Best config for replacing the power lines on boat

Thread Starter

Upex

Joined Nov 10, 2013
76
Hi all,

Thanks for any input, as have been struggling to confirm I'm heading in the right direction.

Have a boat that has been wired with incorrect cables, they are far too small, poorly done etc, so im going to replace all of them so know it's right and clean etc. It's a 12v set up, from 2 leisure batteries (starting is separate battery and wiring and ok, so leaving that alone).

There isn't much to power to be honest, couple of pumps, running lights, cabin lights, few gauges, horn etc. But I want to add 2 spot lights at front (has 1 presently which doesnt work) and some usb sockets in the cabin for phone charging etc.

Batteries are in the stern (back) and spots will be at the very front, which is a run of circa 28ft. The wheel house with switches and gauges is circa 1/3 from back to front, and the usb sockets will be circa 2/3 toward front.

So my issues / questions:

The spot lights are 55w each, so I'm thinking I need heavy 16mm or 25mm cable to ensure low voltage drop (current spot is on 2.5mm, no wonder it doesn't work), is my math correct?

Should I power each spot via separate cables, I.e. 2 pos and 2 neg cables all the way (there is no chassis to ground, so have to run all the way back to batteries), or power both by a single pair of heavier cables?

As I need to 'tap in' to the power (and ground of course) at 2/3 and 1/3 distance, hows best to do this? Can i cut the main power cable at each location and power a fuse box, and simply attach the main power cable to the same bolt lug on the fuse board, so in effect the cable is constant, with the fuel box proving power for the accessories at both locations? If not, what type of fuel box do i need that has main power in and a main power out?

For the ground 'splits' can I do the same but just use a bus bar, so main ground cable is attached at 1 end, and attached at other end to move to next location, whilst providing accessory ground points?

If both the above are ok and how to effectively tap off the + and - will this impact my cable size selection? Ie as add additional resistance.

If can't 'split' a single heavy cable, ill need to install 3 sets, 1 to wheel house, 1 to cabin and 1 to the spot lights, which seems like a lot of cable!

Last one for now, will I need to drive the spot lights from relay(s)? Current draw will only be circa 9.2amp for the pair (110w), but I'm unsure how i could attach 25mm cable through a normal switch which can handle the current, but has standard blade connectors.

Sorry for length, hard to describe in words, especially when do not know correct terminology. But like this ( if shows up correctly):

Batteries + ------ 'split' for fuse box for switches horn etc ----------- 'split' for usb points ------- 2 spot lights
Batteries - -------------------- bus bar for grounds ----------------------------- bus bar for grounds ---------- spot light -

Many thanks.
 

MrChips

Joined Oct 2, 2009
20,189
Don't split any line.

Take a good look at what you have already installed.
Everything starts with the battery and distribution panel, i.e. two bus bars, one for B+ and another for B-.
Make sure there is one properly rated fuse (e.g. 50A) on the B+ line. You should have a battery switch to switch over from one battery to the other.

From the B+ bus bar, all lines should be fused ( 5A or 10A depending on function) followed by a switch.
Every function should have its own fuse and switch, i.e. one switched B+ and one B- for each function.

For' and aft' running lights together would be considered as one function. Don't split the line. Have two separate runs from the switched B+ terminal block and B- bus bar.

55W is a bit excessive for me. Try and replace those with modern LED running lights (10W max).

All cables should be marine grade cables.

If you are unsure on any of the above, there are many marine sites with useful information.
The most important thing to keep in mind is that a simple 12V marine battery can pack a lot of energy. We don't want to see your boat go up in smoke.

https://www.westmarine.com/WestAdvisor/Marine-Wire-Terminal-Tech-Specs

Edit: Sorry. I reread your post and noticed you specified spotlights and not marine nav lights. Just make sure your cabling can handle the amperage. Don't split the cable. Take two separate runs from the distribution panel, one for forward spotlight and one for aft. Never split the B- cables.

2.5mm cable is 10AWG. This is good for 10A on 28' run.
 

Thread Starter

Upex

Joined Nov 10, 2013
76
Ok Mr chips, many thanks for taking time to read my arduous post and give such a good reply.

I'll need to feed the spot lights from battery via 1 or 2 runs, have a seperate run to wheel house and then another to cabin then by sounds of it. So 3 or 4 positive and 3 or 4 negative cables at battery end.

Everything will be fused and powered independently, I was just aafter a way of having one large cable front to back, rather than separate cables for the 3 areas, seemed like a waste as will need 3 different sizes unless go way ott and use the biggest stuff for everything.

So needs to be like this if feeding spots seperately:

B ====@3m====== wheel house (+ fuse box and - bus bar). Need to calc total power draw still.
A ====@6m===== cabin fuse box(+ fuse box and - bus bar). 6amp max, so 4mm or 6mm2 cable?
T ====@9m===== spot light (+ inline fuse). 5amp, so 6mm2 cable?
T ====@9m===== spot light (+ inline fuse). 5amp, so 6mm2 cable?

Or this if feeding both spots via single cable set:

B ====@3m====== wheel house (+ fuse box and - bus bar). Need to calc total power draw still.
A ====@6m===== cabin fuse box(+ fuse box and - bus bar). 6amp max, so 4mm or 6mm2 cable?
T ====@9m===== spot light (+ inline fuse). 10amp, so 10mm or 16mm2 cable?

Thanks for the help, want to ensure its right lol.

Edit: crossed posts there. Is that chart one way length or total + and - distance?

Cheers Mr chips
 

MrChips

Joined Oct 2, 2009
20,189
Chart is round trip.
Hence for 25' boat, double the cable length to 50'.

Hence my correction: 10AWG good for 5A on 60' cable (3% voltage drop).
 

Thread Starter

Upex

Joined Nov 10, 2013
76
Thanks Mr chips. 6awg should allow both spot lights to be run on one feed cable pair if reading diagram right, or as you say, 2 separates of 10awg, so will get searching.

Thanks
 
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Thread Starter

Upex

Joined Nov 10, 2013
76
So, I've decided to move the forward spot lights to be closer to the fuse box, need to measure for sure, but should save about 4 or 5 foot for each of the spots, perhaps more.

With Mr chips input, I'm going to run 1 pair of cables to fuse box and negative bus bar. Then power everything independently from there. Longer cabling for in cabin items but they all reasonably small loads, so not an issue.

However, I'm unsure of the principle of how to confirm correct cable size for the second leg, ie everything that comes off the fuse box.

The battery to fuse box/bus bar cabling will be bigger (to handle all currents combined plus some extra head hight to be safe and allow for bulb changes etc without worry that it pulls too much with everything on) and I'm happy with calculating that first cabling.

But for the fusebox to load legs.... do i need to factor in the length of battery to fuse/bus bar cables, or just calculate fuse to load cables in isolation?

Ie (fictitious figures), if battery to fuse/bus is 10ft and 50a total, I'll use 6 awg cable. Then if fuse/bus to load is another 10f and 5a, is 16 awg ok (5amp at 10f) or does it need to be 14 awg (combined length 20ft at 5a)?

Cheers all, and to Mr chips again for helping me get my head straight and defining my basic set up to be solid.
 
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Thread Starter

Upex

Joined Nov 10, 2013
76
Think I figured out that you use full distance to calc voltage drop of second leg cables, so having a hearty supply from battery to fuse makes no difference and doesn't allow smaller cables from fuse box to load etc.

If this is wrong, please say, as don't want to waste cash on very excessive cable that isn't needed lol.

Cheers
 

wayneh

Joined Sep 9, 2010
16,235
If you switch to LED headlights, all your problems go away and the current wiring is probably adequate. Seems a lot easier, and will extend battery life to boot.

Think I figured out that you use full distance to calc voltage drop of second leg cables, so having a hearty supply from battery to fuse makes no difference and doesn't allow smaller cables from fuse box to load etc.
I'd say that's wrong. Your concern is the total voltage drop. You can calculate the voltage drop due to any load separately in both sections of cable. As long as the total drop is less than the target, you're fine. In your scenario you might calculate 0.1V lost in the run to the fusebox and 0.6V in the longer and higher gauge section.
 

Reloadron

Joined Jan 15, 2015
5,106
If you switch to LED headlights, all your problems go away and the current wiring is probably adequate. Seems a lot easier, and will extend battery life to boot.

I'd say that's wrong. Your concern is the total voltage drop. You can calculate the voltage drop due to any load separately in both sections of cable. As long as the total drop is less than the target, you're fine. In your scenario you might calculate 0.1V lost in the run to the fusebox and 0.6V in the longer and higher gauge section.
You may want to give that some thought. I just went to the "Daymaker" headlight and spots on a 92 Harley Davidson Electraglide and the LED headlight with High/Low beam is amazing considering the old incandescent lamps I replaced. Matter of fact lamp by lamp I have been going LED on the entire bike. Saving all the old stuff but the LEDs are such an improvement and use low power which I like. A Google of Daymaker Headlights should bring up some results.

Ron
 
What your finding in a car now days is multiple locations. At least one in the engine compartment and one inside. One car I had had a fuse for each filament of the headlights.

Somewhat important is to have everything terminate at one place. If you can't do that consider somewhat separating devices that care what the 0V reference point is going to be. i.e. gauges (stuff that uses the engine block for a reference) vs lighting.

It's also possible that not all loads will be on at one time.

There are fuses available for large currents, but there are also fusible links used. They are just as important. They are generally a lighter guage wire in a circuit with a high temperature insulation and in a location where burning in half won't cause any damage. A Toyota that I owned actually had short lengths of wires in a "fuse box" in the engine compartment.

If you do some looking, you can find some really decent low voltage wiring panels.
 

Thread Starter

Upex

Joined Nov 10, 2013
76
Cheers all. Had a look at some led spots, they were still rated at 55w, so current daw on 12v system be same as a 55w halogen no? 4.9amp or so?

May well revisit leds at later date, as my deck lights are all led to reduce current consumption, and they are on a lot more than the spots. But for now, i have the halogen spots, so will go with them. Dropping to lower draw leds later will only benefit, but have the scope in cable for more power as needed.

Thanks for the calculation method Wayne h, ill calc both and add together then, although if use the full length assuming same cable, it'll only build in more headroom, so will see if it's pushing me into too heavy cable first, and check if I can drop a size via your method. Thanks.

Kiss, it'll be fused well, probably more than needed, but I'd rather be safe than sorry etc.

Cheers all
 

ebeowulf17

Joined Aug 12, 2014
3,146
Look at the package carefully. We do have house lamps, sort of labeled as, 60 W equivalent.
I was thinking that too. Lots of LEDs list the wattage of conventional bulb they replace in very large print, and only list their actual power consumption in fine print on the back. I'd expect the LEDs to consume a fraction of the power for equivalent light.
 

Thread Starter

Upex

Joined Nov 10, 2013
76
Screenshot_20170627-071858.png Ha ha, ordered some lower power ones, advertised at 100w, real draw of 12 or 18w, can't recall which. Thanks both, I'd have overlooked that on face value.

If anyone has a spare moment, I'd appreciate any corrections to the above, which is my calculations of what to use for what, fuse and cable wise. You'll note I'm only using 1.5mm or 6mm cable (apart from fuse box supply which is 35mm) as I've ordered rolls of them, so keen to oversize and use what I have, versus buying another size, unless this will cause issues?

Thanks for all the help.
 

ebeowulf17

Joined Aug 12, 2014
3,146
View attachment 129838 Ha ha, ordered some lower power ones, advertised at 100w, real draw of 12 or 18w, can't recall which. Thanks both, I'd have overlooked that on face value.

If anyone has a spare moment, I'd appreciate any corrections to the above, which is my calculations of what to use for what, fuse and cable wise. You'll note I'm only using 1.5mm or 6mm cable (apart from fuse box supply which is 35mm) as I've ordered rolls of them, so keen to oversize and use what I have, versus buying another size, unless this will cause issues?

Thanks for all the help.
Excellent! I'm glad you were able to find more suitable spotlights. Sorry I'm no help on wire gauges.
 

MrChips

Joined Oct 2, 2009
20,189
How did you arrive at 55W from two USB?
20A fuse appears to be excessive. Go with 10A fuse.

No need for five different fuses. 10A and 15A should be sufficient.

(Make sure you carry spare fuses, 10A, 15A, 20A in your emergency kit.)
 

Thread Starter

Upex

Joined Nov 10, 2013
76
I guessed Mr chips, as labelled 2.1amp per plug, so 4.2 max per pair, so 55 seemed to be a nice number that just went past the max power they could take.

In reality, they'll never get near that, probably only 1 in use at a time really, so will drop as you suggest.

Guessed at horn to (single air trumpet horn thing) as no specs on it and sellers all come back with no power or current answer, helpful!

Will have a full set of spare fuses on board, thanks.

Do cable sizes look ok then?

Assume you mean group circuits on same fuse?

Thanks for the help all
 
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