calculating wire length

[Westerly23]

I’m installing new cabin lights in a 23 foot sailboat. I am using 4 halogen lights and 5 L.E.D. spot lights. My total Current is 3.92, however I don't see every light ever being on at once.

My question is regarding calculating the wire length.

If I have one main line say 20 feet, and then two 10 foot lines that branch off in a T, would I call that 30 feet, or do I need to add up the equal line in the other direction and call it 40 feet?

Along the same lines, if one side of the T was 20 feet, and there was 10 feet in the other direction, do I need to add them both or just the longer of the two?

Thanks.

[Tim B]

Wire length is always an issue, whatever field you're working in. You can rest assured that you'll never get it right, but here's a few tips to help you get close.

1) Decide where you want switches/automatic controls etc.

2) Draw out the layout on paper, and decide what the most efficient connection method is. (In some cases using multicore cables can be a real help, especially with low-current DC applications)

3) Measure (as accurately as possible) the length of cable runs, Then add an allowance for curvature, and accessibility at the ends of each cable. About 6 inches for ceiling lights / switches, 1 foot for built-in panels.

4) If it is financially viable, buy a reel of cable (that will be more than you need). It is often cheaper to buy a rell than the same length per meter.

5) Run the cable from the reel/coil. When you're happy with it, cut it to length with enough free to access it easily.

If you have set lengths (often the case in theatrical sound and lighting environments) it pays to have short lengths of cable so that you don't end up with excessive amounts of cable lying around.

Cheers,

Tim B.

[TerryKing]

Question(s): What are you trying to determine:

1. SAFE wire size for your expected current?
2. Allowable voltage drop at the 'end' of each circuit?

With less than 5 amps total, it's unlikely that you will have a problem with "Safe" wire size, assuming you simply go with something rated over the 5 amps, such as #16 (USA).

The other question: What is the supply voltage? 12 or 24Volt, probably.

The issues are also a little different for the Halogen Incandescent lamps than for the LEDs, which have SOME kind of current-limiting built-in (if a complete lamp fixture or direct 12V replacement) and are less sensitive to voltage drop over their operating range. But LED's will go dark completely at some low voltage, whereas incandescents will glow feebly with only 3 volts or so...

Tell us just a little more about your system. It is very reasonable to calculate what you need, and it's not rocket science, just Ohm's Law.. But you need to say where the loads are on this multibranch circuit. Essentially you will calculate each branch separately, and add them (well, the voltage drops in series) up.

This is a typical boat wiring situation, and worth working thru.

[Westerly23]

TerryKing,
Thanks for the reply.
I am trying to size the wire.
I have all the formulas, it’s just figuring out the length I seemed to have trouble with.
If I add in every branch along the way to the farthest light the overall length adds up fast and I find that I may need a large gauge.

But here is what I have.

I made a little sketch. I noticed when I stated sketching that in order to run the LEDs on one switch I would have to have them on to use the halogens (they have a switch on the fixture). I’m working on that problem. Advise? Make 2 circuits, 1 for LED one for halogen?

But, from the fuse it’s about 5 feet to the cabin top. 7 feet to the forward most light, 3 feet over to the port forward light and 6 feet to the aft light.
From the forward port light to the LED in the V berth is 4 feet and then 5 feet to either side are the reading lights.

Everything adds up to 70 feet round trip. Or 48 feet to the farthest light.
Despite the obvious switch flaw, which length of wire would I use for calculating the voltage drop, 70 or 48?

There are 5 LEDs (.12A) and 4 halogens (.83A)
The total draw is 3.92
This is a 12V system.

I’ve got a couple good books, I don’t think I‘ve read this however.
http://farm1.static.flickr.com/152/435538752_f98360b9f8.jpg

[alan white]

To keep things simple, and considering you have about 70 ft of wire to get both poles, one tinned fifty foot 12 ga. red wire and one tinned fifty foot 12 ga. black wire will work fine. This is probably overkill, but maybe someday you will add something.
So you need two 50 ft rolls. If you buy a couple of inline fuses, you can still have lights while you locate the short, because the "smaller" inline fuses will prevent the "big" fuse at the panel from blowing.
At least that's how I'd do it, but then, I'm no electrical engineer. If you've got nothing to do with the excess wire left over, double the leads to the lights closest to the panel, and wrap them together in tape. Not elegant, but doing so will cut your voltage loss where you need to (in half), where the wires will be carrying the most current. Just like a subway system, the most people are closer to the main terminal. So balancing the system would mean bigger trains on the main terminal end. If you have 10 ga. wire (as opposed to using double leads), the 10 ga. will do the same thing, and will look more impressive to the ship's rats.

Alan

[alan white]

By the way, Terry sounds like he knows what he's talking about (more than me), so he'll save you some money on wire I'm sure. I always overkill wiring rather than figuring circuit resistence. I would use the (reasonably) biggest wire I already owned, however inelegant that might seem, but I'd get the smallest wire that still worked well if I had to go out and find it. Marine wire is different, that I know. It is tinned copper, and it is not usually available except at marine supplys.

A.

[TerryKing]

Westerly,

-- Let's figure out the switch question first:

1. Are you planning to be able to turn all the LED lights on at once, to be able to move about the cabin areas?

2. You show a fuse (not a circuit breaker, right?)

3. Do you plan to have a single 'cabin lights master switch' on the whole circuit so you can be sure all lights are off during the daytime?

4. How are the lights "clustered"? In your drawing, the port aft 6 ft section has both an LED and a halogen. Are they both really at the "end" of that run? Similar question on the Starboard side. IF you separate the LEDs, then the wire lengths to each halogen will be clearer.

This is a good example of this boat wiring stuff, so I think it's worthwhile working this out.

I will attach the first drawing you made, with a bunch of arrows and numbers (sounds tekkie, right?). This will actually be easier when we know how the lights are 'clustered'. So this is just an example, IF all the lamps had individual switches.
----------------------------------
How This Works:
- I lied when I said "This is just Ohm's Law" ! This is because there are multiple branches in the circuit.
- The 'other thing we learned' in DC Circuits was "Kirchoff's Laws". See http://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws for a detailed explanation. But all we need to know for this kind of multi-branch circuit is:

The current leaving any junction is equal to the sum of the currents entering that junction.

So they just add up. The attached version of your drawing shows how the currents would add up if all the lamps were ON.

NOTE: The current is different in different parts of the circuit, right? The Voltage Drops (when you finally figure them out) will be the SUM of the drops in the individual sections. Just another little addition problem... and like Alan said, Bigger Wire Is Better.

Once you decide about the LED's and switches, let's make a diagram that shows it all.

Hope you don't think I'm making a big deal out of this, but it's a really good example of boat wiring questions, so it's worthwhile making it clear.

[TerryKing]

Somewhat Different Subject:
I plan to be wiring SOME boat later in the year, in China! So I have been thinking about this, but you got me thinking more:)

My Fantasy of the way my cabin lights would work:

- There would be a 'cabin lights master circuit breaker' which I would make sure was turned off in the daytime so no lights could be left on.

- There would be two kinds of lights:

-- multiple low-current efficient white LEDs that would make it easy to move around the main cabin areas at night. There would be TWO switches that control these: one close to the entryway from the cockpit, so you can see your way in, and one at the Captain's Usual Bunk, to turn them off when going to sleep.

-- Multiple Halogen reading / cooking / charting lamps, each with it's own local switch.

AND: There would be an LED lamp in the Head. There would be two switches that control it, one at the head doorway and one at the Captain's Usual Bunk. For obvious midnight reasons :p

So, it's a little more wire... but only for the low-current LEDs, so it can be smaller.

(I like the word "Fantasy" to define Design and Engineering when "It's only thought-stuff and paper". I used to give a talk called "Fantasy as an Engineering Design Tool" at some colleges and IBM. So I'm a little wacky?? )

As boatdesign.net shows, Boat Fantasies Are The Best!

[alan white]

Just because I enjoy how this is moving along, Terry, be as technical as you like, at least for my benefit. Wiring can be extremely elegant---- not under or overdone. LEDs didn't even exist as lamps back when I last wired a whole boat. I imagine they have revolutionized the way people enjoy good illumination below compared with the incandescent/halogen era. I can remember when I used to turn off the halogens and use an oil lamp to conserve enough current to run the bilge pump.

A.

[Tim B]

Red Lights at night are better than white, there is less glare (this becomes obvious after a night passage).

If you have multiple switches, then it may not be a bad idea to consider using a relay (or MOSFET) to drive the lights, and having signal cable running across the boat to control it.

Cheers,

Tim B.

[TerryKing]

..snip..If you have multiple switches, then it may not be a bad idea to consider using a relay (or MOSFET) to drive the lights, and having signal cable running across the boat to control it. Cheers, Tim B.

Hey, Tim, then I can hook the COMPUTER up to it! Now we're really having fun.!

IsThere a 12V DC Version of X10? No Extra Wires...

Seriously there might be something to this. Newer commercial aircraft have local MosFET switches and only data bus to turn stuff on and off... Maybe AirBusBoat??

[Westerly23]

TerryKing,

1st, thanks for your help. I spent a long while pouring over the books again last night. They are good at stating how things work, and how to trouble shoot, but not from designing from scratch. We bought a bare hull, good thing it was only 23 feet!

My fantasy lights systems would have six lights in the main cabin, 4 LEDs and 2 halogens. The LEDs would be near the 4 corners and the halogens would sit in the centers, both “lines” of lights are about 2 feet from the centerline.

After reading and drawing last night, I came up with a switch plan. I would run the 2 port LEDs on one switch, the 2 STD LEDs on one switch and each halogen on one switch. This would give me 4 switches I could mount near the companionway.

In the V berth, we have one LED in the center. There are halogen reading lights for him and her. The reading lights have a switch on the fixture. The LED will need a switch.

So, to specifically answer your questions;
We want to have options, and not have to run all the LEDs at once.
I show a fuse, the boat had an 8 circuit panel in place that uses fuses rather than breakers. For cost reasons I would like to retain it, but I have been looking at breaker panels.
I don’t think we NEED a master switch to ensure all the lights are off, but it’s a good idea. I believe I could put a master switch just off the panel.

Your notes on the diagram are much appreciated. I’ve made a new diagram, this time with switches.

I calculated the needed wire gauge.
~ 24 feet each for the main cabin LEDs I could run 18g but 16g is the smallest recommended. Thoughts?
The same goes for the main cabin halogens.
For the 48 foot run to the V berth I could run 16g, but the calculator I’m using says to use 14g for 49 feet with a draw of 1.78, so go 14g here right?


http://farm1.static.flickr.com/174/436503835_06cd9bb963.jpg

By the way, has anyone used a single book to design and install a complete electrical system from scratch? I think getting over this hump was the hardest part, thing should be smoother from here.

Everyone’s thoughts and advice are much appreciated.

[Westerly23]

http://farm1.static.flickr.com/173/436549408_8f60d412c0.jpg

Hypothetical question.

When calculating the length of wire, would one add the lengths to the farthest fixture or all fixtures on the circuit? Is the correct answer 30 feet or 25?

[TerryKing]

Westerly said:
----------( COPY )--------------------------
I calculated the needed wire gauge.
~ 24 feet each for the main cabin LEDs I could run 18g but 16g is the smallest recommended. Thoughts?
The same goes for the main cabin halogens.
For the 48 foot run to the V berth I could run 16g, but the calculator I’m using says to use 14g for 49 feet with a draw of 1.78, so go 14g here right?
-----------------( END COPY )----------------------

We're still missing a term in the calculations. What "Voltage Drop" is acceptable?
Does the calculator you use let you set the allowable drop, or is it some fixed assumption?

In your second post, (actually in all cases), the answer is "It Depends" (typical normal Engineering Answer :-) ) What it Depends ON is what we need to know:

Where and how much (current) are the loads in a multibranch circuit?
What is the acceptable voltage drop?

Then R = V / I and
For a particular wire size, R = R (Ohms per foot) times length in Ft.

So for a working example:
1 load of 4 amps
40 feet circuit length (20 ft x 2 )
Acceptable voltage drop = 1.0

R = V / I = 1.0 / 4 = .25 Ohms acceptable resistance

So, what SIZE wire has less than .25 ohms in 40 feet?

Mumble Mumble.. (digging for table.. where the heck's my Radio Amateur's Handbook??) OK, let's use this:
AWG Ohms/Foot
4 .000292
6 .000465
8 .000739
10 .00118
12 .00187
14 .00297
16 .00473 * 40 = .19 (OK, good..) (SAFE 10 Amps)
18 .00751 * 40 = .30 (Close, maybe OK) ( SAFE 6 Amps )
20 .0119
22 .0190
24 .0302
26 .0480
28 .0764

So, we need #16 AMERICAN Wire Gauge (There are others! Can convert)

And so on....

You NEED to know two things to pick a wire size:

1. What is the Maximum SAFE current capacity you need (without overheating).
2. What is the Acceptable voltage drop.

We (whaddaya mean WE??) should start an Electrical Systems page on the WIKI and put some of this stuff up there.... In our Other Spare Time...

So, uh, sorry to go around again on the Guitar (As Arlo Guthrie would say) but, what's the acceptable voltage drop???

[Westerly23]

The calculator I'm using allows you to input a voltage drop. I've been using the default 3% but i can select 10% too.
Both give me a drop of .356volts. (12 volts, 1.78A, 48 feet, using 16G wire).

When you ask, what's the acceptable voltage drop, do you mean how low before the devise fails to operate normally? As in the info the manufacture should supply? I'm using these LEDs; http://www.doctorled.com/p23.htm
They are rated 11-15VDC

I'm afraid I may not understand how to find the acceptable voltage drop.

[TerryKing]

I always did bad in math because I made dumb mistakes. That's why when computers came along, I really liked them. If I got it right once with simple data, it was ALWAYS right. Yahoo! (tm)

OK, I did a quick calculator for this stuff.. You put in:
Length of the wire (Circuit, both out and back)
Current that will flow.

It tells you:
Voltage drop for many different sizes of wire
If a wire size is SAFE for your current

NOTE: Often you will want a larger size than needed to be just Safe, so that the voltage drop isn't too high. Also, you MIGHT add just One More Thing on some wire some time, right???

OK, let me try to get this up here.... (Oh, this is one of those XLS MicroSoft Office Excel SPreadsheet things like all the Hull Design guys use). I think it will work on the GoogleTools thing. I'll try that out and report... I'm NOT a salesman for MS!

NOTE FREE SOFTWARE: You can run this .XLS Excel spreadsheet even if you don't have the Microsoft software, by downloading it and then uploading it to Google Spreadsheets. You need to have/get a free Google Account. I tried it and it runs fine.

[TerryKing]

You Asked, "When you ask, what's the acceptable voltage drop, do you mean how low before the device fails to operate normally? "

Yes, that's the idea. Now, (Here We Go Again!) it's an Engineering Question so, "It Depends"..

- Sometimes the manufacturer gives you a minimum voltage, like 10.5 Volts
(Or like your LED lamps, apparently 11.0V)
- OFTEN you Get To Decide. Like the Halogen lamps... Their light output will drop as the voltage drops, but you have to decide what voltage is OK.

AND you (as the Engineer) also Get To Decide how low your battery voltage will "Normally" go. There has been a lot of discussion here about that VS different battery types. So it's like:

Acceptable Voltage Drop = Minimum Battery Voltage - Minimum Device Operating Voltage.

So, ideally you'd connect your LED and Halogen lamps to a variable voltage and (at night) see how low they can go in voltage at (your call!) acceptable brightness.

SOMEBODY has to make design decisions, and You're IT.

..I'll look at those LED lamps. What are the Halogens??

[Westerly23]

http://www.fisheriessupply.com/online/ln_menu/product.asp/mode/1/product_id/258377/N/8134+4294962427/Ne/6/sID/46085E805A67897F/R/8410/act/A01/catalog_name/FISCO/Ntx/mode+matchpartial+rel+Inactive/Shopby/Shop+By+Department/Ns/P_Sort

That chart is great! Thanks a lot for walking me through this.

[alan white]

What's the average voltage of a battery discharge cycle?

[TerryKing]

This is the information I received from Dr.LED:

This for the Flood Saturn Ring [M100-24W-D-WF] units, but is similar for other units rated "11-15V"

??What is the light output of these units at various voltages?
** the light output is the same from 11 to 15 Volts

?? Does the light output slowly diminish?
** Yes, between 10 and 11 V ONLY.

??Does it quit completely at a certain voltage?
** YES. usually at around 10V.

??Can the lamp be damaged if a battery runs down to 6 volts overnight?
** NO.

[TerryKing]

http://www.fisheriessupply.com/onlin...ment/Ns/P_Sort

Are you using 10 watt lamps in these, as your .84A current suggests? The units pointed to show 20 watts, but I'm sure you can use 10W bulbs...

[Westerly23]

We've ordered them. The in store display stated 10w.

3.45A (2 halogens + 1 led) should do well on 48 foot run in 12g wire according to a calculator I downloaded. Your excel sheet seems to be a little more forgiving.

bigger is better/safer right?

[TerryKing]

We've ordered them. The in store display stated 10w. 3.45A (2 halogens + 1 led) should do well on 48 foot run in 12g wire according to a calculator I downloaded. Your excel sheet seems to be a little more forgiving. bigger is better/safer right?
Right! I see .26V drop which is fine...

SOFTWARE: You can run that .XLS Excel spreadsheet even if you don't have the Microsoft software, by downloading it and then uploading it to Google Spreadsheets. You need to have/get a free Google Account. I tried it and it runs fine.

For an ONLINE wire size calculator, take a look at:
http://editgrid.com/user/boatdesign
where you'll see a couple of tryout online spreadsheets. The BoatWireSizing on will let you change the wire length and the current, and will update to show you the voltage drop for different wire sizes, and what's safe.. Just trying out Online Spreadsheets (editgrid has graphs etc, google does not)..

[VKRUE]

Hey guys:
Not trying to be abusive here but, isn't all this just a bit overkill ?

I'm not an electrical engineer but, having been a truck driver (and owner) for so long I've done my share of wiring. It seems to me that your concern about precise voltage drops is all overkill in this particular situation. I mean, your not wiring a computer or any kind of thermal device or what ever that requires precise wire lengths to provide acurate measurement or readings (unless you plan to install some sonar, radar or other highly teckie thing).

I've completely rewired intire trailers... rewired most of the dash in my old Peterbilt as well as many other trucks and the main thing I've found that is important is this...

I've also had a bit to do with CB radios and their antenna systems... even built my own antennas from junk... and talked around the world... literally !

The main wire coming from the power source supplying the circuit has to be big enough to carry the required current... i.e. all the devices on that circuit presumed to be on (Total Load) and plus some... for good measure. Never build a circuit with "just enough to get by".

Your dealing with a 12 volt system here right.

#12 guage wire will be more than sufficient for most applications.
If you plan on having something with a very large current draw, say a hairdryer or cigarette lighter plugin this would be a minimum of #12 guage... depending on the distance from the power source (battery or breaker box) to the device used (or it's outlet). If your going to have an outlet for your wifes 1850 watt hair dryer in the head... some 20' or so from the power source, then a #10 would be preferable.

The smaller the wire (diameter, not guage #) ... the less current (amps) it can hold without overheating.

Look at the trucks going down the highway at night sometime. You'll notice a lot of what we call "Christmas Trees" out there. Trucks with hundreds of lights shining and looking ever so nice ! These things are using # 12 guage wire for supply, #14 guage wire for most of the lights...and #16 for LEDs because they use such a small current draw. These rigs are 70+ feet long... compared to your 30' boat.

It just seems to me that for what your wanting (lights basically) you can use a # 12 or #14 wire to supply the power (the main line) and use #14 or #16 wire from that main line as individual feeds to each light. Remember too, the main ground must be as big (guage) as the main supply line's guage (or the sum of the power supplies if multiple like in a trailers pigtail).

One of the replies here (sorry I can't get back to find out who) mentioned using a lot longer (50' maybe) wire than necessary and just rolling up the excess wire. According to my radio guy (years ago now) this is a no no ! If there is current "flowing" thru that wire then you've created a "coil" or "loop". This can reduce certian readings like current draw or in the case of a radio signal it will recuce the SWR (Signal to Wave Ratio) but, the test reading is FALSE. It's only an illusion of sorts. Also, this coil or loop creats a magnetic field that can disrupt other electrical devices in the imediate area such as a radio or computer... possibly a depth finder or whatever.

Not trying to overide anybody's ideas... Learning all of this stuff is surely usefull information... I just thought that it seems a little overboard.

You are going to have the lights... right.
Your lights are going to be where you want them... right.
You are going to have to run a certian length of wire to those lights... right.
These are "givins"... right.

Seems that what you really need to be concerned about is "HOW MANY LIGHTS CAN I HAVE ?" and "HOW MANY BATTERIES DO I NEED" and last but not least, "HOW WILL I KEEP THEM BATTERIES CHARGED".

The fact that you've choosen to use LED's is in your favour in that... they have such little power consumption. I'll bet you can have as many of these little buggers as you want. The halogens on the other hand, use considerably more power. I would limit these to necessity.

Am I missing something ?

[TerryKing]

Hey guys: Not trying to be abusive here but, isn't all this just a bit overkill ? ..snip... Am I missing something ?
You ain't missing much, Vic... This is more of a "design it for a new installation" kind of thing. I think there are a few things that are a little different than cars and trucks:
- The weight is important on a light boat, or long-range cruiser
- The batteries do not always have an alternator charging them, and the voltage may drop down to 10 or 11 volts on deep-cycle battery. So a 1 volt drop may not matter with 14 Volts to start, but matter on a deep discharge.
- Being Real Sure of your wire size VS loads and fuses is important when you're sleeping in a wooden boat 100 miles from a fire station. Or shore...

But I've done most of my wiring just like you said, truth to tell :eek:

[alan white]

Hi Vic,
Could you find and quote the thing about the coiled wire?
I think you're a lot like me in the way you wire. But Terry is outlining an ideal plan because that's his expertise, which means he enjoys the heck of of design elegance, which is to say, there is something of value in the excersise itself for Terry, me, the Westerly owner, and anyone who trips across this thread.
It doesn't involve a single wiring job, but everyone's wiring job. As Terry went through his analysis, I went through the same analysis on paper.
I know I could have done the job lickety-split myself, using experience (overdo, and you can't go wrong), and a rough calculation of the parameters.
Then as I tried to pin it down hard, with exact calculations, I found I was having a hard time figuring the resistence per foot of wire, the acceptable voltage drop, the wire/fixture ratio as a total resistence (voltage arrives at fixture minus the voltage lost to the hot wire, but what about the ground wire? I figured it too used voltage, to a small degree, which would drop to exactly zero upon reaching the battery). But I wasn't sure.
How is the resistence of the ground wire calculated? My head was spinning.
It carries the same amperage as the hot side, but a miniscule voltage, which must mean its footage is also calculated in, but only as if it were supplied with a tiny voltage. The voltage would be progressively less until the wire reached the battery. I imagined this could be charted, a downward line showing several sections--- the hot wire, the hot wire after the first branching, after the second branching, the ground back to the second branching, then to the first branching, then the final five feet. Voltage dropping the whole way, in a series of linear angles, from battery to battery. Each fixture would have its own unique graph.
The wire size would be determined by the first five feet from the battery to the first branching. That section would carry the biggest load. Each fixture would have a slightly different voltage too, depending on how it connected to the harness.
The coiled wire you mentioned---- which was a misunderstanding---- would such a low voltage actually create a meaningful field? What would it affect?

Alan

[VKRUE]

Hi Alan:
You ask about the coil issue. I myself cannot answer your question.
My old radio guy... the man that I paid large sums of money to for setting up my radios (C.B. & HAM no, I don't have a Ham ticket) told me about it when we were trying to measure and set the SWR's on my first Ham rig. I had mentioned that I had discovered that if I coil the antenna's excess coax it would significantly lower the SWR's and not need any manual adjustment of the antenna...

He informed me that although I did indeed see this effect on my SWR meter, it was in fact a false reading. He tried to explain how the coil works but that's all over my head. Anyway, he was a very good radio man and I trusted no one else in the country to handle my radios (experience taught me this). As far as the voltage goes...there's not much voltage in a radio signal but, the coil was enough to disturb things !

Wish that I could help more.

Maybe Terry knows !

[TerryKing]

...(snip)... My head was spinning.
It carries the same amperage as the hot side, but a miniscule voltage, which must mean its footage is also calculated in, but only as if it were supplied with a tiny voltage. The voltage would be progressively less until the wire reached the battery. I imagined this could be charted, a downward line showing several sections--- the hot wire, the hot wire after the first branching, after the second branching, the ground back to the second branching, then to the first branching, then the final five feet. Voltage dropping the whole way, in a series of linear angles, from battery to battery. Alan

Cool, Alan, you really thought this through. The other "Kirchoff Law" (I mentioned the Current Law earlier) is "The sum of the voltages in a complete circuit is ZERO". Which is what you figured out! A graph or pictorial, like you suggested, might have the battery vertically on the left, with voltage going from zero at the bottom to +12 at the top. On the right would be the series circuit of the fuse, wire to a device, the voltage drop across the device, and the return 'ground wire'. Each wire would have a small voltage drop, depending on the Current, the Length and the Wire Size. The 'drops' would exactly cancel out the battery voltage and the sum would be Zero.

Like Vic's experience, you can wire a lot based on experience, and the maximum allowable current in a wire size. Like, "The current is definitely less than 20 amps, so AWG #12 is fine". Or, you can draw a diagram showing every wire, it's length, and it's wire size. And you can find the Sum of the currents going into (and thru) each wire in a multibranch circuit. And then you can show the voltage drop in every wire section, and what's "Left Over" for each of the devices you wanted to power in the first place!

Some time, we should start a Boat Wiring section of the Wiki and show both approaches with some guidelines for "Big Enough".

[TerryKing]

--(snip)-- I had mentioned that I had discovered that if I coil the antenna's excess coax it would significantly lower the SWR's and not need any manual adjustment of the antenna...

This is a case of "They're Both Right!" Because they are totally different cases.

The Radio guy was talking about High Frequency effects on an antenna and it's coaxial feed line. A small coil made in the coax line indeed does act as a 'choke' or inductor and can solve some Standing Wave Ratio problems. The "G5RV" antenna I use at home has about 10 turns of coax about 6 inch diameter just below the transition to balanced line.

The DC (or low frequency 50 or 60 Hz) power wiring has no such effects, and 'extra' length on wires simply adds voltage drop and power loss.