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Old 01-12-2018, 13:43   #1
er9
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Pairing circuit breaker to wire to load

Now that iv'e completed my LED lighting upgrade i have a final question.

I have two seperate runs (port & starboard)...each about 75' round trip from DC panel and back. I figure max amperage on either circuit with all lights on at once is 3ish amps.

Each run has been wired with 14 gauge wire. If i understand correctly 14 gauge is safe up to 15 amps which is way more than i probably needed.

Each run is protected by a 15 amp circuit breaker (original breaker for incandescents).

Even though im using wire that is rated far higher than what the circuit would ever see can i leave the 15amp circuit breaker in place or should i downsize it to 10 amp for any safety reason im not aware of?
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Old 01-12-2018, 16:55   #2
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Re: Pairing circuit breaker to wire to load

Size CP to protect the wire.

You can of course go lower if reasons, but no need.
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Old 01-12-2018, 17:33   #3
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Re: Pairing circuit breaker to wire to load

the only reason would be because the light fixture wires you joined too are probably tiny. otherwise I wouldn't be worried about the 14awg. it will take a lot more then 15a. but it's often fused at 15a.

if you had some smaller breakers laying around I would change, but I woudln't go running out to buy some.
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Old 01-12-2018, 17:37   #4
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Re: Pairing circuit breaker to wire to load

I am going to disagree when it comes to interior LED lighting. I've had LED bulbs burn out and not blow the breaker. The smell sets off my alarm. I see no reason for a 10a breaker for a 3a load. The big wire is great for voltage drop but that doesn't mean you should have 10 or 15 amps running around for no reason.
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Old 02-12-2018, 10:42   #5
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Re: Pairing circuit breaker to wire to load

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Originally Posted by Ecos View Post
I am going to disagree when it comes to interior LED lighting. I've had LED bulbs burn out and not blow the breaker. The smell sets off my alarm. I see no reason for a 10a breaker for a 3a load. The big wire is great for voltage drop but that doesn't mean you should have 10 or 15 amps running around for no reason.
Sorry this does not make sense , led bulbs do not blow out , a single diode not working can make then go out.
and there is not 15 amps running through the cable the amps will be dictated by the load asked for , so an led wanting 1 amp will ask for 1 amp (give or take dont need the science)
Also a 15 amp breaker on 15 amp wire is absolutely fine especially at very low amps being asked for, the wire will take more than 15 amps (it is made this way to allow small variations , i,e a small power surge) and the breaker will blow at 15 amps.
there is no need no get worried this is the doom mongers telling stories,
It is physics in its simplest form .
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Old 02-12-2018, 11:07   #6
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Re: Pairing circuit breaker to wire to load

Another way to evaluate this is from a net usable amperage perspective.

Your 75 ft 14 AWG run will have a predicted 5-7% voltage drop @ 3A [assuming a nominal 12V DC source.]

Since the small LED light loads are distributed along the length of the wire run, it may be on the lower side of the calculated voltage drop. [i.e., closer to 5%, or 11.4V from a nominal 12V source; or 12.35V at a nominal 13 V DC source. This measured at the end of the run with all lights on.]

Therefore, unless your nominal voltage is 12.5 or better when you are running the lights totaling 3A, you have already saturated the supportable load [think design/acceptable voltage for devices on the circuit] for your 14 AWG run. Because of this, I would consider a smaller breaker more closely matching the realistic amperage/usable voltage de-rating of your 14 AWG lighting runs. [e.g., 5A breaker]

This would be a conservative approach [lower risk factor/ improved margin of safety] without consequence to your current set-up.

You are still safe and within spec with a 15A breaker. However that would allow more load [Amps] than the wire could support without undesirable voltage drop. [e.g., a 10A load would result in a 17-20% voltage drop resulting in 10.4V on the end of the circuit from a nominal 13V source...]

Something to consider [but not obsess over...] if you prefer conservative approches to electrical systems...

Either way, your lights will work just fine... [If you don't add too many more and burn them all at once...]

Cheers! Bill
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Old 02-12-2018, 11:31   #7
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Re: Pairing circuit breaker to wire to load

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.
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Old 02-12-2018, 12:26   #8
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Re: Pairing circuit breaker to wire to load

[QUOTE=tarian;2773318]Sorry this does not make sense , led bulbs do not blow out


What do you call it when an LED bulb smokes and stops working? I've had several do that.
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Old 02-12-2018, 12:40   #9
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Re: Pairing circuit breaker to wire to load

Ecos. As anyone familiar with old British sports cars with Lucas electrics would know, you have allowed the smoke to escape from your LEDs.

Perhaps the Chinese are using the same smoke in their LED's today as Lucas did in cars. Frankly, I've never understood how LEDs work. Magic smoke might explain it. Here's a kit to replace the smoke in an MG. You could try it on your LED's

Lucas Replacement Smoke Kit
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Old 02-12-2018, 13:36   #10
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Re: Pairing circuit breaker to wire to load

Along the lines of what mr.w. said, I would check to see what voltage you are getting in the bow when all of those lights are on, and your engine and shorepower are off. Rashly presuming that you have LED light fixtures, not raw LEDs, and that they are happy with the voltage you are getting, then the voltage drop caused by the 14g wire won't matter. In fact, it may help, because the driver circuitry in each fixture will have to do less work and will run cooler, good things.

And since the breakers are rated to protect the wires, they'll still do perfectly well. Personally, I prefer to go conservative and say "If there's never going to be more than a 4A load on this circuit, I want a 7.5A or 10A breaker" making the breaker "too small" for the wire size. This ensures that if the wire is overheating and any part of the circuit is drawing more than the usual power, the breaker will trip out FASTER. Now if the breaker was $50...I'd probably ignore it. If it was $15, I'd probably change it, because I can use the old breakers as spares (they do eventually wear out or fail) and again, a breaker that trips faster is a good thing. Few folks are aware of how long it takes a specified overload to cause a breaker to trip--it is nowhere near instantly.
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Old 02-12-2018, 17:29   #11
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Re: Pairing circuit breaker to wire to load

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.
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Old 02-12-2018, 21:46   #12
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Re: Pairing circuit breaker to wire to load

A 15 amp breaker with in-line fuses for each fixture leaves the circuit available to add fans, extra lights, or other fixtures.
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Old 03-12-2018, 08:06   #13
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Re: Pairing circuit breaker to wire to load

As long as the wire is protected near the source,

additional smaller CP can be placed near load devices to protect them.
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Old 03-12-2018, 11:26   #14
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Re: Pairing circuit breaker to wire to load

Fusing each light fixture would be rediculas.
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Old 03-12-2018, 11:51   #15
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Re: Pairing circuit breaker to wire to load

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Fusing each light fixture would be rediculas.
+1
VL = LED voltage (2V, or 4V for blue and white LEDs). The LED current must be less than the maximum permitted for your LED. For standard 5mm diameter LEDs the maximum current is usually 20mA, so 10mA or 15mA are suitable values for many circuits. So minimal power used

it would take a major failing in your wiring or installation to need to protect every light and appliance, this should really only needed on sensitive electronics that do not do well to surges.
If wired correctly and safely , with good connections with the main wire protected at source, i.e panel then you should be absolutely fine, there is nothing wrong with fusing everything to protect it but you have to way on cost, time, and effectiveness, no production boat manufacturer will fuse at the end to protect the (appliance) used to describe most boaty electricials
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