calculating wire length

Calculating this stuff...

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.

 

This is like tennis.. bat that question back!!!!

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??

 

http://www.fisheriessupply.com/onli... Inactive/Shopby/Shop By Department/Ns/P_Sort

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

 

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

 

Info about "Dr.LED" lamps

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.

 

Halogens: 10W or 20W??

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...

 

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)..

 

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 ?

 

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

 

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

 

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 !

 

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".

 

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.