12V - 110 Why some Lights only work on one or the other

[Pparker]

I don’t understand why some of my interior lights only come on when I’m switched to 12V and some of the lights will only work when I switch to 110. I can’t have all my lights in the cabins working at the same time. Cuz the switch is only pointed one way at a time. Does anyone know why and how I can fix this. I just bought this boat and can’t figure this out. Thanks Pat

[Pete7]

With the lights switched off take a look at the bulbs to see if some are 12v and others 110v which might explain what is going on with the wiring.

Pete

[tkeithlu]

Hmmm. If that were the case, I'd expect the 110vac to burn out the 12vdc bulbs.

I think I would start by putting a multimeter on one circuit and the switching to see if indeed 12vdc and 110vac are running in the same wires. I doubt that they are, given the above. Then I'd trace one or the other, and I suggest the 12vdc for safety reasons, back to its source.

Does the switch have two inlets (commons), one 12vdc and one 110vac? Is it possible that it has 12vdc only on the common, and one switch position then sends power to an inverter, hence 110vac, while the other sends power to your 12vdc circuits?

OK, my mind is beginning to center. What is the source of power to the switch? It's likely only one source, and I'm betting on 12vdc from your batteries.

[Bill Seal]

I would guess you have 2 lighting circuits, 110vac and 12VDC. You can't run 12VDC lights on 120vac without smoke.
What's the source of the 110vac?

[S/V Illusion]

It seems obvious some lights are 12VDC and others are 120VAC. Our boat is similarly configured to facilitate lighting from either source. No need for both concurrently.

[AKA-None]

Originally Posted by S/V Illusion

It seems obvious some lights are 12VDC and others are 120VAC. Our boat is similarly configured to facilitate lighting from either source. No need for both concurrently.

But wouldn’t that be at least separate bulbs and I’m not familiar with any fixtures with 12 and 110 bulbs

But then I don’t get out that much

[smac999]

There is no way you have a switch that chooses between 12v or 120v…

Post pictures of switches and panels.

[CaptTom]

Originally Posted by Pparker

Cuz the switch is only pointed one way at a time.

I think that's the answer. Someone put in a selector switch so you can choose between using the 12V lights and the 120V lights. Presumably, you could remove the switch replace it with two switches, so either, both or none can be on.

Without photos, especially of the wiring, it's hard to guess.

[S/V Illusion]

Originally Posted by AKA-None

But wouldn’t that be at least separate bulbs and I’m not familiar with any fixtures with 12 and 110 bulbs

But then I don’t get out that much

He was asking about separate light fixtures.

[Scorpius]

It strikes me that there must be an interlock, either via a selector switch or a relay, that disables at least the 12v lighting circuits when 120v is available. Maybe somebody thought that 12v fixtures should not be used when the 120v could - for whatever reason. Seems weird.

[ranger58sb]

Originally Posted by AKA-None

But wouldn’t that be at least separate bulbs and I’m not familiar with any fixtures with 12 and 110 bulbs

We have both DC and AC interior lighting systems. The AC interior lighting circuits run transformers to low voltage DC. All of the original interior incandescent bulbs were 24VDC.

We replaced all of the original interior incandescent bulbs -- no matter whether on the DC or AC circuits -- with 10-30VDC LEDs.

The boat was obviously expected to be on shore power or generator power 24/7, but it's not obvious to me why the builder implemented lighting in this way.

-Chris

[billgewater]

I really think a visit from someone who knows the basics is needed. Even 110 VAC can kill - or ruin the day with a nice fire.