to the rule or the rule is wrong.

The rule (of thumb) should be, "Fuse for the circuit load or the wire's max capability whichever is lower."

I am a 30-year licensed Aircraft mechanic and graduate engineer - I think I know what to do but a newbie to electricity could get really lost. My project is a practical application of all the opinions around here - I hope these side topics raise useful points for the lurkers (and me...)

I know what I am gonna do - 14g wire and predominantly 5amp fuses - unless the smart guys point out something I am missing.
Here is a great article from Maine Sail on why you don't fuse for the load but fuse for the wire. Too small of a fuse and you get voltage drop. I don't think that would be a significant problem in this case since you have low needs but thought I would share this article.

Fair winds,

Jesse
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Old 20-06-2014, 06:39   #12
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Re: Circuit, Wire, Fuse Size

On another note, you never said what your load actually will be. But looking at this chart from Blue Sea Systems (large pdf at link), at a 40 foot run (round trip) with a 3% voltage drop and a 5 amp load (guessing based on the fuse you said you would use), you should be using 10 gauge wire.

As far as fusing (same chart as you had is in the large chart I linked to) you could then gauge up to 60 amp for single wire or 40 amp if bundled.

Jesse
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Old 20-06-2014, 07:09   #13
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Re: Circuit, Wire, Fuse Size

Quote:
Originally Posted by JK n Smitty View Post
Here is a great article from Maine Sail on why you don't fuse for the load but fuse for the wire. Too small of a fuse and you get voltage drop. I don't think that would be a significant problem in this case since you have low needs but thought I would share this article.

Fair winds,

Jesse
Wow - Votage drop from the fuse - OK only .1V but In big circuits it can make a difference. Never even really thought of that.

I did know about ampacity and V-drop but hadn't really "summarized" it in my head as well as St. Elsewhere did - And to DenverD0n, thanks for you confidence in me - LOL

I don't want to get to far afield topic wise to quickly but I was thinking about the solar.

So over wiring 14g vs. 16g helps with resistance too. And efficient stuff (LEDs) and low resistance have to be good from an overall standpoint.

So the supply to the controller from a 100w on the bimini at 17 volts does not need to be fused right? I mean if shorted (while not a good thing) it's not like suddenly there is 300 amps on the wire. The panel can only produce 6a @ 17v. So appropriately sized wire will never melt.

Another niggling topic. We've all worked on messy systems. I am hoping to make a very "clean" maintainable system, properly labelled wires in conduit so in the future a bad wore can be pulled out (pulled through) etc.

Cabin lights. From a wires usage perspective they are usually wired in a parrallel "chain" one to the next from distribution to bow. Of course a failure in the front of the chain kills all lights and of course you have connections at each fixture buried behind headliner.

I know I am overthinking it but this is the kind of stuff that bugs me.

One idea I am toying with is a "forward Distribution Panel" - A terminal box abeam the mast or so. Everything for the mast and forward stops here with terminals.

So the cabin lights would go -

- Aft DC bus
- Cabin Light Master switch
- Master Fuse
- a single pair to forward dist box
- 5 pairs to 5 (LED) lights - not fused because it's all 14g and master fuse is sized appropriately for 14g (5a)?

Thanks for playing along with me everyone. I guess now, as my dad would say, "You got the talking part done. Now go cut some metal."

I am also trying to document everything as part of the boat maintenance documentation as I go along - The spreadsheet already has the energy budget worksheet started. I added columns for wire run length, AWG size and fuse rating which will help me source the parts. I was originally thinking, "100feet of wire or so." After guestimating each circuit I am up to 350 feet and counting... I'll post it when I am proud of it.

The second tab is the parts list. I have completed most of the parts list which includes source as I will be online ordering most of it and want to keep track of where I got stuff and what got delivered.

I have been on the hunt for a free schematic drawing software - CNET is coming up bupkiss and I may have to do it in powerppoint or some kludge.

Any ideas on that one? (software should be free or nearly so and simple)
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Old 20-06-2014, 07:20   #14
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Re: Circuit, Wire, Fuse Size

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Originally Posted by Ex-Calif View Post
I have been on the hunt for a free schematic drawing software - CNET is coming up bupkiss and I may have to do it in powerppoint or some kludge.

Any ideas on that one? (software should be free or nearly so and simple)
I use Draft Sight.
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Old 20-06-2014, 11:13   #15
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Re: Circuit, Wire, Fuse Size

I don't think voltage drop thru the fuse is an issue most of the time. As mentioned by Maine sail solar circuits where any drop in voltage can prevent your bank from reaching full charge are a special case.
Most of the time you would need a fairly big voltage drop in the wire already for the fuse to push you over the 10%.
I personally hate having little fuses every where in line so I prefer the panel has breakers and or fuses protecting the load (and the wire) In some cases this means having a sub fuse or breaker panel, a common place for this would be to have an electronics fuse panel near the helm to avoid voltage drop concerns as well as providing for easier wire runs.
I'm not sure it was mentioned before but some loads should be wired at 3% drop, ABYC calls out Nav lights, Panel feeders, Bilge blowers, and Electronic equipment. I do know a few boat builders that use the 3% rule for all loads on the boat, and man Ive never seen so much 8 and 6 gauge in my life. I'm guessing it adds well over 100 lbs to the 50' they build.
The idea for the lighting terminal block is a good one and I've seen it used before on high end semi custom boats. They basically use terminal blocks to distribute the lighting loads on one circuit. It provides for more robust wiring as well as easier trouble shooting down the line.

On the amount of wire, your gonna need alot. I was adding some system on a 35' power boat recently. I wasn;t rewiring the whole thing just adding some electronics I brought along one of our shop guys and he had brought about 50' of 12 and 16 gauge with him, I told him we would need more but he didn't believe me, an hour in to the project he had to make a run to West for another 50' (those wire runs thru the hull can be deceivingly long)
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