Pairing circuit breaker to wire to load

[er9]

Originally Posted by patrickbarbary

Is it 12 or 24V? I would measure how much amps max and get a lower fuse. Also, ALWAYS tighten the wires to your fuses VERY tightly and check twice a year. I almost had a fire because of a loose wire in the fuse panel.

12volt

[er9]

Originally Posted by KP44

The smallest wire in the circuit needs to be protected by the breaker.

Even if your main wire is 14 ga, the fixture(s) wires are probably 18 ga and is rated to carry 5 amps before danger of melting or burning. So a 5 amp circuit breaker is correct.

Since you only expect 3 amps, this will not be a problem.

Do not use 15 amp unless 14 ga is the smallest wire in the circuit, including the interior circuits of the lights.

Or you could use a 15 amp breaker and a 3 amp in-line fuse at each light fixture.

Thanks for all the replies and info....much appreciated.

Iv'e been doing a lot more research past few days and this is the conclusion iv'e also arrived at, to protect the smallest wire in the circuit which coming from the fixture is pretty small...i'm guessing 18 GA.

Each fixture has a voltage regulator and will work happily within a fairly wide range of voltage.

The only issue is trying to replace the 15 amp circuit breaker on my 1980 DC panel could prove to be a challenge. If i decided to leave the breaker in could i just put a 5 amp fuse inline on the +wire after it leaves the panel to the lights to protect them at a lower amperage?

[er9]

Originally Posted by KP44

The smallest wire in the circuit needs to be protected by the breaker.

Even if your main wire is 14 ga, the fixture(s) wires are probably 18 ga and is rated to carry 5 amps before danger of melting or burning. So a 5 amp circuit breaker is correct.

Since you only expect 3 amps, this will not be a problem.

Do not use 15 amp unless 14 ga is the smallest wire in the circuit, including the interior circuits of the lights.

Or you could use a 15 amp breaker and a 3 amp in-line fuse at each light fixture.

Has to be at each light? wow i think i would rather replace the entire distribution panel than to remove all those ceiling panels again. I invented new forms of bodily contortion i prefer to never repeat again getting to some fixtures.

[Zil]

After reading here all about voltage drop, wire capacity, and do smaller fuses have more voltage drop? I guess the original lights did not glow.
I would have thought 18AWG marine grade would carry 15 amps with no trouble.

[belizesailor]

Originally Posted by Zil

After reading here all about voltage drop, wire capacity, and do smaller fuses have more voltage drop? I guess the original lights did not glow.
I would have thought 18AWG marine grade would carry 15 amps with no trouble.

Fuses/breakers dont normally create any significant voltage drop. The voltage drop is created by resistance in the wire.

Amps alone is not enough to determine correct wire gauge. You also need to know length of wire run, voltage, and target voltage drop.

Here is an online calculator:

https://www.calculator.net/voltage-d...s=15&x=78&y=28

[john61ct]

The Blue Sea Circuit Wizard app lets you take other relevant variables into account.

Every connection / gadget in the path adds to voltage drop.

Different owners and use cases affect what is "significant."

[hellosailor]

All fuses have some voltage drop, typically so small that for all practical purposes it is ignored.

"If i decided to leave the breaker in could i just put a 5 amp fuse inline on the +wire after it leaves the panel"
You could do it that way, but having fuses/breakers in series that way tends to come back to bite you, when you're trying to figure out why a circuit has gone dead and "the" fuse/breaker is good, and you've forgotten about the other one. Also, the very short length of the 18g section of wire usually means that it can carry a higher load than a longer run of the same gauge wire would, so it should not be a significant worry. But that's your call, either approach would work safely, IMO.

[belizesailor]

Originally Posted by hellosailor

All fuses have some voltage drop, typically so small that for all practical purposes it is ignored.

"If i decided to leave the breaker in could i just put a 5 amp fuse inline on the +wire after it leaves the panel"
You could do it that way, but having fuses/breakers in series that way tends to come back to bite you, when you're trying to figure out why a circuit has gone dead and "the" fuse/breaker is good, and you've forgotten about the other one. Also, the very short length of the 18g section of wire usually means that it can carry a higher load than a longer run of the same gauge wire would, so it should not be a significant worry. But that's your call, either approach would work safely, IMO.

Yeah, I really dislike inline fuses for this reason...and its one more spare to keep on hand.

[hellosailor]

Spares, yeah. I've learned to keep a 7-day pill dispenser, the kind like a strip from an ice cube tray, for those. One for glass, one for ATO/ATC, it makes it easier to keep them all together and sorted that way.

Headed out to a race early one morning on a nearly new boat and the boss asks me how's the fuel. I said the gauge shows empty, and he says impossible. And flipping the breaker does nothing to change that, so I pull a panel and start worming behind a bulkhead to access the back of the gauge, thinking maybe it hadn't been wired up tightly. What do I find? Oh, yeah, about 18" away from the gauge, firmly buried (but unattached, just floating in the air) behind the bulkhead, there's a ****ing glass inline fuse that had failed. Totally out of sight and hey, who the hell would bury a fuse that way?!

Or the double headlight fuse that bit me on an older GM product. One fuse in the car for the headlights, a second fuse under the hood for the "headlights and stuff" circuit. And the one in the car had blown because someone had dropped a spring the headlight switch and just left it in there, so every time it rolled out of the grease...bzzzt! and the fuse would blow for 'no' reason.

I can understand why serial fuses are SOMETIMES a rational idea, sometimes even a good one. But if there's no big red sign that says "HEY, THERE'S A MAN BEHIND THE CURTAIN, LOOK THIS WAY" then they certainly can baffle you.

[Zil]

Yes, fuses do have a voltage drop. Smaller amp fuses have more voltage drop than higher amp. That is another argument for using the larger fuse sized to the gauge of the wire. mainesail has page about this.

[er9]

Originally Posted by hellosailor

All fuses have some voltage drop, typically so small that for all practical purposes it is ignored.

"If i decided to leave the breaker in could i just put a 5 amp fuse inline on the +wire after it leaves the panel"
You could do it that way, but having fuses/breakers in series that way tends to come back to bite you, when you're trying to figure out why a circuit has gone dead and "the" fuse/breaker is good, and you've forgotten about the other one. Also, the very short length of the 18g section of wire usually means that it can carry a higher load than a longer run of the same gauge wire would, so it should not be a significant worry. But that's your call, either approach would work safely, IMO.

much thanks...i think im going to just leave the 15amp breaker in for the time being, no inline fuse. i will be installing a completely new distribution panel some time late next year so i think i will wait and make the switch to a smaller breaker then.

[GordMay]

Greetings and welcome aboard the CF, patrickbarbary.