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.

[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

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]

Originally Posted by Tim B

..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?




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]

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.