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Tapping wire runs for new additions

Geoff W.

Makes Up For It With Enthusiasm
Blogs Author
Hi all,

I'm in the process of installing some new LED lights and a couple fans on my boat, and would like to have everything running off the "Cabin Lights" breaker switch for simplicity. i figured I could just tap into the existing wire run for the dome lights, but how would you recommend adding a 3rd connection to an existing run? I found some tap-looking things at Ace Hardware but am not sure this is what I need. I'd rather not do new individual wire runs back to the panel if not needed.

Thanks!
 

toddster

Curator of Broken Parts
Blogs Author
Perhaps "The Pros" will chime in, but in my opinion, removable equipment (e.g. fans, pumps, etc.) ought to be connected through screw terminals or something similar, so that it can be removed for service & trouble-shooting without cutting and splicing wires. Of course, here we're assuming that the wire size and fusing requirements of your fans are compatible with your lights and the total load on the circuit. If every device has its own switch, and in-line fuse, a simple terminal strip might be the thing.

It kind of brings up a comparison of two alternate visions for wiring a system.

1. All devices wired separately back to the central control panel, which was pretty easy when there were, like, six bulbs and a pump on the boat. On some modern boats, or those owned by gadget-freaks,:rolleyes: this can result in a massive central panel with dozens (!?) of switches and massive bundles of cables squeezed into spaces that were never meant for them. (And we won't even mention the similar horrors of 90's BMW wiring...)

2. Devices in each area of the boat wired to a local sub-panel (which has breakers or fuses for each device as appropriate). Only sub-panels and major equipment are fed directly from the main panel. This can simplify the wiring, even if there end up being more panels to buy. e.g. Instead of having main breakers for "lights," "fans," "12VDC outlets," each with wires snaking all over the boat, you might have "forward cabin," "aft cabin," "nav station." And the lights, fans, and outlets in each of those zones connect to the sub-panels with short wire-runs.

I imagine that it would help if one sat down before doing anything, and figured out which of these visions to follow. Yeah, I can only imagine...
 

Kenneth K

1985 32-3, Puget Sound
Blogs Author
I like using terminal blocks for adding connections. Cut your wire in two, and hook up both sides back up to the terminal block. Then add a jumper bar(s) and you can have the line feed multiple circuits.

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If you're adding to the cabin light circuit, ideally you wouldn't want to exceed the original total amperage load by much. However if you're switching to LED bulbs in all the cabin light fixtures, you're probably saving more than enough amperage to add on the devices you want.
 

Geoff W.

Makes Up For It With Enthusiasm
Blogs Author
Thanks gents - I swapped all the cabin bulbs to LED so I think the net savings is still pretty good. Need to test total draw of the light strings I want to hang.

Dumb question, the overall capacity of the circuit would essentially be the amp on the circuit breaker switch, right?
 

toddster

Curator of Broken Parts
Blogs Author
Yes, but the controlling factor is the size and length of the wire in the circuit. If you put too many amps in (by having the wrong breaker and too many devices) you will start a fire. Likewise, if the wire is too long and skinny so that the voltage drops, you will create resistance and smoke those fans.

Worth reading up on. Or just always use big wires so you don't have to worry about that stuff, within reason.
 

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
DC trivia

Yes, but the controlling factor is the size and length of the wire in the circuit. If you put too many amps in (by having the wrong breaker and too many devices) you will start a fire. Likewise, if the wire is too long and skinny so that the voltage drops, you will create resistance and smoke those fans.

Worth reading up on. Or just always use big wires so you don't have to worry about that stuff, within reason.

I have found that the late-80's (OEM) wiring by EY is mostly #12 or 14, i.e. good enough then and now.
I do tend to upsize to #12 for "cabin" circuits if making any changes or additions. 12 volts DC is not much 'push' and having some excess ampacity is a good thing. I have also replaced all of the inside bulbs with LED's.

Then there are some loads that specify #8 or #10, like the fridge compressor.
 

Tin Kicker

Sustaining Member
Moderator
Thanks gents - I swapped all the cabin bulbs to LED so I think the net savings is still pretty good. Need to test total draw of the light strings I want to hang.

Dumb question, the overall capacity of the circuit would essentially be the amp on the circuit breaker switch, right?

No - but not dumb and a lot of people have the same thought.

The circuit breaker is rated to protect only the wire. Period. To keep it from going up in smoke and igniting things around it or melting into other wires in a common run. The wire itself is sized for the load on the end, be it asingle LED light or a washing machine. If a person puts a washing machine or any other excessive load on a wire intended for a light, the c/b is supposed to trip to protect the wire. A really bad idea is that if a person were to add lights/loads PLUS swap to a bigger breaker.

The rule of thumb is to size the breaker and wire at 3X the load. (Example: 15A breaker for 5A load) This allows for the c/b to age, wire to get a little corrosion over time, knowing that additional minor loads will probably be added int he future, etc. Most (not all) c/b's function by heating a little bit of metal inside, called a bimetallic strip. Running a c/b at near the rated load, especially with a little corrosion anywhere in the circuit frequently results in the c/b having nuisance trips regularly. Having the c/b and wire sized within 2/3 is frequently seen, but now you understand why that is not desirable. (Example: 15A breaker for 10A load.)

Changing all the bulbs to LED means you probably have plenty of capacity for small fans. It's easy to check the specs on the fan and add up the watts (printed on the bulb) for the incandescent bulbs removed. (Volts=Amps * Resistance, so in a 12V system like we have, Amps = Watts / 12 volts)

The Home Depot and automotive splices which cut through the insulation should NOT be used in a corrosive (as in boat) environment. They will allow a source of moisture to get to the copper and cut through the protective tinning or other coating on the copper. They are easy to spot in a short while because they will be black or green with corrosion.

Adding a couple of terminal blocks is accepted practice, easy to do, and you'll probably come to appreciate it in the future. However you want to group loads on a terminal block is up to you, say, putting all the lights on one and fans on another or maybe a section of the boat. But definitely label each end of each wire. When you are trying to troubleshoot in the dark while getting bounced around, wire labels are worth their weight in gold.
 
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Geoff W.

Makes Up For It With Enthusiasm
Blogs Author
Thanks everyone, I will skip the "invasive" wire taps and use some small bus bars here and there. The entire "cabin light" circuit is rated at 15A, which is obviously meant to be more than enough to have every single light on at the same time. Right now, looking @ my battery monitor, if I flip every light on I'm using almost exactly 2A, and a fan at max puts me at 2.4A. I think this is fine for the overall circuit, at least, I feel safe enough with it...

One additional question though is "stepping down" wire size - I have heard this requires a fuse as well. The entire cabin light circuit wire gauge looks almost twice as thick as the fan's little fairy wires. Is this a situation where I should add a fuse to the smaller wire to prevent overload from the larger circuit?
 

Tin Kicker

Sustaining Member
Moderator
It's easy to over-think this stuff. The fan probably came with it's own wire, and that will be sized for the load of the fan on the end of the wire. Even if there were a dead short at the fan, as long as the wire is not laying against something flammable like insulation, you should be fine.
 

bigd14

Contributing Partner
Blogs Author
wire and fuse sizing guide

http://assets.bluesea.com/files/resources/reference/20010.pdf


 
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