Trying to figure wire gauge - SailNet Community

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
 Not a Member? 
  #1  
Old 11-01-2010
Junior Member
 
Join Date: Sep 2010
Posts: 17
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 0
Spoonman is on a distinguished road
Trying to figure wire gauge

I'm rewiring an old boat. In doing so I'm removing all of the existing wire as much of it is corroded and very little is type III tinned copper. I have the formulas and tables for computing the 'wire size in circular mils' and arriving at the right AWG gauge if I know this one thing: the length of the circuit.

It seems to me that computing the length of the circuit for a circuit that goes from the electrical panel to the load and back to the negative terminal block is easy. When the circuit is more complicated it seems tricky. For example, I'm thinking that I'd like to have one breaker on the electrical panel for the cabin lights on the port side and one for the cabin lights on the starboard side. There will probably be as many as 5 cabin lights on each side as the boat is 48-feet long.

I don't think I want to do 5 home-runs from the electrical panel to each of the individual lights and instead should run one wire from the electrical panel to a terminal strip or even a fuse block that is amidships and then run wires from there to each of the lights. What I'm wondering is how to calculate the wire gauge I need.

I'm thinking that it's a thicker wire from the electrical panel to the fuse block that is capable of handling the load of all 5 lights running at once while each wire for each light can be thinner as 1) it only has to carry the load of one light and 2) the distance is shorter for the lights closer to the terminal strip / fuse block.

For example, 5 x 1.5 amp light bulbs times 20 feet from the electrical panel to the fuse block (40 feet round trip) requires a wire of 7764 circular mils or greater which means 10-gauge. The wire from the fuse block to just one 1.5 amp light 10 feet away (20 feet round trip) requires a wire of 896 circular mils which is quite small but since 16-gauge is the smallest one should use, 16-gauge is right.

So, assuming my math is right on the calculations for the circular mils, is it OK to plan on two different thicknesses of wire: thick wire from the electrical panel to the terminal strip / fuse block and thinner wire from the terminal strip / fuse block to the actual load?

Being new to the forums I sure hope I'm doing this posting thing correctly and apologize if I'm not.

Spoon
Reply With Quote Quick reply to this message Share with Facebook
  #2  
Old 11-02-2010
kd3pc's Avatar
Senior Member
 
Join Date: Oct 2006
Location: Callao, VA
Posts: 1,354
Thanks: 0
Thanked 9 Times in 9 Posts
Rep Power: 9
kd3pc is on a distinguished road
spoon

your thought process is good, but for the boat - perhaps too theoretical...

you will have the same voltage present everywhere on the lighting circuit (whether 12VDC or 110VAC) and each will have the same current capacity - ie sparks will fly...if shorted to ground. Any load on the circuit has the ability, and will if given a chance, to sink every AMP available until the fuse blows or the wire melts, or the fixture fails.

the idea of one size out and a smaller size back...you have neglected the safety aspect of the return MAY have to carry the full load at some point. as far as the wire is concerned...use the same size both ways. BTW your theory is correct, just not practical in the real world.

16 would be a tad small for 7.5 amps, safety, line length loss, connector loss, etc. 10 would be overkill...besides it is tough to get 10 in the little light fixture.

And one "homerun" or circuit run to the first light, then to the second and so on will do nicely. You may run into issues fusing the same circuit twice. Having hot terminal strips throughout the boat is not a good idea either...every where you break the circuit will be a potential problem, both electrically and corrosion wise. Not to mention that it must be protected from other systems and human contact so that nothing shorts out what is there.

Buy a roll of #14 and #12 stranded for the cabin loads, and a handful of good name brand connectors, some heatshrink and a decent crimp tool..oh, don't forget the fuses or circuit breakers

for higher capacity loads - check one of the better 12 volt bibles/marine wiring books from the library or buy one and work from there.

You are on track to a successful project, but don't caught in the overthink.
Reply With Quote Quick reply to this message Share with Facebook
  #3  
Old 11-02-2010
Cruisingdad's Avatar
Best Looking MALE Mod
 
Join Date: Jul 2006
Location: Washington State
Posts: 9,904
Thanks: 3
Thanked 110 Times in 53 Posts
Rep Power: 10
Cruisingdad is a jewel in the rough Cruisingdad is a jewel in the rough Cruisingdad is a jewel in the rough
I don't have a wire gauge table, so will use what I saw online for a table. Seems like if you use 5 10w bulbs (typical Xenon) then a 40 foot RT would be about 12 ga. That may not even count fot a 10% voltage drop which is acceptable on lights. So you could pull 12ga for it.

However, how I would (and have) done it is that I like switches. I would probably pull a 10 like you suggested, to a switch/fuse box that was conventiantly located and make sure it was fused. I would then pull my switches for the boat off of that and use a smaller ga (add it up, but I bet you will get away with 14ish). Make sure every leg from the switch is fused seperate.

Also, if you are going to put fans on the circuts, now is the time to plan for them. Many peopleput them on the same circut as the lights. So it might be worth your while to up to 12 ga with the fans. Just another thought.

My opinions.

Brian
__________________
Sailnet Moderator



1987 Tayana Vancouver 42, Credendo Vides, (Mom and Pops boat, F/T Mobile Live Aboards in Puget Sound)

My Website:
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


Follow My Blog at:
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


Follow me on Facebook:

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote Quick reply to this message Share with Facebook
  #4  
Old 11-02-2010
Maine Sail's Avatar
Senior Member
 
Join Date: Jan 2003
Location: Maine Coast
Posts: 5,532
Thanks: 13
Thanked 147 Times in 113 Posts
Rep Power: 15
Maine Sail is just really nice Maine Sail is just really nice Maine Sail is just really nice Maine Sail is just really nice
Wire Size Calculator (LINK)
__________________
______
-Maine Sail / CS-36T


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.




© Images In Posts Property of Compass Marine Inc.


Reply With Quote Quick reply to this message Share with Facebook
  #5  
Old 11-02-2010
Cruisingdad's Avatar
Best Looking MALE Mod
 
Join Date: Jul 2006
Location: Washington State
Posts: 9,904
Thanks: 3
Thanked 110 Times in 53 Posts
Rep Power: 10
Cruisingdad is a jewel in the rough Cruisingdad is a jewel in the rough Cruisingdad is a jewel in the rough
Quote:
Originally Posted by Maine Sail View Post
Nice Link!!!

Brian
__________________
Sailnet Moderator



1987 Tayana Vancouver 42, Credendo Vides, (Mom and Pops boat, F/T Mobile Live Aboards in Puget Sound)

My Website:
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


Follow My Blog at:
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


Follow me on Facebook:

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote Quick reply to this message Share with Facebook
  #6  
Old 11-02-2010
sailingdog's Avatar
Telstar 28
 
Join Date: Mar 2006
Location: New England
Posts: 43,290
Thanks: 0
Thanked 11 Times in 11 Posts
Rep Power: 13
sailingdog is just really nice sailingdog is just really nice sailingdog is just really nice sailingdog is just really nice sailingdog is just really nice
I'd recommend going with LED lighting as well, since it will drop your electrical usage considerably.
__________________
Sailingdog

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Telstar 28
New England

You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

If you're new to the Sailnet Forums... please read this
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
.

Still—DON'T READ THAT POST AGAIN.
Reply With Quote Quick reply to this message Share with Facebook
  #7  
Old 11-03-2010
Stu Jackson's Avatar
Senior Member
 
Join Date: Jul 2001
Posts: 811
Thanks: 0
Thanked 9 Times in 8 Posts
Rep Power: 14
Stu Jackson is on a distinguished road
Here's another wire sizing link: The West Advisor: Marine Wire

Our 34 foot boat has 14 ga wire for lighting. It's too small, 'cuz the lights dim if you turn on more than one.

Also remember to wire in parallel not series, 'cuz if one blows it'll be like old Xmas tree lights!

The negative (return) wire has to be the same size as the positive wires going out - it's all one circuit.
__________________
Stu Jackson, C34, 1986, M25 engine, Rocna 10 (22#)
Reply With Quote Quick reply to this message Share with Facebook
  #8  
Old 11-07-2010
Junior Member
 
Join Date: Sep 2010
Posts: 17
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 0
Spoonman is on a distinguished road
Great replies. thank you everyone. Just so you don't think I'm totally stupid, I never intended to say that I figured the return wire would be smaller. The out and the back need to be the same size as, like you say, they are all on the same circuit.
Reply With Quote Quick reply to this message Share with Facebook
Reply

Quick Reply
Message:
Options

By choosing to post the reply above you agree to the rules you agreed to when joining Sailnet.
Click Here to view those rules.

Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


Similar Threads
Thread Thread Starter Forum Replies Last Post
what gauge wire to use with a Fourwinds 2 wind generator Bene505 Electrical Systems 3 06-09-2010 08:33 PM
Crimping lugs onto 4- and 6-gauge wire LookingForCruiser Gear & Maintenance 5 06-13-2009 05:43 AM
Alternator wire gauge myster Gear & Maintenance 4 10-10-2008 07:04 PM


All times are GMT -4. The time now is 02:34 PM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012

The SailNet.com store is owned and operated by a company independent of the SailNet.com forum. You are now leaving the SailNet forum. Click OK to continue or Cancel to return to the SailNet forum.