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  #1  
Old 02-25-2008
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Wiring cabin lights in Serial

I am starting a project to rewire my boat. I understand the ideal is to wire everything in parallel. Is there a reason I should not wire my cabin lights in parallel?

I would have two runs down each side of the boat. I would use wire for each rated for the max amp draw for all lights at the full distance.

Other components like VHF and GPS would all be wired in parallel with their own fuses etc. through a fuse block.

Is there any reason I should not do the lights in parallel? It just seems to make more sense rather than having bundles of 3-4 duplex wires starting out from the beginning on each side.

Thanks.
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Old 02-25-2008
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Maccauley-

Unless you're planning on wiring a separate circuit for each and every light, you have to run them in parallel. If you run them in series, you couldn't turn on one fixture without having them all on, and the voltage requirements would be added, so five fixtures would require 60 VDC, rather than the 12 VDC you're supplying and five times the current.

It really depends on how much control over the lighting you want, and whether you want to be able to add to the system down the road or not. By ganging all the lights in to a single parallel circuit now, you make it much less likely that you'll be able to tap in to a light fixture for power at the different locations on the boat later, since you'll run a risk of overloading the circuit. If you ran separate wires for each light fixture, then you can always tap into one in the future for small accessories, like cabin fans and such.

Personally, the way I would probably wire the boat, if I were doing it, is to wire the light fixtures independently, and then run the wire back to the electrical panel, but put them into a fused switch block, like the one shown below, and then have the switch block connected to the circuit breaker on the panel. This is especially true if I were going with LED-based lighting. Then I would match the individual switch fuses to that of the load that circuit would generally use. This way, you can also upgrade the fusing on the circuit or split the circuits to separate breakers in the future relatively easily. Having multiple small loads on a single large breaker can be somewhat dangerous from a safety standpoint.

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Last edited by sailingdog; 02-25-2008 at 01:57 PM.
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Old 02-25-2008
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Just make sure the lead wires to the individual fixtures are rated for the max current from the breaker or fuse. Use terminal blocks to branch off of (not those silly three way butt connectors). A port & stbd DC bus reminds me of how a submarine is setup. There shouldn't be anything wrong with little things like lights (esp if you use LED).. just don't start hooking up pumps, fans, DC outlets and stuff.. those sort of things need their own leads from the panel. Both side should have their own breaker/fuse.
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mccauley, the reason behind wiring in parallel is that you will have the same voltage across all fixtures. If you were to wire in series, the first fixture will have the proper voltage, say 12V, then the voltage will start dropping with each additional fixture on the circuit. In other words, the 1st fixture will have 12V, the second might have 11V, then the next one 10V, and so on. This means that your fixtures will be dimmer that the previous one on the circuit, with the last one on the circuit being the dimmest.

Sailingdog comment is based on wanting 12V at the very last fixture, so you would have to raise the voltage at the begining and have some blindingly bright & hot lamps . As he said, this will also consume more amps.

When wiring in parallel, you take the positive wire and connect it to the positive terminals of ALL fixtures on the circuit. The you take the negative wire and connect it to the negative terminals of ALL fixtures. In the end, you will have one positive and one negative wire going to your fuse, for each circuit.



This would be a series circuit, and like I said, voltage would go down from 12V at the first fixture, and go down with each additional fixture.


Also, in series, depending on the fixture itself, if you turn off a fixture (you are opening the circuit), the rest of the fixtures on the circuit will go off. Sort of like the older Christmas lights, that once one goes bad, the rest also turn off. However, there are ways around that with fixtures designed specifically for series connection.

I agree with sailboy, I would go with one stardboard and one port circuit
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Last edited by xtatico1404; 02-25-2008 at 02:11 PM. Reason: adding info
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xtatico1404 !
Not quite.
If you wire in series as you described then the first light will not have 12V. It will only have 6V if both ligths are the same. The second will also have 6V.
So it will not work at all unless you use 6V bulbs.
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SD, to control each of the lights I have fixtures that each have a switch. The switch on the main panel would be used to energize the cabin light circuit and I can then turn each one on/off at the fixture.

Forgive my ignorance but why does it work for my house to be wired in series?. My living room for example, all lights and fixtures, are on a single circuit, wired back to a single switch on the main breaker panel. That circuit is a single wire run first to one outlet, then to another, and so on terminating on one of those. The single breaker switch energizes that whole circuilt and I turn on those I want. I would think for the boat it should work to do it the same way.

My cabin lights are all 10 watts which I think draws about an amp. If I wire the port circuit assuming 5 amps of power draw for 5 lights shouldn't I have enough for each one to work?
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Quote:
Originally Posted by tomaz_423 View Post
xtatico1404 !
Not quite.
If you wire in series as you described then the first light will not have 12V. It will only have 6V if both ligths are the same. The second will also have 6V.
So it will not work at all unless you use 6V bulbs.
Tomaz, you are right, there will be 12V from the begining to the end of the circuit, so with two fixtures of the same wattage, each will have 6V, and if you add more fixtures, the votlage will keep going down. Thanks for the correction, for some stupid reason I was not assuming the same resistance per fixture
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If your house is wired in series, you're in deep ****... the electrical system in a house is wired in parallel... never in series, so I hope you didn't do any of the wiring if you can't tell the difference.

Yes, if each fixture draws an amp, and you have the wiring and breaker heavy enough to support the load, you should be fine. BTW, if the light fixtures draw 1 amp each, and you have five of them, you don't want to use a 5 amp breaker, but something like a 7.5 amp breaker. This is because most fuses and breakers are designed to operate at about 80% of their rated capacity full-time. In other words, you don't want to run a five amp load on a five amp breaker... it isn't a good idea.

The main reason for the switch panel, in my case, is that some of the light fixtures I have are custom made by a friend of mine and didn't include light switches on them... but they're far too useful to not have aboard.
Quote:
Originally Posted by maccauley123 View Post
SD, to control each of the lights I have fixtures that each have a switch. The switch on the main panel would be used to energize the cabin light circuit and I can then turn each one on/off at the fixture.

Forgive my ignorance but why does it work for my house to be wired in series?. My living room for example, all lights and fixtures, are on a single circuit, wired back to a single switch on the main breaker panel. That circuit is a single wire run first to one outlet, then to another, and so on terminating on one of those. The single breaker switch energizes that whole circuilt and I turn on those I want. I would think for the boat it should work to do it the same way.

My cabin lights are all 10 watts which I think draws about an amp. If I wire the port circuit assuming 5 amps of power draw for 5 lights shouldn't I have enough for each one to work?
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Quote:
Originally Posted by maccauley123 View Post
Forgive my ignorance but why does it work for my house to be wired in series?. My living room for example, all lights and fixtures, are on a single circuit, wired back to a single switch on the main breaker panel. That circuit is a single wire run first to one outlet, then to another, and so on terminating on one of those. The single breaker switch energizes that whole circuilt and I turn on those I want. I would think for the boat it should work to do it the same way.
macauley, that is a common misconception, but your whole house is wired in parallel. Hopefully this will clear things up

As I said, the hot wire from your panel goes to the positive terminal of ALL fixtures in the circuit, and the negative wire goes to the negative of all fixtures and you only have one positive and one negative going back to your panel or fuse.

This is the only diagram that I found and it shows just two "fixtures" on the circuit. If you were to put 5 in the same manner, you would still only have one positive and one negative going back to the panel (battery). That is the reason why a lot of people tend to think that the house is wired in series, rather than in parallel, but the truth is that all fixtures share the same positive and the same negative wire.

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Quote:
Originally Posted by sailingdog View Post
so I hope you didn't do any of the wiring if you can't tell the difference.
maccauley, take that recommendation seriously. Not trying to put you down or anything, but even though we are probably talking 12V, electricity is serious business and without the proper knowledge, you can get hurt, damage a lot of equipment, etc. Again, based on the conversation so far, hire an electrician....just my 2 cents.
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